An Impact of Humans on Evolution

In Chapter 22 of Human Biology 5th ed. (p. 513) we make the point that evolutionary processes may be impacted by human activities, such as trophy hunting for big-game animals. The evidence comes from a study of bighorn sheep. (See “Undesirable Evolutionary Consequences of Trophy Hunting”, Nature 426: 655-658, 2003). The article documents a substantial decline in body size, horn size, and reproductive success among four-year-old (young breeding age) male bighorn sheep in one location in Alberta, Canada over 30 years as the result of the hunting practices that targeted primarily the prime breeding-age males.

Is there a solution to this problem that would still allow trophy hunting? Well, yes there is, and it lies in a key concept of evolution - that the death of an individual animal after his/her reproductive years cannot affect heritable processes. Therefore, hunters should be allowed to harvest only the truly old males; males so old that they are unlikely to be competing successfully with younger, more fit males for females. In bighorn sheep this age is probably about 8 years, when the horns generally form a complete curl. Wildlife management experts are taking heed. Over the years the legal minimum horn length for bighorn sheep has risen in most wildlife management areas from 3/4 curl, to 4/5 curl, and now to full curl. The number of male bighorn sheep harvested each year has dropped, but that’s the price that has to be paid. Sound wildlife management practices based on good science will benefit us all, whether or not you hunt.

Incidentally, a more widespread problem may be commercial fishing. When minimum size limits are set, the surviving female fish are smaller, reproduce at a younger age, and produce fewer eggs. In other words, when humans harvest the big fish, the puny fish are more likely to survive to pass on their genes.