All organisms store their biological history in their DNA. Therefore, evolutionary changes over time should involve a continued accumulation of genetic changes in the DNA. As a result, organisms that are more distantly related should have accumulated a greater number of these genetic differences than organisms that are more closely related.
Comparing Genomes
Evolutionary biologists believe that all living creatures have descended from simple cellular organisms that arose about 2.5 billion years ago. Some of the characteristics of those earliest organisms have been preserved in all living things alive today. By comparing the genomes (sequence of all the genes) of different groups of animals, we can specify the degree of relationship among the groups more precisely than by any other means.
The genome of gorillas, which the fossil record indicates diverged from humans between 6 and 8 million years ago, differs from the genome of humans by 1.6% of its DNA. However, the genome of chimpanzees, which diverged about 5 million years ago, differs by only 1.2%.
Comparing Protein Sequences
These echoes of the evolutionary past also show up protein sequences, such as hemoglobin, the oxygen-binding component of red blood cells. Humans and macaques, both primates, show less difference in their hemoglobin proteins than do more distantly related mammals, such as dogs. Nonmammalian vertebrates, such as birds, reptiles, and frogs, differ even more.
A Molecular Clock?
Genes evolve at different rates because, although mutation is a random event, some proteins are more tolerant of changes in their amino acid sequence that are other proteins. For this reason, the genes that encode these more tolerant, less constrained proteins evolve faster. The average rate at which a particular kind of gene or protein evolves gives rise to the concept of a molecular clock. Molecular clocks run rapidly for less constrained proteins and slowly for more constrained proteins, though they all time the same evolutionary events.
The concept of a molecular clock is useful for two purposes. It determines evolutionary relationships among organisms, and it indicates the time in the past when species started to diverge from one another. However, it should be noted that the idea of a molecular clock ticking away in proteins is a controversial concept not totally embraced and supported by all scientists.
Even Junk DNA Yields Clues
Another interesting line of evidence supporting evolution involves sequences of DNA known as pseudogenes. Pseudogenes are remnants of genes that no longer function but continue to be carried along in DNA as excess baggage (junk DNA). Pseudogenes also change over time, as they are passed from ancestors to descendants, and they offer an especially useful way of reconstructing evolutionary relationships. Since they perform no function, the degree of similarity between pseudogenes must simply reflect their evolutionary relatedness. The more remote the last common ancestor of two organisms, the more dissimilar their pseudogenes will be.
The Molecular Evidence Continues to Grow
The evidence for evolution from molecular biology is overwhelming and is growing steadily. In some case, this molecular evidence transcends paleontological evidence. Take whales for example. Anatomical and paleontological evidence indicates that the whale’s closest living land relatives seem to be the even-toed hoofed mammals (cattle, sheep, camels, goats, etc.). Recent analysis of some milk protein genes have confirmed this relationship and have suggested that the closest land-bound living relative of the whales may be the hippopotamus.
Evidence of Evolution
The molecular record is one of several lines of evidence that reveal the ongoing and dynamic nature of the process of evolution. Other pieces of evidence are biogeography, comparative anatomy, and the fossil record.
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