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There are many forms of evidence for evolution. One of the strongest forms of evidence is comparative anatomy; comparing structural similarities of organisms to determine their evolutionary relationships. Organisms with similar anatomical features are assumed to be relatively closely related evolutionarily, and they are assumed to share a common ancestor. As a result of the study of evolutionary relationships, anatomical similarities and differences are important factors in determining and establishing classification of organisms. Some organisms have anatomical structures that are very similar in embryological development and form, but very different in function. These are called homologous structures. Since these structures are so similar, they indicate an evolutionary relationship and a common ancestor of the species that possess them. A clear example of homologous structures is the forelimb of mammals. When examined closely, the forelimbs of humans, whales, dogs, and bats all are very similar in structure. Each possesses the same number of bones, arranged in almost the same way. While they have different external features and they function in different ways, the embryological development and anatomical similarities in form are striking. By comparing the anatomy of these organisms, scientists have determined that they share a common evolutionary ancestor and in an evolutionary sense, they are relatively closely related. Other organisms have anatomical structures that function in very similar ways, however, morphologically and developmentally these structures are very different. These are called analogous structures. Since these structures are so different, even though they have the same function, they do not indicate an evolutionary relationship nor that two species share a common ancestor. For example, the wings of a bird and dragonfly both serve the same function; they help the organism to fly. However, when comparing the anatomy of these wings, they are very different. The bird wing has bones inside and is covered with feathers, while the dragonfly wing is missing both of these structures. They are analogous structures. Thus, by comparing the anatomy of these organisms, scientists have determined that birds and dragonflies do not share a common evolutionary ancestor, nor that, in an evolutionary sense, they are closely related. Analogous structures are evidence that these organisms evolved along separate lines. Vestigial structures are anatomical features that are still present in an organism (although often reduced in size) even though they no longer serve a function. When comparing anatomy of two organisms, presence of a structure in one and a related, although vestigial structure in the other is evidence that the organisms share a common evolutionary ancestor and that, in an evolutionary sense, they are relatively closely related. Whales, which evolved from land mammals, have vestigial hind leg bones in their bodies. While they no longer use these bones in their marine habitat, they do indicate that whales share an evolutionary relationship with land mammals. Humans have more than 100 vestigial structures in their bodies. Comparative anatomy is an important tool that helps determine evolutionary relationships between organisms and whether or not they share common ancestors. However, it is also important evidence for evolution. Anatomical similarities between organisms support the idea that these organisms evolved from a common ancestor. Thus, the fact that all vertebrates have four limbs and gill pouches at some part of their development indicates that evolutionary changes have occurred over time resulting in the diversity we have today.
This drawing was created in 1848, but it's likely that you recognize the animal it depicts as a horse. Although horses haven't changed that much since this drawing was made, they have a long evolutionary history during which they changed significantly. How do we know? The answer lies in the fossil record. 
Fossils are a window into the past. They provide clear evidence that evolution has occurred. Scientists who find and study fossils are called paleontologists. How do they use fossils to understand the past? Consider the example of the horse, outlined in figure \(\PageIndex{2}\). Fossils spanning a period of more than 50 million years show how the horse evolved. The oldest horse fossils show what the earliest horses were like. They were only 0.4 m tall, or about the size of a fox, and they had four long toes. Other evidence shows they lived in wooded marshlands, where they probably ate soft leaves. Over time, the climate became drier, and grasslands slowly replaced the marshes. Later fossils show that horses changed as well.
Scientists can learn a great deal about evolution by studying living species. They can compare the anatomy, embryos, and DNA of modern organisms to help understand how they evolved.
