Sunday, 24 October 2021

Evolution News - Observed Rapid Evolution in African Elephants

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Female elephants in Gorongosa National Park in Mozambique evolved to be tuskless in response to intense hunting.
Credit: Joyce Poole/ElephantVoices. Source: Science
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Elephants in Gorongosa National Park, Mozambique, declined by 90% during the 20-year civil war because of ivory poaching. This spurred the rapid evolution of tuskless females, such as this female Loxodonta africana.
Credit: ElephantVoices. Source: Science
Civil war drove these elephants to lose their tusks—through evolution | Science | AAAS

Since evolution by natural selection is driven primarily by the selecting environment, rapid change in the environment can produce rapid evolution. If the selection pressure is strong enough, this intense selection can also result in a linked deleterious gene increasing in the population by being dragged up the fitness landscape by a strongly advantageous gene to which it is linked by close proximity on a chromosome.

This was the case in Mozambique, where during a 15-year-long civil war when there was intense poaching of elephants for their ivory to finance the armies of both sides and because any wild-life protection was largely absent or non-functional. In the Gorongosa National Park for example, the elephant population fell by 90% in 20 years with predation heavily selecting against those with tusks. Consequently, the gene for tusklessness in females increased in the population. Over a period of 28 years which included the 15 years of civil war, the proportion of tuskless females increased from 18.5% (n = 52) to 50.9% (n = 108), but so did the proportion of females in the population and the pre-term deaths of male calves!

This was due to an increase in the genes AMELX and MEP1a, which are associated in mammals with normal tooth formation. AMELX is linked by its proximity on the X chromosome to a dominant male-lethal syndrome in humans that also diminishes the growth of maxillary lateral incisors (homologous to elephant tusks). In other words, as AMELX increases in the population, so does a male-lethal gene that also reduces the growth of tusks in elephants. The result is an absolute increase in the proportion of live births of females and of tuskless females in particular.

In the abstract to their recent paper, a team led by Princeton scientists Shane C. Campbell-Staton of the Department of Ecology and Evolutionary Biology and Brian J. Arnold of Department of Computer Science, Princeton University, Princeton, NJ 08540, USA. said:

Understanding the evolutionary consequences of wildlife exploitation is increasingly important as harvesting becomes more efficient. We examined the impacts of ivory poaching during the Mozambican Civil War (1977 to 1992) on the evolution of African savanna elephants (Loxodonta africana) in Gorongosa National Park. Poaching resulted in strong selection that favored tusklessness amid a rapid population decline. Survey data revealed tusk-inheritance patterns consistent with an X chromosome–linked dominant, male-lethal trait. Whole-genome scans implicated two candidate genes with known roles in mammalian tooth development (AMELX and MEP1a), including the formation of enamel, dentin, cementum, and the periodontium. One of these loci (AMELX) is associated with an X-linked dominant, male-lethal syndrome in humans that diminishes the growth of maxillary lateral incisors (homologous to elephant tusks). This study provides evidence for rapid, poaching-mediated selection for the loss of a prominent anatomical trait in a keystone species.
Over the short term, this attrition of males is not overly detrimental to elephants because herds are matriarchal with a single male able to serve multiple females and 'spare' males forming non-breeding 'batchelor' herds, so the population can remain stable or even increase with relatively few males.

What we can observe in this population then is a rapid evolution of a strong advantageous gene, in that tusklessness makes females less likely to be poached, with the 'opportunistic' increase in a deleterious gene which kills pre-term male calves by being linked physically to a strongly advantageous one, the deleterious effect in the population being more than compensated for by the overall advantageous effect.

Over the longer term, and with control of poaching, whatever environmental pressures led to the evolution of tusks in females originally is likely to reassert itself and the balance should shift back towards tusked females and more male live births. An beautiful example of how a changing environment changes the meaning of the same genetic information without any change in that information itself. And no magic deities involved anywhere in that process.

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