Evidence for evolution
We have discussed why evolutionary theory arose, the mechanisms that may be responsible for evolution, and a mathematical theory to identify when evolution is occurring. But we still have not investigated one important question: How do we know evolution occurred? Most evidence supporting evolution can be classified in one of the following categories:
- Comparative anatomy: This involves careful comparison of body parts in different organisms. For example, all mammals have hair or fur and have mammary glands used to produce milk to feed their young. Further investigation reveals that mammals also have similar bone structures. These data imply that all mammals had an ancestral relationship. If we further examine the bone structure in mammals, birds, reptiles and amphibians, we can also see amazing similarities, again indicating that even these diverse organisms came from a common ancestral species.
- Comparative biochemistry: Recent advances in biochemical analysis, including protein and DNA sequencing, have allowed for detailed investigations into comparisons of species at the molecular level. This type of comparison reveals differences in sequences that can be used to determine how similar two species are. The rate of change in sequences has also been used as a molecular clock to calculate the time of divergence between two species.
- Vestigial structures: Organs or body parts that have no functional value to an organism are called vestigial structures. Since they are present but have no function, they are probably remnants of structures from an ancestral species. For instance, the appendix in humans is an example of a vestigial structure. It is the remains of the cecum, a structure that acts as a storage chamber in animals that eat a diet high in cellulose. Another example is the pelvic girdle found in whales. This structure has no function and, with time, has been reduced in size and even disconnected from the spine. However, it remains, and gives us evidence that ancestral whales were once land dwelling creatures with hind limbs.
- Embryonic development: This is perhaps the most curious and fascinating category we will discuss. When vertebrates are examined, the stages of embryonic development are strikingly similar. Even in organisms as different as turtles and chickens and humans, the form of the developing embryo, and the structures of the embryo, are almost identical. In fact, at one point in human development, gill slits are apparent. It is argued that these similarities are evidence for a common ancestor for all vertebrates.
- Natural distribution of living things: By examining where living things exist in the world, we can compare similarities and differences among the species. This has lead to some interesting findings.
- Fossil evidence: Probably the most important category of evidence for evolution is the fossil record. Various methods, most notably radioactive dating, can determine the age of particular rocks. Embedded in these rocks are fossils, preserved specimens of once living organisms. What we know from fossils is that organisms that were alive in the past do not closely resemble organisms alive today, and many species are extinct. What we can see in these fossils are similarities with plants and animals that are alive today, but that these creatures have changed over time. One of the best examples of this is the horse. Fossil evidence over the past 60 million years documents the changes in the horse, including its overall size, hoof structure and jaw and tooth patterns.
- Selective breeding (artificial selection): Humans have been cultivating plants and animals for thousands of years. We have selected certain traits and attributes in these organisms and breed organisms to retain and enhance traits that are useful and beneficial to us. This process of selective breeding or artificial selection is quite amazing.