The evidence for evolution

The fossil record is really much better for establishing that the history of life is one of evolving life than people realize. It is a sampling of the species of life that have lived on this planet. It is a biased sample with respect to the distribution of categories of species. However it is good enough to establish a number of things:

(a) The species in one era of time will resemble more closely those in nearby eras of time than those in more distant eras of time.

(b) Patterns of replacement of species are common. That is, we commonly see transitions in which entire communities of life are replaced by other communities. At first sight this suggests that catastrophism ala Cuvier in which the biosphere was recreated every so often. However the fossil record is more than adequate to establish that these replacements were local (only occurred in particular areas and were not global in scope) and were asynchronous, i.e., occurred at random times with respect to each other.

(c) In those case where there is a relatively complete fossil record (principally the sedimentary marine record) smoothly graded sequences are observed. It is such sequences that led Lamarck to propose his theory of evolution two hundred years ago.

An entirely separate line of evidence is biogeography, the study of the diversity of life as a function of geography. As a general rule geographical barriers are also species barriers. Island species tend to be similar to but not the same as mainland species. The longer they were isolated, the greater the differences. Species with broad geographical distributions tend to be diversified across their ranges.

An interesting example of this are certain South American monkeys in the Brazilian rain forest. Before serious studies of them were undertaken they were assumed to be all one species. DNA studies (and observation of breeding choices) revealed that there were several species, most of whom were divided by river barriers. However the DNA studies also suggested that there was a deep evolutionary divide of about six million years between the monkeys in one area and those in another area. (Molecular clock arguments.) This suggested that in the past the two areas had been separated by a geographical barrier. Geologists investigated and discovered that there had been a hitherto unknown uplift that had occurred about six million years ago that had separated the two areas, an uplift that had subsequently eroded away. In this instance the pattern of biogeographical diversity was strong enough to predict past geology.

There is an enormous amount of supporting evidence that has come out of modern molecular studies. The older sorts of evidence were already so strong when Darwin wrote (almost 150 years ago!) that the question was not seriously in doubt.

The nature of the evidence is not generally one of killer experiments or smoking guns. Rather it is a large collection of bits of information that all fit together. This is characteristic of the reconstruction of history. Any one small bit of evidence can be ambiguous or, in the case of human history, a deliberate forgery or fiction. The evidences are more of the kind “If A happened then we would see B and we don’t” and “If X happened we would see Y and we do”. Broad class of explanation are ruled out because the evidence doesn’t fit.