The evidence for evolution, ranging from inferences based the fossil record to direct observations, is overwhelming. This page represents a necessarily brief summary, divided into 5 broad categories, with selected examples.

Paleontology
Comparative Anatomy
Biogeography
Embryology
Molecular Biology

Moreover, the fossil record contains many examples of transitional forms[#](Benton 2001)(Gould 1981). Intermediate forms have been discovered between fish and amphibians[#](Edwards 1989)(Daeschler, et al. 2006)(Shubin, et al. 2006), between amphibians and reptiles[#](Benton 1990)(Kardong 2002), and between reptiles and mammals[#](Kemp 1982)(Luo, et al. 2001)(Vaughan, et al. 2000)(Wang, et al. 2001). We can trace the evolution of whales from terrestrial mammalian ancestors through several intermediate stages[#](Thewissen, et al. 2001), and the evolution of birds from small running dinosaurs[#](Dingus & Rowe 1998)(Forster, et al. 1998)(Burnham, et al. 2000)(Sereno 1999)(Xu, et al. 2000)(Norell, et al. 2002).

The mammalian ear and jaw provide an excellent example, complete with transitional stages from the fossil record[#](Vaughan, et al. 2000) (see Figure 1). The lower jaws of mammals contain only one bone, whereas those of reptiles have several. The bones now found in the mammalian ear are homologous with the additional bones in the reptile jaw. Paleontologists have discovered intermediate forms of mammal-like reptiles with a double jaw joint - one composed of the bones that persist in mammalian jaws, the other consisting of bones that eventually became the hammer and anvil of the mammalian ear.

The limbs of vertebrates provide another example of homologous structures (Figure 2). All of these limbs have similar structures that perform different functions, suggesting they have common ancestors that had these structures. This conclusion is supported by independent evidence from the fossil record including a general chronology of intermediate forms between dinosaurs and modern birds, in which theropod structures were modified into modern bird structures [#](Carroll 1997)(Sereno 1999).

Additionally, all organisms carry useless remnants of formerly functional structures that make no sense except as holdovers from different ancestral states. Whales and dolphins - which evolved from terrestrial mammals - possess vestiges of leg bones hidden inside their bodies[#](Thewissen, et al. 2001)(Vaughan, et al. 2000). The same is true of many snake species, which evolved from reptilian ancestors with legs[#](Romer & Parsons 1977)(Raynaud 1990).

For example, marsupials (“pouched” mammals such as kangaroos, koalas and opossums) are found only in Australia and South America[#](Patterson & Pascual 1972)(Woodburne & Case 1996), although the earliest ancestors of modern marsupials are actually found on North America. (Opossums have moved back into North America, but only after the rise of the Isthmus of Panama connected North and South America)[#](Marshall, et al. 1982)(Webb 1976). Placental mammals (other than those introduced by humans) occur everywhere but Australia.

A look at the movements of continents (and their timing) explains these patterns (see Figure 3). South America, Australia, Africa, and Antarctica once made up the continent of Gondwanaland. They split apart 180 million years ago, which is also when marsupial and placental mammals diverged[#](Vaughan, et al. 2000). Similarly, lungfishes, ratite (ostrich-like) birds, and leptodactylid frogs are found nowhere but Australia and southern South America and Africa[#](National Academy of Science, 1999).

In addition, if Australian marsupials are evolutionary related to South American marsupials, fossils of common ancestors should be found dating from before these two landmasses separated during the late Cretaceous. And in fact, fossil marsupials are found on Antarctica dating to this period [#](Woodburne & Case 1996).

Another important aspect of biogeography is the distribution of species relative to the distribution of suitable habitat. It is possible that species are simply found where there is suitable habitat and the geographic distributions of species could be explained without evolution. However, there are many instances in which suitable habitat lacks species that would thrive there. This is because species evolve in a single location even though they may spread elsewhere, and geographical barriers like oceans, rivers and mountain ranges often restrict species’ movements.

For example, the deserts of North America, Africa and Australia are very similar habitats, and plants from one grow well in the other. However, cacti (in the family Cactaceae) only inhabit the Americas, while Saharan and Australian vegetation is very distantly related (mostly Euphorbiaceae)[#](Futuyma 1998).

Because geographic isolation prevents many species from reaching areas[#](MacArthur & Wilson 1967), isolated islands provide further examples of evolution[#](National Academy of Science, 2004)(Freed, et al. 1987). When the Hawaiian Islands first rose from the sea, they were barren of plants and animals. Over time, wind- and water-borne seeds reached the islands, as did some birds and flying insects. But all sorts of organisms (potential competitors and predators) never reached the islands, because of their geographic isolation. Those species that did reach the islands diversified over time because of the absence of related organisms that would compete for resources. In all, at least 71 endemic (found nowhere else) species of Hawaiian birds evolved from species that arrived from elsewhere. From a single successful colonization of the Hawaiian Archipelago by an ancestral species from North America, Hawaiian honeycreepers evolved[#](National Academy of Science, 1999). They include a diverse array of species including seed-eating birds, insectivorous birds (some with woodpecker-like adaptations), and nectar-feeding birds. In addition to the honeycreepers, endemic Hawaiian birds included three seabirds, several waterfowl, two raptors, and many passerine (perching) birds including descendants of Old World flycatchers, honeyeaters, and thrushes[#](Freed, et al. 1987).

