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teacher zone...

Need a resource to accompany this game for you next lesson?

Check out this natural selection and evolution webquest that supplements the game for meaningful learning fun! Your students will learn about natural selection and answer questions while viewing the Natural Selection Amoeba Sisters video on YouTube. They then will play Who Wants to Live a Million Years game where students select adaptations for a species to ensure its survival in changing environments over time.

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With this interactive Natural Selection Game, students learn about Darwin's Theories and about Natural Selection. Easy to use for teachers and FUN for students.

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What is natural selection? What factors cause a population of animals to thrive or die out over generations? Is it better to be fast, big, or have stronger senses in a particular environment?

In Primer's fifth video on evolution, Justin Helps creates a blender and python-powered natural selection simulation, testing for variations on food availability along with the creatures' speed, sizes, and their ability to sense food. The only predators within the simulation are creatures whose larger size allows them to eat an average-size creature.

Natural selection is a key component of evolution. Which traits and combinations will win out in this simulated environment? A quick explanation from PBS.org:

In the process of natural selection, individuals in a population who are well-adapted to a particular set of environmental conditions have an advantage over those who are not so well adapted. The advantage comes in the form of survival and reproductive success. For example, those individuals who are better able to find and use a food resource will, on average, live longer and produce more offspring than those who are less successful at finding food. Inherited traits that increase individuals' fitness are then passed to their offspring, thus giving the offspring the same advantages.

Learn more about evolution and natural selection at Khan Academy, where Helps worked on the content team for five years.

Watch more videos about evolution, including PBS' Evolution 101 & how natural selection works and It's Okay to Be Smart's The 12 Days of Evolution.

via The Awesomer.

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Simulating Natural Selection | The Kid Should See This

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What is natural selection? What factors cause a population of animals to thrive or die out over generations? Is it better to be fast, big, or have stronger senses in a particular environment? In Primer's fifth video on evolution, Justin Helps creates a blender and python-powered natural selection simulation, testing for variations on food availability along with the creatures' speed, sizes, and their ability to sense food.

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This is an animation by Glencoe to show genetic variety in a species.

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The Origins of Life - Australian Academy of Science

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The Origins of Life - Australian Academy of Science

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This article made by an academy in Australia shows how life begun

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Everything we know about life comes from a sample size of one: life here on Earth. And the fact is, we don't really know exactly how life arose from inorganic matter all those billions of years ago ... and who's to say that different processes might not have taken place on different planets out there in the universe? Perhaps there are more questions to ask, more possibilities to consider. But let's start with what we know about some of the very first living things on Earth.

Cyanobacteria, also known as blue-green algae, started out on Earth quite a while ago. Possible fossil examples have been found in rocks that are around 3500 million years old, in Western Australia.

Although commonly referred to as blue-green algae, cyanobacteria are not actually algae. Cyanobacteria, and bacteria in general, are prokaryotic life forms. This basically means that their cells don't have organelles (tiny structures inside cells that carry out specific functions) and do not have distinct nuclei-their genetic material mixes in with the rest of the cell. This characteristic is distinctive of bacteria and archaea; all other life forms on Earth, including real algae, consist of eukaryotic cells with organelles and with genetic material contained in one place (the nucleus).

The streaky artwork of masses of cyanobacteria (blue-green algae). Image adapted from: Bobby McKay; CC BY ND 2.0

Bacteria (and archaea) are hardy creatures. They thrive in hot, cold, salty, acidic and alkaline environments in which most eukaryotes would perish. Despite this, they have a bad image: after all, bacteria cause many diseases in humans.

Yet without them we may not be here at all. Prokaryotes were the earliest life forms, simple creatures that fed on carbon compounds that were accumulating in Earth's early oceans. Slowly, other organisms evolved that used the Sun's energy, along with compounds such as sulfides, to generate their own energy. Cyanobacteria then went a step further: they started to utilise water during photosynthesis, releasing oxygen as a by-product. Over time, enough oxygen accumulated in Earth's atmosphere to allow for the evolution of oxygen-metabolising organisms.

When we go out into the universe searching for life beyond our home planet, we think we're most likely to find it lurking somewhere where there's water ...

