Quote from Behave: the biology of humans at our best and worst by Robert M. Sapolsky (my bullets and highlights)
Evolution rests on three steps: (a) certain biological traits are inherited by genetic means; (b) mutations and gene recombination produce variation in those traits; (c) some of those variants confer more “fitness” than others. Given those conditions, over time the frequency of more “fit” gene variants increases in a population.
We start by trashing some common misconceptions.
First, that evolution favors survival of the fittest. Instead evolution is about reproduction, passing on copies of genes. An organism living centuries but not reproducing is evolutionarily invisible.* The difference between survival and reproduction is shown with “antagonistic pleiotropy,” referring to traits that increase reproductive fitness early in life yet decrease life span. For example, primates’ prostates have high metabolic rates, enhancing sperm motility. Upside: enhanced fertility; downside: increased risk of prostate cancer. Antagonistic pleiotropy occurs dramatically in salmon, who epically journey to their spawning grounds to reproduce and then die. If evolution were about survival rather than passing on copies of genes, there’d be no antagonistic pleiotropy.
Another misconception is that evolution can select for preadaptations—neutral traits that prove useful in the future. This doesn’t happen; selection is for traits pertinent to the present. Related to this is the misconception that living species are somehow better adapted than extinct species. Instead, the latter were just as well adapted, until environmental conditions changed sufficiently to do them in; the same awaits us. Finally, there’s the misconception that evolution directionally selects for greater complexity. Yes, if once there were only single-celled organisms and there are multicellular ones now, average complexity has increased. Nonetheless, evolution doesn’t necessarily select for greater complexity—just consider bacteria decimating humans with some plague.
The final misconception is that evolution is “just a theory.” I will boldly assume that readers who have gotten this far believe in evolution. Opponents inevitably bring up that irritating canard that evolution is unproven, because (following an unuseful convention in the field) it is a “theory” (like, say, germ theory). Evidence for the reality of evolution includes:
· Numerous examples where changing selective pressures have changed gene frequencies in populations within generations (e.g., bacteria evolving antibiotic resistance). Moreover, there are also examples (mostly insects, given their short generation times) of a species in the process of splitting into two.
· Voluminous fossil evidence of intermediate forms in numerous taxonomic lineages.
· Molecular evidence. We share ~98 percent of our genes with the other apes, ~96 percent with monkeys, ~75 percent with dogs, ~20 percent with fruit flies. This indicates that our last common ancestor with other apes lived more recently than our last common ancestor with monkeys, and so on.
· Geographic evidence. To use Richard Dawkins’s suggestion for dealing with a fundamentalist insisting that all species emerged in their current forms from Noah’s ark—how come all thirty-seven species of lemurs that made landfall on Mt. Ararat in the Armenian highlands hiked over to Madagascar, none dying and leaving fossils in transit?
· Unintelligent design—oddities explained only by evolution. Why do whales and dolphins have vestigial leg bones? Because they descend from a four-legged terrestrial mammal. Why should we have arrector pili muscles in our skin that produce thoroughly useless gooseflesh? Because of our recent speciation from other apes whose arrector pili muscles were attached to hair, and whose hair stands up during emotional arousal.