Introduction to evolution

This article is a non-technical introduction to the subject. For the main encyclopedia article, see Evolution.

To see a brief description of evolution in simpler language, visit the Simple Wikipedia article on evolution

Overview

Life forms reproduce to make offspring.

The offspring differs from the parent in minor random ways.

If the differences are helpful, the offspring is more likely to survive and reproduce.

This means that more offspring in the next generation will have the helpful difference.

These differences accumulate resulting in changes within the population.

Over time, this process gradually leads to entirely new types of life.

This process is responsible for the many diverse life forms in the world today.

Evolution is the ongoing process of change that has transformed life on earth from its simple beginnings into its present diversity. Evolution occurs through changes in genes, the "recipes" for constructing organisms. When an organism reproduces, small, random changes in its genes make the offspring different from the parents. Sometimes these changes increase the chances of an offspring surviving to reproduce. When this happens, the genes for the beneficial traits are passed on again, becoming more common in the next generation.

Genes that do not help organisms reproduce may become rarer or be eliminated from the population. This is called natural selection. Through natural selection, populations of organisms slowly change over time as they adapt to changes in their environments.

The theory of evolution is the explanation for how evolution occurs. This states that all living things are descended from a single common ancestor, which lived at some point in the distant past. This idea is called common descent.

Since the beginning of life, evolution has transformed the first species into a very large number of different species, as life has found a variety of ways to survive and flourish. This has produced the variety of organisms that exist today. The theory of evolution does not address how life originated (abiogenesis), only how life has changed since its inception.

Evolutionary biology, the scientific study of evolution, has itself undergone changes over time. For example, Gregor Mendel contributed a clear understanding of the role of genetics in variation between organisms. The field of population genetics came from a merger between evolutionary biology and genetics and addresses how new species develop. This process is called speciation and involves new species developing from ancestral forms.

Evolution is a well-supported explanation for a given set of data, not a mere hypothesis. The theory of evolution is supported by an immense body of scientific evidence, just like the "theory of gravity". The fields of molecular biology, paleontology, and taxonomy all contribute evidence that illuminates the evolutionary process. There is no serious disagreement among biologists that evolution does occur, and more than 99.9% of all professional biological scientists support evolution. It is the foundation of the research conducted in all fields of biology. However, some debate remains about some of the details of how evolution occurs, and how important different mechanisms and processes are in producing evolution.

Haeckel's Paleontological Tree of Vertebrates (c. 1879). The evolutionary history of species has been described as a "tree", with many branches arising from a single trunk. Each branch indicates a species, and each fork in a branch represents the ancestor diverging in form, becoming multiple new forms of life. While Haeckel's tree is somewhat outdated, it illustrates clearly the principles that more complex modern reconstructions can obscure.

Darwin's idea

Charles Darwin proposed the theory of evolution by natural selection.

File:Mendel.png Gregor Mendel's work on the inheritance of traits laid the foundation for genetics.

Further information: Charles Darwin, Natural Selection, and Common Descent

Charles Darwin (1809-1882) proposed that there is unity in all life. Darwin viewed the history of life as a tree, each fork in the tree’s limbs representing shared ancestry. The tips of the limbs represent the modern species present today. This was especially controversial because humans did not receive a special place at the top of the evolutionary tree; they were merely one of many branches. To explain these relationships, he proposed that all living things are related and have descended from a common ancestor in a process he described as "descent with modification". The popular press of his day interpreted Darwin's views to imply that humans were descended from monkeys. In fact, however, evolution states that humans and present-day monkeys share a common ancestor.

Darwin's explanation of the mechanisms of evolution relies on his theory of natural selection, a theory he presented in the famous text The Origin of Species (published in 1859). The modern theory of natural selection incorporates five basic ideas:

1. Organisms will produce more offspring than their habitat can sustain. There will be a "struggle to survive".
2. Not all the offspring will be identical.
3. Some of the differences between the offspring will be due to variations in their genetic makeup, the "code" that determines each organism's inherited traits.
4. Genetic variations that help an organism to survive and reproduce are more likely to be passed on to the next generation than genetic variations that are unhelpful.
5. Over time, helpful genetic variations will accumulate until a new species results.

