Natural Selection as a Mechanism of Evolution
The Best Adapted and Most Reproductively Fit Individuals Survive
Jun 12, 2009
Darwin’s grand idea of evolution by natural selection is relatively simple, but often misunderstood. This most famous component of Darwin’s theory of evolution holds that all members of a species vary slightly from each other and thus have different structural, behavioral, and/or physiological traits. Those organisms with variation in traits that permit them to best exploit their environment will preferentially survive and pass these beneficial traits on to future generations.
Over long periods of time, the accumulation of such favorable variations produces new organismal characteristics and eventually new species. This proposition has been popularly termed “survival of the fittest” but might be more accurately described as “survival of the best adapted and most reproductively fit.”
Natural Selection in Action
Imagine a population of small fish in a remote lake. Suppose this fish population exhibits:
- Variation in traits. Some of the fish are a bright silver color on top (dorsal side) while others are a darker grayish color
- Differential reproduction due to advantageous adaptation. No environment can support unlimited population growth so not all individuals can reproduce to their full potential. If the bright colored fish in the general population of that species are more easily seen by predators and thus tend to get eaten more often, the dark colored fish will survive in greater numbers and therefore, produce more offspring.
- Heredity. The surviving dark colored fish pass this advantageous color trait (genes) on to their offspring. This was the missing piece Darwin could not explain in his theory. He knew that offspring tended to possess the traits of their parents; he just lacked a mechanism to explain it.
According to Darwin the long-term results of such a situation would be that the more advantageous trait, dark coloration, which allows the fish that possess the trait to have more offspring, becomes more common (higher gene frequency) in the population. If this process continues, eventually all (or at least most) of the fish in that population will be dark colored.
A living example of what appears to be natural selection in action can be seen in the peppered moth (Biston betularia) of Britain. The moths come in two color variations or subspecies — light colored or typica and dark colored or carbonaria. Originally, the vast majority of peppered moths had the light coloration, which effectively camouflaged them against the light-colored lichen-covered trees on which they rested.
However, with the onset of the Industrial Revolution and the burning of massive quantities of coal, the lichens died and the trees were blackened by soot. What had been an advantage now became a disadvantage as the typica moths became more visible against the trees. As predators found and ate more of the typica moths, the carbonaria moths flourished as their dark color now gave them the advantage.
With improved environmental standards, light-colored peppered moths have again become more common. These dramatic changes over relatively short periods of time have remained a subject of much interest and continued study and strongly implicate birds as the agent of selection in the case of the peppered moths.
Natural selection acts on the phenotype, or the observable physical characteristics of an organism, but the genetic (heritable) basis of any phenotype which gives a reproductive advantage will increase in frequency over the following generations. Over time, this process can result in adaptations that specialize organisms for particular ecological niches and may eventually result in the emergence of new species. In other words, natural selection is an important process (though not the only process) by which evolution takes place within a population of organisms.
Mechanisms of Evolution
Evolution encompasses changes on two vastly different scales – from an increase in the frequency of a gene for colored spots on the feathers of a bird (microevolution) to something as grand in scale as the evolution of the entire bird lineage (macroevolution). Despite the scale on which it happens, evolution at both levels is driven by the primary mechanisms of natural selection mutation, genetic drift, and gene flow.
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