But we may owe bacteria more than the air we breathe. It is likely that eukaryotic cells, of which humans are made, evolved from bacteria about two billion years ago. One theory is that eukaryotic cells evolved via a symbiotic relationship between two independent prokaryotic bacteria. A single bacterium was engulfed by another one, and the smaller cell continued to exist inside the other, which was beneficial to both. They evolved to become the more advanced eukaryotic cell, with its membrane-enclosed nucleus.

Regardless of how it happened, the evolution of eukaryotic cells was a significant milestone in the history of life on Earth. As conditions became more favourable, more complex organisms began to evolve.

Some of the oldest evidence of life on Earth is 3.49-billion-year-old fossilised remains of microbial mat structures, which look like wrinkle marks in rocks, found in the Pilbara region of Western Australia. Also found in the Pilbara region are fossilised remains of stromatolites. These are also mat-like structures of microbes that live in shallow marine environments and are still around today. Sand accumulates on top of the microbial mats, and the microbes move up towards the surface to get to the light again, making distinctive bulbous-shaped layers that eventually solidify into rocks.

Stromatolites in Hamelin Pool, Shark Bay, Western Australia Image adapted from: Paul Morris; CC BY SA 2.0

As we look elsewhere in the universe for life beyond our home planet, we think we're most likely to find it lurking somewhere where there's water, and where there's a heat source to make the water warm. Although we know that some living things thrive in more extreme conditions, the combination of warmth and water seem to be the most likely requirements for creating an environment that can support some kind of life-at least, the kinds of life forms similar to what we find on Earth. But who knows what other kinds of living things might exist?

This article was adapted from Academy website content and reviewed by the following expert: Professor Malcolm Walter AM FAA Australian Centre for Astrobiology, School of Biological Earth and Environmental Sciences, UNSW Sydney

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Everything we know about life comes from a sample size of one: life here on Earth. And the fact is, we don't really know exactly how life arose from inorganic matter all those billions of years ago ... and who's to say that different processes might not have taken place on different planets out there in the universe?

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An Overview of History Of lIfe on astrobio.net!

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A new study led by scientists from the University of Bristol has used a combination of genomic and fossil data to explain the history of life on Earth, from its origin to the present day.

Palaeontologists have long sought to understand ancient life and the shared evolutionary history of life as a whole.

However, the fossil record of early life is extremely fragmented, and its quality significantly deteriorates further back in time towards the Archaean period, more than 2.5 billion years ago, when the Earth's crust had cooled enough to allow the formation of continents and the only life forms were microbes.

Holly Betts, lead author of the study, from the University of Bristol's School of Earth Sciences, said: "There are few fossils from the Archaean and they generally cannot be unambiguously assigned to the lineages we are familiar with, like the blue-green algae or the salt-loving archaebacteria that colours salt-marshes pink all around the world.

"The problem with the early fossil record of life is that it is so limited and difficult to interpret - careful reanalysis of the some of the very oldest fossils has shown them to be crystals, not fossils at all."

Fossil evidence for the early history of life is so fragmented and difficult to evaluate that new discoveries and reinterpretations of known fossils have led to a proliferation of conflicting ideas about the timescale of the early history of life.

Co-author Professor Philip Donoghue, also from Bristol's School of Earth Sciences, added: "Fossils do not represent the only line of evidence to understand the past. A second record of life exists, preserved in the genomes of all living creatures."

Co-author Dr Tom Williams, from Bristol's School of Biological Sciences, said: "Combining fossil and genomic information, we can use an approach called the 'molecular clock' which is loosely based on the idea that the number of differences in the genomes of two living species (say a human and a bacterium) are proportional to the time since they shared a common ancestor."

By making use of this method the team at Bristol and Mark Puttick from the University of Bath were able to derive a timescale for the history of life on Earth that did not rely on the ever-changing age of the oldest accepted fossil evidence of life.

Co-author Professor Davide Pisani said: "Using this approach we were able to show that the Last Universal Common Ancestor all cellular life forms, 'LUCA', existed very early in Earth's history, almost 4.5 Billion years ago - not long after Earth was impacted by the planet Theia, the event which sterilised Earth and led to the formation of the Moon.

"This is significantly earlier than the currently accepted oldest fossil evidence would suggest.

"Our results indicate that two "primary" lineages of life emerged from LUCA (the Eubacteria and the Archaebacteria), approximately one Billion years after LUCA.

