Artificial selection: are humans driving non-intentional evolution in animals?

For years, fish scientists have alerted about the dangers of establishing a minimum fish size for a given species, as a way to assure that fish makes it at least through its first reproduction, disregarding regulations about a maximum size as well. But the topic has finally made the news. In an interview to BBC,Professor Adam Hart explained that we have for millennia intentionally selected size and other features of our domesticated livestock and plants, which has led to descendent organisms that differ genetically from their ancestors. This change in gene frequency is evolution, and, in this human-induced case, we call it artificial selection. However, 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, which has been labeled as "unnatural selection".

As we let it slip at the beginning of our post, one such example of unnatural selection is what we have been doing to our fish under the auspices of our fishing regulations. We have all (at least the ones reading this post) come to believe that it is wrong to eat small juvenile fish: we should let them reproduce first to assure that we will have more fish in the future. Indeed, there is nothing wrong with such reasoning, the problem is that such measures addresses only half of the problem. As the preferred choice of both fishermen and consumers, the policy of catching large fish and releasing the small ones has been an important tool for managing fish stocks since the 1880’s. With time, policies and regulations have advised the use of larger mesh sizes in fishing nets, as a way of selecting larger fish whilst letting the little ones pass through the gaps. Under most national policies, fishermen have been legally obliged to target large fish and release the smaller ones. Size limits, of course, vary according to the target species.

Photo by Laura Honda

Despite its good intention, targeting the largest fish has consequences: "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" said Dr Eric Palkovacs from the University of California Santa Cruz in the same interview to BBC. In other words, the genes for "smallness" prosper while genes for "largeness" are selectively removed by fishing.

In addition to the fact that fish are evolving to be smaller, they are also become sexually mature at a younger age. This is because those fish that have genes causing later maturity are likely to be the largest ones, and, therefore harvested before they have the chance to breed, removing those genes from the population.

Photo by Laura Honda

Photo by Laura Honda

An example of this issue is the North Atlantic Cod, perhaps the most emblematic species in the fisheries literature. This species can reach up to 2 or more meters in length and these giants were common in the 19th century. Nowadays, cods of this size are virtually extinct, they rarely grow beyond 1 meter.

However, developing regulations and technology that give both the juveniles and the larger specimens a chance to survive is not straightforward. We could think of regulations with slots of capture, with a minimum and a maximum size allowed. But how feasible is that? Or perhaps, we could use our understanding of the high fish mortality in the early phases of fish development and target a proportion of juveniles only. But then again: do we really know enough about all fish targets to establish juvenile quotas? Or more seriously: can countries, especially developing ones, really enforce such quotas?

While we wait for the right answers or right regulations, we are probably only worsening a "Darwinian debt" for generations to come.

By Maria Grazia Pennino