The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve "progress" towards an ultimate goal; in fact, it is not goal-driven. Evolution does not necessarily make life forms more advanced, more intelligent, or more sophisticated. For example, fleas (wingless parasites) are descended from a winged, ancestral scorpionfly, and snakes are lizards that have lost the use for limbs. Organisms are merely the outcome of mutations that succeed or fail, dependent upon the environmental conditions at that time. In reality, when the environment changes, most species fail to adapt and become extinct. This reality is evident in the struggles species are currently facing as environments change because of global warming.

Mendel’s contribution

Further information: Gregor Mendel and Genetics

Darwin’s theory of natural selection laid the groundwork for evolutionary theory, but he lacked an accurate explanation for the source of variations within the population. Like many of his predecessors, Darwin incorrectly deduced that heritable traits were a product of the environment. Such a view of evolution assumed that characteristics acquired during an organism's lifetime could be passed on to its offspring (e.g. giraffes stretching for leaves on higher branches would give birth to offspring with longer necks). This misconception (of the inheritance of acquired characters) became known as Lamarckism, after one of its primary supporters, Jean-Baptiste Lamarck (1744-1829). The suggestion that body structures used extensively to cope with the environment become better developed, while structures not used deteriorate, is not in fact supported by evidence.

The missing information necessary to help explain the emergence of new traits in the offspring was provided by developments in the field of genetics, pioneered by Gregor Mendel (1822-1884). Mendel’s experiments with breeding pea plants demonstrated that heredity in sexual reproduction works by reshuffling and recombining factors (genes) during sexual reproduction. Genes are the basic units of heredity in living organisms. A gene is a segment of a DNA molecule on a chromosome that directs the physical development and behavior of the organism. It is this reshuffling of the genetic code that ensures that no two individuals will be exact copies. The merging of Darwin's theory with an understanding of heredity led to the birth of the field of science called population genetics.

Population genetics

Further information: Population Genetics and Hardy-Weinberg

Some definitions

White Peppered Moth

Black Peppered Moth

From a genetic viewpoint, evolution is a generation-to-generation change in the frequencies of alleles within a population that shares a common gene pool. An allele is one specific form of a gene; for example, a gene for coloration in moths may have two alleles, black and white. A population is a localized group of individuals belonging to the same species. For example, all the trout of the same species sharing a single stream is a population. A gene pool is the complete set of alleles in a single population. Each allele occurs a certain number of times in the gene pool. The fraction of genes that belong to a given allele is called the "allele frequency". Therefore, if half of the body-color genes are genes for black moths, then the black-body allele frequency is 0.50. Evolution consists of changes in the frequencies of alleles within a group of interbreeding organisms. Populations evolve, not individuals: evolution does not describe changes in individuals (such as how a child matures into an adult, or how a tree grows from a seed), but changes from one generation to another within a population.

Hardy-Weinberg equilibrium

Gametes fuse during fertilization

A theory known as the "Hardy-Weinberg principle" states that the frequencies of alleles in a sufficiently large population will not change over time if the only forces acting on the population are:

1. random reshuffling of alleles during the formation of the gametes, such as the sperm and egg.
2. random combination of the genes in these sex cells during fertilization, the process in which the egg and sperm combine to form a new cell.
   

An example

White and black mice

Suppose a group of mice inhabit a barn. In this population, there are only two versions of the gene that controls fur color. One allele produces black fur and accounts for 75% of the genes, the other produces white fur and makes up the remaining 25% of the genes. If an allele’s chance of being passed on to the next generation is due entirely to random processes (the shuffling and combining that takes place in the formation of sex cells and fertilization), the allele frequencies will stay the same and the composition of the gene pool remains 75% black-coding genes and 25% white-coding genes. Since there is no change in the allelic frequencies, there is no evolutionary change in fur color. This population is in "Hardy-Weinberg equilibrium".

