In the mathematics of evolutionary change, the selection advantage is a key mathematical quantity that determines the rate at which a favorable mutation spreads through the population. The selection advantage is a quantification of the net result of advantages that a variety of a species gains by virtue of its fertility and fecundity and longevity. As we saw before, even a small selection advantage can lead to rapid spread of the mutation.
One of the interesting things that occurs is that since the entire organism is subjected to the same environment, a selection advantage for one feature can act simultaneously on many different features and can lead to a group of changes that might seem on the surface to be unrelated. Thus one can have many simultaneous changes in an organism, a process known as coherence.
Jerry Coyne (a professor in the Department of Ecology and Evolution at the University of Chicago) describes how this works.
Coherence is precisely the product of natural selection working with mutation. Yes, mutations are random in the sense I have described, but to say that an evolutionary step taken by an organism is unconnected to its predecessor completely ignores the fact that during evolution organisms are adapting to something in their environment, and that this adaptation can involve a coherent, coordinated response of many features. Consider the evolution of whales from terrestrial animals, now documented by a superb fossil record. The fossils show a wolf-like creature gradually becoming aquatic, with the hind limbs being reduced and finally lost, the forelimbs transformed into flippers, and the nostrils gradually moving atop the head to form the blowhole. How can anyone say that these changes (which of course look planned at the end) are unconnected or incoherent? They represent a case of natural selection eventually turning a land animal into a well-adapted aquatic one.
In Almost Like a Whale (p. 36), Steve Jones says that when some wolves were domesticated to become companions to humans and became the dogs we now have, its other features also changed, although these were not deliberately sought for.
Its ears, once pricked, are floppy, and the sounds of the world dulled. Its sharp eyes are blurred by a fringe of hair and can no longer stare an opponent into submission. The lupine tail, an expression of rage or delight, is in many breeds so curled as to bear no message at all. Most pets cannot even raise their hackles in anger as their hair is too long. All this comes from an unconscious preference by man for an animal that knows its place.
What was once done without thought has been echoed by science. In the 1950 Russia, silver foxes were farmed for fur. They were savage, suspicious and liable to die from anxiety. On a certain collective, in an attempt to improve matters, only those willing to accept human company were chosen as parents. Within twenty years and a mere ten thousand foxes, the farmers saw a great shift in their charges. The ranch was filled with well-behaved animals that looked more like dogs, with lowered tails and drooping ears. Many had piebald coats, quite unlike their unrestrained kin, and the females reproduced – like dogs – twice rather than once each year. To breed for tameness was enough to make the change. The other characteristics followed.
There are other factors that increase the selection advantage and thus speed up evolution considerably. The powerful driver that is sexual selection in the wild (usually females selecting males for mating based on certain qualities) prevents random mating and can result in generating significant selection advantages, and is believed to be the source of the exotic and elaborate plumage and songs of some male birds and the gaudiness of flowers (Jones p. 102). Animals tend to prefer to mate with others that either look like themselves or with those animals that look like the ones that raised them, if they had foster mothers (Jones p. 49). Darwin himself first emphasized the importance of sexual selection (On the Origin of Species, 1859, p. 87).
All kinds of factors can come into play that result in one variety of a species separating from the rest, and evolving into a new species as a result. As Jones says (p. 231):
Species are divided from each other in many ways – by space, by time, by mating preference, by the inability to fertilize an egg or produce healthy young, or by the sterility of offspring. The hurdles at which the sexual athletes fail are as varied as life itself. Those involved may never meet, or may mate at their own special time or season. Males and females of different kinds may choose not to pair, or may – with more or less enthusiasm – mate but fail to make a fertile egg. The geographical checks can be as narrow as the few inches between different orchids upon which certain bees feed or as wide as the ocean that separates American and European species of gull. When it comes to time, some flies mate in the morning and some in the evening and some crickets in the spring and others in the autumn; but two kinds of cicadas in North America emerge and mate every thirteen or seventeen years. The difference ensures that they almost never get together (in spite of a certain confusion every couple of centuries).
Colour, song, scent and more all play a part in settling who is, and who is not acceptable.
As soon as one group within a species separates out from the rest and breeds within itself in a new environment, differences get accentuated leading to the eventual formation of new species with new characteristics. But some aspects of the environment (like gravity) are the same everywhere on the Earth and have remained unchanged for a long time. For example, the radiation from the Sun has not changed, and the visible spectrum of light that reaches the surface of the Earth is pretty much the same everywhere on the Earth and has been stable for a long time. Hence it should not surprise us that the ability to see has a huge selection advantage and has evolved independently over 40 times in evolutionary history, although the resulting eyes differ in detail.
Thus if we could run the evolutionary clock all over again, while some things will be quite different, certain features like the eye are likely to recur in some form simply because the environment that makes them advantageous is stable and unchanged by the life forms that happen to come into being. Such a process is called convergent evolution.
All these things constitute evidence that evolution, far from being purely random, is a strongly constrained and law-like process.
POST SCRIPT: Religion as divider
In the film Deconstructing Harry Woody Allen discusses the role that religion plays in creating us-them distinctions.