"This result is testament to the power of genomic information, as it is impossible, based on the available fossil information, to discriminate between the oldest eubacterial and archaebacterial fossil remains."

The study confirms modern views that the eukaryotes, the lineage to which human life belongs (together with the plants and the fungi, for example), is not a primary lineage of life. Professor Pisani added: "It is rather humbling to think we belong to a lineage that is billions of years younger than life itself."

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A timescale for the origin and evolution of all of life on Earth - Astrobiology Magazine

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A new study led by scientists from the University of Bristol has used a combination of genomic and fossil data to explain the history of life on Earth, from its origin to the present day. Palaeontologists have long sought to understand ancient life and the shared evolutionary history of life as a whole.

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Evolution and the history of life on earth

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Evolution and the history of life on earth

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The evidence is overwhelming that all life on Earth has evolved from common ancestors in an unbroken chain since its origin. Darwin's principle of evolution is summarized by the following facts. All life tends to increase: more organisms are conceived, born, hatched, germinated from seed, sprouted from spores, or produced by cell division (or other means) than can possibly survive. Each organism so produced varies, however little, in some measurable way from its relatives. In any given environment at any given time, those variants best suited to that environment will tend to leave more offspring than the others. Offspring resemble their ancestors. Variant organisms will leave offspring like themselves. Therefore, organisms will diverge from their ancestors with time. The term natural selection is shorthand for saying that all organisms do not survive to leave offspring with the same probability. Those alive today have been selected relative to similar ones that never survived or procreated. All organisms on Earth today are equally evolved since all share the same ancient original ancestors who faced myriad threats to their survival. All have persisted since roughly 3.7 billion to 3.5 billion years ago during the Archean Eon (4 billion to 2.5 billion years ago), products of the great evolutionary process with its identical molecular biological bases. Because the environment of Earth is so varied, the particular details of any organism's evolutionary history differ from those of another species in spite of chemical similarities.

Everywhere the environment of Earth is heterogeneous. Mountains, oceans, and deserts suffer extremes of temperature, humidity, and water availability. All ecosystems contain diverse microenvironments: oxygen-depleted oceanic oozes, sulfide- or ammonia-rich soils, mineral outcrops with a high radioactivity content, or boiling organic-rich springs, for example. Besides these physical factors, the environment of any organism involves the other organisms in its surroundings. For each environmental condition, there is a corresponding ecological niche. The variety of ecological niches populated on Earth is quite remarkable. Even wet cracks in granite are replete with "rock eating" bacteria. Ecological niches in the history of life have been filled independently several times. For example, quite analogous to the ordinary placental mammalian wolf was the marsupial wolf, the thylacine (extinct since 1936) that lived in Australia; the two predatory mammals have striking similarities in physical appearance and behaviour. The same streamlined shape for high-speed marine motion evolved independently at least four times: in Stenopterygius and other Mesozoic reptiles; in tuna, which are fish; and in dolphins and seals, which are mammals. Convergent evolution in hydrodynamic form arises from the fact that only a narrow range of solutions to the problem of high-speed marine motion by large animals exists. The eye, a light receptor that makes an image, has evolved independently more than two dozen times not only in animals on Earth but in protists such as the dinomastigote Erythropsodinium. Apparently eyelike structures best solve the problem of visual recording. Where physics or chemistry establishes one most efficient solution to a given ecological problem, evolution in distinct lineages will often tend toward similar, nearly identical solutions. This phenomenon is known as convergent evolution.

Life ultimately is a material process that arose from a nonliving material system spontaneously-and at least once in the remote past. How life originated is discussed below. Yet no evidence for spontaneous generation now can be cited. Spontaneous generation, also called abiogenesis, the hypothetical process by which living organisms develop from nonliving matter, must be rejected. According to this theory, pieces of cheese and bread wrapped in rags and left in a dark corner were thought to produce mice, because after several weeks mice appeared in the rags. Many believed in spontaneous generation because it explained such occurrences as maggots swarming on decaying meat.

By the 18th century it had become obvious that plants and animals could not be produced by nonliving material. The origin of microorganisms such as yeast and bacteria, however, was not fully determined until French chemist Louis Pasteur proved in the 19th century that microorganisms reproduce, that all organisms come from preexisting organisms, and that all cells come from preexisting cells. Then what evidence is there for the earliest life on Earth?

