Balanced polymorphism (A way to preserve two different alleles)
Sources of Variation
Mutations
The origin of all variation in a population
Note: Lateral Gene Transfer (aka Horizontal Gene Transfer) from other organisms is another way to get completely new genetic material into a particular population of organisms, but it isn't in the textbooks yet!
Responsible for the creation of new alleles in the genome
Everything else is simply rearranging this new material!
Sexual Reproduction
The combining of genetic material from two different individuals creates individuals with new combinations of alleles.
This has several different names:
Recombination
Sexual recombination
Genetic recombination
Three different ways genetic material is shuffled:
Independent assortment
Crossing over
Random joining of gametes
Unknown source.
More Detail on Sources of Variation in Sexual Reproduction
Diploid organisms are ones that have two copies of each chromosome in their cells (i.e., they have homologous chromosomes).
This becomes significant in a heterozygous individual.
Remember that an individual heterozygous for a particular trait is one with two different alleles for that trait at a particular gene locus (as illustrated below).
This allows the recessive allele to “hide” from natural selection.
As a result, variation persists in the population since it is not “selected” out.
Haploid organisms—ones that don't have homologous chromosomes—don't have as much variability because every allele is expressed and therefore can be acted on by natural selection.
The maintenance of different phenotypes in a population.
Word derivation:
Poly: many
Morpho: form
Polymorphism: many forms
The idea is that two different phenotypic forms of a single trait are maintained in a population, i.e. there is a “balance” of the two forms—neither of the forms is “selected” out.
Balanced polymorphism is maintained in three ways:
Heterozygote advantage
Individuals that are heterozygous at a particular gene locus (e.g. Pp) may survive and have better reproductive success than either of the homozygotes (e.g. AA or aa).
When inbred individuals are crossbred, the hybrids produced are much more vigorous than either inbred parent.
This is probably due to harmful recessives that were homozygous in the parents becoming heterozygous in the offspring.
This also means the heterozygote advantage may manifest itself.
Example: when crossbreeding of corn plants, inbred varieties are produced specifically so their offspring will be more vigorous.
Frequency-dependent selection
The reproductive success of one variant may decline if its phenotype becomes too common in the population.
Example:
Certain butterflies maintain several different phenotypic appearances that all look noxious to predators (even though they're not).
If they were all the same, predators would figure out much faster that they were only faking noxiousness—an increased frequency of the same phenotype will be selected against.
Modes of Selection
Natural selection will affect the frequency of a hereditable trait in a population in three different ways, depending on which phenotypes are favored.
The diagram below shows how an original snail population can be changed in different ways depending on which type of selection acts on the population.
Keep in mind that the basic mechanism of natural selection is the same, even if it has different “modes”: certain hereditable traits are favored based on their reproductive success.
Example: stabilizing selection keeps human birth weights around 3–4 kg; any babies much smaller or larger than this are selected against (die).
Directional selection
Favors deviation from the average, selecting for traits that are (at least initially) relatively rare.
Most common during periods of environmental change or adjustment to a new habitat.
Example: the average size of black bears increased with each ice age, and decreased during the intermediate periods: each time a rarer trait was selected for because it was more favorable.
Disruptive selection
Favors extreme phenotypes as opposed to intermediate, or “average,” phenotypes.
Occurs when environmental conditions are varied to support extremes—specialness in one way or specialness in another—but “just average” traits don't cut it.
Example: both short and long beak sizes in finches have certain advantages, but medium-sized beaks aren't all that helpful.
Sexual Selection
What creates sexual dimorphism—the difference in secondary sex characteristics in males and females.
Secondary sex characteristics are any differences between males and females besides their reproductive organs.
An example in humans is average body size: females are lighter, on average, but this trait is not related to their reproductive organs.
This is actually a general trend: males are usually larger, and often showier (antlers on male deer, plumage on male birds, etc.)
This is in part because having these traits makes the males more “masculine” (by definition), and thus more likely to attract females.
These are not necessarily favored because they are advantageous to living, but instead because they are advantageous to breeding.
For example, in birds, showier plumage may attract predators—but if it gets more chicks, that's what matters!
Basically, the rational behind these traits is: “hey, if I have showy tailfaithers, I might have more sex before I die from tripping over them and being eaten!”
Often, the ultimate evolutionary outcome is a compromise between sexual selection and ordinary “survival of the fittest.”
However, sometimes the line is blurred: e.g. in deer, antlers are more attractive to females, but also help the male deer fight off predators.
