[Viewpoint] Change we can believe in

A recent symposium on evolution in Montreal posed to high-school students and university professors the following question: “Do you think that humans are still evolving?” Approximately 80 percent of the audience answered “no.” Indeed, there is an almost universal belief that, with multifaceted cultures and intricate technology, humans have freed themselves from the pressures of natural selection. Recent findings, however, show otherwise. Far from providing immunity against evolutionary pressures, culture often creates new ones. For example, the genes associated with digestion of lactose are more prevalent in populations that have traditionally bred cattle and consumed milk.

In scientific reviews in Nature Genetics and Proceedings of the National Academy of Sciences of the United States of America, the evolutionary biologist Stephen Stearns and his colleagues set out to demonstrate that natural selection operates on contemporary humans. Supported by extensive genealogies, including centuries of church and national health registries, their argument is convincing.

Indeed, contrary to the widely held assumption that evolution takes millennia to manifest itself, recent evidence suggests that its effects can become visible as quickly as in a few generations. Rapid evolutionary change, or “contemporary evolution,” is not drastic; humans are not likely to sprout wings a few generations down the road. Rather, these evolutionary effects are difficult to detect, as they are reflected in a population’s genetic composition.

Contemporary evolution requires a specific set of conditions. First, the population must comprise individuals with varying characteristics, or traits. Moreover, members of the population must differ in terms of survival rates, including, most important, lifetime reproductive success (LRS) - the total number of offspring that they produce over a lifetime. These conditions are fundamentally linked: different values for a particular trait translate into diverse survival rates.

This crucial link facilitates change in a trait’s average value over successive generations. For example, if larger individuals produce more children than smaller ones, the number of larger individuals would grow, thereby increasing the average size of individuals in the population. The most significant changes occur when individuals at one end of a trait’s value are heavily favored, pushing the entire population in that direction.

Prevalent human traits are often strongly associated with LRS. For example, people who first reproduce at a younger age tend to have more children, so selection generally favors those who become parents earlier. Tall women’s LRS tends to be lower, while tall men’s is higher.

But translating selection pressure into evolutionary change requires another crucial ingredient: the variation observed in the trait should be caused at least in part by genetic differences. Indeed, evolution is possible only if the resemblance between related individuals has a genetic basis, and is not simply a reflection of a shared environment.

For example, siblings do not have to grow up together for their resemblance to be evolutionarily meaningful, owing to their common genes. Even when it comes to morphology, personality, and life-history traits - such as age at sexual maturity and fertility - related individuals’ likeness often has a genetic basis.

The conditions required for rapid evolutionary change to occur in human populations exist. But, given that measurable alterations in a trait over time can occur for many reasons - including those related to shared culture or environment - or randomly (“genetic drift”), studies demonstrating this phenomenon are rare. Today’s challenge is to isolate the diverse sources of change.

Recently developed statistical tools have finally made this possible. With more sophisticated methods, my colleagues and I were reliably able to distinguish genetic changes, and thus to demonstrate an evolutionary shift toward a younger age at first birth in a small, insular population in Quebec over the last 140 years.

This discovery challenges another fundamental myth: evolutionary change necessarily benefits the species. In fact, evolution simply increases the average individual’s reproductive success - with potentially damaging demographic consequences. So, while contemporary evolution is occurring, adaptation may not always lead to a better life.

* The author is chair of research in behavioral ecology, department of biological sciences, University of Quebec.

by Denis Reale