Genetics and Evolution - Variation And Adaptation

Effective adaptations and variations are perpetuated in a species and tend to be incorporated into the normal or predominant phenotype for most individuals in the species. Variation persists, but it ranges around an evolutionarily determined norm. This is called adaptive radiation. For example, over time Darwin's finches have developed beaks best suited to their functions. The finches that eat grubs have long, thin beaks to enter holes in the ground and pull out the grubs. Finches that eat buds and fruit have claw-like beaks to grind their food, giving them a survival advantage in environs where buds are the only available food source. In another example of adaptive radiation, present-day giraffes have necks of varying lengths, but most tend to be long.

Ancient humans underwent many evolutionary changes. An example of adaptive radiation in humans is the development and refinement of the upper limbs to perform fine motor skills necessary to make and use complex tools. Another adaptation is the quantity of the pigment melanin present in the skin. People from areas near the equator have more melanin, an adaptation that darkens their skin and protects them from the sun. Anatomical structure also appears to have responded to the environment. Those who thrive in colder climates produce offspring that are shorter and broader than those who live in warmer climates. This adaptive stature, with its relatively low surface area, enables those people to conserve rather than lose body heat. For example, native Alaskans, generally of short stature, are well suited to their cold climate.

Natural selection does not produce uniformity or perfection. Instead, it generates variability that persists when it helps a species to adapt to, and thrive in, its environment. It acts on phenotypes, not on genotypes, and it is not a process that always discards individual genes in favor of others that might produce traits better suited for survival. For polygenic traits, many different combinations of gene pairs may produce the same or comparable phenotypes. Multiple phenotypes may be beneficial or even neutral in terms of survival in a given environment, and there is no reason for such variation to be eliminated by natural selection.

Furthermore, natural selection does not completely or rapidly eliminate genes that produce traits unsuited for adaptation or survival. While some individuals with harmful traits die young or do not reproduce, some do reproduce and pass their genes and traits to the next generation. Culling out these genes may require several generations. In other instances, seemingly harmful genes may be retained in the gene pool because there may be circumstances or environments in which their presence would improve as opposed to compromise survival. For example, the recessive allele that causes sickle-cell diseases (sickle-cell anemia and sickle B-thalassemia, in which the red blood cells contain abnormal hemoglobin) may have had a role in survival in some parts of the world. Although people who are pure recessive for this trait become ill and die prematurely, those who are hybrid for the trait may have retained a survival advantage in areas where malaria was present because people who have the sickle-cell trait or anemia are immune to the effects of malaria. This phenomenon is called balanced polymorphism and is yet another example of the seemingly counterintuitive actions of natural selection. While the sickle-cell trait is not beneficial on its own, it was advantageous in areas where malaria was a greater threat to survival than sickle-cell diseases.

By definition, natural selection is an unending, continuous process. The popular understanding of natural selection as "survival of the fittest" is somewhat misleading because organisms and species with phenotypes most suited to survive in their environments are not necessarily the "fittest." Present-day examples of natural selection include the evolution of bacteria that are antibiotic resistant and insects that resist extermination with pesticides.

Increasing and Decreasing Genetic Variation

A gene pool comprises the alleles for all the genes in a population. Gene pools in natural populations contain considerable variation, and for evolution to proceed there must be mechanisms to create and increase genetic variation. Mutation, a change in a gene, serves to create or increase genetic variation. Recombination, a process that creates new alleles and new combinations of alleles, also increases genetic variation. Another way for new alleles to enter a gene pool is by migrating from another population. In closely related species new organisms can enter a population, mate within it, and produce fertile hybrids. This action is known as "gene flow."

The actions that create and increase genetic variation are balanced by mechanisms that decrease it. Specific actions of natural selection serve to reduce genetic variation. One way it prevents new alleles from increasing in frequency is by varying the reproductive capability of organisms in a population. For example, it can weed out harmful alleles in an effort to remove unfit variants from the population.

The tendency toward increased genetic variation within a population is balanced by other mechanisms that act to decrease it. For example, some variations have the effect of limiting an organism's ability to reproduce. Any such variations, as well as those incidentally paired with them, will be nonadaptive since they will have a lower probability of being passed to the next generation. Under such circumstances, the process of natural selection serves to reduce genetic variation. In fact, differences in reproductive capability are often called natural selection, Whenever natural selection weeds out nonadaptive alleles by limiting an organism's reproductive capability, it is depleting genetic variation within the population.

Genetic Drift

Other evolutionary mechanisms contribute to genetic variation. Genetic drift is random change in the genetic composition of a population. It may occur when two groups of a species are separated and as a result cannot reproduce with each other. The gene pool of these groups will naturally differ over time. When the two groups are reunited and reproduce, gene migration occurs as their genetic differences combine, serving to increase genetic diversity. If they remain separate, their genetic differences may become so great that they develop into two separate species.

In small populations genetic drift can cause relatively rapid change because each individual's alleles constitute a large proportion of the gene pool and when an individual does not reproduce, the results are felt more acutely in smaller, rather than large, populations. When small populations are affected by genetic drift, they may suffer a loss of valuable diversity. For this reason scientists and others involved in conservation such as zoo curators of endangered species make every effort to ensure that populations are large enough to withstand the effects of genetic drift.