Geologic record

Past time on Earth, as inferred from the rock record, is divided into four immense periods of time called eons. These are the Hadean (4.6 billion to 4 billion years ago), the Archean (4 billion to 2.5 billion years ago), the Proterozoic (2.5 billion to 541 million years ago), and the Phanerozoic (541 million years ago to the present). For the Hadean Eon, the only record comes from meteorites and lunar rocks. No rocks of Hadean age survive on Earth. In the figure, eons are divided into eras, periods, and epochs. Such entries in the geologic time scale are often called "geologic time intervals."

Among the oldest known fossils are those found in the Fig Tree Chert from the Transvaal, dated over three billion years ago. These organisms have been identified as bacteria, including oxygenic photosynthetic bacteria (cyanobacteria)-i.e., prokaryotes rather than eukaryotes. Even prokaryotes, however, are exceedingly complicated organisms that grow and reproduce efficiently. Structures of communities of microorganisms, layered rocks called stromatolites, are found from more than three billion years ago. Since Earth is about 4.6 billion years old, these finds suggest that the origin of life must have occurred within a few hundred million years of that time.

Chemical analyses on organic matter extracted from the oldest sediments show what sorts of organic molecules are preserved in the rock record. Porphyrins have been identified in the oldest sediments, as have the isoprenoid derivatives pristane and phytane, breakdown products of cell lipids. Indications that these organic molecules dating from 3.1 billion to 2 billion years ago are of biological origin include the fact that their long-chain hydrocarbons show a preference for a straight-chain geometry. Chemical and physical processes alone tend to produce a much larger proportion of branched-chain and cyclic hydrocarbon molecular geometries than those found in ancient sediments. Nonbiological processes tend to form equal amounts of long-chain carbon compounds with odd and even numbers of carbon atoms. But products of undoubted biological origin, including the oldest sediments, show a distinct preference for odd numbers of carbon atoms per molecule. Another chemical sign of life is an enrichment in the carbon isotope C ¹², which is difficult to account for by nonbiological processes and which has been documented in some of the oldest sediments. This evidence suggests that bacterial photosynthesis or methanogenesis, processes that concentrate C ¹² preferentially to C ¹³, were present in the early Archean Eon.

The Proterozoic Eon, once thought to be devoid of fossil evidence for life, is now known to be populated by overwhelming numbers of various kinds of bacteria and protist fossils-including acritarchs (spherical, robust unidentified fossils) and the entire range of Ediacaran fauna. The Ediacarans-large, enigmatic, and in some cases animal-like extinct life-forms-are probably related to extant protists. Almost 100 species are known from some 30 locations worldwide, primarily sandstone formations. Most Ediacarans, presumed to have languished in sandy seaside locales, probably depended on their internal microbial symbionts (photo- or chemoautotrophs) for nourishment. No evidence that they were animals exists. In addition to the Ediacarans, acritarchs, and other abundant microfossils, clear evidence for pre-Phanerozoic, or Precambrian, life includes the massive banded-iron formations (BIFs). Most BIFs date from 2.5 billion to 1.8 billion years ago. They are taken as indirect evidence for oxygen-producing, metal-depositing microscopic Proterozoic life. Investigations that use the electron microprobe (an instrument for visualizing structure and chemical composition simultaneously) and other micropaleontological techniques unfamiliar to classical geology have been employed to put together a much more complete picture of pre-Phanerozoic life.

The earliest fossils are all of aquatic forms. Not until about two billion years ago are cyanobacterial filaments seen that colonized wet soil. By the dawn of the Phanerozoic Eon, life had insinuated itself between the Sun and Earth, both on land and in the waters of the world. For example, the major groups of marine animals such as mollusks and arthropods appeared for the first time about 541 million years ago at the base of the Cambrian Period of the Phanerozoic Eon. Plants and fungi appeared together in the exceptionally well-preserved Rhynie Chert of Scotland, dated about 408 million-360 million years ago in the Devonian Period. Solar energy was diverted to life's own uses. The biota contrived more and more ways of exploiting more and more environments. Many lineages became extinct. Others persisted and changed. The biosphere's height and depth increased, as did, by implication, the density of living matter. The proliferation and extinctions of a growing array of life-forms left indelible marks in the sedimentary rocks of the biosphere ( see evolution: The concept of natural selection).