For this equilibrium to persist and no evolution to occur, the following conditions must be met:

1. The population must be large. Small populations can have chance fluctuations in the gene pool, a condition known as genetic drift.
2. There must be no exchange of members between populations. Such migration between populations is called gene flow.
3. The mutation rate must be insignificant. Mutations can create new alleles and thus change the frequency of the pre-existing alleles.
4. Mating must be random. There can be no preference for any particular allele during mate selection.
5. Natural selection must not occur. The chance that each allele has for survival and reproduction must be the same.

It is very rare, if not impossible, for all these conditions to be met in a natural population. Therefore, frequencies of alleles in a gene pool are always changing, resulting in evolution of populations over successive generations.

Evidence of evolution

During the voyage of the Beagle, naturalist Charles Darwin collected fossils in South America, and found fragments of armor like giant versions of the scales on the modern armadillos living nearby. On his return, the anatomist Richard Owen showed that the fragments were from gigantic extinct glyptodons, related to the armadillos. This was one of the patterns of distribution that helped Darwin to develop his theory.

Further information: Paleontology, Fossils, and Gradualism

Science has provided a wide range of evidence for evolution, with fossil records being most prominent. In addition, studies of the anatomical and genetic similarities between present-day species serve as additional evidence for evolution.

French naturalist and zoologist Georges Cuvier studied fossils.

Archaeopteryx fossil.

The fossil record

Paleontology, the study of fossils, supports the idea that all living creatures are related. Fossils also provide evidence that accumulated changes over long periods led to the diverse forms of life we see today. The fossil itself reveals the organism's structure, and the age of the fossil reveals when the species existed. From this, relationships can be established between present and extinct species, allowing paleontologists to construct family trees that link all life forms.

Modern paleontology began with the work of Georges Cuvier (1769-1832). Cuvier noted that, in sedimentary rock, each layer contained a specific group of fossils. The deeper (older) the layer, the simpler the life forms. He also noted that many forms of life from the past are no longer present today. Cuvier proposed the idea of catastrophism, which explained the fossil record in the light of the theological views of his time. He proposed that catastrophes had occurred in localized areas throughout the earth’s history. Such areas were then repopulated by species that migrated in from nearby locations.

Nowadays, many more fossils have been discovered and identified. These fossils serve as a chronological record, documenting the emergence of new, more complex species from simpler ancestral forms. The fossil record also provides examples of transitional species that provide evidence of ancestral links between species that exist today. One such transitional fossil is Archaeopteryx, an ancient creature that had the distinct characteristics of a reptile, yet clearly possessed the feathers of a bird. The implication from such a find is that modern reptiles and birds arose from a common ancestor.

Comparative anatomy

Further information: Convergent evolution and Divergent evolution

Taxonomy is the branch of biology that names and classifies all living things. The founder of the science of taxonomy was Carolus Linnaeus (1707-1778). Linnaeus placed organisms into categories based on similar physical features (sometimes called their "morphology"). He never suggested that organisms which fell into similar groups were related; rather he followed the conventions of his time, that all species were uniquely created and remained fixed and unchanging.

Today, scientists still use morphological similarities to assist them in categorizing life forms. However, these groupings are based on ancestral relationships. For example, orangutans, gorillas, chimpanzees, and humans all belong to the same taxonomic grouping referred to as a family – in this case the family called "Hominidae". These animals are grouped together because of similarities in morphology that come from common ancestry. A similarity in anatomical features because of shared ancestry is called homology.

Homologous structures. Note how the same basic design appears repeatedly in different types of forelimbs of different species.

Strong evidence for evolution comes from analysis of "homologous" structures that no longer perform the same task. One example is the forelimbs of mammals. The forelimbs of a human, cat, whale, and bat all have strikingly similar bone structures. However, each of these four species' forelimbs performs a different task. Such a "design" makes little sense if they are unrelated and uniquely constructed for their particular tasks. The theory of evolution explains these homologous structures: all four animals shared a common ancestor, and each has undergone change over many generations. These changes in structure have produced forelimbs adapted for different tasks. This is what Darwin described as "descent with modification".

