# data[].attributes.headline
Website: Guides to the Six Kingdoms of Life

# data[].attributes.subject
Website: Guides to the Six Kingdoms of Life

# data[].attributes.body
https://sccpss.itslearning.com/LearningToolElement/ViewLearningToolElement.aspx?LearningToolElementId=1315786

# data[].attributes.body
https://sciencetrek.org/sciencetrek/topics/kingdoms/facts.cfm

# data[].attributes.body
https://docs.google.com/presentation/d/1_vDEEmug9-d6rvRT1vLAMJ7YcPI5IMsrP4mDiistcUs/edit?usp=sharing

# data[].attributes.body
https://docs.google.com/document/d/1JTaazCHwVEFgMQNiC2TbulXDRw52cbQ_ygSZyJPSZeY/edit?usp=sharing

# data[].attributes.headline
Basic Taxonomy-6 Kingdoms of Life-Classification

# data[].attributes.subject
Basic Taxonomy-6 Kingdoms of Life-Classification

# data[].attributes.attachment_link.title
Basic Taxonomy-6 Kingdoms of Life-Classification

# data[].attributes.attachment_link.description
Explaining Charles Darwin’s finches and how the study of them on the Galapagos Islands and South American mainland led to the theory of evolution.

# data[].attributes.headline
Galapagos Finch Evolution - Speciation by Gradualism vs. Punctuated Equilibrium

# data[].attributes.subject
Galapagos Finch Evolution - Speciation by Gradualism vs. Punctuated Equilibrium

# data[].attributes.attachment_link.title
Galapagos Finch Evolution — HHMI BioInteractive Video

# data[].attributes.body
Lots of definitions and quick explanations

# data[].attributes.attachment_link.description
What is evolution and how does it work? Evolution 101 provides the nuts-and-bolts on the patterns and mechanisms of evolution. You can explore the following sections: An introduction to evolution Evolution briefly defined and explained The history of life: looking at the patterns How does evolution lead to the tree of life?

# data[].attributes.headline
Vestigial Structures: Proof of Evolution on Your Body

# data[].attributes.subject
Vestigial Structures: Proof of Evolution on Your Body

# data[].attributes.attachment_link.title
Proof of evolution that you can find on your body

# data[].attributes.headline
Homologous Structures vs Analogous Structures | Key Differences

# data[].attributes.subject
Homologous Structures vs Analogous Structures | Key Differences

# data[].attributes.attachment_link.title
Homologous Structures vs Analogous Structures | Key Differences

# data[].attributes.attachment_link.description
Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.

# data[].attributes.headline
Myths and Misconceptions about Evolution

# data[].attributes.subject
Myths and Misconceptions about Evolution

# data[].attributes.attachment_link.title
Myths and misconceptions about evolution - Alex Gendler

# data[].attributes.headline
Ecological Succession: Primary and Secondary

# data[].attributes.subject
Ecological Succession: Primary and Secondary

# data[].attributes.body
https://tinyurl.com/disturbances-succession

# data[].attributes.headline
Ecological Succession: Nature's Great Grit

# data[].attributes.subject
Ecological Succession: Nature's Great Grit

# data[].attributes.attachment_link.title
Ecological Succession: Nature's Great Grit

# data[].attributes.body
https://tinyurl.com/j-curves-and-s-curves

# data[].attributes.body
https://tinyurl.com/population-capacity

# data[].attributes.headline
Exponential and Logistic Growth in Populations

# data[].attributes.subject
Exponential and Logistic Growth in Populations

# data[].attributes.attachment_link.title
Exponential and logistic growth in populations  | Ecology | Khan Academy

# data[].attributes.body
https://tinyurl.com/ecological-limits

# data[].attributes.body
https://tinyurl.com/there-are-limit

# data[].attributes.body
https://tinyurl.com/indirect-effects

# data[].attributes.headline
Keystone Species: Predators, Ecosystem Engineers, and Mutualists

# data[].attributes.subject
Keystone Species: Predators, Ecosystem Engineers, and Mutualists

# data[].attributes.body
https://tinyurl.com/Key-species

# data[].attributes.headline
Notes: Tropic Cascades and Keystone Species

# data[].attributes.subject
Notes: Tropic Cascades and Keystone Species

# data[].attributes.body
https://tinyurl.com/Ecosystem-Engineering

# data[].attributes.headline
Vocabulary: Community and Population Ecology

# data[].attributes.subject
Vocabulary: Community and Population Ecology

# data[].attributes.body
https://tinyurl.com/community-population

# data[].attributes.body
https://tinyurl.com/symbiosis-basics

# data[].attributes.body
https://tinyurl.com/10-per-rule

# data[].attributes.headline
Flow of Energy and Matter Through Ecosystems

# data[].attributes.subject
Flow of Energy and Matter Through Ecosystems

# data[].attributes.attachment_link.title
Flow of energy and matter through ecosystems (video) | Khan Academy

# data[].attributes.attachment_link.description
See how energy flows and matter is recycled in ecosystems.

# data[].attributes.headline
Food Webs and Energy Pyramids: Bedrocks of Biodiversity

# data[].attributes.subject
Food Webs and Energy Pyramids: Bedrocks of Biodiversity

# data[].attributes.attachment_link.title
Food Webs and Energy Pyramids: Bedrocks of Biodiversity

# data[].attributes.body
https://tinyurl.com/habitat-niche

# data[].attributes.body
https://tinyurl.com/vocab-ecology

# data[].attributes.body
https://docs.google.com/document/d/1UAtby32_0W6u0f-T-1TiMAPza1JVa_y5p2pwlCNjrMU/edit?usp=sharing

# data[].attributes.body
https://docs.google.com/document/d/1NXc07odN7H5D_Qx0zSVzqrG78VpGG6l3Jwv0BFmgtxA/edit?usp=sharing

# data[].attributes.headline
Non-Mendelian Traits: Incomplete Dominance - KEY

# data[].attributes.subject
Non-Mendelian Traits: Incomplete Dominance - KEY

# data[].attributes.body
https://docs.google.com/document/d/1mPU00tGizZXZVZqwaj-EI_CetRVZwxyraXBsJpmjuUQ/edit?usp=sharing

# data[].attributes.body
https://docs.google.com/document/d/1LobYWwRRHEoCsSm1kG7fBUe7n0Wydu9eM-0CXeXn0dw/edit?usp=sharing

# data[].attributes.body
https://docs.google.com/document/d/11WFRFjEWBTDsSKY9sJ1Pz1Yqep523k2mN4KOiJ_FkX8/edit?usp=sharing

# data[].attributes.body
https://docs.google.com/document/d/17eMYSmXeqTXFKqv7OnEYXkpWYA_RenndK5kZCF1a0YI/edit?usp=sharing

