Evolution: Natural Selection

WebLecture Topics

Factors in evolution
Selections
Genetic Diversity

The Mechanisms of Natural Selection

Natural selection is the process where the expression of characteristics already available to a species species shifts, with some becoming more popular and some less. If a particular trait gives individuals a survival advantage, these individuals will mature and reproduce, increasing the appearance of their version of genetic material in the population. If particular traits do not give a survival advantage, they may die out entirely.

Factors in microevolution

Five different causes are usually listed as sources of microevolutionary change. Notice that some factors increase genetic diversity while other factors reduce it.

Genetic drift (including the bottleneck and founder effects) is the loss of alleles because the frequency of carriers drops so low that a time comes when the last carrier is killed before it can reproduce. Genetic drift always reduces variation in the gene pool.
- The special case of the genetic bottleneck occurs when a small population in which most variation has disappeared suddenly grows larger. In this case, most individuals in the current population will differ from the average population prior to the bottleneck stage, but will be very similar to each other. Some biologists speculate that this is what happened to the cheetah, producing the modern population that is genetically very similar.
- The founder effect occurs when a small group of individuals is isolated geographically from the larger population. Because the isolated group has a smaller gene pool than the larger group, it will have a very different allele distribution--especially if genetic drift has eliminated certain alleles altogether. The aboriginal peoples of Australia are considered an example of the founder effect, since they do not carry the I^B allele for blood type and hence have no individuals with type B or type AB blood.
Gene flow is the introduction of new genetic material into a previously isolated population as a result of the migration of individuals into the population. Gene flow increases the genetic variation of the population. The most dramatic example of gene flow is the modern migration of previously isolated human populations all over the world.
Mutations are permanent, unpredicatable changes in the nucleotide sequences or in the gene sequences themselves. Only changes that occur in reproductive cells will be passed on to offspring. Most mutations that have any effect (substitutions into active genes that result in a major change in amino acid, for example) are usually harmful, so mutations have only minor effects on a population unless the population is tracked for a very long time.
Nonrandom mating occurs when individual organisms selectively breed with only a portion of the population. This can be the result of geographical isolation, or it can be the result of social factors (especially in human populations). Such inbreeding usually decreases genetic diversity over the long term by eliminating gene flow into the pool. It may also emphasize retention of particular recessive traits.
Natural selection is the non-random preservation over generations of particular traits that increase survivability. Natural selection generally decreases genetic variation, especially if the population is small enough for genetic drift of less-successful alleles to cause their elimination altogether.

Selection and phenotypes

Natural selection works on the expressed trait, the phenotype, which is the factor that increases or decreases survivability, not directly on the genotype. But because natural selection eliminates whole organisms (along with their entire genetic treasurehouse), it also affects the distribution genotypes in the population.

Three different kinds of distribution changes are possible with natural selection.

Stabilizing selection occurs with the extremes of a particular trait are eliminated. For example, infants with very high (over 10 lbs) and very low (less than 4.5 lbs) birth weights are less likely to survive than infants with a median birthweight of around 7.5 lbs. The variation in birth weights is reduced where this characteristic is an inheritable factor.
Directional selection shifts the distribution of a phenotype toward an extreme. One of the major health concerns facing us today is the use of antibiotics which, over time, have selected for bacteria that are resistant to the antibiotic by killing of those that are not. Your chances of running into resistant bacteria are much higher now than they were two generations ago, because more of the bacteria population is resistant.
Disruptive selection eliminates the middle characteristics in favor of the extremes. This occurs when the environment changes to favor the extreme characteristics while penalizing the one that hitherto had been the best adaption. If the resulting extremes are different enough, they may cease to interbreed and become separate biological species in nature (even though breeding in captivity would still be possible).

Genetic diversity

If more than two alleles for a gene are present in the population, the gene is said to be polymorphic. If a trait is programmed by multiple genes, each of which has several alleles, the range of expression for that trait can be very wide. Humans possess proteins called human leukocyte antigens that identify cells as "local" and trigger immune reactions to anything foreign that doesn't match the local pattern. At least seven different loci (gene locations on chromosomes) code for HLA proteins, and each loci has multiple regulatory regions and multiple allele possibilities. The combination of HLA proteins for a given human is not unique, but will occur very infrequently in the general population.

Polymorphism is "balanced" when several alleles persist in the population over a long period of time. If a heterozygous pairing (Bb) is more advantageous than either homozygonous pairing (bb or BB), then both B and b will persist in the population. One example of heterozygous advantage is the persistence of the sickle-cell anemia recessive gene in the population, which occurs because those who are heterozygous for sickle cell anemia not only do not suffer from the disease, but have a greater resistence to malaria than those who are homozygous for non-sickle cells. In this particular instance, the conditions resulting from a mild case of one disease provides a level of immunity to another disesease.

Another factor which preserves polymorphism is frequency-dependent selection, which occurs when a particular and popular trait becomes dangerous. The most frequent examples of frequency-dependent selection involve coloration. If, over time, a particular color pattern is preferentially hunted by a species' predator, it will decrease in frequency as part of the population. The predator, which has previously ignored the less popular color, may start hunting it in preference to its previous prey. The formerly "less advantageous" allele now becomes advantageous--at least for a time--and both alleles are preserved. One of the most studied examples of frequency-dependent polymorphism is the peppered-moth population of England, which underwent a color shift as smokey residue during the industrial revolution covered the white stones against which they rested.

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