... if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.
Charles Darwin, On the origin of Species, 18591
Another way to express what Darwin meant is that individuals with characteristics that are useful to them in their struggle to survive are more likely to produce offspring with the same characteristics. One straightforward consequence of Darwin's idea is that animals that are sick or injured will be less successful to defend themselves against predators or to escape from them. Therefore, such individuals are more likely to be killed by predators before they have a chance to reproduce. This is so obvious that it should hardly need to be proven. Nevertheless, it has been demonstrated to take place in the wild many times. One example involving African wild dogs (Lycaon pictus) and their prey impala (Aepycerus melampus) in Zimbabwe was published recently2.
African wild dogs
African wild dogs (also called African hunting dogs) hunt in packs and share their kill. One large animal that wild dogs prefer to hunt is impala. Once a pack finds an impala, or any other prey, they begin to chase it until the prey gets tired and they catch up with it. Such chases can apparently last for several kilometers at high speeds. Obviously, this hunting method is energetically very costly. Imagine yourself having to run, say, 2 kilometers (about 1.3 miles) as fast as you can before your every meal. Just to obtain enough energy to be able to run 3x2 fast kilometers a day, you would probably have to eat an extra meal every day, but that would make it necessary to run 2 additional kilometers!
This being the case, which impalas would a pack of wild dogs rather go after to minimize their energy expenditure during a hunt? Undoubtedly, the weak and the sick ones, because they will be slower than the healthier animals, and by chasing the slower impalas the wild dogs will spend less energy.
Impalas in Africa
To determine if this was indeed what was happening in the wild, British scientists2 collected bone marrow from one group of impalas that had been killed by wild dogs and another group that had been killed non-selectively by humans. They knew from previous studies that impalas in poor condition had very little fat in their bone marrows. Therefore, as a measure of the physical condition of each impala they calculated the amount of fat it had in its bone marrow at the time of its death. The graph below shows their results.
The impalas that had been killed by wild dogs (bottom curve) had significantly less marrow fat than the impalas that had been killed by humans (top curve). Therefore, the authors of the study concluded that wild dogs selectively prey on impalas that are in poorer condition.
A more general result that we can derive is that the weaker impalas are less likely to live long enough to reproduce, while the stronger ones are more likely to escape from wild dogs (and other predators) and live long enough to reproduce. What does this mean in terms of evolution? It means that the healthier and stronger impalas are more likely to pass the genes that contribute to their good health and physical strength onto their offspring. In turn, their offspring will be more likely to be healthy and so on. This is basically how natural selection works.
Dinner is being served
For more information follow these links.
Natural selection 1
Natural selection 2
African wild dog 1
African wild dog 2
1. Charles Darwin, On the Origin of Species, 1859. full text
2. Alistair Pole, Iain J. Gordon and Martyn L. Gorman. African wild dogs test the 'survival of the fittest' paradigm. Proc. R. Soc. Lond. B (Suppl.) Biology Letters 270, S57 (2003).
Wild dog and impala pictures were downloaded from the University of Michigan Museum of Zoology Animal Diversity Web
Cross-posted at Transitions