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Life - Evolution and the history of life on Earth

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The evidence is overwhelming that all life on Earth has evolved from common ancestors in an unbroken chain since its origin. Darwin's principle of evolution is summarized by the following facts. All life tends to increase: more organisms are conceived, born, hatched, germinated from seed, sprouted from spores, or produced by cell division (or other means) than can possibly survive.

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This Video shows how a species goes EXTINCT in a video. Lets get sad about the dead species.

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Table of Contents

This article was most recently revised and updated by

natural selection, process that results in the adaptation of an organism to its environment by means of selectively reproducing changes in its genotype, or genetic constitution.

A brief treatment of natural selection follows. For full treatment, see evolution: The concept of natural selection.

In natural selection, those variations in the genotype (the entire complex of genes inherited from both parents) that increase an organism's chances of survival and procreation are preserved and multiplied from generation to generation at the expense of less advantageous variations. Evolution often occurs as a consequence of this process. Natural selection may arise from differences in survival, in fertility, in rate of development, in mating success, or in any other aspect of the life cycle. All such differences result in natural selection to the extent that they affect the number of progeny an organism leaves.

Gene frequencies tend to remain constant from generation to generation when disturbing factors are not present. Factors that disturb the natural equilibrium of gene frequencies include mutation, migration (or gene flow), random genetic drift, and natural selection. A mutation is a spontaneous change in the gene frequency that takes place in a population and occurs at a low rate. Migration is a local change in gene frequency when an individual moves from one population to another and then interbreeds. Random genetic drift is a change that takes place from one generation to another by a process of pure chance. Mutation, migration, and genetic drift alter gene frequencies without regard to whether such changes increase or decrease the likelihood of an organism surviving and reproducing in its environment. They are all random processes.

Natural selection moderates the disorganizing effects of these processes because it multiplies the incidence of beneficial mutations over the generations and eliminates harmful ones, since their carriers leave few or no descendants. Natural selection enhances the preservation of a group of organisms that are best adjusted to the physical and biological conditions of their environment and may also result in their improvement in some cases. Some characteristics, such as the male peacock's tail, actually decrease the individual organism's chance of survival. To explain such anomalies, Darwin posed a theory of " sexual selection." In contrast to features that result from natural selection, a structure produced by sexual selection results in an advantage in the competition for mates.

The Editors of Encyclopaedia Britannica John P. Rafferty.

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Natural selection | Definition & Processes

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natural selection, process that results in the adaptation of an organism to its environment by means of selectively reproducing changes in its genotype, or genetic constitution. A brief treatment of natural selection follows. For full treatment, see evolution: The concept of natural selection.

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An Video by BrainPOP shows things about Human Evolution!

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This short video shows a time lapse of the organisms as they "appeared" on Earth, and also shows how short of a time humans have been on Earth.

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0:00 / 0:15•Watch full video

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A introduction to human evolution

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The Smithsonian Institution's Human Origins Program

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Human evolution

Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

One of the earliest defining human traits, bipedalism -- the ability to walk on two legs -- evolved over 4 million years ago. Other important human characteristics -- such as a large and complex brain, the ability to make and use tools, and the capacity for language -- developed more recently. Many advanced traits -- including complex symbolic expression, art, and elaborate cultural diversity -- emerged mainly during the past 100,000 years.

Humans are primates. Physical and genetic similarities show that the modern human species, Homo sapiens, has a very close relationship to another group of primate species, the apes. Humans and the great apes (large apes) of Africa -- chimpanzees (including bonobos, or so-called "pygmy chimpanzees") and gorillas -- share a common ancestor that lived between 8 and 6 million years ago. Humans first evolved in Africa, and much of human evolution occurred on that continent. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa.

Most scientists currently recognize some 15 to 20 different species of early humans. Scientists do not all agree, however, about how these species are related or which ones simply died out. Many early human species -- certainly the majority of them - left no living descendants. Scientists also debate over how to identify and classify particular species of early humans, and about what factors influenced the evolution and extinction of each species.

Early humans first migrated out of Africa into Asia probably between 2 million and 1.8 million years ago. They entered Europe somewhat later, between 1.5 million and 1 million years. Species of modern humans populated many parts of the world much later. For instance, people first came to Australia probably within the past 60,000 years and to the Americas within the past 30,000 years or so. The beginnings of agriculture and the rise of the first civilizations occurred within the past 12,000 years.

