Are humans driving evolution in animals?

By Prof Adam Hart
University of Gloucestershire

Image source, EPA
Image caption,
Bigger fish are preferentially removed from the sea, enabling genes for "smallness" to prosper

Are humans inadvertently driving evolution in other species? Mounting evidence suggests activities such as commercial fishing, angling and hunting, along with the use of pesticides and antibiotics, are leading to dramatic evolutionary changes.

Sitting down to a roast chicken dinner doesn't seem like an obvious opportunity to consider evolution. But it is.

Think about it: those big tasty carrots, that plump, tender chicken and those handsome potatoes all differ markedly from their natural ancestors.

A wild carrot is barely more than a slightly enlarged purple tap-root and red jungle fowl certainly don't have the extravagant cleavages found on modern broiler chickens.

The intentional selection of the qualities we like (such as flavour and size) in domesticated livestock and cultivated crops has led to descendent animals and plants that differ genetically from their ancestors. This change in gene frequency is evolution, and in this case has come about by a process called artificial selection.

Natural selection is basically the same process. The difference is that instead of humans selecting individuals to breed, natural selection pressures such as predation, or the reluctance of females to mate with lower quality males, cause some individuals in a population to prosper and produce offspring while others fare poorly, leaving fewer offspring.

If the trait that caused the parents to prosper has a genetic basis, then the offspring will inherit that trait and likewise prosper, changing the frequency of genes in the population.

Image source, Getty Images
Image caption,
Cultivated crops differ markedly from their natural ancestors

Not all human selection pressures are as intentional as those imposed by plant and animal breeders. Recent research is revealing that many of our activities exert significant unintentional selection on organisms. Such "unnatural selection", as it has been termed, is causing evolution in those populations as the inevitable logic of Darwinian selection kicks in.

Perhaps the best and most important example of unintentional evolution arising from our activity is antibiotic resistance. Antibiotics impose an immense selection pressure on bacteria and there is a huge advantage to any that can resist. Likewise, pesticides select for pesticide resistance.

Some well understood examples of unnatural selection and evolution come from commercial fisheries. It is the larger fish that are usually targeted and those that remain are consequently smaller. But crucially this effect isn't just a demographic change.

Dr Eric Palkovacs from the University of California Santa Cruz explains: "We have removed the large fish and that has a direct effect on the size structure of a population. Subsequent populations will feel that impact because those smaller fish contribute more genes to the population." In other words, the genes for "smallness" prosper while genes for "largeness" are selectively removed by fishing.

Not only are the fish evolving to be smaller but they also are evolving to become sexually mature at a younger age. This is because those fish that have genes causing later maturity are likely to be harvested before they have the chance to breed, removing those genes from the population.

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Urbanization selects for species able to tolerate the environments we create

The selection pressure and evolution caused by fishing can have wider ecosystem consequences. Atlantic cod that used to be several metres long are now only a metre or so, which, points out Palkovacs, means "we basically have an organism that once was top predator in the system and now serves as prey to other organisms".

Our relentless pursuit of the biggest individuals is causing evolutionary change in another harvested species; big horn sheep living on the appropriately named Ram Mountain in Alberta, Canada.

Trophy hunters pay large sums to hunt these animals and they are after the biggest and most impressive males. Big males with big horns can fight successfully against other males and thereby mate with far more females than smaller males with less impressive horns.

Professor David Coltman, at the University of Alberta, and colleagues have been studying the sheep on Ram Mountain for over 40 years, following individuals throughout their lives. The males grow their horns quite quickly when they're young, but a successful ram has to live long enough to become socially dominant. Once he does, those big horns pay off and he can have a lot of offspring.

Hunters set up a strong selection pressure on these big males. Suddenly the advantages of being big (more mates, more offspring) are countered by a rather big disadvantage (being shot and mounted on a wall).

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Hunting has exerted a strong selection pressure on big horn sheep

This pressure is leading to smaller horns but it's not just that there are more smaller-horned males around because the larger-horned males are being removed. As in fisheries, hunting is causing evolutionary changes in the genetic make-up of the population, in this case resetting the baseline of horn size to a lower level.

Prof Coltman explains: "The crux here is that the horns develop to the length where they can be legally harvested several years before they achieve social dominance. So in effect they're being harvested from the population before they can reproduce and pass their genes on."

As a consequence, big horn sheep horns have evolved to be as much as 25% smaller.

When we fish and hunt we aren't acting like natural predators. We are relentless, ruthlessly efficient "super predators" taking out the biggest and best. Similarly, when we change the environment we do so on a grand scale.

Urbanization continues apace and selects for species able to tolerate the environments we create. At a global scale we are causing climate change that imposes further selection pressures that we still don't fully understand.

It seems that virtually everything we do can have an accidental evolutionary consequence and scientists are already devising evolutionarily sustainable management plans for harvested resources.

This is just as well, because if we aren't prudent in managing our unnatural selection pressures we will be paying a "Darwinian debt" for generations to come.

Prof Adam Hart presents Unnatural Selection on BBC Radio 4 at 11:00 GMT on Tuesday 2 February.

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