Every few years it seems that the British biologist Steve Jones declares the death of evolution by natural selection in the human species. The logic here is simple even to a schoolboy: evolution requires variation in fitness, and with declining risk by death during our reproductive years humans have abolished the power of selection. But this confuses the symptom for the disease. Death is simply one way that natural selection can occur. Michelle Duggar has 19 children. The average American woman has around two by the end of her reproductive years. It doesn’t take a math whiz to figure out that Michelle Duggar is more “fit” in the evolutionary sense than the average bear. Even without high rates of death, some people have more children than other people, and if those people who have more children than those who do not are different from each other in inherited traits, evolution must occur. Q.E.D.
But you probably shouldn’t be convinced by logic alone. Science requires theory, experiment, and observation. (If you’re talking humans, you can remove the second from the list of possibilities: there are certain unavoidable ethical obstacles to experimenting on human evolution—plus we take far too long to reproduce.) But humans sometimes have something which bacteria can not boast: pedigrees! Not all humans, of course. Like most of the world’s population I don’t have much of a pedigree beyond my great-grandparents’ generation. But luckily for biologists, the Catholic Church has long taken a great interest in life events such as baptism, marriage, and death, and recorded this info parish by parish. With these basic variables, demographers can infer the the rough life histories of many local populations over the centuries. In many European nations, these databases can go more than 10 generations back. And some aspects of human evolution are revealed by these records.
What aspects am I talking about? Reproduction itself. Not only is variation in fitness one of the primary ways by which evolution occurs, but it is also a trait upon which evolution operates! How else are there rabbits which breed like…rabbits, and pandas…which don’t. There is often variation within species for the odds of multiple births, age at first reproduction, and lifespan, depending upon the differences in selection pressures over a population. And that seems to be exactly what occurs in human beings. There is interesting evidence for evolution of reproductive patterns from populations as diverse African pygmies and Finns, but more recently some researchers have been plumbing the depths of the records of the Roman Catholic Church in Quebec, and they’ve come back with gold.