Paleoanthropology

Paleoanthropology is the scientific study of human evolution. Paleoanthropology is a subfield of anthropology, the study of human culture, society, and biology. The field involves an understanding of the similarities and differences between humans and other species in their genes, body form, physiology, and behavior. Paleoanthropologists search for the roots of human physical traits and behavior. They seek to discover how evolution has shaped the potentials, tendencies, and limitations of all people. For many people, paleoanthropology is an exciting scientific field because it investigates the origin, over millions of years, of the universal and defining traits of our species. However, some people find the concept of human evolution troubling because it can seem not to fit with religious and other traditional beliefs about how people, other living things, and the world came to be. Nevertheless, many people have come to reconcile their beliefs with the scientific evidence.

Early human fossils and archeological remains offer the most important clues about this ancient past. These remains include bones, tools and any other evidence (such as footprints, evidence of hearths, or butchery marks on animal bones) left by earlier people. Usually, the remains were buried and preserved naturally. They are then found either on the surface (exposed by rain, rivers, and wind erosion) or by digging in the ground. By studying fossilized bones, scientists learn about the physical appearance of earlier humans and how it changed. Bone size, shape, and markings left by muscles tell us how those predecessors moved around, held tools, and how the size of their brains changed over a long time. Archeological evidence refers to the things earlier people made and the places where scientists find them. By studying this type of evidence, archeologists can understand how early humans made and used tools and lived in their environments.

The process of evolution

The process of evolution involves a series of natural changes that cause species (populations of different organisms) to arise, adapt to the environment, and become extinct. All species or organisms have originated through the process of biological evolution. In animals that reproduce sexually, including humans, the term species refers to a group whose adult members regularly interbreed, resulting in fertile offspring -- that is, offspring themselves capable of reproducing. Scientists classify each species with a unique, two-part scientific name. In this system, modern humans are classified as Homo sapiens.

Evolution occurs when there is change in the genetic material -- the chemical molecule, DNA -- which is inherited from the parents, and especially in the proportions of different genes in a population. Genes represent the segments of DNA that provide the chemical code for producing proteins. Information contained in the DNA can change by a process known as mutation. The way particular genes are expressed - that is, how they influence the body or behavior of an organism -- can also change. Genes affect how the body and behavior of an organism develop during its life, and this is why genetically inherited characteristics can influence the likelihood of an organism's survival and reproduction.

Evolution does not change any single individual. Instead, it changes the inherited means of growth and development that typify a population (a group of individuals of the same species living in a particular habitat). Parents pass adaptive genetic changes to their offspring, and ultimately these changes become common throughout a population. As a result, the offspring inherit those genetic characteristics that enhance their chances of survival and ability to give birth, which may work well until the environment changes. Over time, genetic change can alter a species' overall way of life, such as what it eats, how it grows, and where it can live. Human evolution took place as new genetic variations in early ancestor populations favored new abilities to adapt to environmental change and so altered the human way of life.

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Human evolution is the lengthy process of change by which people originated from apelike ancestors. Scientific evidence shows that the physical and behavioral traits shared by all people originated from apelike ancestors and evolved over a period of approximately six million years.

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An Video by BrainPOP shows what natural selection is in a fun way!

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Many people think evolution requires thousands or millions of years, but biologists know it can happen fast. Now, thanks to the genomic revolution, researchers can actually track the population-level genetic shifts that mark evolution in action-and they're doing this in humans. Two studies presented at the Biology of Genomes meeting here last week show how our genomes have changed over centuries or decades, charting how since Roman times the British have evolved to be taller and fairer, and how just in the last generation the effect of a gene that favors cigarette smoking has dwindled in some groups.

"Being able to look at selection in action is exciting," says Molly Przeworski, an evolutionary biologist at Columbia University. The studies show how the human genome quickly responds to new conditions in subtle but meaningful ways, she says. "It's a game-changer in terms of understanding evolution."

Evolutionary biologists have long concentrated on the role of new mutations in generating new traits. But once a new mutation has arisen, it must spread through a population. Every person carries two copies of each gene, but the copies can vary slightly within and between individuals. Mutations in one copy might increase height; those in another copy, or allele, might decrease it. If changing conditions favor, say, tallness, then tall people will have more offspring, and more copies of variants that code for tallness will circulate in the population.