# data[].attributes.body
https://tinyurl.com/body-layers

# data[].attributes.headline
All About Different Body Systems (see links on right)

# data[].attributes.subject
All About Different Body Systems (see links on right)

# data[].attributes.body
http://www.biology4kids.com/files/systems_main.html

# data[].attributes.body
https://tinyurl.com/body-systems-overview

# data[].attributes.headline
Biological Levels in Biology: The World Tour

# data[].attributes.subject
Biological Levels in Biology: The World Tour

# data[].attributes.attachment_link.title
Biological Levels in Biology: The World Tour

# data[].attributes.attachment_link.title
Levels of organisation an organism | Cells | Biology | FuseSchool

# data[].attributes.body
https://tinyurl.com/pedigree-review

# data[].attributes.body
https://tinyurl.com/understanding-pedigree

# data[].attributes.body
https://biologydictionary.net/codominance/

# data[].attributes.body
https://biologydictionary.net/incomplete-dominance/

# data[].attributes.headline
Co-Dominance and Incomplete Dominance (Khan Academy)

# data[].attributes.subject
Co-Dominance and Incomplete Dominance (Khan Academy)

# data[].attributes.attachment_link.title
Co-dominance and Incomplete Dominance | Biomolecules | MCAT | Khan Academy

# data[].attributes.headline
Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

# data[].attributes.subject
Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

# data[].attributes.attachment_link.title
Incomplete Dominance, Codominance, Polygenic Traits, and Epistasis!

# data[].attributes.body
https://tinyurl.com/inheritance-patterns

# data[].attributes.headline
Monohybrids and the Punnett Square Guinea Pigs

# data[].attributes.subject
Monohybrids and the Punnett Square Guinea Pigs

# data[].attributes.attachment_link.title
Monohybrids and the Punnett Square Guinea Pigs

# data[].attributes.headline
How Mendel's Pea Plants Helped us Understand Genetics

# data[].attributes.subject
How Mendel's Pea Plants Helped us Understand Genetics

# data[].attributes.attachment_link.title
How Mendel's pea plants helped us understand genetics - Hortensia Jiménez Díaz

# data[].attributes.body
https://tinyurl.com/probability-heredity

# data[].attributes.headline
DNA, Chromosomes, Genes, and Traits: An Intro to Heredity

# data[].attributes.subject
DNA, Chromosomes, Genes, and Traits: An Intro to Heredity

# data[].attributes.attachment_link.title
DNA, Chromosomes, Genes, and Traits: An Intro to Heredity

# data[].attributes.body
https://tinyurl.com/mendels-rules

# data[].attributes.body
https://tinyurl.com/cells-gone-wild

# data[].attributes.body
Mitosis is Also Explained in this Video

# data[].attributes.headline
Image Comparing Phases of Mitosis and Meiosis - Diploid (2n) vs. Haploid (n)

# data[].attributes.subject
Image Comparing Phases of Mitosis and Meiosis - Diploid (2n) vs. Haploid (n)

# data[].attributes.body
https://tinyurl.com/the-differnces

# data[].attributes.headline
Video: Mitosis vs. Meiosis: Side by Side Comparison

# data[].attributes.subject
Video: Mitosis vs. Meiosis: Side by Side Comparison

# data[].attributes.attachment_link.title
Mitosis vs. Meiosis: Side by Side Comparison

# data[].attributes.headline
Website: Difference Between Haploid And Diploid

# data[].attributes.subject
Website: Difference Between Haploid And Diploid

# data[].attributes.body
https://tinyurl.com/lets-count-chromosomes

# data[].attributes.headline
Video: Chromosome Numbers During Division: Demystified!

# data[].attributes.subject
Video: Chromosome Numbers During Division: Demystified!

# data[].attributes.attachment_link.title
(OLD VIDEO) Chromosome Numbers During Division: Demystified!

# data[].attributes.attachment_link.readable_text
The endosymbiotic theory is the accepted mechanism for how eukaryotic cells evolved from prokaryotic cells. It involves a cooperative relationship between two cells which allow both to survive-and eventually led to the development of all life on Earth.

Endosymbiotic Theory History

First proposed by Boston University biologist Lynn Margulis in the late 1960s, the Endosymbiont Theory proposed that the main organelles of the eukaryotic cell were actually primitive prokaryotic cells that had been engulfed by a different, bigger prokaryotic cell.

Margulis' theory was slow to gain acceptance, initially facing ridicule inside mainstream biology. Margulis and other scientists continued work on the subject, however, and now her theory is the accepted norm within biological circles.

During Margulis' research on the origin of eukaryotic cells, she studied data on prokaryotes, eukaryotes, and organelles, finally proposing that similarities between prokaryotes and organelles, combined with their appearance in the fossil record, was best explained by something called "endosymbiosis" (meaning "to cooperate inside.")

Whether the larger cell provided protection for the smaller cells, or the smaller cells provided energy to the larger cell, this arrangement seemed to be mutually beneficial to all of the prokaryotes.

While this sounded like a far-fetched idea at first, the data to back it up is undeniable. The organelles that seemed to have been their own cells include the mitochondria and, in photosynthetic cells, the chloroplast. Both of these organelles have their own DNA and their own ribosomes that do not match the rest of the cell. This indicates that they could survive and reproduce on their own.

In fact, the DNA in the chloroplast is very similar to photosynthetic bacteria called cyanobacteria. The DNA in the mitochondria is most like that of the bacteria that causes typhus.

Before these prokaryotes were able to undergo endosymbiosis, they first most likely had to become colonial organisms. Colonial organisms are groups of prokaryotic, single-celled organisms that live in close proximity to other single-celled prokaryotes.

Advantage to Colony

Even though the individual single-celled organisms remained separate and could survive independently, there was some sort of advantage to living close to other prokaryotes. Whether this was a function of protection or a way to get more energy, colonialism has to be beneficial in some manner for all of the prokaryotes involved in the colony.

Once these single-celled living things were within close enough proximity to one another, they took their symbiotic relationship one step further. The larger unicellular organism engulfed other, smaller, single-celled organisms. At that point, they were no longer independent colonial organisms but instead were one large cell.

When the larger cell that had engulfed the smaller cells started to divide, copies of the smaller prokaryotes inside were made and passed down to the daughter cells.

Eventually, the smaller prokaryotes that had been engulfed adapted and evolved into some of the organelles we know of today in eukaryotic cells such as the mitochondria and chloroplasts.

Other Organelles

Other organelles eventually arose from these first organelles, including the nucleus where the DNA in a eukaryote is housed, the endoplasmic reticulum and the Golgi apparatus.

In the modern eukaryotic cell, these parts are known as membrane-bound organelles. They still do not appear in prokaryotic cells like bacteria and archaea but are present in all organisms classified under the Eukarya domain.

# data[].attributes.attachment_link.description
The endosymbiotic theory is the accepted mechanism for how eukaryotic cells evolved from prokaryotic cells. It involves a cooperative relationship between two cells which allow both to survive-and eventually led to the development of all life on Earth.

# data[].attributes.body
Video (3:33)

# data[].attributes.headline
Prokaryotic vs. Eukaryotic Cells: What's the Difference?

# data[].attributes.subject
Prokaryotic vs. Eukaryotic Cells: What's the Difference?

# data[].attributes.attachment_link.title
Prokaryotic vs. Eukaryotic Cells: What's the Difference?

# data[].attributes.attachment_link.description
Discover the structural and functional difference between prokaryotic and eukaryotic cells

# data[].attributes.headline
Prokaryote vs. Eukaryote Graphic Organizer

# data[].attributes.subject
Prokaryote vs. Eukaryote Graphic Organizer

# data[].attributes.attachment_link.description
Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.

# data[].attributes.body
Video (7:42)

# data[].attributes.attachment_link.description
Explaining Charles Darwin’s finches and how the study of them on the Galapagos Islands and South American mainland led to the theory of evolution.

