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Water - Liquid Awesome: Crash Course Biology #2

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Water's Polarity

One of water's important properties is that it is composed of polar molecules: the hydrogen and oxygen within water molecules (H ₂ O) form polar covalent bonds. While there is no net charge to a water molecule, the polarity of water creates a slightly positive charge on hydrogen and a slightly negative charge on oxygen, contributing to water's properties of attraction. Water's charges are generated because oxygen is more electronegative than hydrogen, making it more likely that a shared electron would be found near the oxygen nucleus than the hydrogen nucleus, thus generating the partial negative charge near the oxygen.

As a result of water's polarity, each water molecule attracts other water molecules because of the opposite charges between water molecules, forming hydrogen bonds. Water also attracts or is attracted to other polar molecules and ions. A polar substance that interacts readily with or dissolves in water is referred to as hydrophilic (hydro- = "water"; -philic = "loving"). In contrast, non-polar molecules such as oils and fats do not interact well with water, as shown in Figure \(\PageIndex{1}\) and separate from it rather than dissolve in it, as we see in salad dressings containing oil and vinegar (an acidic water solution). These nonpolar compounds are called hydrophobic (hydro- = "water"; -phobic = "fearing").

Figure \(\PageIndex{1}\): Oil and water do not mix. As this macro image of oil and water shows, oil does not dissolve in water but forms droplets instead. This is due to it being a nonpolar compound. (credit: Gautam Dogra).

Water's States: Gas, Liquid, and Solid

The formation of hydrogen bonds is an important quality of the liquid water that is crucial to life as we know it. As water molecules make hydrogen bonds with each other, water takes on some unique chemical characteristics compared to other liquids and, since living things have a high water content, understanding these chemical features is key to understanding life. In liquid water, hydrogen bonds are constantly formed and broken as the water molecules slide past each other. The breaking of these bonds is caused by the motion (kinetic energy) of the water molecules due to the heat contained in the system. When the heat is raised as water is boiled, the higher kinetic energy of the water molecules causes the hydrogen bonds to break completely and allows water molecules to escape into the air as gas (steam or water vapor). On the other hand, when the temperature of water is reduced and water freezes, the water molecules form a crystalline structure maintained by hydrogen bonding (there is not enough energy to break the hydrogen bonds) that makes ice less dense than liquid water, a phenomenon not seen in the solidification of other liquids.

Water's lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes: the water molecules are pushed farther apart compared to liquid water. With most other liquids, solidification when the temperature drops includes the lowering of kinetic energy between molecules, allowing them to pack even more tightly than in liquid form and giving the solid a greater density than the liquid.

The lower density of ice, illustrated and pictured in Figure \(\PageIndex{2}\), an anomaly, causes it to float at the surface of liquid water, such as in an iceberg or in the ice cubes in a glass of ice water. In lakes and ponds, ice will form on the surface of the water creating an insulating barrier that protects the animals and plant life in the pond from freezing. Without this layer of insulating ice, plants and animals living in the pond would freeze in the solid block of ice and could not survive. The detrimental effect of freezing on living organisms is caused by the expansion of ice relative to liquid water. The ice crystals that form upon freezing rupture the delicate membranes essential for the function of living cells, irreversibly damaging them. Cells can only survive freezing if the water in them is temporarily replaced by another liquid like glycerol.

Figure \(\PageIndex{2}\): Hydrogen bonding makes ice less dense than liquid water. The (a) lattice structure of ice makes it less dense than the freely flowing molecules of liquid water, enabling it to (b) float on water. (credit a: modification of work by Jane Whitney, image created using Visual Molecular Dynamics (VMD) software¹; credit b: modification of work by Carlos Ponte)

Link to Learning

Video: Click here to see a 3-D animation of the structure of an ice lattice. (Image credit: Jane Whitney. Image created using Visual Molecular Dynamics VMD software.²)

Water's High Heat Capacity

Water's high heat capacity is a property caused by hydrogen bonding among water molecules. Water has the highest specific heat capacity of any liquids. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie. It therefore takes water a long time to heat and long time to cool. In fact, the specific heat capacity of water is about five times more than that of sand. This explains why the land cools faster than the sea. Due to its high heat capacity, water is used by warm blooded animals to more evenly disperse heat in their bodies: it acts in a similar manner to a car's cooling system, transporting heat from warm places to cool places, causing the body to maintain a more even temperature.