In some cases, anatomical comparison of structures in the embryos of two or more species provides evidence for a shared ancestor that is not obvious in the adult forms. Such homologies might be lost or take on different functions as the embryo develops. For example, part of the basis of classifying the vertebrate group (which includes humans), is the presence of a tail and pharyngeal gill slits. Because of the morphological similarities present in embryos of different species during development, it would be easy to assume that organisms re-enact their evolutionary history as an embryo – for example, human embryos passing through an amphibian then a reptilian stage before completing their development as mammals. This misconception is known as “ontogeny recapitulates phylogeny”. Such a re-enactment of evolution during embryonic development is not supported by scientific evidence.

Vestigial structures

Whale skeleton showing vestigial hind leg bones

Homology also includes a unique group of shared structures referred to as "vestigial structures". The term "vestigial" refers to anatomical parts that are of minimal, if any, value to the organism that possesses them. These apparently illogical structures are remnants of organs that played an important role in ancestral forms. For example, whales still possess small vestigial leg bones which appear to be remnants of the legs that their ancestors used to walk on land.

Humans also have many vestigial structures, including the ear muscles, the wisdom teeth, the appendix, the tail bone, body hair (including goose bumps), and the semilunar fold in the corner of the eye.

Convergent evolution

Whitetip shark (left) compared to the bottlenose dolphin

Anatomical comparisons can also be misleading. Organisms which share similar environments will often develop similar physical features. This is known as "convergent evolution". For example, both sharks and dolphins have similar body forms, yet are only distantly related – sharks are fish and dolphins mammals. Such similarities are a result of both populations being exposed to the same selective pressures. Within both groups, changes that aid swimming would be favored. Thus, over time, they develop similar morphology, even though they are not closely related.

Artificial selection

The results of artificial selection: Great Danes and Chihuahuas

Artificial selection is the controlled breeding of domestic plants and animals. In controlled breeding, humans determine which animals will reproduce, and to some degree, which alleles will be passed to future generations. The process of artificial selection has a significant impact on the evolution of domestic animals. For example, people have produced different types of dogs by controlled breeding. The differences between the Chihuahua and the Great Dane are the result of artificial selection. Despite the dramatic differences in physical appearance, they share a recent common ancestor.

Darwin drew much of his support for natural selection from observing the outcomes of artificial selection. Much of his book On the Origin of Species was based on his observations of the diversity in domestic pigeons arising from artificial selection. Darwin proposed that if dramatic changes in domestic plants and animals could be achieved by humans in short periods, then natural selection, given millions of years, could produce the differences between living things today. In fact, there is no real difference in the genetic processes underlying artificial and natural selection. As in natural selection, the variations are a result of random mutations; the only difference is that in artificial selection, humans select which organisms will be allowed to breed.

Molecular biology

A section of DNA

Every living organism contains molecules of DNA, RNA, and protein. If two organisms are closely related, these molecules will be very similar. On the other hand, if the organisms are distant relations, these molecules will be more different. For example, siblings share the closest relationship possible, and thus have very similar DNA sequences. The field of molecular systematics focuses on working out evolutionary relationships by measuring similarities in these molecules. Such molecular comparisons have allowed biologists to build a "relationship tree" of the evolution of various organisms. Scientists have made great strides in analyzing these molecules, particularly the DNA that makes up organisms' genes. The exact form of these genes is called the genotype, which influences the morphology (or phenotype) of an organism; thus, analyzing genes provides a clear understanding of the relationships between species.

Comparing both DNA and proteins has been extremely useful when studying species that are so closely related that there are no obvious anatomical differences. The extent of their relationship can be determined from how similar these molecules are.

Genetic comparisons also allow scientists to draw conclusions about organisms whose common ancestors lived such a long time ago that morphological similarities are not apparent. For example, comparison of the DNA in chimpanzees with that of gorillas and humans demonstrated that chimpanzees share more genetic similarities with humans than with gorillas. Some studies suggest as much as 96% similarity between the genes of humans and chimps. Therefore, this implies that these two species, humans and chimpanzees, share a closer evolutionary relationship as well.

Co-evolution

Co-evolution is a process in which two or more species influence the evolution of each other. All organisms are influenced by life around them; however, to meet the definition of "co-evolution", there must be evidence that some genetically determined traits in each species result from the interaction between the two organisms.