# data[].attributes.headline
Galapagos Finch Evolution - Speciation by Gradualism vs. Punctuated Equilibrium

# data[].attributes.subject
Galapagos Finch Evolution - Speciation by Gradualism vs. Punctuated Equilibrium

# data[].attributes.attachment_link.title
Galapagos Finch Evolution — HHMI BioInteractive Video

# data[].attributes.body
Lots of definitions and quick explanations

# data[].attributes.attachment_link.description
What is evolution and how does it work? Evolution 101 provides the nuts-and-bolts on the patterns and mechanisms of evolution. You can explore the following sections: An introduction to evolution Evolution briefly defined and explained The history of life: looking at the patterns How does evolution lead to the tree of life?

# data[].attributes.headline
Divergent vs Convergent Evolution - Homologous vs Analogous Structure with Examples

# data[].attributes.subject
Divergent vs Convergent Evolution - Homologous vs Analogous Structure with Examples

# data[].attributes.attachment_link.title
Homologous vs Analogous Structure, Divergent vs Convergent Evolution with examples

# data[].attributes.headline
Vestigial Structures: Proof of Evolution on Your Body

# data[].attributes.subject
Vestigial Structures: Proof of Evolution on Your Body

# data[].attributes.attachment_link.title
Proof of evolution that you can find on your body

# data[].attributes.headline
Homologous Structures vs Analogous Structures | Key Differences

# data[].attributes.subject
Homologous Structures vs Analogous Structures | Key Differences

# data[].attributes.attachment_link.title
Homologous Structures vs Analogous Structures | Key Differences

# data[].attributes.attachment_link.title
Genotype vs Phenotype | Understanding Alleles

# data[].attributes.attachment_link.title
Evidence for evolution (article) | Khan Academy

# data[].attributes.attachment_link.description
Learn for free about math, art, computer programming, economics, physics, chemistry, biology, medicine, finance, history, and more. Khan Academy is a nonprofit with the mission of providing a free, world-class education for anyone, anywhere.

# data[].attributes.attachment_link.readable_text
Explain the theory of evolution

All species of living organisms, from bacteria to baboons to blueberries, evolved at some point from a different species. Although it may seem that living things today stay much the same, that is not the case-evolution is an ongoing process.

The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask questions about the living world. Its power is that it provides direction for predictions about living things that are borne out in experiment after experiment. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that "nothing makes sense in biology except in the light of evolution." He meant that the tenet that all life has evolved and diversified from a common ancestor is the foundation from which we approach all questions in biology.

Learning Objectives

Define natural selection

Describe the basis of the present-day theory of evolution

Describe how the theory of evolution by natural selection is supported by evidence

Refute common misconceptions about evolution

Natural Selection

Darwin and Descent with Modification

Charles Darwin is best known for his discovery of natural selection. In the mid-nineteenth century, the actual mechanism for evolution was independently conceived of and described by two naturalists: Charles Darwin and Alfred Russel Wallace. Importantly, each naturalist spent time exploring the natural world on expeditions to the tropics. From 1831 to 1836, Darwin traveled around the world on H.M.S. Beagle, including stops in South America, Australia, and the southern tip of Africa. Wallace traveled to Brazil to collect insects in the Amazon rainforest from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. Darwin's journey, like Wallace's later journeys to the Malay Archipelago, included stops at several island chains, the last being the Galápagos Islands west of Ecuador. On these islands, Darwin observed species of organisms on different islands that were clearly similar, yet had distinct differences. For example, the ground finches inhabiting the Galápagos Islands comprised several species with a unique beak shape (Figure 1).

The species on the islands had a graded series of beak sizes and shapes with very small differences between the most similar. He observed that these finches closely resembled another finch species on the mainland of South America. Darwin imagined that the island species might be species modified from one of the original mainland species. Upon further study, he realized that the varied beaks of each finch helped the birds acquire a specific type of food. For example, seed-eating finches had stronger, thicker beaks for breaking seeds, and insect-eating finches had spear-like beaks for stabbing their prey.

Wallace and Darwin both observed similar patterns in other organisms and they independently developed the same explanation for how and why such changes could take place. Darwin called this mechanism natural selection. Natural selection, also known as "survival of the fittest," is the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits; this leads to evolutionary change.

For example, a population of giant tortoises found in the Galapagos Archipelago was observed by Darwin to have longer necks than those that lived on other islands with dry lowlands. These tortoises were "selected" because they could reach more leaves and access more food than those with short necks. In times of drought when fewer leaves would be available, those that could reach more leaves had a better chance to eat and survive than those that couldn't reach the food source. Consequently, long-necked tortoises would be more likely to be reproductively successful and pass the long-necked trait to their offspring. Over time, only long-necked tortoises would be present in the population.

Natural selection, Darwin argued, was an inevitable outcome of three principles that operated in nature. First, most characteristics of organisms are inherited, or passed from parent to offspring. Although no one, including Darwin and Wallace, knew how this happened at the time, it was a common understanding. Second, more offspring are produced than are able to survive, so resources for survival and reproduction are limited. The capacity for reproduction in all organisms outstrips the availability of resources to support their numbers. Thus, there is competition for those resources in each generation. Both Darwin and Wallace's understanding of this principle came from reading an essay by the economist Thomas Malthus who discussed this principle in relation to human populations. Third, offspring vary among each other in regard to their characteristics and those variations are inherited. Darwin and Wallace reasoned that offspring with inherited characteristics which allow them to best compete for limited resources will survive and have more offspring than those individuals with variations that are less able to compete. Because characteristics are inherited, these traits will be better represented in the next generation. This will lead to change in populations over generations in a process that Darwin called descent with modification. Ultimately, natural selection leads to greater adaptation of the population to its local environment; it is the only mechanism known for adaptive evolution.

Papers by Darwin and Wallace (Figure 2) presenting the idea of natural selection were read together in 1858 before the Linnean Society in London. The following year Darwin's book, On the Origin of Species, was published. His book outlined in considerable detail his arguments for gradual changes and adaptive survival by natural selection.

Demonstrations of evolution by natural selection are time consuming and difficult to obtain. One of the best examples has been demonstrated in the very birds that helped to inspire Darwin's theory: the Galápagos finches. Peter and Rosemary Grant and their colleagues have studied Galápagos finch populations every year since 1976 and have provided important demonstrations of natural selection. The Grants found changes from one generation to the next in the distribution of beak shapes with the medium ground finch on the Galápagos island of Daphne Major. The birds have inherited variation in the bill shape with some birds having wide deep bills and others having thinner bills. During a period in which rainfall was higher than normal because of an El Niño, the large hard seeds that large-billed birds ate were reduced in number; however, there was an abundance of the small soft seeds which the small-billed birds ate. Therefore, survival and reproduction were much better in the following years for the small-billed birds. In the years following this El Niño, the Grants measured beak sizes in the population and found that the average bill size was smaller. Since bill size is an inherited trait, parents with smaller bills had more offspring and the size of bills had evolved to be smaller. As conditions improved in 1987 and larger seeds became more available, the trend toward smaller average bill size ceased.

Theory of Evolution

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis; otherwise, the selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons such as an individual being taller because of better nutrition rather than different genes.

Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes caused by mutation can have one of three outcomes on the phenotype. A mutation affects the phenotype of the organism in a way that gives it reduced fitness-lower likelihood of survival or fewer offspring. A mutation may produce a phenotype with a beneficial effect on fitness. And, many mutations will also have no effect on the fitness of the phenotype; these are called neutral mutations. Mutations may also have a whole range of effect sizes on the fitness of the organism that expresses them in their phenotype, from a small effect to a great effect. Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each of the offspring.

A heritable trait that helps the survival and reproduction of an organism in its present environment is called an adaptation. Scientists describe groups of organisms becoming adapted to their environment when a change in the range of genetic variation occurs over time that increases or maintains the "fit" of the population to its environment. The webbed feet of platypuses are an adaptation for swimming. The snow leopards' thick fur is an adaptation for living in the cold. The cheetahs' fast speed is an adaptation for catching prey.

Whether or not a trait is favorable depends on the environmental conditions at the time. The same traits are not always selected because environmental conditions can change. For example, consider a species of plant that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed, and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators (Figure 3).

In other cases, similar phenotypes evolve independently in distantly related species. For example, flight has evolved in both bats and insects, and they both have structures we refer to as wings, which are adaptations to flight. However, the wings of bats and insects have evolved from very different original structures. This phenomenon is called convergent evolution, where similar traits evolve independently in species that do not share a recent common ancestry. The two species came to the same function, flying, but did so separately from each other.

These physical changes occur over enormous spans of time and help explain how evolution occurs. Natural selection acts on individual organisms, which in turn can shape an entire species. Although natural selection may work in a single generation on an individual, it can take thousands or even millions of years for the genotype of an entire species to evolve. It is over these large time spans that life on earth has changed and continues to change.

Our Modern Understanding

The mechanisms of inheritance, or genetics, were not understood at the time Charles Darwin and Alfred Russel Wallace were developing their idea of natural selection. This lack of understanding was a stumbling block to understanding many aspects of evolution. In fact, the predominant (and incorrect) genetic theory of the time, blending inheritance, made it difficult to understand how natural selection might operate. Darwin and Wallace were unaware of the genetics work done by Austrian monk Gregor Mendel, which was published in 1866, not long after publication of Darwin's book, On the Origin of Species.

Mendel's work was rediscovered in the early twentieth century at which time geneticists were rapidly coming to an understanding of the basics of inheritance. Initially, the newly discovered particulate nature of genes made it difficult for biologists to understand how gradual evolution could occur. But over the next few decades genetics and evolution were integrated in what became known as the modern synthesis-the coherent understanding of the relationship between natural selection and genetics that took shape by the 1940s and is generally accepted today.

In sum, the modern synthesis describes how evolutionary processes, such as natural selection, can affect a population's genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species. The theory also connects this change of a population over time, called microevolution, with the processes that gave rise to new species and higher taxonomic groups with widely divergent characters, called macroevolution.

Evidence of Evolution

The evidence for evolution is compelling and extensive. Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species, to identifying patterns in nature that were consistent with evolution, and since Darwin, our understanding has become clearer and broader.

Physical Evidence

Fossils

Fossils provide solid evidence that organisms from the past are not the same as those found today, and fossils show a progression of evolution. Scientists determine the age of fossils and categorize them from all over the world to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years (Figure 4a). For example, scientists have recovered highly detailed records showing the evolution of humans and horses (Figure 4b).

Anatomy and Embryology

Another type of evidence for evolution is the presence of structures in organisms that share the same basic form. For example, the bones in the appendages of a human, dog, bird, and whale all share the same overall construction (Figure 5) resulting from their origin in the appendages of a common ancestor. Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout. Scientists call these synonymous parts homologous structures.

Some structures exist in organisms that have no apparent function at all, and appear to be residual parts from a past common ancestor. These unused structures without function are called vestigial structures. Some examples of vestigial structures are wings on flightless birds, leaves on some cacti, and hind leg bones in whales.

Another evidence of evolution is the convergence of form in organisms that share similar environments. For example, species of unrelated animals, such as the arctic fox and ptarmigan, living in the arctic region have been selected for seasonal white phenotypes during winter to blend with the snow and ice (Figure 6). These similarities occur not because of common ancestry, but because of similar selection pressures-the benefits of not being seen by predators.

Embryology, the study of the development of the anatomy of an organism to its adult form, also provides evidence of relatedness between now widely divergent groups of organisms. Mutational tweaking in the embryo can have such magnified consequences in the adult that embryo formation tends to be conserved. As a result, structures that are absent in some groups often appear in their embryonic forms and disappear by the time the adult or juvenile form is reached. For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in their early development. These disappear in the adults of terrestrial groups but are maintained in adult forms of aquatic groups such as fish and some amphibians. Great ape embryos, including humans, have a tail structure during their development that is lost by the time of birth.

Biological Evidence

Biogeography

The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time. Broad groups that evolved before the breakup of the supercontinent Pangaea (about 200 million years ago) are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana. The presence of members of the plant family Proteaceae in Australia, southern Africa, and South America is best due to their appearance prior to the southern supercontinent Gondwana breaking up.

The great diversification of marsupials in Australia and the absence of other mammals reflect Australia's long isolation. Australia has an abundance of endemic species-species found nowhere else-which is typical of islands whose isolation by expanses of water prevents species migration. Over time, these species diverge evolutionarily into new species that look very different from their ancestors that may exist on the mainland. The marsupials of Australia, the finches on the Galápagos, and many species on the Hawaiian Islands are all unique to their one point of origin, yet they display distant relationships to ancestral species on mainlands.

Molecular Biology

Like anatomical structures, the structures of the molecules of life reflect descent with modification. Evidence of a common ancestor for all of life is reflected in the universality of DNA as the genetic material and in the near universality of the genetic code and the machinery of DNA replication and expression. Fundamental divisions in life between the three domains are reflected in major structural differences in otherwise conservative structures such as the components of ribosomes and the structures of membranes. In general, the relatedness of groups of organisms is reflected in the similarity of their DNA sequences-exactly the pattern that would be expected from descent and diversification from a common ancestor.

DNA sequences have also shed light on some of the mechanisms of evolution. For example, it is clear that the evolution of new functions for proteins commonly occurs after gene duplication events that allow the free modification of one copy by mutation, selection, or drift (changes in a population's gene pool resulting from chance), while the other copy continues to produce a functional protein.

Evolution-It's a Thing

This video defines evolution and discusses several varieties of evidence that support the Theory of Evolution:

Misconceptions of Evolution

Although the theory of evolution generated some controversy when it was first proposed, it was almost universally accepted by biologists, particularly younger biologists, within 20 years after publication of On the Origin of Species. Nevertheless, the theory of evolution is a difficult concept and misconceptions about how it works abound.

Evolution Is Just a Theory

Critics of the theory of evolution dismiss its importance by purposefully confounding the everyday usage of the word "theory" with the way scientists use the word. In science, a "theory" is understood to be a body of thoroughly tested and verified explanations for a set of observations of the natural world. Scientists have a theory of the atom, a theory of gravity, and the theory of relativity, each of which describes understood facts about the world. In the same way, the theory of evolution describes facts about the living world. As such, a theory in science has survived significant efforts to discredit it by scientists. In contrast, a "theory" in common vernacular is a word meaning a guess or suggested explanation; this meaning is more akin to the scientific concept of "hypothesis." When critics of evolution say evolution is "just a theory," they are implying that there is little evidence supporting it and that it is still in the process of being rigorously tested. This is a mischaracterization.