Water's Heat of Vaporization

Water also has a high heat of vaporization, the amount of energy required to change one gram of a liquid substance to a gas. A considerable amount of heat energy (586 cal) is required to accomplish this change in water. This process occurs on the surface of water. As liquid water heats up, hydrogen bonding makes it difficult to separate the liquid water molecules from each other, which is required for it to enter its gaseous phase (steam). As a result, water acts as a heat sink or heat reservoir and requires much more heat to boil than does a liquid such as ethanol (grain alcohol), whose hydrogen bonding with other ethanol molecules is weaker than water's hydrogen bonding. Eventually, as water reaches its boiling point of 100° Celsius (212° Fahrenheit), the heat is able to break the hydrogen bonds between the water molecules, and the kinetic energy (motion) between the water molecules allows them to escape from the liquid as a gas. Even when below its boiling point, water's individual molecules acquire enough energy from other water molecules such that some surface water molecules can escape and vaporize: this process is known as evaporation.

The fact that hydrogen bonds need to be broken for water to evaporate means that a substantial amount of energy is used in the process. As the water evaporates, energy is taken up by the process, cooling the environment where the evaporation is taking place. In many living organisms, including in humans, the evaporation of sweat, which is 90 percent water, allows the organism to cool so that homeostasis of body temperature can be maintained.

Water's Solvent Properties

Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can readily dissolve in it. Therefore, water is referred to as a solvent, a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules will form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, as illustrated in Figure \(\PageIndex{3}\) and serves to keep the particles separated or dispersed in the water.

When ionic compounds are added to water, the individual ions react with the polar regions of the water molecules and their ionic bonds are disrupted in the process of dissociation. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na⁺ and Cl - ions, and spheres of hydration form around the ions, illustrated in Figure \(\PageIndex{3}\). The positively charged sodium ion is surrounded by the partially negative charge of the water molecule's oxygen. The negatively charged chloride ion is surrounded by the partially positive charge of the hydrogen on the water molecule.

Figure \(\PageIndex{3}\): When table salt (NaCl) is mixed in water, spheres of hydration are formed around the ions.

Water's Cohesive and Adhesive Properties

Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass.

Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (heavier) than the water. Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top. It's even possible to "float" a needle on top of a glass of water if it is placed gently without breaking the surface tension, as shown in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\): The weight of the needle is pulling the surface downward; at the same time, the surface tension is pulling it up, suspending it on the surface of the water and keeping it from sinking. Notice the indentation in the water around the needle. (credit: Cory Zanker)

These cohesive forces are related to water's property of adhesion, or the attraction between water molecules and other molecules. This attraction is sometimes stronger than water's cohesive forces, especially when the water is exposed to charged surfaces such as those found on the inside of thin glass tubes known as capillary tubes. Adhesion is observed when water "climbs" up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle. This is because the water molecules are attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it. This type of adhesion is called capillary action, and is illustrated in Figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): Capillary action in a glass tube is caused by the adhesive forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves. (credit: modification of work by Pearson-Scott Foresman, donated to the Wikimedia Foundation)

Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants. These forces create a "pull" on the water column. This pull results from the tendency of water molecules being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water from their roots to their leaves. Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider, shown in Figure \(\PageIndex{6}\), use the surface tension of water to stay afloat on the surface layer of water and even mate there.

Figure \(\PageIndex{6}\): Water's cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat. (credit: Tim Vickers)

pH, Buffers, Acids, and Bases

The pH of a solution indicates its acidity or alkalinity.

\[\ce{H_2O(I) \leftrightharpoons H^+ (aq) + O^- (aq)} \nonumber\]

litmus or pH paper, filter paper that has been treated with a natural water-soluble dye so it can be used as a pH indicator, to test how much acid (acidity) or base (alkalinity) exists in a solution. You might have even used some to test whether the water in a swimming pool is properly treated. In both cases, the pH test measures the concentration of hydrogen ions in a given solution.

