Genetic Variation and Natural Selection: Natural Selection

Natural Selection

As discussed in Specialized Cell Structure and Function, sexual reproduction and DNA mutation are the two primary processes that increase genetic variability. Although mutation is the only source of new alleles, the potential for a new allele combination is increased with every sexual reproduction from three primary processes:

• Random union of sperm and egg
• Crossing over during meiosis 1
• Independent assortment of homologous chromosomes

Although most genetic recombinations involving a recessive gene are biologically neutral, the potential for that gene to be expressed in the future still survives in that organism and therefore in that breeding population. Increasing genetic variation and the environmental effects on that variation form the basis for natural selection. Natural selection is a theory that states that those individuals who are best adapted to live in an area will survive and reproduce. Within a given population, there exists a normal degree of genetic variation that may or may not make an individual more adapted to the environment or, more importantly, changes in the environment. A species with a great deal of genetic variability is more likely to survive as a species in a changing environment than a species with limited variability.

Darwin and Natural Selection

The theories of Charles Darwin still form the foundation for our understanding of natural selection. It is still widely upheld that natural selection is based on five factors:

• Within a species, individual variations exist naturally. Some organisms are faster, more colorful, bigger, or smarter than others in their species. Individual variations sometimes are helpful, neutral, or harmful.
• More offspring are produced than can survive, and their success in their life struggle to gain food, shelter, and a mate determines their ability to successfully reproduce and pass on their genetic complement.
• The reproductive rates of individuals are unequal, or favored based on environmental pressures.
• Environmental conditions determine the reproductive success of certain individuals because they possess a trait that allows them an advantage in that environment.
• Individuals who are able to provide the necessary food, shelter, and avoid predation reproduce more successfully than others. Over generations, the characteristics of the population change as those individuals more successful in reproduction populate the species. Less-successful types do not pass on as many traits because they have fewer descendants. Over time, failure to reproduce at a rate equal to or greater than the mortality rate leads to extinction. The process whereby the frequency of certain traits within a species change by uneven reproduction rates caused by natural selection is called evolution.

In Darwin's model, natural selection did not necessarily prohibit an individual from reproducing, but rather favored the most adapted individuals with a greater chance for reproduction, the possibility of greater birth numbers, and the increased development and survival of the offspring. So natural selection allows the total number of certain members of a population to decline or become extinct because of environmental pressure, while others increase accordingly.

The classic case study of natural selection was recorded by H. B. D. Kettlewell in 1952. In a most interesting experiment, he concluded that an environmental influence, predation by birds, affected the total number of reproducing moths depending on their color. Kettlewell was working at Oxford University at the time and discovered a shard of information from the 1840s, 100 years earlier, that noted the first appearance of a dark morph of the peppered moth. Until that time, only white or pepper-colored moths had been observed. He connected the dates with the onset of heavy industrial output in that area. He also knew that the factories at that time and in his time produced voluminous daily clouds of black smoke heavily laden with soot. He also knew that the peppered moths were common all over England; they were nocturnal and hid on tree trunks during the day; they were preyed upon by many species of birds. He hypothesized that the birds were preying upon the moths that were less camouflaged and therefore easier to see. In so doing, they were favoring the one morph type over another, which created uneven reproductive rates that favored an increase in one type of moth over the others.

In his experiment, he released light-colored moths in soot-covered forests and an equal number of dark moths in normal forests. He tagged each moth with a touch of paint and then set traps for their recapture. In the soot-covered dark forest, he recaptured mostly dark moths; in the lighter forest, mostly light moths. He concluded the uneven recapture rates were based on uneven predatory rates. To confirm his suspicion, he observed and took pictures of birds preying upon moths whose body color contrasted with the environment while apparently overlooking the more camouflaged moth. Those moths best adapted to the environment survived and reproduced; the others did not. Kettlewell's experiment is an example of how environmental pressures can determine the characteristics of species.

At this time in history, Darwin's theories met great opposition from other scientists and religious leaders because of their newness and related controversial nature. Much of the criticism appeared to stem from a misunderstanding of his ideas.