Individuals Evolve

Evolution is the change in genetic composition of a population over time, specifically over generations, resulting from differential reproduction of individuals with certain alleles. Individuals do change over their lifetime, obviously, but this is called development and involves changes programmed by the set of genes the individual acquired at birth in coordination with the individual's environment. When thinking about the evolution of a characteristic, it is probably best to think about the change of the average value of the characteristic in the population over time. For example, when natural selection leads to bill-size change in medium-ground finches in the Galápagos, this does not mean that individual bills on the finches are changing. If one measures the average bill size among all individuals in the population at one time and then measures the average bill size in the population several years later, this average value will be different as a result of evolution. Although some individuals may survive from the first time to the second, they will still have the same bill size; however, there will be many new individuals that contribute to the shift in average bill size.

Organisms Evolve on Purpose

Statements such as "organisms evolve in response to a change in an environment" are quite common, but such statements can lead to two types of misunderstandings. First, the statement must not be understood to mean that individual organisms evolve. The statement is shorthand for "a population evolves in response to a changing environment." However, a second misunderstanding may arise by interpreting the statement to mean that the evolution is somehow intentional. A changed environment results in some individuals in the population, those with particular phenotypes, benefiting and therefore producing proportionately more offspring than other phenotypes. This results in change in the population if the characteristics are genetically determined.

It is also important to understand that the variation that natural selection works on is already in a population and does not arise in response to an environmental change. For example, applying antibiotics to a population of bacteria will, over time, select a population of bacteria that are resistant to antibiotics. The resistance, which is caused by a gene, did not arise by mutation because of the application of the antibiotic. The gene for resistance was already present in the gene pool of the bacteria, likely at a low frequency. The antibiotic, which kills the bacterial cells without the resistance gene, strongly selects individuals that are resistant, since these would be the only ones that survived and divided. Experiments have demonstrated that mutations for antibiotic resistance do not arise as a result of antibiotic.

In a larger sense, evolution is not goal directed. Species do not become "better" over time; they simply track their changing environment with adaptations that maximize their reproduction in a particular environment at a particular time. Evolution has no goal of making faster, bigger, more complex, or even smarter species, despite the commonness of this kind of language in popular discourse. What characteristics evolve in a species are a function of the variation present and the environment, both of which are constantly changing in a non-directional way. What trait is fit in one environment at one time may well be fatal at some point in the future. This holds equally well for a species of insect as it does the human species.

Evolution Explains the Origin of Life

It is a common misunderstanding that evolution includes an explanation of life's origins. Conversely, some of the theory's critics believe that it cannot explain the origin of life. The theory does not try to explain the origin of life. The theory of evolution explains how populations change over time and how life diversifies the origin of species. It does not shed light on the beginnings of life including the origins of the first cells, which is how life is defined. The mechanisms of the origin of life on Earth are a particularly difficult problem because it occurred a very long time ago, and presumably it just occurred once. Importantly, biologists believe that the presence of life on Earth precludes the possibility that the events that led to life on Earth can be repeated because the intermediate stages would immediately become food for existing living things.

However, once a mechanism of inheritance was in place in the form of a molecule like DNA either within a cell or pre-cell, these entities would be subject to the principle of natural selection. More effective reproducers would increase in frequency at the expense of inefficient reproducers. So while evolution does not explain the origin of life, it may have something to say about some of the processes operating once pre-living entities acquired certain properties.

In Summary: Misconceptions of Evolution

Many misconceptions exist about the theory of evolution-including some perpetuated by critics of the theory. First, evolution as a scientific theory means that it has years of observation and accumulated data supporting it. It is not "just a theory" as a person may say in common vernacular.

Another misconception is that individuals evolve, though in fact it is populations that evolve over time. Individuals simply carry mutations. Furthermore, these mutations neither arise on purpose nor do they arise in response to an environmental pressure. Instead, mutations in DNA happen spontaneously and are already present in individuals of a population when a selective pressure occurs. Once the environment begins to favor a particular trait, then those individuals already carrying that mutation will have a selective advantage and are likely to survive better and outproduce others without the adaptation.

Finally, the theory of evolution does not in fact address the origins of life on this planet. Scientists believe that we cannot, in fact, repeat the circumstances that led to life on Earth because at this time life already exists. The presence of life has so dramatically changed the environment that the origins cannot be totally produced for study.

Check Your Understanding

Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.

Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.

# data[].attributes.attachment_link.description
All species of living organisms, from bacteria to baboons to blueberries, evolved at some point from a different species. Although it may seem that living things today stay much the same, that is not the case-evolution is an ongoing process.

# data[].attributes.attachment_link.description
Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.

# data[].attributes.attachment_link.description
"5 Processes That Make Evolution RANDOM" is published by Ernest Wolfe in countdown.education.

# data[].attributes.headline
Genetic Drift - Founder Effect and Bottleneck Effect

# data[].attributes.subject
Genetic Drift - Founder Effect and Bottleneck Effect

# data[].attributes.attachment_link.title
Genetic Drift | Founder Effect and Bottleneck Effect Explained

# data[].attributes.headline
The Evolution of Populations: Natural Selection, Genetic Drift, and Gene Flow

# data[].attributes.subject
The Evolution of Populations: Natural Selection, Genetic Drift, and Gene Flow

# data[].attributes.attachment_link.title
The Evolution of Populations: Natural Selection, Genetic Drift, and Gene Flow

# data[].attributes.attachment_link.readable_text
The Hardy-Weinberg equilibrium principle says that allele frequencies in a population will remain constant in the absence of the four factors that could change them. Those factors are natural selection, mutation, genetic drift, and migration (gene flow). In fact, we know they are probably always affecting populations.

Natural Selection

Natural selection has already been discussed. Alleles are expressed in a phenotype. Depending on the environmental conditions, the phenotype confers an advantage or disadvantage to the individual with the phenotype relative to the other phenotypes in the population. If it is an advantage, then that individual will likely have more offspring than individuals with the other phenotypes, and this will mean that the allele behind the phenotype will have greater representation in the next generation. If conditions remain the same, those offspring, which are carrying the same allele, will also benefit. Over time, the allele will increase in frequency in the population.

Mutation

Mutation is a source of new alleles in a population. Mutation is a change in the DNA sequence of the gene. A mutation can change one allele into another, but the net effect is a change in frequency. The change in frequency resulting from mutation is small, so its effect on evolution is small unless it interacts with one of the other factors, such as selection. A mutation may produce an allele that is selected against, selected for, or selectively neutral. Harmful mutations are removed from the population by selection and will generally only be found in very low frequencies equal to the mutation rate. Beneficial mutations will spread through the population through selection, although that initial spread is slow. Whether or not a mutation is beneficial or harmful is determined by whether it helps an organism survive to sexual maturity and reproduce. It should be noted that mutation is the ultimate source of genetic variation in all populations-new alleles, and, therefore, new genetic variations arise through mutation.

