How might homologous structures show that evolution has occurred?

Question

How can biologists use the idea of homologous structures to determine whether particular species are closely related?

Answer 1

A homologous structure is a structure shared between two species (or between the male and female of the same species) , but with a different function in each.

Examples are: the same bones in the same relative positions in primate hands, bat wings, whale and dolphin flippers, pterosaur wings, horse legs, the forelimbs of moles, and webbed amphibian legs. The same five bones in the mammalian inner ear correspond to the five bones in the hinged jaws of reptiles. Testicles in males correspond to ovaries in females.

A homologous structure is evidence that the two organisms are related by evolution because it is evidence of a common ancestor that had this structure, but it became "repurposed" for a different function. In other words, it provides an explanation of why two radically different functions as those served by the human hand and a bat's wing, would have the same underlying bone structure ... because they are related by ancestry, they were working with the same raw materials inherited from that ancestor.

The more homologous structures shared between two organisms, the stronger the evidence of common ancestry.

It is also evidence against intelligent design, because these are examples where the structure preceded the function (the same structure modified for two different functions) ... whereas in design, function precedes structure (a designer sees a function, and then designs the most optimal structure to serve that function).

Answer 2

Evolution is generally defined by whether or not a new trait confers survival. If you consider survival a good thing, then evolution is good. So, Evolution=survival. I think survival is a "pro". Of course, evolution process can theoretically work in a "reverse" (unexpected) manner. What if we evolved to simply have an improved survival rate during a global viral epidemic? Then, what if another virus came along which then exploited our "evolved trait", and (worst-case) killed everyone? Would that be "good" evolution? I think not!