Forced evolution

Gomphothere, the now-extinct relative of elephants, used to have four tusks. The largest-known tusk belonging to the woolly mammoth stretched 4.2 metres and weighed 91 kg. The longest African elephant tusk measured around 3.5 metres and weighed close to 100 kg. Elongated incisors have been among the most distinctive features for millions of years for proboscideans, after a long proboscis or trunk, of course. But the modern-day jumbos are now shedding this spectacular trait, thanks to human greed.

In Mozambique, past hunting pressure has led to an increase of naturally tuskless elephants, according to a study conducted by Shane Campbell-Staton of Princeton University and colleagues. The researchers, who carried out their work at Gorongosa National Park, recently reported in Science that video and photographic records show as the number of elephants plummeted during the Mozambican Civil War (1977-1992) when armies hunted these giants for ivory, the proportion of tuskless female African savanna elephants (Loxodonta africana) jumped from 18 per cent to 51 per cent.

According to them, decades of poaching have made tusklessness relatively advantageous from an evolutionary standpoint in Gorongosa, pushing the proliferation of tuskless females with mutations in two tooth genes. It’s estimated that the population of large herbivores during the years of intense fighting dropped by 90 per cent.

In humans, disruption in one of the two aforementioned genes lead to the absence of a pair of upper incisors, which are “anatomical equivalent of tusks,” said Campbell-Staton. He further said: “(Tusks are) not just ornamental. They serve a purpose”, detailing how these pachyderms use tusks to dig for water and strip tree bark for food. “If an elephant doesn’t have the tool to do those things, then what happens? (www.sciencenews.org)”.

This is not the first time that human's killer instinct has led to evolutionary changes in members of species. A rapid decrease in horn size of bighorn sheep, a prime target for trophy hunting in North America, has been noticed. High harvesting rates of larger fish have resulted in significant decreases in body size and earlier age of maturation in several species like Atlantic cod, Atlantic salmon, and North Sea plaice (Genetic effects of harvest on wild animal populations by Fred W Allendor et al). In the case of the red kangaroo, hunters target the larger individuals in a group, and there is evidence that the average size has declined.

And many a time, our activities lead to our own suffering — drug-resistant superbugs. Antibiotics impose extreme selection pressure on microbes, fungus and parasites, and there is an immense advantage to any that can resist. Likewise, pesticide resistance: DDT-resistant mosquitoes, for example. Earlier, the mosquito population had more DDT-sensitive and fewer DDT-resistant mosquitoes. But when the use of DDT as an insecticide started in the 1940s, the resistant variety had a competitive advantage over their counterparts and now mosquitoes are largely immune to the chemical.

Many evolutionary experts are dubbing these forced changes in traits “unnatural selection" or "exploitative selection” — analogous to artificial selection, a term that Charles Darwin used for the intentional selection of certain traits in domestic animals and plants. Trophy hunting, poaching, and fishing target individuals with certain desirable phenotypes or observable physical properties of an organism. The result is increased mortality of those individuals with phenotypes desired by humans. Consequently, those features will decrease in frequency.

So, humans are not only driving extinction but also speciation. Accor­ding to a 2012 study by E P Palkovacs et al, human activity is involved in “162 of the 198 study systems in which contemporary trait change has been documented in the wild”.

In the short term, exploitative selection may appear to be an escape from extinction but there are multiple drawbacks. One key drawback is the loss of genetic variation and useful traits. Worse, small populations are most likely to be affected by the loss of genetic variation due to excessive harvest because of their smaller effective population size.

Citing Atlantic cod, the size of which has shrunk from several metres to only about a metre, Palkovacs points out that "we basically have an organism that once was the top predator in the system and now serves as prey to other organisms (BBC)".

Though these forced evolutions may appear to be taking place in select pockets, at a global scale, humans are causing climate change that leads to even more selection pressure that our species can’t fathom.

Even as we face several challenges in developing management plans to minimise the risk of harmful genetic changes caused by the exploitation of wild species, the application of genomic technology holds great potential. Until then and even beyond, we must avoid levels of exploitation that require several years of evolutionary recovery. We must avoid “Darwinian Debt”.