Killed people's natural selection

The human being as the engine of evolution

Animals are hunted, fished or become victims of traffic accidents. Depending on how one-sided the pressure is on the species that is built up in this way, this can have evolutionary consequences for the species concerned.

Cities, environmental pollution, climate, agriculture and forestry - humans have changed their environment enormously in a very short time. And with it the living conditions of the living beings that populate the earth with it. For many wild species, this changes the selection pressure that has acted on them for millennia and guided their development. For some species, these changes came too quickly or were too fundamental to allow their survival, as evidenced by the high number of plant and animal species that have died out in modern times. Others still find enough habitat to survive unchanged. And some change.

Swallow versus car

Often times, the pressures that humans exert in other ways are indirect. For example, animals that are transported to foreign areas can affect native species there or even make them disappear. Sometimes, however, the influence of humans is also very direct: that is when certain animals are deliberately removed from populations. This selection does not have to be made consciously, as a recent study from the American state of Nebraska showed. Charles Brown and Mary Bomberger Brown have been studying pale-forehead swallows there for decades. The animals nest in colonies that like to be under motorway bridges, as the researchers write in the journal Current Biology.

If you look at the number of birds that are killed each year in collisions with cars - an estimated 80 million in the United States alone, according to scientists - you would actually expect that natural selection would prefer animals that could avoid such collisions, it said. In fact, during their investigations, the ornithologists found that the number of swallows killed by cars has decreased significantly over the past 30 years, with the same or increased traffic. What also changed are the wings of the birds. They became shorter in the total population over the years. Those of the animals that ended up as “roadkills” on the street diverged more and more from the average over the same period: They became longer and longer in comparison.

The scientists conclude from this that car traffic disadvantages swallows with longer wings - possibly because they could soar less steeply away from the road than animals with short wings. Accordingly, they assume that the traffic exerts a selection pressure on the animals, which is involved in the creation of the shorter wings. Not alone, however: the researchers identified cold waves and a changed spectrum of prey insects (also caused by humans) as additional factors in this development.

The fact that swallows with long wings of all things are "sorted out" in the population examined by the Browns is a side effect of traffic, not a targeted removal. The case was different with a herd of Canadian bighorn sheep that Marco Festa-Bianchet from the Université de Sherbrook in Quebec and his colleagues examined. His study by Ram Mountain, published ten years ago, is considered to be one of the best evidence that an intensive and targeted removal of only certain individuals from a population can lead to changes on an evolutionary level. Bighorn sheep are hunted almost exclusively because of the twisted horns of the rams; Females and young animals are usually not shot. Before a ram can be hunted as a “trophy ram”, its horns must have reached a certain size, which happens sooner for some animals, later or never for others.

Big horns - big success

Usually, the rams with the largest horns are particularly strong and most successful in reproduction - when they reach a certain age. At Ram Mountain, however, it was precisely these animals that were shot years before they reached this age and, according to Festa-Bianchet's study, contributed less to the survival of the population than would have been the case under natural circumstances. As it turned out, this was evidently not without consequences: the horns of the bighorn sheep on Ram Mountain became smaller over the years.

A study on a mouflon population in southern France found similar effects. In this intensely hunted population, according to the authors, both the quality of the horns of the rams and the body size of both sexes decreased over the course of almost 30 years. The researchers attribute this to the fact that the animals with the largest horns were given preference and that horn and body size were closely related. Here, too, males whose horns were large at a young age were apparently particularly at risk of being shot early. This meant that they could only contribute little to the gene pool of subsequent generations.

A recently published study from the USA also suggests that selective and intensive hunting can show "side effects". Terry Bowyer of the University of Idaho and colleagues compared over 22,000 trophies of North American native animals shot between 1900 and 2008. In their evaluation, the researchers found a trend towards a reduction in the size of these structures in 11 of 17 antlers and 3 of 8 horn-bearers. This find could be explained with two hypotheses. The better supported is that intensive hunting of the males has led to a rejuvenation of the male population structure, says Bowyer. It would be possible, but less well established, that the finding is based on a selective hunt for the largest trophies.

European red deer

Atle Mysterud from the University of Oslo and his colleagues examined red deer antlers that had been hunted in Hungary over a period of about a hundred years. They found individual periods in which the trophy quality decreased. Mysterud attributes this to excessive hunting pressure during the relevant times, which - similar to what was assumed in the American trophy study - led to a strong rejuvenation of the population. In contrast to his American colleagues, however, he found no tendency to steadily reduce the size of the trophies. He explains this with the fact that the deer in his field of study are not hunted exclusively from the point of view of the trophy size, but according to very different criteria. This prevents one-sided selection pressure. In addition, when regulating hunting, measures were taken decades ago to prevent such effects, he writes.

There are also regulations in Switzerland to prevent negative genetic consequences of hunting, as Martin Baumann from the Hunting, Fishing, Forest Biodiversity Section of the Federal Office for the Environment says. The cantons planned the hunt in such a way that the remaining stock of the animals would remain as natural as possible. In particular, the hunt for males is restricted so that enough old males live in the herd. According to Baumann, for example, neither yearlings nor adult deer with particularly strong antlers are allowed to be shot in Graubünden. In this way, the canton protects those males from excessive hunting who are likely to reproduce most successfully even under natural circumstances.

Another hot topic is the pressure that fisheries have on removing animals from fish stocks on a large scale - and depending on the equipment used, with varying degrees of selectivity. It is known that this can lead to changes within populations. In a long-term laboratory experiment, for example, the removal of the largest fish in each population resulted in the animals not only being on average smaller after five generations, but also showing a number of other changes, such as a lower egg volume or a changed feed intake. When the researchers stopped the selective extraction, some of these properties (after five generations) returned to the original "mean" - but not all. In nature, the effects of fishing on growth and maturation processes have been demonstrated, for example in the case of cod. In the Barents Sea, this fish becomes sexually mature around 2.5 years earlier and is smaller than it was a few decades ago.

In most cases - not only in the case of fish - it is not entirely clear to what extent the changes are genetically determined and thus evolutionary and to what extent they are based on phenotypic plasticity, i.e. within the framework of what its genetic composition gives an individual as a reaction space makes his living conditions available. This basically requires genetic tests, but these are still rare. For cod off Iceland, researchers were able to show that the frequency of the different variants of a gene - at the same time as changes in the fishery - have changed in recent decades. The changes shown in Bowyer's and Mysterud's studies, on the other hand, seem to be based on phenotypic effects, according to the researchers.

Presumably, says the population geneticist and evolutionary biologist Fred Allendorf from the University of Montana, both processes are involved in the vast majority of cases. It is important to be aware of the species and the selection pressure that humans exert on the species through hunting and fishing, and to closely monitor the condition of the populations affected.