An extensively documented example of co-evolution involves the relationship between an ant called Pseudomyrmex and the acacia, a plant that the ant uses for food and shelter. The relationship between the two is so intimate that it has led to the evolution of special structures and behaviors in both organisms. The ant defends the acacia against herbivores and removes parasitic fungi from its leaves. In return, the plant has evolved swollen thorns which the ants use as shelter, and special flower parts which the ants eat. Such co-evolution does not imply that the ants and the tree somehow choose to behave in such an altruistic (selfless concern for the welfare of others) manner for each others' benefit. Rather, across a population small genetic changes in both ant and tree benefited each on its own accord. The benefit gave a slightly higher chance of the characteristic being passed on to the next generation, where it gave the new tree (and the new ant colony) a greater chance of survival. Over time, successive mutations created the relationship we observe today.

Species

File:Mayr.jpg

Ernst Mayr

Further information: Species, Speciation, and Phylogenetics

Given the right circumstances, and enough time, evolution leads to the emergence of new species. Scientists have struggled to find a precise and all-inclusive definition of a species. The classic definition, used here, was developed by Ernst Mayr (1904-2005). Mayr defined a species as a population or group of populations whose members have the potential to interbreed naturally with one another to produce viable, fertile offspring. Also, the members of a species cannot produce viable fertile offspring with members of other species.

Speciation is the lineage-splitting event that results in two separate species forming from a single common ancestral population. The most widely accepted method of speciation is called "allopatric speciation". This requires the geographic separation of a population. Separation may be due to a variety of geological forces such as the emergence of mountain ranges or the formation of canyons. For speciation to occur, separation must be complete to the point that genetic exchange between the two populations is completely disrupted. In their separate environments, the genetically isolated groups follow their own unique evolutionary pathways. Each group will accumulate different mutations as well as be subjected to different selective pressures. The accumulated genetic changes may result in separated populations that can no longer interbreed if they are reunited. If interbreeding is no longer possible, then they would be considered different species. A common criticism from those who reject evolution as a viable theory is that speciation has never been observed. Speciation has been observed in several groups of organisms, including bacteria, round worms, insects, and fish; as well as in several groups of plants. In addition, past speciation events are recorded in fossils.

There are numerous species of cichlids which demonstrate dramatic variations in morphology

For example, scientists have documented the formation of five new species of cichlid fishes from common ancestry since they were isolated less than 4000 years ago from the parent stock, in Lake Nagubago. The basis for speciation in this case was morphology (physical appearance) and lack of natural interbreeding. These fish have complex mating rituals and different coloration, which with only slight modifications would change the mate selection process. The five forms that arose could not be convinced to interbreed.

Inter-species barriers

Reproductive barriers that prevent interbreeding can be classified as either prezygotic barriers or postzygotic barriers.

Prezygotic barriers

Different species of Firefly do not recognize each others' mating signals, and as a result do not generally interbreed.

Prezygotic barriers prevent mating between species or prevent the fertilization of the egg if the species attempt to mate. Some examples are:

• Temporal isolation - Occurs when species mate at different times. Populations of the western spotted skunk (Spilogale gracilis) overlap with the eastern spotted skunk (Spilogale putorius) yet remain separate species because the former mates in summer and the latter in late winter.

• Behavioral isolation - Signals that elicit a mating response may be sufficiently different to prevent a desire to interbreed. The rhythmic flashing in male fireflies is species-specific and thus serves as a prezygotic barrier.

• Mechanical isolation - Anatomical differences in reproductive structures may prevent interbreeding. This is especially true in flowering plants that have evolved specific structures adapted to certain pollinators. Mechanical barriers often contribute to reproductive isolation of flowers that are pollinated by insects. This has been well documented in the orchid family.

• Gametic isolation - The gametes of the two species are chemically incompatible, thus preventing fertilization. Gamete recognition may be based on specific molecules on the surface of the egg that attach only to complementary molecules on the sperm. Such mechanisms are common in fish species.

