Natural Selection: A Two Step Process
- Natural selection is a mechanism of change.
- Variation must exist in the population.
- Natural selection can then act on this variation to select the “most fit” individuals to survive and reproduce.
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Sources of Variation
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Preservation of Variation
- Diploidy (a way to hide recessive alleles)
- Balanced polymorphism (A way to preserve two different alleles)
Sources of Variation
Mutations
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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!
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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.
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This has several different names:
- Recombination
- Sexual recombination
- Genetic recombination
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Three different ways genetic material is shuffled:
- Independent assortment
- Crossing over
- Random joining of gametes
More Detail on Sources of Variation in Sexual Reproduction
Summary and Contrast
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Sexual reproduction really does “mix it up”:
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Asexual reproduction has no variation!
- There is no new variation in the population unless there is a mutation.
- Asexual populations are just clones—they are all identical.
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Example:
- Take the classroom population: you can see the variation between each person.
- If the classroom were a petri dish and the students were all bacteria, they would all look identical!
Independent Assortment of Homologous Chromosomes
- Occurs during meiosis.
- Homologous chromosomes independently assort such that the resulting gametes will have different combinations of chromosomes.
Crossing Over
- Occurs during meiosis.
- Homologous chromosomes exchange DNA.
- Result: the final chromosomes in the sperm or egg are a mixture of DNA from that individual's mother and father!
- Note: two or three crossing over events happen with each pair of chromosomes. In other words, crossing over is a common event!
Random Joining of Gametes
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One set of homologous chromosomes comes from each parent.
- One from the male
- One from the female
- It is a totally random process as to which sperm and egg meet.
Preservation of Variation
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There are two primary ways that variation is preserved in populations:
Diploidy
- Diploid organisms are ones that have two copies of each chromosome in their cells (i.e., they have homologous chromosomes).
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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.
Balanced Polymorphism
- The maintenance of different phenotypes in a population.
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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.
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Balanced polymorphism is maintained in three ways:
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Heterozygote advantage
- Individuals that are heterozygous at a particular gene locus (e.g. Aa) may survive and have better reproductive success than either of the homozygotes (e.g. AA or aa).
- Classic example: hemoglobin and sickle-cell disease
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Hybrid vigor
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When inbred individuals are crossbred, the hybrids produced are much more virorous 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 hereozygote advantage may manifest itself.
- Example: when crossbreeding of corn plants, inbred varieties are produced specifically so their offspring will be more vigorous.
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Frequency-dependent selection
- The reproductive success of one variant may decline if its phenotype becomes too common in the population.
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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.
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Stabilizing selection
- Acts against extreme phenotypes.
- Favors the more common, intermediate, variants.
- Example: stabilizing selection keeps human birth weights around 3–4 kg; any babies much smaller or larger than this are selected against (die).
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Direcitonal 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.
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Diversifying 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.
- Can result in balanced polymorphism.
- Example: both short and long beak sizes in finches have certain advantages, but medium-sized beaks aren't all that helpful.
Sexual Selection
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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.
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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.
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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!”
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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.