- Species called ring species exist. A ring species is a species with a circular geographic distribution (a distribution around something) in which individuals from neigboring populations can reproduce with each other EXCEPT at ONE point. An example is in Ensatina salamanders, shown here:
- Another line of evidence that species have evolved from other species is the existence of homology: similarity among species that is apparently inherited from a common ancestor. So, for example, vertebrates that dwell on land have very similar limb structure with a single large upper limb bone, two lower limb bones, and five fingers. This similarity suggests that all these species evolved from a common ancestor which had this limb structure; the reason all these species have this structure is that their ancestor did. Examples of homology occur throughout all life. Some important points about homology include:
- the pattern of homology is hierarchical -- that is, larger groups of organisms that share some common (homologous) feature can be divided into smaller groups that share more and more features in common. For example, all vertebrates have a vertebral column. This large group includes the smaller group of land-dwelling vertebrates which have a vertebral column AND the common limb structure described above. This group includes the still smaller group of the mammals, which have a vertebral column, common limb structure, AND have fur (hair) and mammary glands. Thus, the pattern of homology is not random. This provides evidence that mammals came from a distant ancestor that was also the ancestor to the rest of the vertebrates and had a vertebral column (that's why all vertebrates have the vertebral column -- they inherited it from that ancestor), and more recently within that group came from an ancestor to all the land-dwelling vertebrates, which evolved the limb structure, and then still more recently came from a common ancestor to all mammals which evolved fur and mammary glands. This hierarchical pattern is thus exactly what is predicted if species evolve, through speciation, from pre-existing species.
- similarity of characters among species is particularly good evidence for evolution if other hypotheses that could explain the similarity can be ruled out. A main alternate hypothesis for similarity is that the structures are required for similar function. Similarities in structures that are modified to be used for different functions, or similarities existing for no functional reason, are thus strong evidence for evolution. For example, the five-toed limb of land vertebrates does not have an apparent functional reason and occurs in species that use their limbs in very different ways (bats use it to support wings, monkeys use it to climb, salamanders to walk.) Thus, common function can be ruled out as a possible explanation for the similarity. More examples of this are given below.
- some homology exists in the form of vestigial organs -- structures that resemble structures of other species but are much reduced in size and may have little or no apparent function. For example, some snakes have tiny bones that resemble leg and pelvic girdle bones of other reptiles. Since snakes do not have legs, these are apparently functionless. The fact that they exist suggests that snakes evolved from a common ancestor with other reptiles, an ancestor that had legs.
- some homology is common to all life and provides evidence that all living things came from a common ancestor. An important example of this is the universality of almost all of the genetic code. Remember that the genetic code is the system of three base sequences in DNA and RNA such that each three letter sequence codes for a specific amino acid in the protein that is made based on the information in DNA and RNA. With a couple of minor exceptions, every organisms has the same code. Since there is apparently no chemical reason that the code has to be what it is, the near universality of the code provides evidence that all species evolved from a common ancestor, which had this genetic code.
- The evidence noted above all comes from comparing currently living species. A large body of evidence for evolution comes from fossils -- the fossil record shows many cases in which, through time, forms occur that have apparently changed slightly from previous forms and which then apparently change into other forms.
These salamanders live around the edges of the central valley of California. They vary in color from place to place, as shown. At most areas where different color forms meet, they reproduce with each other. However, at the bottom of the ring (indicated by the red arrow) where the yellow spotted form on the right meets the brown form that is red-orange underneath on the left, the two forms do not reproduce with each other.
The fact that ring species such as this exist suggests that evolutionary change within populations (presumably what created the color differences between salamander populations) can accumulate to the point where some different populations are as different as are different species -- they can no longer reproduce with each other. The fact that most CAN reproduce suggests that this is all one species, but the populations that can NOT reproduce with each other are as different as two different species.