Genetic Drift

Another way a population's allele frequencies can change is genetic drift (Figure 11.2.1), which is simply the effect of chance. Genetic drift is most important in small populations. Drift would be completely absent in a population with infinite individuals, but, of course, no population is this large. Genetic drift occurs because the alleles in an offspring generation are a random sample of the alleles in the parent generation. Alleles may or may not make it into the next generation due to chance events including mortality of an individual, events affecting finding a mate, and even the events affecting which gametes end up in fertilizations. If one individual in a population of ten individuals happens to die before it leaves any offspring to the next generation, all of its genes-a tenth of the population's gene pool-will be suddenly lost. In a population of 100, that 1 individual represents only 1 percent of the overall gene pool; therefore, it has much less impact on the population's genetic structure and is unlikely to remove all copies of even a relatively rare allele.

Imagine a population of ten individuals, half with allele A and half with allele a (the individuals are haploid). In a stable population, the next generation will also have ten individuals. Choose that generation randomly by flipping a coin ten times and let heads be A and tails be a. It is unlikely that the next generation will have exactly half of each allele. There might be six of one and four of the other, or some different set of frequencies. Thus, the allele frequencies have changed and evolution has occurred. A coin will no longer work to choose the next generation (because the odds are no longer one half for each allele). The frequency in each generation will drift up and down on what is known as a random walk until at one point either all A or all a are chosen and that allele is fixed from that point on. This could take a very long time for a large population. This simplification is not very biological, but it can be shown that real populations behave this way. The effect of drift on frequencies is greater the smaller a population is. Its effect is also greater on an allele with a frequency far from one half. Drift will influence every allele, even those that are being naturally selected.

ART CONNECTION

Figure 11.2.1: Genetic drift in a population can lead to the elimination of an allele from a population by chance. In each generation, a random set of individuals reproduces to produce the next generation. The frequency of alleles in the next generation is equal to the frequency of alleles among the individuals reproducing.

Do you think genetic drift would happen more quickly on an island or on the mainland?

Genetic drift can also be magnified by natural or human-caused events, such as a disaster that randomly kills a large portion of the population, which is known as the bottleneck effectthat results in a large portion of the genome suddenly being wiped out (Figure 11.2.2). In one fell swoop, the genetic structure of the survivors becomes the genetic structure of the entire population, which may be very different from the pre-disaster population. The disaster must be one that kills for reasons unrelated to the organism's traits, such as a hurricane or lava flow. A mass killing caused by unusually cold temperatures at night, is likely to affect individuals differently depending on the alleles they possess that confer cold hardiness.

Figure 11.2.2: A chance event or catastrophe can reduce the genetic variability within a population.

Another scenario in which populations might experience a strong influence of genetic drift is if some portion of the population leaves to start a new population in a new location, or if a population gets divided by a physical barrier of some kind. In this situation, those individuals are unlikely to be representative of the entire population which results in the founder effect. The founder effect occurs when the genetic structure matches that of the new population's founding fathers and mothers. The founder effect is believed to have been a key factor in the genetic history of the Afrikaner population of Dutch settlers in South Africa, as evidenced by mutations that are common in Afrikaners but rare in most other populations. This is likely due to a higher-than-normal proportion of the founding colonists, which were a small sample of the original population, carried these mutations. As a result, the population expresses unusually high incidences of Huntington's disease (HD) and Fanconi anemia (FA), a genetic disorder known to cause bone marrow and congenital abnormalities, and even cancer.¹

CONCEPT IN ACTION

Visit this site to learn more about genetic drift and to run simulations of allele changes caused by drift.

Gene Flow

Another important evolutionary force is gene flow, or the flow of alleles in and out of a population resulting from the migration of individuals or gametes (Figure 11.2.3). While some populations are fairly stable, others experience more flux. Many plants, for example, send their seeds far and wide, by wind or in the guts of animals; these seeds may introduce alleles common in the source population to a new population in which they are rare.

Figure 11.2.3: Gene flow can occur when an individual travels from one geographic location to another and joins a different population of the species. In the example shown here, the brown allele is introduced into the green population.

Summary

There are four factors that can change the allele frequencies of a population. Natural selection works by selecting for alleles that confer beneficial traits or behaviors, while selecting against those for deleterious qualities. Mutations introduce new alleles into a population. Genetic drift stems from the chance occurrence that some individuals have more offspring than others and results in changes in allele frequencies that are random in direction. When individuals leave or join the population, allele frequencies can change as a result of gene flow.

Art Connections

Figure 11.2.1 Do you think genetic drift would happen more quickly on an island or on the mainland?

Genetic drift is likely to occur more rapidly on an island, where smaller populations are expected to occur.

Multiple Choice

Galápagos medium ground finches are found on Santa Cruz and San Cristóbal islands, which are separated by about 100 km of ocean. Occasionally, individuals from either island fly to the other island to stay. This can alter the allele frequencies of the population through which of the following mechanisms?

A. natural selection
B. genetic drift
C. gene flow
D. mutation

C

In which of the following pairs do both evolutionary processes introduce new genetic variation into a population?

A. natural selection and genetic drift
B. mutation and gene flow
C. natural selection and gene flow
D. gene flow and genetic drift

B

Free Response

Describe natural selection and give an example of natural selection at work in a population.

The theory of natural selection stems from the observation that some individuals in a population survive longer and have more offspring than others, thus passing on more of their genes to the next generation. For example, a big, powerful male gorilla is much more likely than a smaller, weaker gorilla to become the population's silverback, the pack's leader who mates far more than the other males of the group. The pack leader will, therefore, father more offspring, who share half of his genes, and are thus likely to also grow bigger and stronger like their father. Over time, the genes for bigger size will increase in frequency in the population, and the population will, as a result, grow larger on average.

Footnotes

1 A. J. Tipping et al., "Molecular and Genealogical Evidence for a Founder Effect in Fanconi Anemia Families of the Afrikaner Population of South Africa," PNAS 98, no. 10 (2001): 5734-5739, doi: 10.1073/pnas.091402398.

Glossary

bottleneck effect

the magnification of genetic drift as a result of natural events or catastrophes

founder effect

a magnification of genetic drift in a small population that migrates away from a large parent population carrying with it an unrepresentative set of alleles

Contributors and Attributions

gene flow

the flow of alleles in and out of a population due to the migration of individuals or gametes

Template:ContribOpenStaxConcepts

# data[].attributes.attachment_link.description
The Hardy-Weinberg equilibrium principle says that allele frequencies in a population will remain constant in the absence of the four factors that could change them. Those factors are natural selection, mutation, genetic drift, and migration (gene flow). In fact, we know they are probably always affecting populations. Natural selection has already been discussed.

# data[].attributes.attachment_link.readable_text
In asexual reproduction, one individual produces offspring that are genetically identical to itself. Reproduction is a marvelous culmination of individual transcendence in that organisms "transcend" time through the reproduction of offspring. In animal organisms, reproduction can occur by two primary processes: asexual reproduction and sexual reproduction.

Organisms produced by asexual reproduction are the product of mitosis. In this process, a single parent replicates body cells and divides into two individuals. Many invertebrates, including sea stars and sea anemones, reproduce in this manner. Common forms of asexual reproduction include: budding, gemmules, fragmentation, regeneration, binary fission, and parthenogenesis.

Budding: Hydras

Hydras exhibit a form of asexual reproduction called budding. In this form of asexual reproduction, an offspring grows out of the body of the parent, then breaks off into a new individual. In most instances, the budding is restricted to certain specialized areas. In some other limited cases, buds may come from any number of places on the body of the parent. The offspring typically remain attached to the parent until it is mature.

