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The bottleneck effect. Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population.
By chance, blue alleles are over-represented in the new population and gold alleles are lost.

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The bottleneck effect.
Bottlenecking a population of organisms tends to reduce genetic variation in a population, as in these northern elephant seals in California that were once hunted nearly to extinction.

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One species, two populations. These 2 caribou populations in the Yukon are not totally isolated - their ranges overlap. Nonetheless, a caribou is more likely to breed with members of its own population than with members of the other population. Thus the gene pool (total aggregate of alleles) of each population is isolated.

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One consequence of intersexual selection is sexual dimorphism, where males exhibit showy phenotypes to attract females.

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Gene flow due to immigration of people throughout the world has reduced the genetic differences between the U.S. population and other populations. The computer-generated image on this magazine cover illustrates the associated reduction in phenotypic variation.

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Genetic drift.This small wildflower population has a stable size of 10 plants. Only 5 plants (those in white boxes) of generation 1 produce fertile offspring. By chance, only 2 plants of generation 2 manage to leave fertile offspring. The C^W allele first increases in generation 2, then falls to zero in generation 3. Genetic variation on this population has been reduced.

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An individual such as this Medium Ground Finch does not evolve, but populations evolve over time by natural selection.

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Heterozygote advantage and balanced polymorphism. The recessive sickle-cell allele produces mutant hemoglobin with reduced capacity to carry oxygen. This mutation also confers malaria resistance in heterozygotes. This heterozygote advantage leads to a larger proportion of the mutant allele than usual in areas where malaria is widespread. These populations exhibit balanced polymorphism between the mutant and wild-type alleles.

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The Hardy-Weinberg theorem.
The gene pool of this wildflower population remains constant from one generation to the next. The allele frequencies (p and q) and genotype frequencies do not change from Mendelian processes alone.

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Mendelian inheritance preserves allele frequencies from one generation to the next.
This incomplete dominance of flower color gene preserves the ratio of C^R and C^W alleles in both the diploid adults and haploid gametes assuming random fusion of the gametes in the population.

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Individuals are selected; populations evolve. The bent grass (Agrostis tenuis) in the foreground is growing on the tailings of an abandoned mine.
These plants tolerate concentrations of heavy metals that are toxic to other plants of the same species.
Many seeds from the pasture drift onto the tailings, but only those with genes that enable them to tolerate metallic soil survive and reproduce. The allele frequencies of the gene pool in this population changes over time: microevolution.

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Independent assortment of chromosomes and crossing over during meiosis result in sexual recombination and provides the basis for enormous variation among the offspring.

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Directional selection shifts the overall makeup of the population by favoring variants at one extreme of the distribution. In this case, darker mice are favored because they live among dark rocks and a darker fur color conceals them from predators.

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Disruptive selection favors variants at both ends of the distribution. These mice have colonized a patchy habitat made up of light and dark rocks, with the result that mice of an intermediate color are at a disadvantage.

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Stabilizing selection removes extreme variants from the population and preserves intermediate types. If the environment consists of rocks of an intermediate color, both light and dark mice will be selected against.

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Natural selection for mating success may result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics. Many species exhibit intersexual selection (mate choice), when individuals of one sex (usually females) are choosy in selecting their mates. Often this leads to intrasexual selection, when individuals of one sex (usually males) compete for mates of the opposite sex. Such competition often decreases his chances of survival. _Vid_Campbell7e/WolvesAgonistic-V.swf

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Mutations are the source of all heritable variation.
The diverse coat colors of these wild mustangs are the products of past mutations over many generations.