• Geographic/habitat isolation - Geographic: The two species are separated by large-scale physical barriers, such as a mountain or large body of water, and therefore cannot mate with each other. This is illustrated in two separate species of antelope squirrels, genus Ammospermophilus, which inhabit opposite sides of the Grand Canyon. Habitat: The two species prefer different habitats, even if they live in the same general area, and therefore do not encounter each other. For example, two different species of garter snakes in the genus Thamnophis occur in the same area but one prefers the water while the other prefers dry land.

File:100 0637.jpg

The Mule is a hybrid of a horse and a donkey, and is usually infertile

Postzygotic barriers

Postzygotic barriers occur after fertilization, usually resulting in the formation of a hybrid zygote that is neither viable nor fertile. This is typically a result of incompatible chromosomes in the zygote. Some examples include:

• Reduced hybrid viability - A barrier between species occurs after the formation of the zygote, resulting in incomplete development and death of the offspring.

• Reduced hybrid fertility - Even if two different species successfully mate, the offspring produced may be infertile. Crosses of horse species within the genus Equus tend to produce viable but sterile offspring. For example, crosses of zebra x horse and zebra x donkey produce sterile zorses and zedonks. Horse-donkey crosses produce sterile mules. Very rarely, a female mule may be fertile.

• Hybrid breakdown - Some hybrids are fertile for a single generation but then become weak or inviable.

Different perspectives on the mechanism of evolution

James Hutton

File:NaturalhistoryMag.jpg

Stephen Jay Gould

Richard Dawkins

Further information: Creation-evolution controversy, Level of support for evolution, and Punctuated equilibrium

The theory of evolution is widely accepted among the scientific community. It serves to link the diverse specialty areas of biology. Evolution provides all of biology with a solid scientific base. The significance of evolutionary theory is best described by the title of a paper by Theodosius Dobzhansky (1900-1975), published in American Biology Teacher; "Nothing in Biology Makes Sense Except in the Light of Evolution."

Nevertheless, the theory of evolution is not dogmatic. In fact, there is much discussion within the scientific community concerning the mechanisms behind the evolutionary process. Two such areas of debate are:

• The rate at which evolution occurs
• The unit of evolutionary change, the organism or the genes

Rate of change

Two views exist concerning the rate of evolutionary change. Darwin and his contemporaries viewed evolution as a slow and gradual process. Evolutionary trees are based on the idea that profound differences in species are the result of many small changes that accumulate over great periods.

This view had its basis in the works of the geologist James Hutton (1726-1797) and his theory called "Gradualism". Hutton's theory suggests that profound geological change is the cumulative product of slow, continuous processes. A similar perspective was adopted for biological changes.

Such a view fails to explain the fossil record, which shows evidence of new species appearing suddenly, then persisting in that form for long periods. The paleontologist Stephen Jay Gould (1940-2002) developed a model that suggests that evolution continues through periods of rapid change alternating with periods of relative stability, a model called "Punctuated Equilibrium".

Unit of change

It is generally accepted amongst biologists that the unit of selection in evolution is the organism, and that natural selection serves to either enhance or reduce the reproductive potential of an individual. Reproductive success, therefore, can be measured by the volume of an organism's surviving offspring. This view has been challenged by a variety of biologists, as well as philosophers. Richard Dawkins, for example, proposes that much insight can be gained if we look at evolution from the gene's point of view, and view natural selection as selecting amongst genes, in addition to organisms. This perspective is captured in the following quotation from his book, The Selfish Gene:

Individuals are not stable things, they are fleeting. Chromosomes too are shuffled to oblivion, like hands of cards soon after they are dealt. But the cards themselves survive the shuffling. The cards are the genes. The genes are not destroyed by crossing-over; they merely change partners and march on. Of course they march on. That is their business. They are the replicators and we are their survival machines. When we have served our purpose we are cast aside. But genes are denizens of geological time: genes are forever.

Others view selection working on many levels, not just at a single level of organism or gene; for example, Stephen Jay Gould called for a hierarchical perspective on selection.