Gemmules (Internal Buds): Sponges

Sponges exhibit a form of asexual reproduction that relies on the production of gemmules or internal buds. In this form of asexual reproduction, a parent releases a specialized mass of cells that can develop into offspring. These gemmules are hardy and can be formed when the parent experiences harsh environmental conditions. The gemmules are less likely to become dehydrated and in some cases may be able to survive with a limited oxygen supply.

Fragmentation: Planarians

Planarians exhibit a form of asexual reproduction known as fragmentation. In this type of reproduction, the body of the parent breaks into distinct pieces, each of which can produce an offspring. The detachment of the parts is intentional, and if thy are large enough, the detached parts will develop into new individuals.

Regeneration: Echinoderms

Echinoderms exhibit a form of asexual reproduction known as regeneration. In this form of asexual reproduction, a new individual develops from a part of another. This typically happens when a part, like an arm, becomes detached from the parent's body. The separated piece can grow and develop into a completely new individual. Regeneration can be thought of as a modified form of fragmentation.

Parthenogenesis

Parthenogenesis involves the development of an egg that has not been fertilized into an individual. Most organisms that reproduce through this method can also reproduce sexually. Animals like water fleas reproduce by parthenogenesis. Most kinds of wasps, bees, and ants (which have no sex chromosomes) also reproduce by parthenogenesis. Additionally, some reptiles and fish are capable of reproducing in this manner.

Advantages and Disadvantages of Asexual Reproduction

Asexual reproduction can be very advantageous to certain higher animals and protists. Organisms that remain in one particular place and are unable to look for mates would need to reproduce asexually. Another advantage of asexual reproduction is that numerous offspring can be produced without "costing" the parent a great amount of energy or time. Environments that are stable and experience very little change are the best places for organisms that reproduce asexually.

One major disadvantage of this type of reproduction is the lack of genetic variation. All of the organisms are genetically identical and therefore share the same weaknesses. A gene mutation can persist in the population as it is continuously repeated in the identical offspring. Since organisms produced asexually grow best in a stable environment, negative changes in the environment can have deadly consequences for all individuals. Due to the high numbers of offspring that can be produced in a relatively short period of time, population explosions often occur in favorable environments. This extreme growth may lead to rapid depletion of resources and an exponential death rate in the population.

Asexual Reproduction in Other Organisms

Animals and protists are not the only organisms that reproduce asexually. Yeast, fungi, plants, and bacteria are capable of asexual reproduction as well. Yeast reproduce most commonly by budding. Fungi and plants reproduce asexually through spores. Plants can also reproduce by the asexual process of vegetative propagation. Bacterial asexual reproduction most commonly occurs by binary fission. Since the bacterial cells produced through this type of reproduction are identical, they are all susceptible to the same types of antibiotics.

# data[].attributes.attachment_link.description
Asexual reproduction involves producing progeny that are genetic clones of the parent. This can be done by regeneration, budding, and binary fission.

# data[].attributes.attachment_link.readable_text
Vegetative propagation or vegetative reproduction is the growth and development of a plant by asexual means. This development occurs through the fragmentation and regeneration of specialized vegetative plant parts. Many plants that reproduce asexually are also capable of sexual propagation.

The Process of Vegetative Propagation

Vegetative reproduction involves vegetativeor non-sexual plant structures, whereas sexual propagation is accomplished through gamete production and subsequent fertilization. In non-vascular plants such as mosses and liverworts, vegetative reproductive structures include gemmae and spores. In vascular plants, vegetative reproductive structures include roots, stems, and leaves.

Vegetative propagation is made possible by meristem tissue, commonly found within stems and leaves as well as the tips of roots, that contains undifferentiated cells. These cells actively divide by mitosis to allow widespread and rapid primary plant growth. Specialized, permanent plant tissue systems also originate from meristem tissue. It is the ability of meristem tissue to continually divide that allows for plant regeneration required by vegetative propagation.

Advantages and Disadvantages

Because vegetative propagation is a form of asexual reproduction, plants produced through this system are genetic clones of a parent plant. This uniformity has advantages and disadvantages.

One advantage of vegetative propagation is that plants with favorable traits are repeatedly reproduced. Commercial crop growers can employ artificial vegetative propagation techniques to ensure advantageous qualities in their crops.

A major disadvantage, however, of vegetative propagation is that it does not allow for any degree of genetic variation. Plants that are genetically identical are all susceptible to the same viruses and diseases and crops produced through this method are, therefore, easily wiped out.

Types of Vegetative Propagation

Vegetative propagation may be accomplished by artificial or natural means. Though both methods involve the development of a plant from parts of a single mature part, the way that each is carried out looks very different.

Artificial Vegetative Propagation

Artificial vegetative propagation is a type of plant reproduction that involves human intervention. The most common types of artificial vegetative reproductive techniques include cutting, layering, grafting, suckering, and tissue culturing. These methods are employed by many farmers and horticulturists to produce healthier crops with more desirable qualities.

Cutting: A part of a plant, typically a stem or leaf, is cut off and planted. Adventitious roots develop from the cuttings and a new plant forms. Cuttings are sometimes treated with hormones before being planted to induce root development.

Grafting: In grafting, a desired cutting or scion is attached to the stem of another plant that remains rooted in the ground. The tissue systems of the cutting become grafted into or integrated with the tissue systems of the base plant over time.

Layering: This method involves bending plant branches or stems so that they touch the ground. The portions of branches or stems in contact with the ground are then covered with soil. Adventitious roots or roots that extend from structures other than plant roots develop in the parts covered by soil and the attached shoot (branch or stem) with new roots is known as a layer. This type of layering also occurs naturally. In another technique called air layering, branches are scraped and covered with plastic to reduce moisture loss. New roots develop where the branches were scraped and the branches are removed from the tree and planted.

Suckering: Suckers attach to a parent plant and form a dense, compact mat. Since too many suckers can lead to smaller crop size, excess numbers are pruned. Mature suckers are cut away from a parent plant and transplanted to a new area where they sprout new plants. Suckering has the dual purpose of growing new shoots and removing nutrient-sucking buds that prohibit a main plant from growing.

Tissue Culture: This technique involves the culturing of plant cells that may be taken from different parts of a parent plant. The tissue is placed in a sterilized container and nurtured in a special medium until a mass of cells known as a callus is formed. The callus is then cultured in a hormone-laden medium and eventually develops into plantlets. When planted, these mature into fully grown plants.

Natural Vegetative Propagation

Natural vegetative propagation happens when plants grow and develop naturally without human intervention. An important ability that is key to enabling natural vegetative propagation in plants is the ability to developadventitious roots.

Through the formation of adventitious roots, new plants may sprout from stems, roots, or leaves of a parent plant. Modified stems are most often the source of vegetative plant propagation. Vegetative plant structures that arise from plant stems include rhizomes, runners, bulbs, tubers, and corms. Tubers can also stretch from roots. Plantlets emerge from plant leaves.

Plant Structures That Enable Natural Vegetative Propagation

# data[].attributes.attachment_link.title
Natural and Artificial Methods of Propagating Plants

# data[].attributes.attachment_link.description
Vegetative propagation or vegetative reproduction is the growth and development of a plant by asexual means. This development occurs through the fragmentation and regeneration of specialized vegetative plant parts. Many plants that reproduce asexually are also capable of sexual propagation. Vegetative reproduction involves vegetativeor non-sexual plant structures, whereas sexual propagation is accomplished through gamete production and subsequent fertilization.

