One Way Biologists Use DNA to Answer Questions About Evolution
Here’s an analogy, using words, of how evolutionary biologists and population geneticists use DNA evidence to answer questions about evolutionary history.
Let’s imagine that the words “night,” “sight,” and “fight” are like animals found in South America, Madagascar, and Australia, respectively. And you have a fossil word, light, found in oceanic deposits. Let’s also imagine that it’s easier for evolution to change or add one letter in a word at a time than to change multiple letters. You could make a hypothesis that light was the ancestor of night, sight, and fight and each evolved separately, by a one letter change. To tie this back to the crayfish story, this would be analogous to three separate colonizations of land by a single marine ancestor.
You could also, however, hypothesize that what happened was that light became night, which became sight, which became fight. This would be analogous to a single colonization, with subsequent evolution on land.
In this case, both hypotheses explain the pattern equally well.
But what if the words you found on South America, Madagascar and Australia were “fight,” “fright,” and “freight”? In that case, it’s easy to imagine that light became fight, which became, by one letter addition, fright, which, by a similar process, became freight. The other hypothesis, the one analogous to multiple colonizations would be more difficult to swallow, since it would require that light became fight, light became fright (a one letter change and a one letter addition), and light became freight (a one letter change and two additions).
The evidence would point strongly to a single colonization by light to fight to fright to freight.
DNA is not words, of course. It's a large molecule with a series of four different base pairs. We designate those base pairs A, T, C, and G. And each organism’s DNA contains millions of these As, Ts, Cs and Gs. By looking at the differences in the sequences of base pairs in the DNA of crayfish, computers can do an analysis similar to what we did with words, and give us answers as to which hypothesis is more likely to be correct.