Natural selection Reading
Charles Darwin didn’t just propose that organisms evolved. If that had been the beginning and end of his theory, he wouldn’t be in as many textbooks as he is today! Instead, Darwin also proposed a mechanism for evolution: natural selection. This mechanism explained how populations could evolve in such a way that they became better suited to their environments over time.
Darwin’s concept of natural selection was based on several key observations:
- Traits are often heritable. In living organisms, many characteristics are inherited, or passed from parent to offspring. (Darwin knew this was the case, even though he did not know that traits were inherited through genes.)
- More offspring are produced than can survive. Organisms are capable of producing more offspring than their environments can support. Thus, there is competition for limited resources in each generation.
- Offspring vary in their heritable traits. The offspring in any generation will be slightly different from one another in their traits (color, size, shape, etc.), and many of these features will be heritable.
Based on these simple observations, Darwin concluded the following:
- In a population, some individuals will have inherited traits that help them survive and reproduce. The individuals with the helpful traits will leave more offspring in the next generation than their peers, since the traits make them more effective at surviving and reproducing.
- Because the helpful traits are heritable, and because organisms with these traits leave more offspring, the traits will tend to become more common in the next generation.
- Over generations, the population will become adapted, or become adjusted to new conditions or the environment.
Darwin’s model of evolution by natural selection allowed him to explain the patterns he had seen during his travels. For instance, if the Galápagos finch (a type of bird) species shared a common ancestor, it made sense that they should broadly look like one another. If groups of finches had been isolated on separate islands for many generations, however, each group would have been exposed to a different environment in which different heritable traits might have been favored; for example different sizes and shapes of beaks for using different food sources. These factors could have led to the formation of clear differences of species on each island.
Here are explanations which may help you get a better sense of how, when, and why natural selection takes place.
Natural selection depends on the environment
Natural selection favors traits that are beneficial that is, they help an organism survive and reproduce more effectively than its peers in a specific environment. Traits that are helpful in one environment might actually be harmful in another.
Natural selection acts on existing heritable variation
For natural selection to act on a feature, there must already be variation, or differences among individuals for that feature. Also, the differences have to be heritable, determined by the organisms’ genes.
Heritable variation comes from random mutations
The original source of the new gene variants that produce new heritable traits, such as fur colors, is random mutation.