The modern theory of evolution is easy to grasp. It can be summarized in a single (albeit slightly long) sentence: Life on earth evolved gradually beginning with one primitive species—perhaps a self-replicating molecule—that lived more than 3.5 billion years ago; it branched out over time, throwing off many new and diverse species; and the mechanism for most (but not all) of evolutionary change is natural selection.
When you break that sentence down, you find that it really consists of six components: evolution, gradualism, speciation, common ancestry, natural selection, and nonselective mechanisms for evolutionary change.
Evolution: a species undergoes genetic change over time. Over many generations, a species can evolve into something quite different, and these differences are based on changes in DNA, which originate as mutations. Although all species evolve, they don’t do so at the same rate. [The rate of change] depends on the evolutionary pressures they experience.
Gradualism: it takes many generations to produce a substantial evolutionary change , such as the evolution of birds from reptiles. The evolution of new features, like the teeth and jaws that distinguish mammals from reptiles, does not occur in just one or a few generations, but usually over hundreds or thousands—or millions—of generations. Some change can occur very quickly. Populations of microbes have very short generations, some as brief as twenty minutes. This means that these species can undergo a lot of evolution in a short time, accounting for the depressingly rapid rise of drug resistance in disease-causing bacteria and viruses. But when talking about really big change, we’re usually referring to change that requires many thousands of years.
Speciation (splitting): the evolution of different groups within a species to the point where they can no longer interbreed. The ancestor species splits into two separate species; individuals of the first new species are incapable of breeding with individuals of the second new species. For example, two populations of a single reptilian species, probably living in different places, begin to evolve slight differences from each other. Over a long time, these differences gradually grew larger. Eventually the two populations would have evolved sufficient genetic differences that members of the different populations could not interbreed. (There are many ways this can happen: members of different animal species may no longer find each other attractive as mates, or if they do mate with each other, offspring could be sterile.)
The birth of the ancestor of all birds wouldn’t have looked so dramatic at the time. We wouldn’t have seen the appearance of flying creatures from reptiles. but merely two slightly different populations of the same [ancestor species], probably no more different than members of diverse human populations are today. But species don’t have to split. Whether they do depends on whether circumstances allow populations to evolve enough differences that they are no longer able to interbreed. The vast majority of species—more than 99% of them—go extinct without leaving any descendants.
Although speciation is slow, it happens sufficiently enough, over such long periods of history, that it can easily explain the stunning diversity of living plants and animals on earth.
Common Ancestry: we can always look back in time, using either DNA sequences or fossils, and find descendants joining at their ancestors. Creatures with recent common ancestors share many traits, while those whose ancestors lay in the distant past are more dissimilar. This phenomenon, called the nested arrangement of life, had been noticed long before Darwin. This nested arrangement of life is itself strong evidence for evolution. DNA sequencing has confirmed pre-DNA “family trees” of evolutionary relationships among species.
Natural Selection: If individuals within a species differ genetically from one another, and some of those differences affect an individual’s ability to survive and reproduce in its environment, then in the next generation the “good” genes that lead to higher survival and reproduction will have relatively more copies than the “not so good” genes. Over time, the population will gradually become more and more suited to its environment as helpful mutations arise and spread through the population, while deleterious ones are weeded out. Ultimately, this process produces organisms that are well adapted to their habitats and way of life.
Example: The wooly mammoth whose remains have been found in North Eurasia and North America. These creatures probably descended from mammoth ancestors that had little hair—like the modern elephant. Mutations led to some individual mammoths being more hairy than others. When the climate became cold or the species spread to more northern regions, the hirsute [hairy] individuals were better able to tolerate their frigid surroundings and left more offspring than their balder counterparts. This enriched the population in genes for hairiness. In the next generation, the average mammoth will be slightly hairier. Let this process continue over some thousands of generations, and your smooth mammoth gets replaced by a shaggy one. The process is remarkably simple. And since many traits can affect an individual’s adaptation to its environment (its “fitness”), natural selection can, over eons, sculpt an animal or plant into something that looks designed.
Processes other than natural selection can cause evolutionary change: The most important is the simple random changes in the proportion of genes caused by the fact that different families have different numbers of offspring. This leads to evolutionary change that, being random, has nothing to do with adaptation. (see genetic drift)
— Excerpts from Why Evolution is True, chapter 1, by Jerry Coyne, professor of Evolution, University of Chicago
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