This “adaptive radiationThe rapid speciation from a single or a few species to many, filling many open ecological niches, such as following colonization of new islands.” can also be seen in Darwin’s Finches in the Galápagos Islands[#](Grant 1999) (Figure 4), Anolis lizards on Carribean islands[#](Losos, et al. 1997) and cichlid fishes in Africa’s Great Lakes (Victoria, Tanganyika and Malawi) which contain more species of fish than any other lakes in the world. Nearly 2,000 unique species have evolved in the last 10 million years. In each of these lakes, one or a few species have initiated rapid adaptive radiations, resulting in flocks of several hundred closely related but phenotypically diverse species. Lake Tanganyika has nearly 300 species and Lake Victoria more than 600 species. Lake Malawi alone has nearly 1,000 species of cichlid; all but 2 evolved in Lake Malawi and are found nowhere else[#](Kocher 2004) (Figure 5).

Reptiles and birds lay eggs, while placental mammals (which evolved from reptiles) do not. However, primitive mammals called monotremes (platypuses and echidnas) do lay eggs. In marsupials - more modern than monotremes but more primitive than placentals - an eggshell forms transiently and then is reabsorbed before live birth[#](Tyndale-Biscoe & Renfree 1987)(Vaughan, et al. 2000). In reptiles, the emerging young use an 'egg-tooth' to cut through the leathery eggshell. Placental mammals, having lost the eggshell, have lost the egg-tooth. However, monotremes have an egg-tooth. Most strikingly, several marsupial newborns (such as koalas and bandicoots) retain a vestigial eggtooth-like structure called a caruncle, evidence of their reptilian ancestry[#](Tyndale-Biscoe & Renfree 1987).

In addition, a wide variety of animals, from invertebrates like flies and worms to vertebrates including humans, have very similar sequences of genes that are active early in development[#](National Academy of Science, 1999). These genes influence body segmentation or orientation in all these diverse groups. The presence of such similar genes doing similar things across such a wide range of organisms is best explained by their having been present in a common ancestor.

In order to perform the functions necessary for life, organisms must catalyze chemical reactions. In all known organisms, enzymatic catalysis is performed by protein molecules constructed from the same 20 amino acids. There are nearly 400 naturally occurring amino acids[#](Futuyma 1998).

Molecular techniques have also been used to construct phylogenetic treesA diagram illustrating the evolutionary relationships among species with common ancestry. (see Figure 6). Because of mutations, the sequence of nucleotides in a gene gradually changes over time. Evolutionary theory predicts that the more closely related two organisms are, the less different their DNA will be. More importantly, phylogenetic trees derived from molecular sequences (DNA) should match trees constructed independently from morphologyThe form and structure of an organism. or paleontology (the probability of finding two similar independently-derived trees by chance is extremely small[#](Faith & Cranston 1991)). Many molecular studies have confirmed phylogenetic relationships derived from paleontology and anatomy[#](Benton 1998)(Benton, et al. 1999)(Clyde & Fisher 1997)(Norell & Novacek 1992).

For example, genetic sequences of the proteins myoglobin and hemoglobin were determined for dozens of mammals, birds, reptiles, amphibians, fish, worms, and molluscs. The differences in sequences among different organisms was used to construct a family tree of hemoglobin and myoglobin variation among organisms. This tree agreed completely with trees constructed from the fossil record and comparative anatomy.

Similar family histories have been obtained from the three-dimensional structures and amino acid sequences of other proteins, such as cytochrome cA small protein found loosely associated with the inner membrane of the mitochondrion.It is an essential component of the cell's energy production system. and the digestive proteins trypsin and chymotrypsin[#](National Academy of Science, 1999).

Molecular studies can also isolate the genes responsible for various traits and how they have changed. For example, recent work has shown that the variation in beak shapes in Galápagos Finches is associated with expression patterns of various growth factors, in particular the expression of a gene called Bmp4 in species comprising the genus Geospiza and the timing and spacial expression of a gene called calmodulin[#](Abzhanov, et al.2004).

One possible explanation for the relative similarity between genes from different organisms is that their ways of life are similar. For example, otter and mink genes are more similar than those of otter and rabbit genes. One could argue this is because otters and mink share more similar habitats and behaviors than do otters and rabbits.

But this possible explanation can apply only to functioning genes, however. It does not work for pseudogenesGenes that have lost their protein-coding ability or are otherwise no longer expressed but remain part of the DNA., since they perform no function. Pseudogenes are sort of a molecular version of vestigialSomething that is a vestige (remnant) of a primitive, homologous structure,which has lost all or most of its original function. structures like whale legs. Like functioning genes, pseudogenes also change through time and at a predictable rate, due to random mutations. Since pseudogenes serve no purpose, the degree of similarity between them must simply reflect their evolutionary relatedness. And, as predicted by evolutionary theory, the more remote the last common ancestor of two organisms, the more dissimilar their pseudogenes are[#](Ohta & Nishikimi. 1999).

Benton, MJ. 2005. Fossil Record: Quality. In: Encyclopedia of Life Sciences. Chichester: John Wiley & Sons, Ltd.

Shubin, NH, EB Daeschler, FA Jenkins. 2006. The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440:764-771

Thewissen, JG, EM Williams, LJ Roe & ST Hussain. 2001. Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature. 413: 277-281.

Forster, CA, SD Sampson, LM Chiappe, & DW Krause. 1998. The theropod ancestry of birds: new evidence from the Late Cretaceous of Madagascar. Science 279:1915-1922.