Contents Updated: Tuesday, August 24, 1999
You might be wondering how the environment brings about changes, convergent or otherwise, in animals. It is helpful to know the gist of how evolution works. There are three driving forces of evolution.
From these you can see that an animal born into a particular environment with certain characteristics inherited from its parents and adequately suited to its environment will survive to breed and give birth to a new generation which will inherit its characteristics. The cycle continues—the species survives.
Another animal is badly adapted to it environment, say a black polar bear or a mute blackbird. It is unlikely to breed successfully and the variant feature that led to its failure will not have the chance of appearing again in the next generation. Black polar bears and mute blackbirds quickly die out.
These examples illustrate selection against gross differences in features but Darwin regarded evolution as occurring through an accumulation of small changes caused by the selection of small variations inherited from parents. Such small variations do not prevent breeding, but tip the scales slightly. A grey polar bear might be able to pass on its greyness for many generations because ninety nine times out of a hundred it is as successful in catching seals as its white rivals. But that one time out of a hundred that the white bears are more successful than the grey ones will ensure that the population of polar bears will eventually be all white. Over generations of natural selection that tiny difference favors the white variety.
In Darwinian evolutionary theory, evolution should occur gradually and the gradual changes should be visible in the fossil record. They rarely are. Darwin was unhappy that there was not a smoother fossil record. Species seemed rather to live unchanged for a long time then change suddenly to something new. Eldredge and Gould's theory of punctuated evolution explained this apparent anomaly, extending Falconer's ideas of a hundred years before. When it is well adapted to a stable environment a species can experience a long period of stability. But then it suddenly evolves very quickly, perhaps within ten to a hundred generations. Since geologists can rarely measure intervals in the rocks of closer than 100,000 years, such a rapid change occurs in too short a period to leave a fossil record. It seems as though one species had suddenly given way to the newer one.
Dawkins calculates that in 60,000 years an animal the size of a mouse (40g) could evolve into an animal the size of an elephant (6,000,000g). In 12,000 generations of an average five years per generation, a rate of growth of 0.02 per cent per generation would effect the change yet would be too small for any contemporary observer to notice. Yet in the geological record mice will have given way to elephants in adjacent strata. The change would appear instantaneous in the rocks.
According to Ernst Mayr, it is isolation that allows one branch to evolve quickly in response to the new set of conditions while the larger original group remains fairly stable. Mayr defined a species as a group of interbreeding natural populations that are reproductively isolated from other such groups. Polar bears and brown bears can allegedly breed together to give birth to fertile offspring. In this respect they are both of the same species. But, of course, they never meet in the wild because they live in widely different habitats—they are reproductively isolated and are classified as different species.
Reproductive isolation does not necessarily mean that mating never occurs between the two relatively isolated groups of animals but its frequency must be low. Eventually, as speciation progresses, the offspring become infertile even when mating does occur. Polar bears might occasionally meet brown bears, in Alaska perhaps, and they could interbreed on those occasions. But the frequency of such occurrences is low indeed. Eventually brown and polar bears will not yield fertile young even if they are able to crossbreed. Then the two lines of bears will have been forever separated.
Horses and donkeys mating to give birth to mules is another example. Donkeys and horses have a common ancestor but donkeys specialized for life in rocky deserts while horses specialized in temperate grasslands. Their common ancestor was probably a horse so donkeys are horses that have begun to evolved differently because of their different environment. Now, they cannot interbreed because the product of their union is the sterile mule or hinny. Some of the minority and the main populations, chancing to meet, might attempt to reproduce by mating but the next generation is infertile so reproductive isolation is complete.
A group of individuals isolated from the main population for long enough eventually becomes a new species. Suppose the new species again came into contact with the parent population—perhaps the sea level had risen isolating some animals on a small island but then it fell again. If the new animals had advantages over the old that had evolved in isolation, then they would rapidly dominate the parent population, possibly pushing them to extinction for their conservatism. The original isolated group would have left only a localized fossil record of their evolution on that small island. In later years, unless by luck that particular region were prospected, no fossils showing their evolution would be found.
The main group, on the other hand, being widespread, would have left fossils widely dispersed and easy to discover. Having overwhelmed the old, the new variety would become widespread and its fossils common. Geological strata will show the new species replacing the old instantly in the fossil record. Alternatively the, formerly stable, parent population might be forced to evolve because competition with the invaders is an environmental factor they had not previously encountered. Because competition had forced the original population to change rapidly, the fossil record will again show an apparently discontinuous change.
Darwin recognized from his studies on the Galapagos Islands that chains of islands are perfect for isolating populations to allow speciation to occur. High sea levels cutting off tracts of land provide the nurseries of species. This possibly happened in the evolution of man in the last few million years and it must have happened to some of the dinosaurs in the Cretaceous period when sea levels were high.
One wonders however whether speciation can occur even when populations are not physically isolated in any way. Reproductive isolation is needed. What could promote some of a population to eschew breeding with the rest, if they are not physically isolated?
Could there be an incest gene? By conditioning its owner to prefer sex with close relatives and its children could it allow speciation via sexual isolation within a breeding group? The incest gene would make an animal prefer its own kin, perhaps by linking with a gene which expresses itself in some subtle physical feature, recognizable even at a subliminal level by parents and siblings, a pheromone perhaps. If, by breeding together, an incestuous family retains some advantageous characteristic, the incest gene will spread and will ultimately determine a new species to replace the previous one. The wolf and coyote in North America are interesting examples of close but different species. The wolf commonly hunts in packs whereas the coyote is more solitary. Though present in the same territory, in the wild they do not interbreed and so are classed as different species but in captivity they can be made to interbreed and produce what Bakker describes as healthy hybrids. Was the coyote an incestuous wolf in earlier times?
First, an example of how convergence theory has been used by some unorthodox thinkers to account for the mysterious gap from eight million to four million years ago in the emergence of man.