Hydrogen ions are spontaneously generated in pure water by the dissociation (ionization) of a small percentage of water molecules into equal numbers of hydrogen (H⁺) ions and hydroxide (OH⁻) ions. While the hydroxide ions are kept in solution by their hydrogen bonding with other water molecules, the hydrogen ions, consisting of naked protons, are immediately attracted to un-ionized water molecules, forming hydronium ions (H ₃0⁺). Still, by convention, scientists refer to hydrogen ions and their concentration as if they were free in this state in liquid water.

The concentration of hydrogen ions dissociating from pure water is 1 × 10⁻⁷ moles H⁺ ions per liter of water. Moles (mol) are a way to express the amount of a substance (which can be atoms, molecules, ions, etc), with one mole being equal to 6.02 x 10 ²³ particles of the substance. Therefore, 1 mole of water is equal to 6.02 x 10 ²³ water molecules. The pH is calculated as the negative of the base 10 logarithm of this concentration. The log10 of 1 × 10⁻⁷ is -7.0, and the negative of this number (indicated by the "p" of "pH") yields a pH of 7.0, which is also known as neutral pH. The pH inside of human cells and blood are examples of two areas of the body where near-neutral pH is maintained.

Non-neutral pH readings result from dissolving acids or bases in water. Using the negative logarithm to generate positive integers, high concentrations of hydrogen ions yield a low pH number, whereas low levels of hydrogen ions result in a high pH. An acid is a substance that increases the concentration of hydrogen ions (H⁺) in a solution, usually by having one of its hydrogen atoms dissociate. A base provides either hydroxide ions (OH ⁻) or other negatively charged ions that combine with hydrogen ions, reducing their concentration in the solution and thereby raising the pH. In cases where the base releases hydroxide ions, these ions bind to free hydrogen ions, generating new water molecules.

The stronger the acid, the more readily it donates H⁺. For example, hydrochloric acid (HCl) completely dissociates into hydrogen and chloride ions and is highly acidic, whereas the acids in tomato juice or vinegar do not completely dissociate and are considered weak acids. Conversely, strong bases are those substances that readily donate OH - or take up hydrogen ions. Sodium hydroxide (NaOH) and many household cleaners are highly alkaline and give up OH ⁻ rapidly when placed in water, thereby raising the pH. An example of a weak basic solution is seawater, which has a pH near 8.0, close enough to neutral pH that marine organisms adapted to this saline environment are able to thrive in it.

The pH scale is, as previously mentioned, an inverse logarithm and ranges from 0 to 14 (Figure \(\PageIndex{7}\)). Anything below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 (from 7.1 to 14.0) is alkaline. Extremes in pH in either direction from 7.0 are usually considered inhospitable to life. The pH inside cells (6.8) and the pH in the blood (7.4) are both very close to neutral. However, the environment in the stomach is highly acidic, with a pH of 1 to 2. So how do the cells of the stomach survive in such an acidic environment? How do they homeostatically maintain the near neutral pH inside them? The answer is that they cannot do it and are constantly dying. New stomach cells are constantly produced to replace dead ones, which are digested by the stomach acids. It is estimated that the lining of the human stomach is completely replaced every seven to ten days.

Figure \(\PageIndex{7}\): The pH scale measures the concentration of hydrogen ions (H⁺) in a solution. (credit: modification of work by Edward Stevens)

Link to Learning

Watch this video for a straightforward explanation of pH and its logarithmic scale.

So how can organisms whose bodies require a near-neutral pH ingest acidic and basic substances (a human drinking orange juice, for example) and survive? Buffers are the key. Buffers readily absorb excess H⁺ or OH ⁻, keeping the pH of the body carefully maintained in the narrow range required for survival. Maintaining a constant blood pH is critical to a person's well-being. The buffer maintaining the pH of human blood involves carbonic acid (H ₂CO ₃), bicarbonate ion (HCO ₃⁻), and carbon dioxide (CO ₂). When bicarbonate ions combine with free hydrogen ions and become carbonic acid, hydrogen ions are removed, moderating pH changes. Similarly, as shown in Figure \(\PageIndex{8}\), excess carbonic acid can be converted to carbon dioxide gas and exhaled through the lungs. This prevents too many free hydrogen ions from building up in the blood and dangerously reducing the blood's pH. Likewise, if too much OH ⁻ is introduced into the system, carbonic acid will combine with it to create bicarbonate, lowering the pH. Without this buffer system, the body's pH would fluctuate enough to put survival in jeopardy.

