4 - Common Ancestry

Summary

● Darwin thought that (a) similarities (“matches”) are evidence for common ancestry, but that (b) adaptive similarities provide only weak evidence while neutral or deleterious similarities provide evidence that is stronger. These two ideas about evidence are assessed by using the Law of Likelihood, which is explained and defended. ● Standard assumptions about the evolutionary process suffice to provide a justification for (a). ● Models of different types of evolution process are considered that allow conclusions to be drawn about Darwin’s (b); an example is described in which an adaptive similarity provides stronger evidence for common ancestry than a neutral similarity provides. ● The Law of Likelihood has a symmetry property: if matches favor common ancestry over separate, then “mismatches” must have the opposite evidential significance. This raises the question of how similarities and differences can simultaneously be taken into account. ● An answer is developed by using the concept of correlation, once Reichenbach’s principle of the common cause is shown to be too strong.