An Introduction to the Invertebrates, Second Edition - tiera.ru

18 THE PATTERN OF EVOLUTION: METHODS OF INVESTIGATIONsuggest much earlier dates for the establishment of animals thanfossil evidence can verify. A study of 18 genes placed the divisionof the main groups of animals as early as 670 Ma, with the earliestdivergence between the earliest animals and these main groups1000 Ma or more. Is this discrepancy because molecular changemay not accurately measure the passage of time, or is it becauseearlier fossils have not been found? Evolutionary lines could haveseparated long before morphological differences became detectablein fossils; we await more evidence. At present fossil evidence startslife too late, perhaps due to missing fossils, and molecular evidencestarts life too early, perhaps due to statistical bias. The two estimatesare converging, but seem unlikely to meet firmly.In conclusion, clearly fossils must be included in phylogeneticreconstruction, although cladistic analysis of fossils is especiallyvulnerable to lack of information about characters and to underestimationof convergence. We do risk overemphasising charactersfossilised by chance but, as with any method, we have to workwith what we have got. We do risk interpreting fossils accordingto our preconceptions but, again, we can do that in any form ofbiological enquiry. At least we can now compare the evidence fromthree sources (fossils, morphology and molecules) to assess therelationships of present-day animals.2.5 How can we use molecules to trace phylogeny?Recently, interest in phylogeny has received a new impetus, as ourrapidly growing knowledge of genetics and molecular evolution isproviding a new approach, a new source of evidence about thecourse 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 twospecies is slight, they are liable to be closely related, and the degreeof genetic difference will indicate the closeness of that relationship.The practical difficulties are enormous because most animals haveso many genes, and there are theoretical difficulties also. To baseour study on the genotype itself seems very attractive, but we mustas always be wary of reduction to the simplest units, as this willeliminate essential information that depends on the organisationand interaction of those units. The sheer presence of particulargenes does not define their effects: the action of genes dependsupon other genes present, and a small change in genes can makea big change in animals. That we share about 99% of our geneswith chimpanzees at once illustrates this point. Even though 1%may be a large number of genes, we are not as similar to chimpanzeesas the figures suggest. As will be explained, the rate of geneticchange is not the same as the rate of species change, and neitherrate has remained constant during evolution.