Genetic Variation, The Substrate For Natural Selection

By Haylee Sonnenberg

Genetic variation occurs within and between populations

Individual variation occurs in populations of all species of sexually reproducing organisms.

However, not all the variation we observe in a population is heritable. Phenotype is the cumulative product of an inherited genotype and a multitude of environmental influences.

Remember: only the genetic part of variation can have evolutionary consequences as a result of natural selection, because it is the only part that goes beyond generations.

Genetic variation occurs within and between populations continued

Variation Within Populations Both quantitative and discrete characters contribute to variation within a

population. Quantitative characters vary along a continuum [plant height], while discrete

characters can be classified on an either-or basis [either red or white flowers].Polymorphism

When 2 or more forms of a discrete character are presented in a population, the contrasting forms are called morphs [again, like red or white flowers].

A population is polymorphic for a character if 2 or more distinct morphs are each represented in high enough numbers to be noticeable.

Polymorphism: the existence of polymorphic characters. An example in humans is the ABO blood groups [4 morphs]. Polymorphism applies only to discrete characters.

Polymorphism

Genetic variation occurs within and between populations continued

Variation Within Populations ContinuedMeasuring Genetic Variation

Population biologists use several quantitative definitions of genetic variation. 2 of the most common are the percentage of gene loci represented by two or more alleles in a population, and the average percentage of loci that are heterozygous in the individuals of a population.

Although much of this variation is invisible, it is displayed by molecular differences.

Gel electrophoresis can be used to study these differences.

Genetic variation occurs within and between populations continued

Variation Between Populations Most species exhibit geographical variation, differences in genetic structure

between populations. Because at least some environmental factors are likely to be different from 1

place to another, natural selection can contribute to geographical variation. Genetic drift can also cause chance variations among different populations. A cline: a graded change in some trait along a geographical axis. A cline may represent:

A graded area of overlap where individuals of neighboring populations are interbreeding.

A gradation in some environmental factor.

Clinal variation in a plant

Mutation and sexual recombination generate genetic variation

Mutation New alleles originate only by mutation, or change in the nucleotide sequence

of DNA. Most mutations occur in somatic cells and are lost when an individual dies.

Only mutations that occur in cell lines that produce gametes can be passed along to offspring.

Chance determines where a mutation will strike and how it will alter a gene. Most mutations are harmless, but single point mutations can have significant

impacts on phenotypes. A mutation that alters a protein enough to affect its function is more often

harmful that beneficial. On rare occasions, however, a mutant allele may actually fit its bearer to the

environment better and enhance the reproductive success of the individual. However, this isn’t very likely in stable environments.

Mutation and sexual recombination generate genetic variation continued

Mutation Continued Because chromosomal mutations usually affect many gene loci, they are

almost certain to disrupt the development of the organism. But even rearrangements of chromosomes may in rare instances bring benefits.

Duplications of chromosome segments, like other chromosomal mutations, are nearly always harmful. However, if this duplicated segment doesn’t hurt the genetic balance, it can continue over generations and provide an enlarged genome with excess loci that may eventually take on new functions as a result of mutation. New genes also might arise.

For bacteria and other microorganisms that have very short generation spans, mutation can have a noticeable effect on a population’s variation in a short time.

If a single individual in the population happens to harbor a mutation that renders it resistant to a poison, in just a few hours there may be millions of resistant bacteria.

Mutation and sexual recombination generate genetic variation continued

Sexual Recombination Members of a sexually reproducing population owe nearly all their genetic

differences to the unique recombinations of existing alleles each individual receives from the gene pool.

Sex shuffles alleles and deals them at random to determine individual genotypes.

A population, of course, contains a huge number of possible mating combinations, each bringing together the gametes of individuals that are likely to have different genetic backgrounds.

Sexual reproduction recombines old alleles into fresh assortments every generation.

Diploidy and balanced polymorphism preserve variation

Diploidy The diploid nature of most eukaryotes hides a considerable amount of genetic

variation from selection in the form of recessive alleles in heterozygotes. The recessive allele is exposed to selection only when both parents carry it

and combine 2 copies in 1 zygote. Heterozygote protection maintains a huge pool of alleles that may not be

suitable for present conditions but that could bring new benefits when the environment changes.

Diploidy and balanced polymorphism preserve variation continued

Balanced Polymorphism Selection itself may preserve variation at some gene loci. This ability of natural selection to maintain diversity in a population is called

balanced polymorphism. One of the mechanisms for this preservation of variation is heterozygous

advantage. If individuals who are heterozygous at a particular locus have greater survivorship and reproductive success than any type of homozygote, then 2 or more alleles will be maintained at that locus by natural selection.

Crossbreeding between 2 different inbreed varieties often produces hybrids that are much more healthy than either parent stock.

This hybrid vigor is probably due to 2 factors: The segregation of harmful recessives that were homozygous in the inbreed

varieties. The heterozygote advantage at many loci in the hybrids.

Diploidy and balanced polymorphism preserve variation continued

Balanced Polymorphism Continued A patchy environment, where natural selection favors different phenotypes in

different subregions within a population’s geographical boundaries, can also result in balanced polymorphism.

[Protective coloration suited to different backgrounds may help explain the morphs of those snakes we saw earlier.]

A balanced polymorphism in a finch population

Diploidy and balanced polymorphism preserve variation continued

Balanced Polymorphism Continued Still another [yes, there’s more!] cause of balanced polymorphism is

frequency-dependent selection, in which the reproductive success of any 1 morph declines if that phenotypic form becomes too common in the population.

Frequency-dependent selection

Diploidy and balanced polymorphism preserve variation continued

Neutral Variation Some of the genetic variations observed in populations are probably

unimportant in their impact of reproductive success. Neutral variation: variation that confers no selective advantage for some

individuals over others. The relative frequencies of neutral variations will not be affected by natural

selection; some neutral alleles will increase in the gene pool, and others will decrease by the chance effects of genetic drift.

Variations appearing to be neutral may, in fact, influence survival and reproductive success in ways that are difficult to measure. Variation may be neutral in 1 environment and not in another.

But, even if only a fraction of the extensive variation in a gene pool significantly affects the organisms, that is still an enormous reservoir of raw material for natural selection and the adaptive evolution it causes.

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