Recently, interest in phylogeny has received a new impetus, as our rapidly growing knowledge of genetics and molecular evolution is providing a new approach, a new source of evidence about the course of evolution.
Molecules can be used in different ways. For example, theoretically, the total genes of two different species can be compared.
The underlying idea is that if the genetic difference between two species is slight, they are liable to be closely related, and the degree of genetic difference will indicate the closeness of that relationship. The practical difficulties are enormous because most animals have so many genes, and there are theoretical difficulties also. To base our study on the genotype itself seems very attractive, but we must as always be wary of reduction to the simplest units, as this will eliminate essential information that depends on the organization and interaction of those units. The sheer presence of particular genes does not define their effects: the action of genes depends upon other genes present, and a small change in genes can make a big change in animals. That we share about 99% of our genes with chimpanzees at once illustrates this point. Even though 1% may be a large number of genes, we are not as similar to chimpanzees as the figures suggest. As will be explained, the rate of genetic change is not the same as the rate of species change, and neither
rate has remained constant during evolution.
Using particular molecules as though they were morphological characters presents fewer difficulties: here the idea is that if a molecule (a gene or the immediate product of a gene) changes slowly during evolution, comparison of the amount of change in that molecule will reveal the closeness of the relationship of different groups of animals. For example, a particular molecule might be compared in a range of insects, shrimps and spiders. Comparison should first show that different insects resembled each other more closely than any of them resembled shrimps or spiders, and might then indicate which two of these three groups are most closely related. In short, molecular characters can be substituted for morphological characters to assess the relatedness of animals.