Comparative anatomy is the study of the similarities and differences in the structures of different species. Similar body parts may be homologous structures or analogous structures. Both provide evidence for evolution. Homologous structures are structures that are similar in related organisms because they were inherited from a common ancestor. These structures may or may not have the same function in the descendants. Figure \(\PageIndex{3}\) shows the upper appendages of several different mammals. They all have the same basic pattern of bones, although they now have different functions. All of these mammals inherited this basic bone pattern from a common ancestor. Analogous structures are structures that are similar in unrelated organisms. The structures are similar because they evolved to do the same job, not because they were inherited from a common ancestor. For example, the wings of bats and birds, shown in the figure that follows, look similar on the outside and have the same function. However, wings evolved independently in the two groups of animals. This is apparent when you compare the pattern of bones inside the wings.
Comparative embryology is the study of the similarities and differences in the embryos of different species. Similarities in embryos are likely to be evidence of common ancestry. All vertebrate embryos, for example, have gill slits and tails. All of the embryos in Figure \(\PageIndex{4}\), except for fish, lose their gill slits by adulthood, and some of them also lose their tail. In humans, the tail is reduced to the tail bone. Thus, similarities organisms share as embryos may no longer be present by adulthood. This is why it is valuable to compare organisms in the embryonic stage.
Structures like the human tail bone are called vestigial structures. Evolution has reduced their size because the structures are no longer used. The human appendix is another example of a vestigial structure. It is a tiny remnant of a once-larger organ. In a distant ancestor, it was needed to digest food, but it serves no purpose in the human body today. Why do you think structures that are no longer used shrink in size? Why might a full-sized, unused structure reduce an organism’s fitness?
Darwin could compare only the anatomy and embryos of living things. Today, scientists can compare their DNA. Similar DNA sequences are the strongest evidence for evolution from a common ancestor. Look at the diagram in Figure \(\PageIndex{5}\). The diagram is a cladogram, a branching diagram showing related organisms. Each branch represents the emergence of new traits that separate one group of organisms from the rest. The cladogram in the figure shows how humans and apes are related based on their DNA sequences.
Biogeography is the study of how and why organisms live where they do. It provides more evidence for evolution. Let’s consider the camel family as an example.
Today, the camel family includes different types of camels (Figure \(\PageIndex{6}\)). All of today’s camels are descended from the same camel ancestors. These ancestors lived in North America about a million years ago. Early North American camels migrated to other places. Some went to East Asia via a land bridge during the last ice age. A few of them made it all the way to Africa. Others went to South America by crossing the Isthmus of Panama. Once camels reached these different places, they evolved independently. They evolved adaptations that suited them for the particular environment where they lived. Through natural selection, descendants of the original camel ancestors evolved the diversity they have today.
The biogeography of islands yields some of the best evidence for evolution. Consider the birds called finches that Darwin studied on the Galápagos Islands (Figure \(\PageIndex{7}\))). All of the finches probably descended from one bird that arrived on the islands from South America. Until the first bird arrived, there had never been birds on the islands. The first bird was a seed eater. It evolved into many finch species, each adapted for a different type of food. This is an example of adaptive radiation. This is the process by which a single species evolves into many new species to fill available ecological niches.
In the 1970s, biologists Peter and Rosemary Grant went to the Galápagos Islands to re-study Darwin’s finches. They spent more than 30 years on the project, but their efforts paid off. They were able to observe evolution by natural selection actually taking place. While the Grants were on the Galápagos, a drought occurred, so fewer seeds were available for finches to eat. Birds with smaller beaks could crack open and eat only the smaller seeds. Birds with bigger beaks could crack open and eat seeds of all sizes. As a result, many of the smaller-beaked birds died in the drought, whereas birds with bigger beaks survived and reproduced. As shown in Figure \(\PageIndex{8}\), within 2 years, the average beak size in the finch population increased. In other words, evolution by natural selection had occurred.
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The Galapagos finches remain one of our world's greatest examples of adaptive radiation. Watch as these evolutionary biologists detail their 40-year project to document the evolution of these famous finches: LICENSED UNDER |
What do homologous structures suggest about the evolutionary relationship between organisms?
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