With the help of giant genomic data sets, scientists can now track these evolutionary shifts in allele frequencies over short timescales. Jonathan Pritchard of Stanford University in Palo Alto, California, and his postdoc Yair Field did so by counting unique single-base changes, which are found in every genome. Such rare individual changes, or singletons, are likely recent, because they haven't had time to spread through the population. Because alleles carry neighboring DNA with them as they circulate, the number of singletons on nearby DNA can be used as a rough molecular clock, indicating how quickly that allele has changed in frequency.

Pritchard's team analyzed 3000 genomes collected as part of the UK10K sequencing project in the United Kingdom. For each allele of interest in each genome, Field calculated a "singleton density score" based on the density of nearby single, unique mutations. The more intense the selection on an allele, the faster it spreads, and the less time there is for singletons to accumulate near it. The approach can reveal selection over the past 100 generations, or about 2000 years.

Stanford graduate students Natalie Telis and Evan Boyle and postdoc Ziyue Gao found relatively few singletons near alleles that confer lactose tolerance-a trait that enables adults to digest milk-and that code for particular immune system receptors. Among the British, these alleles have evidently been highly selected and have spread rapidly. The team also found fewer singletons near alleles for blond hair and blue eyes, indicating that these traits, too, have rapidly spread over the past 2000 years, Field reported in his talk and on 7 May in the preprint server bioRxiv.org. One evolutionary driver may have been Britain's gloomy skies: Genes for fair hair also cause lighter skin color, which allows the body to make more vitamin D in conditions of scarce sunlight. Or sexual selection could have been at work, driven by a preference for blond mates.

Other researchers praise the new technique. "This approach seems to allow much more subtle and much more common signals of selection to be detected," says evolutionary geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

In a sign of the method's power, Pritchard's team also detected selection in traits controlled not by a single gene, but by tiny changes in hundreds of genes. Among them are height, head circumference in infants, and hip size in females-crucial for giving birth to those infants. By looking at the density of singletons flanking more than 4 million DNA differences, Pritchard's team discovered that selection for all three traits occurred across the genome in recent millennia.

Joseph Pickrell, an evolutionary geneticist at New York Genome Center in New York City, has used a different strategy to put selection under an even keener microscope, detecting signs of evolution on the scale of a human lifetime. He and Przeworski took a close look at the genomes of 60,000 people of European ancestry who had been genotyped by Kaiser Permanente in Northern California, and 150,000 people from a massive U.K. sequencing effort called the UK Biobank. They wanted to know whether genetic variants change frequency across individuals of different ages, revealing selection at work within a generation or two. The biobank included relatively few old people, but it did have information about participants' parents, so the team also looked for connections between parental death and allele frequencies in their children.

In the parents' generation, for example, the researchers saw a correlation between early death in men and the presence in their children (and therefore presumably in the parents) of a nicotine receptor allele that makes it harder to quit smoking. Many of the men who died young had reached adulthood in the United Kingdom in the 1950s, a time when many British men had a pack-a-day habit. In contrast, the allele's frequency in women and in people from Northern California did not vary with age, presumably because fewer in these groups smoked heavily and the allele did not affect their survival. As smoking habits have changed, the pressure to weed out the allele has ceased, and its frequency is unchanged in younger men, Pickrell explains. "My guess is we are going to discover a lot of these gene-by-environment effects," Przeworski says.

Indeed, Pickrell's team detected other shifts. A set of gene variants associated with late-onset menstruation was more common in longer-lived women, suggesting it might help delay death. Pickrell also reported that the frequency of the ApoE4 allele, which is associated with Alzheimer's disease, drops in older people because carriers died early. "We can detect selection on the shortest timeframe possible, an individual's life span," he says.

Signs of selection on short timescales will always be prey to statistical fluctuations. But together the two projects "point to the power of large studies to understand what factors determine survival and reproduction in humans in present-day societies," Pääbo says.

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Humans are still evolving-and we can watch it happen

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Many people think evolution requires thousands or millions of years, but biologists know it can happen fast. Now, thanks to the genomic revolution, researchers can actually track the population-level genetic shifts that mark evolution in action-and they're doing this in humans.