I have used the metaphor of a detective, coming on the scene of a crime after it is all over and reconstructing from the surviving clues what must have happened. But perhaps I was too ready to concede the impossibility of viewing evolution as an eye witness. Although the vast majority of evolutionary change took place before any human being was born, some examples are so fast that we can see evolution happening with our own eyes during one human lifetime.
There's a plausible indication that this may have happened even with elephants, which Darwin himself picked out as one of the slowest-reproducing animals, with one of the longest generation turnovers. One of the main causes of mortality among African elephants is humans with guns hunting ivory, either for trophies or to sell for carving. Naturally the hunters tend to pick on the individuals with the largest tusks. This means that, at least in theory, smaller-tusked individuals will be at a selective advantage. As ever with evolution, there will be conflicting selection pressures, and what we see evolving will be a compromise. Larger tuskers doubtless have an advantage when it comes to competition with other elephants, and this will be balanced against their disadvantage when they encounter men with guns. Any increase in hunting activity, whether in the form of illegal poaching or legal hunting, will tend to shift the balance of advantage towards smaller tusks. All other things being equal, we might expect an evolutionary trend towards smaller tusks as a result of human hunting, but we'd probably expect it to take millennia to be detectable. We would not expect to see it within one human lifetime. Now let's look at some figures.
The graph above shows data from the Uganda Game Department, published in 1962. Referring only to elephants legally shot by licensed hunters, it shows mean tusk weight in pounds (that dates it) from year to year between 1925 and 1958 (during which time Uganda was a British protectorate). The dots are annual figures. The line through the dots is drawn not by eye but by a statistical technique called linear regression.* You can see that there is a decreasing trend over the thirty-three years. And the trend is highly statistically significant, which means that it is almost certainly a real trend, not a random chance
The fact that there is a statistically significant trend towards shrinking tusks doesn't necessarily mean it is an evolutionary trend. If you were to plot a graph of mean height of 20-year-old men, from year to year during the twentieth century, you'd see in many countries a significant trend towards getting taller. This is normally reckoned to be not an evolutionary trend, but rather an effect of improved nutrition. Nevertheless, in the case of the elephants we have good reason to suspect the existence of strong selection against large tusks. Reflect that, although the graph refers to tusks obtained from licensed kills, the selection pressure that produced the trend could well have come mostly from poaching. We must seriously entertain the possibility that it is a true evolutionary trend, in which case it is a remarkably rapid one. We must be cautious before concluding too much. It could be that we are observing strong natural selection, which is highly likely to result in changes in gene frequencies in the population, but such genetic effects have not so far been demonstrated. It could be that the difference between large-tusked and small-tusked elephants is a non-genetic difference. Nevertheless, I am inclined to take seriously the possibility that it is a true evolutionary trend.
More to the point, my colleague Dr Iain Douglas-Hamilton, who is the world authority on wild African elephant populations, takes it seriously and believes, surely rightly, that it needs looking into more
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