The idea of changes in the composition of a popula Essays - Biology

The idea of changes in the composition of a population being attributable to changes in its environment has been an extremely useful. With this perspective, we can coherently account for the distribution of forms in the fossil record, for example. However, great interest also lies in speciation , which occurs when populations that for some reason cease to interbreed become different enough to lose all ability to interbreed. (In actual life, the boundaries between species are not always absolute varieties only gradually evolve into species, after all and it has been found that occasional fertile crosses between closely simi lar species do occur.) The rule-of-thumb of fertile offspring as defining species remains the way most biologists think of it, but the possible exceptions have generated complexities in classifica tion and in modeling gene flow. A p opulation split in half by a n un crossable barrier c an over time develop huge differences between its two parts. The most famous example of this process, called "allopatric" "different country" speciation, is the squirrels of the Grand Canyon, who have diverged into different species on the two sides of the Canyon. But w hen the genotype of a single, inbreeding population changes gradually over time, at what point should we say it has turned into a new species? Our "rule of thumb" about producing fertile offspring can hardly be applied to two different points in the history of the same population. And yet if the differences between early and late specimens are substantial, can we be confident in calling them the same species? As an intellectual matter, it is difficult to know when one continuously changing population should be regarded as a different species from its remote ance stors. As a practical matter, it is also difficult to know how specimens available for study are actually related to each other anyway. If two fossil snake teeth look similar, but neither looks exactly like any modern snake, were they separate species, or were they merely two slightly different variants of the same species? And is either of them ancestral to any modern snake? Many specialists in prehistoric life paleontologists tend to assume that speciation occurs relatively easily when plant or animal communities are separated, and they argue that the default assumption should be that two specimens should be assumed to be different species until they can be convincingly shown to be the same (which is hard to demonstrate with extinct forms). Others argue that speciation is in fact relatively difficult. Barring intervention with atomic bombardment, we have not succeeded in changing a genotype sufficiently to make it a whole new species just through selective breeding alone. (That is why all dogs are still the same species.) The best assumption when looking at prehistoric forms should therefore be that two similar specimens should be considered to belong to the same species until shown to have more variation between them than is exhibited in a modern population of similar animals, if there is one. All of this becomes particularly controversial in the case of proto-humans. Darwin did not explore the evolution of humans indeed only one clearly pre-human hominid fossil had been discovered when he published The Origin of Species Through Natural Selection in 1859. Since humans look a lot like modern Great Apes, it seemed logical that we should have a common ancestor with them, but that was about as much as one could say. In some respects, the exact ways in which we draw the lines among species may not make much difference. The more we learn about genetics, the more obvious it becomes that what matters most is not the label given to a form, but our understanding of the over-all process of evolutionary transformation that is continually going on. Geneticists are ever refining more detailed "family trees" of biological forms based on the inheritance of specific gene mutations . In these family trees, it makes no significant difference where one species stops and the next begins. What matters is where mutations occur that are inherited by future generations.