Between 1977 and 1992, a civil war in Mozambique killed over a million people, displaced five million more, and destroyed roads, schools, and hospitals. It also killed a lot of elephants. Armies need money to fight, and ivory from elephant tusks was a way to get it. The protected elephant population in Gorongosa National Park in Mozambique was decimated by ivory poachers. Before the war in 1972, the park was home to 2542 elephants. By the year 2000, that number had fallen to just 242. Now, recent research shows that poaching during this period caused rapid evolution in the elephant population, specifically, the rise of tuskless elephants.
Where's the evolution?
As adults, most male and female elephants have tusks. Of course, the tusks are sometimes broken off, but many observers had noticed that some females never develop tusks in the first place. When the team behind the new research, led by Shane Campbell-Staton, studied the elephants in Gorongosa National Park, they found that tuskless individuals had become much more common over the period of the civil war.
The scientists hypothesized that this shift was the result of straightforward natural selection. Tusked elephants were targets for ivory poachers, while tuskless elephants were not. Hence, any elephant that carried a genetic variant (i.e., an allele) that caused it to be tuskless would be more likely to survive to reproduce than a tusked elephant. In this way, elephants carrying tuskless gene variants would leave behind more offspring than tusked elephants, those offspring would pass on the tuskless gene variant, and over generations, tusklessness would become more and more common.
It sure seemed like selection in action, but the team needed more evidence. One of their tests checked to see if the change observed in the elephant population could have been caused by chance, instead of by natural selection. When populations get small (as happened to the Gorongosa elephant population during the civil war), it’s easy for some gene variants to be over- or under- represented from one generation to the next just by chance alone (i.e., through a population bottleneck and genetic drift). Was tusklessness actually helpful for survival – or did it become more common just by chance when so many elephants were killed? To find out, the researchers ran simulations (i.e., a model) to see how a population of 2542 elephants, of which 18.5% of females were tuskless (the situation in 1972), was likely to evolve over 28 years if tuskless and tusked elephants were equally likely to survive. They set up the model so that the population decreased to just 242 individuals (the situation in the year 2000). The simulation predicted that the population would almost always wind up with less than 30% tuskless females if tusked and tuskless elephants had the same chances of survival. But in actuality, the researchers had found that over half the female elephants in Gorongosa were tuskless in the year 2000. This and several other lines of evidence convinced the team that their hypothesis was correct: natural selection favoring tuskless elephants was to blame for the shift towards tusklessness.
Based on their data, the researchers estimated that tuskless elephants were more than five times as likely to survive to reproduce as tusked individuals during the period of intense poaching. That is a big evolutionary advantage! In biological terms, we would say that during intense poaching, tuskless elephants have a much higher evolutionary fitness than tusked elephants.
It might be tempting to think, “Wow, evolution really solved that problem: if tusks are a risk factor, evolution can just get rid of them, and everything works out.” But of course, there is more to this story.
The researchers only identified tuskless female elephants, never males. Why? The team figured out that the trait of tusklessness is likely caused by a gene version on the X chromosome that is lethal in males. In elephants, as in other mammals, the presence of X and Y chromosomes strongly influence genetic sex. It seems that an XX female elephant, in which one X chromosome carries the tuskless gene version, will grow into a healthy but tuskless adult. But an XY male whose X chromosome carries the tuskless gene version will die in utero, before he is ever born.
The rise of tusklessness does indeed help solve the problem of some elephants being killed for ivory. But it leaves adult male elephants (which always develop tusks) vulnerable to poachers, and presumably causes the early death of many males (those who unluckily inherited the tuskless trait from their mothers) in an already small population. As solutions go, this seems like a less than perfect one. But that is often what you get with evolution. Evolution by natural selection is not intentional, it does not “plan ahead”, and it cannot simply grant what a species needs. Natural selection acts on whatever variation is in a population, increasing the frequency of traits that boost survival and reproduction at the current moment – even if that means fewer male elephants in the future.
Shared ancestry shows in teeth and tusks
The part of the X chromosome that the researchers think causes tuskless female elephants and kills male elephants is also known from a syndrome in humans. This syndrome causes reduced tooth growth in XX people and is lethal to XY individuals. Even in mammals as different as elephants and humans, our shared evolutionary ancestry underlies much of our basic biology – and crops up in sometimes surprising ways.
- Campbell-Staton, S. C., Arnold, B. J., Gonçalves, D., Granli, P., Poole, J., Long, R. A., and Pringle, R. M. (2021). Ivory poaching and the rapid evolution of tusklessness in African elephants. Science. 374: 483-487. Read it »
Understanding Evolution resources:
- How did the number of tuskless elephants in Gorongosa National Park change over the period of the civil war? Over how many years did this change occur?
- In your own words, explain what the researchers’ hypothesis was to explain the increase in the frequency of tuskless elephants in Gorongosa National Park.
- The researchers did not identify any tuskless male elephants in the park, though there were many tuskless females. What is the hypothesized explanation for this?
- Read about the concept of evolutionary fitness. Imagine that friend reads about the tuskless elephants and says that they couldn’t be very fit in comparison to tusked individuals because elephants use tusks for defense and to get food. How would you explain the concept of evolutionary fitness as it relates to tuskless elephants to your friend?
- Imagine that a friend points out the headline of a news story about this research that reads “Elephants are evolving in order to avoid ivory poachers.” The friend goes on to say, “Species that work hard can find ways to survive.” How would you respond to explain how evolution really works?
- Advanced: Imagine that the hypothesis that tusklessness is caused by a single X-linked, dominant allele that is lethal in males were true. X+ is the unaffected allele. X– is the tusklessness allele. Y is the Y chromosome.
- Draw a Punnett square to work out the frequency of different genotypes and phenotypes in the offspring of a cross between a tusked female (X+X+) and tusked male (X+Y). What are the expected genotypic ratios among offspring? What are the expected phenotypic ratios among offspring? What is the expected ratio of males to females among the viable offspring?
- Draw a Punnett square to work out the frequency of different genotypes and phenotypes in the offspring of a cross between a tuskless heterozygous female (X–X+) and tusked male (X+Y). What are the expected genotypic ratios among offspring? What are the expected phenotypic ratios among offspring? What is the expected ratio of males to females among the viable offspring?
- The researchers observed that 91.3% of the daughters from the cross described in item a above were two-tusked, and 54.2% of offspring were female. Are these findings consistent with the hypothesis for the genetic basis of tusklessness described above? Explain your answer.
- The researchers observed that 40.9% of the daughters from the cross described in item b above were two-tusked, 44.7% of daughters were tuskless, and 65.7% of offspring were female. Is this consistent with the hypothesis for the genetic basis of tusklessness described above? Explain your answer.
- Teach about how harvesting of wild populations can cause evolution. This research profile for grades 9-16 follows scientist David O. Conover as he investigates the impact of our fishing practices on fish evolution and discovers what happened to the big ones that got away.
- Teach about selection. In this classroom activity for grades 6-16, students learn how artificial selection can be used to develop new dog breeds with characteristics that make the dogs capable of performing a desirable task.
- Teach about Mendelian genetics and evolution. This case study for grades 13-16 examines the evolution of the wrinkled pea from its ancestral round pea shape.
- Campbell-Staton, S. C., Arnold, B. J., Gonçalves, D., Granli, P., Poole, J., Long, R. A., and Pringle, R. M. (2021). Ivory poaching and the rapid evolution of tusklessness in African elephants. Science. 374: 483-487.