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Chapter 13: Microevolution and Population Change

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Evolution: Natural Selection

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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.

Population Change

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.

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.