# data[].attributes.attachment_link.description
Create a new document and edit with others at the same time -- from your computer, phone or tablet. Get stuff done with or without an internet connection. Use Docs to edit Word files. Free from Google.

# data[].attributes.headline
Notes on Asexual and Sexual Reproduction

# data[].attributes.subject
Notes on Asexual and Sexual Reproduction

# data[].attributes.attachment_link.description
Get access to files anywhere through secure cloud storage and file backup for your photos, videos, files and more with Google Drive.

# data[].attributes.headline
Understanding and Building Phylogenetic Trees

# data[].attributes.subject
Understanding and Building Phylogenetic Trees

# data[].attributes.attachment_link.readable_text
(light music) - [Instructor] We're looking at the main types of biodiversity, genetic, ecological, and evolutionary. Here we want to really focus on the latter, talk about evolutionary lineages. Something that I live and breathe in my own scientific research. The study of evolutionary lineages is nothing less than analyzing biodiversity over time. We always want to know where did biodiversity come from, where is it going and what is the role of humans in the future of these patterns. Lineages allow us to look at the evolutionary history of different species, so the study of evolutionary lineages is also the study of the tree of life. Scientists use that tree metaphor to depict actual relationships among organismal groups in a branching diagram, but how do you make that diagram, what do you need to know that will allow us to assemble that tree of life? What we're talking about is a science known as phylogenetic systematics. Phylogenetic patterns are made up of characters, features you can observe in organisms. Sounds pretty simple but what it really means is that a unique feature of an organism represents a unique event in the evolutionary history of that organism an event that marks the first appearance of that feature. And if you look at several organisms and list the unique features of those organisms, you are actually looking at the unique events in their histories that tell you something about their relationships. This is because these events can be shared with other organisms, if they are shared, their histories are shared. In other words the species are related. Think about sea urchins for example, who wouldn't want to think about sea urchins, they're pretty neat animals, they have long spines, nice rounded bodies supporting those spines, but did you know that sand dollars are basically flat sea urchins that have adapted to life on the beach, which I think is pretty nice work if you can get it. So if sand dollars are sea urchins, can we identify some evolutionary novelty that joins all the sand dollars together to the exclusion of all other types of sea urchins? Can we put them together in a single lineage? Here's our sea urchin and here is our sand dollar, but here's another type of sand dollar, the obvious feature that links these two guys is that they are as we said, really flat but no other sea urchins are flat like this, that's a character that uniquely connects all of the sand dollars together to the exclusion of all other sea urchins. A feature that's arisen only once in the evolutionary history of the sea urchins that led to the sand dollars. The suggestion is that these two guys share common ancestry, a common history because right at this point they evolved as characteristic of being flat. With the sand dollar group, all of which we now know share a common ancestry, you can have further elaborations on this flat form. For example, some will have weird holes through their bodies that also represent unique evolutionary events within the sand dollar grouping. And this is another critical thing to recognize about the tree of life. Every group is nested or included within another group, nature is a hierarchy that can be represented by these branching diagrams, diagrams known as cladograms or phylogenetic trees. But things can be really complicated, we know that there is some 250 living species of sand dollars and over 750 extinct fossil species. We also know there's a single phylogenetic tree for sand dollars showing how they're all related one to another. But we don't know the precise shape the branching order also known as the topology of that tree. Each sub-grouping can be supported by a unique evolutionary novelty or even several if you have lots of data. The aim is to best arrange all the unique characters to resolve or support all of the relationships. Ultimately the idea is to make a branch point for every single one of the relationships so we can document the unique evolutionary events or characters among all the species being studied. One thing that emerges from all of this is the simple fact that some species have appeared on earth more recently than others, you can read this from the topology of the tree when you realize that there's a time axis along here, oldest to most recent. The important thing is the relative branching order, the topology of the tree. This branch point occurred before that one and that branch point occurred before both of those. But let's face it, with ten million species on earth, the tree of life can get pretty complicated which is why it takes a lot of data and a lot of studies to place as accurately as possible each group of organisms on that big tree. So for phylogenetic systematics, you need to use a computer, taking lots and lots of character data, coding character traits for every species in your analysis and feeding that into a computer program. There are mathematical processes to produce trees that can be tested and tested again with new characters with the aim of arriving at the most supportable hypothesis for the topology of the tree. Sometimes these new characters can lead to changes in the branching order of the tree. And there's something relatively new on the scene to help phylogeneticists test these hypotheses of relationships and that of course is DNA. Evolutionary pathways and novel features are recorded just as much in DNA as they might be in the physical traits of an organism that you can see with your eyes. Phylogeneticists rely more and more on the analysis of large amounts of molecular data to develop trees. With the grand view of these trees in hand, you get an even more powerful way of looking at how evolution happens you can actually read life's history book which I think is one of the most exciting things you can possibly do in this field of study. For example, in the case of the sand dollars, I can explore why they got flat or why they have these bizarre holes in them, using the tree to tell a story of evolution, it's what I live for in my science. One of the first ever cladograms appeared in the work of none other than Charles Darwin, who recognized the importance of evolutionary trees. I always think of old Chuck when I think of this grand picture of life, this grand picture of biodiversity and how trees show that. Darwin knew that these trees reveal lineages and big picture biodiversity against a background of enormous time. What he didn't know was just how powerful our modern techniques could become in telling a spectacular story and how useful these trees are in helping us protect earth's amazing but threatened biodiversity. (light music)

# data[].attributes.attachment_link.title
Discovering the tree of life (video) | Khan Academy

# data[].attributes.attachment_link.description
(light music) - [Instructor] We're looking at the main types of biodiversity, genetic, ecological, and evolutionary. Here we want to really focus on the latter, talk about evolutionary lineages. Something that I live and breathe in my own scientific research. The study of evolutionary lineages is nothing less than analyzing biodiversity over time.

# data[].attributes.attachment_link.readable_text
Understanding phylogenies

Understanding a phylogeny is a lot like reading a family tree. The root of the tree represents the ancestral lineage, and the tips of the branches represent the descendants of that ancestor. As you move from the root to the tips, you are moving forward in time.

When a lineage splits (speciation), it is represented as branching on a phylogeny. When a speciation event occurs, a single ancestral lineage gives rise to two or more daughter lineages.

Phylogenies trace patterns of shared ancestry between lineages. Each lineage has a part of its history that is unique to it alone and parts that are shared with other lineages.

Similarly, each lineage has ancestors that are unique to that lineage and ancestors that are shared with other lineages — common ancestors.

# data[].attributes.attachment_link.description
Understanding phylogenies Understanding a phylogeny is a lot like reading a family tree. The root of the tree represents the ancestral lineage, and the tips of the branches represent the descendants of that ancestor. As you move from the root to the tips, you are moving forward in time.

# data[].attributes.headline
Difference between Allopatric and Sympatric Speciation

# data[].attributes.subject
Difference between Allopatric and Sympatric Speciation

# data[].attributes.attachment_link.title
Difference between Allopatric and Sympatric speciation Simple 3 min video

# data[].attributes.headline
Types of Natural Selection = Directional, Stabilizing, and Disruptive

# data[].attributes.subject
Types of Natural Selection = Directional, Stabilizing, and Disruptive