Summary

Evolution in popular culture

As Darwin's work spread and became better known, references to it began appearing in the popular culture of the day. Some of the better-known Victorian references to it include:

"Survival of the fittest" - used by Herbert Spencer in Principles of Biology (1864)

"Nature, red in tooth and claw" - from Alfred Lord Tennyson's In Memoriam A.H.H. (1849)

It even merited a song in Gilbert and Sullivan's 1884 opera, Princess Ida, which concludes: "Darwinian man, though well behaved, at best is only a monkey shaved!"

Evolution explains the variety of biological species. Evolution is the result of two basic mechanisms:

• Evolution requires genetic variation within the population. Offspring are not perfect copies of their parents, or each other. If they were, the only factor determining survival and reproductive success would be random chance.
• Some offspring, by chance, have features that allow them to survive and thrive better than others. The offspring that survive will be more likely to have offspring of their own. Some of these useful features are passed along to new generations.

Evolution is not a random process for creating new life forms. Mutations are (partly) random, but natural selection is far from random. Therefore, evolution is an inevitable result of imperfectly copying, self-replicating machines reproducing over billions of years under the selection pressure of the environment. There are many misconceptions about evolution, that can lead to unnecessary confusion and objections to the theory of evolution.

The theory of evolution is supported by a tremendous amount of evidence. Evolution has been observed in the laboratory. Domesticated animals evolve as we selectively breed them for certain traits. The records of past evolution are found in fossils as well as in our fundamentally similar genetic codes, demonstrating common ancestry of all organisms, both surviving and extinct.

Evolution, although disputed in some quarters, is one of the most successful scientific theories ever produced and is universally accepted by biological scientists. An understanding of evolution underlies all biological sciences and much of medicine.

Further reading

• Berra TM (1990). Evolution and the myth of creationism: a basic guide to the facts in the evolution debate. Stanford, Calif: Stanford University Press. ISBN 0-8047-1548-3.
• Darwin C (1979). The illustrated Origin of species (abridged & introduced by Richard E. Leakey; consultants: Bynum WF, Barrett JA). London: Faber and Faber. ISBN 0-571-11477-6.
• Dawkins R (1995). River Out of Eden: a Darwinian view of life. New York: Basic Books. ISBN 0-465-01606-5.
• Dawkins R (1996). Climbing Mount Improbable. New York: W.W. Norton. ISBN 0-393-03930-7.
• Dawkins R. The Selfish Gene. Oxford University Press, USA. ISBN 0-19-929114-4.
• Gould SJ (1980). The Panda's Thumb: more reflections in natural history. New York: Norton. ISBN 0-393-01380-4.
• Gould SJ (1995). Dinosaur in a haystack: reflections in natural history. New York: Harmony Books. ISBN 0-517-70393-9.
• Gould SJ (1989). Wonderful life: the Burgess Shale and the nature of history. New York: W.W. Norton. ISBN 0-393-02705-8.
• Mayr E (2001). What evolution is. New York: Basic Books. ISBN 0-465-04425-5.
• Ridley M (2003). The Red Queen: Sex and the Evolution of Human Nature. New York, NY: Perennial. ISBN 0-06-055657-9.
• Sagan C, Druyan A (1992). Shadows of forgotten ancestors: a search for who we are. New York: Random House. ISBN 0-394-53481-6.
• Sis P (2003). The tree of life: a book depicting the life of Charles Darwin, naturalist, geologist & thinker. New York: Farrar Straus Giroux. ISBN 0-374-45628-3.

External links

Evolution websites

• Understanding Evolution from University of California, Berkeley
• Howstuffworks.com — How Evolution Works
• The Talk Origins Archive: Exploring the Creation/Evolution Controversy
• Evolution by Natural Selection - An outline of the chain of reasoning behind the theory of natural selection
• evowiki - A reader-built encyclopedia of evolution, biology, and origins

Videos about evolution

• Evolution (provided by PBS)
• Evolution of Life Explained by Carl Sagan
• Natural Selection Explained by Carl Sagan

Printable Introduction to evolution

• The Big Picture on Evolution (pdf), with description, from the Wellcome Trust

Genetics

• University of Utah Genetics Learning Center animated tour of the basics of genetics