Figure \(\PageIndex{8}\): This diagram shows the body's buffering of blood pH levels. The blue arrows show the process of raising pH as more CO₂ is made. The purple arrows indicate the reverse process: the lowering of pH as more bicarbonate is created.

Other examples of buffers are antacids used to combat excess stomach acid. Many of these over-the-counter medications work in the same way as blood buffers, usually with at least one ion capable of absorbing hydrogen and moderating pH, bringing relief to those that suffer "heartburn" after eating. The unique properties of water that contribute to this capacity to balance pH-as well as water's other characteristics-are essential to sustaining life on Earth.

Summary

Water has many properties that are critical to maintaining life. It is a polar molecule, allowing for the formation of hydrogen bonds. Hydrogen bonds allow ions and other polar molecules to dissolve in water. Therefore, water is an excellent solvent. The hydrogen bonds between water molecules cause the water to have a high heat capacity, meaning it takes a lot of added heat to raise its temperature. As the temperature rises, the hydrogen bonds between water continually break and form anew. This allows for the overall temperature to remain stable, although energy is added to the system. Water also exhibits a high heat of vaporization, which is key to how organisms cool themselves by the evaporation of sweat. Water's cohesive forces allow for the property of surface tension, whereas its adhesive properties are seen as water rises inside capillary tubes. The pH value is a measure of hydrogen ion concentration in a solution and is one of many chemical characteristics that is highly regulated in living organisms through homeostasis. Acids and bases can change pH values, but buffers tend to moderate the changes they cause. These properties of water are intimately connected to the biochemical and physical processes performed by living organisms, and life would be very different if these properties were altered, if it could exist at all.

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The polarity of the water molecule and its resulting hydrogen bonding make water a unique substance with special properties that are intimately tied to the processes of life. Life originally evolved ...

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Genetics is the study of heredity

DNA and the Laws of inheritance

Alternatives to the true dominant recessive inheritance

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Genetics is the branch of biology that explores how traits and characteristics are passed from one generation to the next. It dives into the molecular blueprint DNA that determines everything from eye color to susceptibility to certain diseases.

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🧬 DNA: The Blueprint of Life

DNA (Deoxyribonucleic Acid) is the molecule that carries genetic instructions for building and maintaining living organisms. It’s composed of:

Nucleotides: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G)

Double helix structure: Two strands twisted around each other, held together by base pairing (A-T, C-G)

Each gene is a segment of DNA that codes for a specific protein, and these genes are located on chromosomes humans have 23 pairs.

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📜 Mendel’s Laws of Inheritance

Gregor Mendel’s experiments with pea plants laid the groundwork for classical genetics. His three laws explain how traits are inherited:

1. Law of Segregation

Each individual has two alleles for each gene (one from each parent).

These alleles separate during gamete formation, so each gamete carries only one allele.

Example: A plant with genotype Aa will produce gametes with either A or a.

2. Law of Independent Assortment

Genes for different traits are inherited independently of one another (if they’re on different chromosomes).

This explains the genetic variation seen in offspring.

Example: In a dihybrid cross (e.g., seed color and shape), the inheritance of one trait doesn’t affect the other.

3. Law of Dominance

When two different alleles are present, one may mask the expression of the other.

The dominant allele is expressed in the phenotype, while the recessive is hidden unless both alleles are recessive.

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Incomplete Dominance

Definition: Neither allele is completely dominant; the heterozygote shows a blend.

Example: In snapdragons, red (RR) crossed with white (rr) yields pink (Rr) flowers.

Genotype vs. Phenotype: Each genotype has a distinct phenotype no masking.

Codominance

Definition: Both alleles are fully expressed in the heterozygote.

Example: Human ABO blood type type AB expresses both A and B antigens.

Key Feature: No blending; both traits appear simultaneously.

Multiple Alleles

Definition: More than two alleles exist for a gene in the population.

Example: ABO blood group has three alleles IA, IB, and i.

