In an earlier post I mentioned that natural selection (hunting pressure) had the potential to increase the proportion of tusklessness in African elephants. But I also noted that this was probably not the full story! And in fact it turns out to be quite a complex tale.
For a start, male & female African elephants (Elephas maximus) differ in tusk development. Most males have tusks, while many females are tuskless, or have small tusks. (This suggests that the genetic contol of tusk development must be quite complex.) And tusk size increases with age, because these modified teeth grow throughout the animal's life: this means that broken tusks can regrow. However, in South Africa's Addo Elephant National Park, 98% of females are tuskless (Whitehouse, 2001).
Because elephants have been extensively hunted for ivory for more than 300 years, natural selection is often proposed as the underlying mechanism for an increase in the proportion of tuskless or small-tusked elephants in a population. After all, it does sound right: if animals with small or no tusks are more likely to survive, & their relative lack of tusk is heritable, then you'd expect that trait to spread through the population. Whitehouse tested this hypothesis in the Addo elephant population - & rejected it.
To begin with, if natural selection (hunting) did have this effect, then you'd expect that tusklessness would be a feature of both male & female Addo elephants. But in 2000, when Whitehouse surveyed the population, female tusklessness was at 98% but all the males had tusks. (Tusklessness in populations elsewhere in Africa - generally attributed to selective hunting - had also been noted as increasing, but not to the levels observed in Addo.)
Whitehouse began by looking at the history of the Addo elephants. While elephants were widespread in South Africa prior to European settlement, by 1900 their numbers had dropped precipitously right across the country. The largest remaining population, of around 130 animals, was in Eastern Cape Province. This was further reduced during 1919-20 by a professional hunter, who'd been contracted to kill the animals by local farmers. By 1931 only 11 elephants remained in the Province, and the Addo park was created in an attempt to preserve that population. Mortality remained relatively high until an elephant-proof fence was built in 1954 to keep the animals within the reserve, & by December 2000 the population had grown to 324 animals. However, there's been no immigration into the Addo herd since 1931.
In other words, the Addo elephants have been through a substantial population bottleneck. That founding population of 11 comprised 8 females & 3 males, but only one adult male (which had tusks) bred with the females. Of those 8 founding females, 4 (50%) were tuskless & 3 had at least one tusk; Whitehouse could find no details of the 8th. This compares with a tuskless frequency, in populations elsewhere in Africa, of 2-20%.
Whitehouse comments that the tusked/tuskless state is likely to be sex-linked, given the strong difference between the sexes. But she also notes that the actual pattern of inheritance is likely to be complex, given that there are several possible phenotypes involved (tusked, having a single tusk [right or left], and tuskless), and that the few tusked mothers in Addo tend to have tuskless calves.
She proposed four hypotheses to explain the high proportion of tuskless females in Addo. The first relates to the vegetation of the area - it's mainly shrubs & succulent plants. The elephants may not need tusks to feed on this sort of vegetation. But the time for which this population has been isolated (no more than 300 years) hardly seems long enough for a lack of selection pressure to produce the observed result.
The second possibility is also related to the vegetation in the area: there could be a link between the availability of nutrients & tusk development. But as Whitehouse comments, since this hypothesis is based on dietary influence on tusk development, the absence and size of tusks would primarily be acquired traits, and so would not be inherited.
The third option is the result of natural selection (selective hunting). Whitehouse notes there is evidence for this from studies of elephant populations elsewhere in Africa. But her comparison of tuskless trends and the hunting pressure faced by the Addo herd failed to support this hypothesis for the Addo animals. For example, they've been fully protected from hunting since 1954, while tusklessness has continued to increase. And while the elephants were hunted between 1919 and 1954, records suggest this was not selective: the animals were generally killed to reduce their impact on farming, & not directly for their ivory.
Which brings us to hypothesis #4. Both the rate of genetic drift and the frequency of inbreeding increase at small population sizes. This final hypothesis suggests that tusklessness may primarily result from population bottlenecks, and non-selective genetic changes occurring due to the subsequent small population size (Whitehouse, 2001). Remember that initial population: 11 animals, with only 8 females and a single breeding male. This must have resulted in a high degree of inbreeding, and that will have continued given the absence of immigration. Whitehouse concludes that relatively high rates of genetic drift and inbreeding occurring due to small pre-1919 population size are primarily responsible for the observed high level of tusklessness in the 1919 Addo elephant population, and that the extreme 1931 bottleneck only made this effect even more pronounced.
So - while natural selection is an important mechanism driving genetic change in populations, it's not the only such mechanism. And it's likely that genetic drift - via the Founder effect - has also been an important player in the evolution of many of New Zealand's plants & animals.
A.M. Whitehouse (2001) Tusklessness in the elephant population of the Addo Elephant National Park, South Africa. Journal of Zoology 257: 249-254