Impact: Expands possible genotype and phenotype combinations.

Polygenic Inheritance

Definition: Traits controlled by multiple genes, each contributing a small effect.

Example: Skin color, height, and intelligence.

Result: Continuous variation across a spectrum, often forming a bell curve.

Pleiotropy

Definition: One gene influences multiple, seemingly unrelated traits.

Example: Marfan syndrome mutation in the FBN1 gene affects connective tissue, eyes, heart, and skeleton.

Epistasis

Definition: One gene masks or modifies the expression of another gene.

Example: Coat color in Labrador retrievers one gene determines pigment, another controls deposition.

Sex-Linked Inheritance

Definition: Genes located on sex chromosomes (usually X).

Example: Hemophilia and color blindness more common in males due to XY configuration.

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DNA (deoxyribonucleic acid) is the hereditary material in humans and most other organisms. It is composed of four chemical bases/letters called adenine (A), guanine (G), cytosine (C) and thymine (T). The human genome is made of over 3 million letters of DNA. These letters are arranged in various sequences and length to make up genes.

Genes are the basic physical and functional units of heredity. We have more than 20,000 genes, some of which provide instructions to cells in to make specific proteins that control the structure and function of our bodies. However, most genes do not code for any proteins and we are still trying to learn about their functions. Genes are located in small thread-like structures called chromosomes, which in turn are located inside the nucleus of our cells.

Basics of genetics presented by the National Geographic channel

Most cells in our body contain 23 pairs of chromosomes. In each pair, we inherit one chromosome from each parent. We therefore have two copies of most of our genes. In contrast, our germ cells (egg and sperm cells) only have one set of chromosomes (and therefore one set of genes) due to a division process called meiosis. When the egg is fertilised by a sperm cell, the fertilised egg will contain two sets of chromosomes, one from each parent.

22 of these pairs are identical in males and females and these are called autosomes. The 23 rd pair are the sex chromosomes and they determine gender. Females have two copies of X chromosome (one from each parent) while males have one X chromosome (from the mother) and one Y chromosome (from the father).

All 23 pairs of chromosomes in males (XY chromosome is present)
Credit: US National Library of Medicine

How are genetic disorders inherited?

A genetic disorder is usually due to changes (mutations) or "spelling mistakes" in a single gene. These changes can either be inherited from parent(s) or they can be " de novo " (i.e. due to a new change in that person). The faulty gene can be located in one of the 22 autosomes, in the X chromosome or even in a structure called the mitochondria. Mitochondria are small structures present in most cells in our body, providing energy for these cells to function normally (much like a battery).

Genetic conditions can be inherited in different ways depending on where the faulty gene is located and how many copies of the faulty gene are required to cause disease. These include:

Autosomal dominant

Autosomal recessive

X-linked

Mitochondrial

In autosomal dominant conditions, the faulty gene is in one of the autosomes (chromosomes 1-22) and only one copy of the faulty gene is required to cause disease. This can be inherited from either parent.

There is a 50% chance of an affected individual passing the faulty gene on to a child and a 50% chance of passing on their normal gene. The risk is the same whether the child is male or female. This is a random process that occurs for each pregnancy.

Occasionally, conditions inherited in this manner have variable expressivity and incomplete penetrance. Variable expressivity means a range of severity from very mild to very severe while incomplete penetrance means that some people with the faulty gene have symptoms and other people with the same exact genetic change have no symptoms at all.

Autosomal dominant inheritance

In autosomal recessive conditions, two copies of the faulty gene are required to cause disease, one from each parent. A parent who has one faulty gene copy and one normal gene copy is known as a heterozygote or a carrier and they usually do not have any symptoms. Carriers are usually not aware of their status until they have an affected child or have undergone genetic testing. For a child to be affected, both parents would need to be carriers of a mutation in the same gene.

If both parents are carriers (most likely scenario):

25% chance (one in four) that a child will be affected

25% chance (one in four) that a child will be unaffected and not a carrier

50% chance (one in two) that a child will be an unaffected carrier

Autosomal recessive inheritance

If one parent is affected and the other is not a carrier:

All children will be carriers

If one parent is affected and the other is a carrier (common with inter-marriages or in small isolated populations):

50% chance that a child is affected

50% chance that a child is a carrier

This is a random process and these risks apply to each new pregnancy.

The faulty gene is located in the X chromosome in such instances. If a female has a faulty gene on one of her two X chromosomes, she is considered a heterozygote carrier and usually does not show any symptoms. This is because of a process called X-inactivation which ensures that females, like males, only have one functional copy of the X chromosome in each cell. X-inactivation is a random process and therefore, the X chromosome inherited from the mother may be active in some cells while in others the active X chromosome is inherited from the father.

On the other hand, if a male has a faulty gene on his single X chromosome, he is termed "hemizygous" and will show symptoms as he does not have another X chromosome to compensate. Generally, X-linked recessive conditions only manifest in males but a minority of female carriers can be affected (usually to a milder extent) due to non-random X-inactivation.

If the mother is a carrier and the father is healthy:

Each son has a 50% chance of being affected

Each daughter has a 50% chance of being a carrier like the mother

X-linked recessive inheritance

If the father is affected and the mother is healthy:

None of his sons will be affected

All of his daughters will be carriers

This is because an affected male cannot transmit the disorder to his male offspring who inherit his Y chromosome but all his daughters will receive his X chromosome.

To summarise, in X-linked recessive inheritance:

The disorder mostly affects males

Healthy carrier females transmit the condition to their sons

There is no male-to-male transmission

Less common than X-linked recessive inheritance

Both males and females can be affected as only one faulty gene copy on the X chromosome is required to cause disease

Females are usually less severely affected than males because of random X-inactivation

Affected females can pass the condition on to their daughters as well as their sons

Affected males will still only transmit the condition to their daughters but not to their sons

X-linked dominant inheritance with an affected mother

X-linked dominant inheritance with an affected father

Mitochondria are energy producing structures inside our cells which have their own DNA with 37 genes. Mutations in mitochondrial genes are passed on to the next generation through the mother (maternal inheritance). This is because only the egg cells but not the sperm cells contribute mitochondria to the developing embryo. Both males and females can be affected with mitochondrial inheritance.

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Genetics basics and inheritance pattern - Gene Vision

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DNA (deoxyribonucleic acid) is the hereditary material in humans and most other organisms. It is composed of four chemical bases/letters called adenine (A), guanine (G), cytosine (C) and thymine (T). The human genome is made of over 3 million letters of DNA. These letters are arranged in various sequences and length to make up genes.

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How is genetic material inherited? | Definition of inheritance

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Inheritance is the way that genetic information is passed from a parent to a child. As humans, we have two sets of 23 chromosomes in most of our cells. We inherit one set from each of our biological parents.

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How did the biological concept of evolution develop over time?

A historical look at adaptation and evolution

The idea that life has changed over Earth’s history was first proposed by early Greek philosophers (Xenophanes, Empedocles, and Aristotle). They recognized fossils as evidence for former life that was destroyed by a natural catastrophe.

With the rise of the Catholic Church, this evolutionary thought declined among scholars who largely accepted the biblical account of earth’s creation which required no mechanistic explanation.

In the early 1600s A.D., Archbishop James Usher calculated based on the bible that the earth was formed on Sunday, October 23rd 4004 B.C.

This became widely accepted amongst church officials and further declined evolutionary thought labeling it as heretical (which at the time carried a death sentence if found guilty).

By the 1700s the church's influence had declined enough that naturalists could again speculate on the evolution of animals.

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Understanding Evolution - Your one-stop source for information on evolution

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Biological evolution, simply put, is descent with inherited modification. This definition encompasses everything from small-scale evolution (for example, changes in the frequency of different gene versions in a population from one generation to the next) to large-scale evolution (for example, the descent of different species from a shared ancestor over many generations). Evolution helps us to understand the living world around us, as well as its history.

The explanation

Biological evolution is not simply a matter of change over time. Many things change over time: caterpillars turn into moths, trees lose and regrow their leaves, mountain ranges rise and erode, but they aren't examples of biological evolution because they don't involve descent with inherited modifications.

All life on Earth shares a common ancestor, just as you and your cousins share a common grandmother. Through the process of descent with modification, this common ancestor gave rise to the diverse species that we see documented in the fossil record and around us today. Evolution means that we're all distant cousins: humans and oak trees, hummingbirds and whales.

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An introduction to evolution - Understanding Evolution

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The definition Biological evolution, simply put, is descent with inherited modification. This definition encompasses everything from small-scale evolution (for example, changes in the frequency of different gene versions in a population from one generation to the next) to large-scale evolution (for example, the descent of different species from a shared ancestor over many generations).

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Jean Babtiste de Lamark ( 1744-1829) came up with the first hypothesis for how evolution worked. He successfully explained that fossils were remains of extinct animals and proposed evolution works by inheritance of acquired characteristics. This hypothesis states that organisms would acquire adaptations based on their environment and then pass the newly acquired trait to their offspring (e.g. neck elongation in Giraffes). The process was considered Transformational whereby individuals transform their characteristics through the uses and disuse of body parts and these traits can be passed on

Charles Lyell (1797-1875)- established uniformitarianism- (1) laws of physics and chemistry have not changed throughout earth’s history (2) past geological events occurred by natural processes similar to those that we observe today.

His work went on to show that the earth was more likely millions of years old not thousands and that these changes in the earth have no inherent directionality.

Teleology is the mistaken notion that the evolution of living organisms is guided by purpose toward an optimal design rather than cause

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Natural selection as the mechanism of evolution

Characteristics of organisms are inherited

More offspring are produced than are able to survive, thus there is competition for those resources in each generation

Offspring vary among each other in regard to their characteristics

From this Darwin and Wallace reasoned that offspring that allow for them to best compete for these resources will survive and have more offspring

Example of natural selection - Darwin’s finches

Darwin noticed while on the Galapagos that the Finches had beaks that varied in shape and size

Each species specialized in specific foods (big beaks for big seeds, small beaks for small beaks, etc…)

These birds have been recorded every year since.

In 1977 the Galapagos when through a major drought, which eliminated small seed-producing plants. This caused small beaked individuals to go be greatly reduced in number.

This shows that Natural selection only takes place when there is variation among individuals in a population

Variation only comes from mutations or sexual reproduction.

Mutations are the ultimate source of new alleles in any population

A heritable trait that aids the survival and reproduction of an organism are called an adaptation

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What do we currently know about the process of evolution?

Patterns of evolution

Evolution has caused large amounts of variation in form and function

When two species evolve in a different direction from a common ancestor this is called Divergent evolution

Other times independent species evolve similar phenotypes separately

Flight has evolved separately in insects, birds, and bats but there is no ancestor that had flight that unites these species. This is called convergent evolution

The wings of a bat and of a bird are called analogous structures

The wings of two bird’s species are called homologous structures

Evolution is constantly-changing populations, the gradual change is referred to as microevolution, while the rise of new species and higher taxonomic groups is referred to as macroevolution

Population genetics

A group of the same species of organisms interacting is a population

Populations are what evolution act upon

Within a population, there are multiple alleles (all combined are called the gene pool) and the frequency that those alleles lead to reproduction is called fitness

Hardy-Weinberg Equilibrium

If there was no evolutionary pressure the frequency of alleles would remain constant

However, we now know that there are 4 important evolutionary forces that disrupt the equilibrium

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What evidence exists that supports evolution?

Evidence of evolution

The fossil record

A fossil is a remnant from past life uncovered from the earth.

The fossil record is the sequence in which fossils appear in rock strata.

Based on this sequence and the ages of rocks and fossils geologists have established a geologic record.

Ages of rocks or fossils can be determined using radiometric dating. This is a technique that looks at the ratio of an element’s stable form (ex. carbon-12) with its radioactive form (ex. carbon-14).

Anatomy and Embryology

The presence of structures in organisms that share the same basic form

Appendages

Vestigial structures

Convergence of form in organisms that share a similar environment (e.g. artic animals)

Embryo development (e.g. Presence of gill slits and tails in vertebrates)

Biogeography is the distribution of organisms being congruent with tectonic movement and evolution

Molecular biology - molecular life reflects descent with modification

Speciation - is the formation of 2 species from one original species.

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Frequently asked questions about evolution.