6 Evolutionary Theory

variability

variability

due to genetics and environment, but only the genetic component is useful in selection (natural or artificial)

population

a group of interbreeding individuals of the same species that inhabit the same space at the same time

hardy weinberg law

if certain criteria are met, the frequency of alleles in a population will not change (i.e. there will be no evolution). In one generation, populations would assort into stable genotypic and phenotypic frequencies

• Let p = frequency of A allele

• Let q = frequency of a allele

• If these are the only two alleles, p + q = 1

• p will be the frequency of A in all gametes in population

• q will be the frequency of a in all gametes in population

• We can predict the frequency of homozygotes and heterozygotes at equilibrium (takes one generation)

• % AA = p2        % Aa = 2pq       % aa = q2

heritability

genetic variability ➗ total variability

total variability

environmental variability + genetic variability

assumptions

of Hardy-Weinberg:

• Infinitely large population: to avoid random fluctuations. In reality population bottlenecks are common, so we have genetic drift.

• Random mating: inbreeding decreases heterozygosity. Outbreeding or “non-assortative mating” increases heterozygosity. Both occur frequently in nature.

• No migration AND no new mutations: otherwise p+q does not equal 1 and allele frequencies change arbitrarily. In reality, new mutations constantly occur and migrations are common.

• No advantage/disadvantage to alleles: or you get natural selection.

genetic drift

permits a population’s allele frequencies to change (evolution) even when the allele increasing in frequency does not affect fitness

• an important driving force in evolution when there are small populations (population bottlenecks)

natural selection

occurs on individuals, whereas evolution occurs on populations (successful individuals contribute more offspring to the next generation than unsuccessful individuals; the allele frequencies in the population change over time)

evolution

the accumulation of changes in the genes of populations over time

• relies on random events and on a definition of fitness that is always relative, always context-dependent, and always changing

• has no goal

microevolution

subtle changes in a population, often over relatively short periods of time

macroevolution

changes in populations that result in the formation of new species or other groups

stabilizing selection

when individuals in the middle of the range of variation in a population are the most fit

speciation

occurs when populations cannot successfully interbreed; requires “reproductive isolation.”

• Once populations no longer exchange genes with each other, they have __ed. From then on, they evolve separately.

genetic incompatability

causes speciation, stems from:

1. Within populations, some combinations of alleles of different genes are unfavorable even if each allele on its own is not. If allele a becomes frequent in one population, that drives allele b to become rare in that population.

2. New and different mutations (including chromosomal rearrangements) occur in each isolated population.

3. Different selection pressures in the populations mean that hybrids might not be suited to either context.

disruptive selection

selection in which individuals at either extreme of the range of a trait do better than individuals in the middle

altruism

when one individual helps another, even at some personal cost

explanations:

1. Mutual benefit (but what about cheaters?)

2. Helping relatives spreads your own genes

3. Population selection

will be selected if r x b > c

• r = chance that aided individual carries allele

• b = the benefit to reproductive fitness of aided individual

• c = the cost to reproductive fitness of altruistic individual

• If aiding several individuals at once, add up all r X b

tit for tat

• In some species, individuals help each other and form stable relationships where favors are reciprocated.

• Not all individuals are equally helpful, but individuals who don’t return favors are punished: the other members of the group stop helping them.

• This only works in intelligent species.

kin selection

• Explained by Dawkins’ selfish gene perspective.

• Relatives are genetically similar: parents and children are 50% related. So are siblings. First cousins are 12.5% related.

• If you carry an allele, there is a 50% chance that your brother or sister does as well.

• An allele that encourages you to die in order to save three or more siblings will likely increase in frequency because it is probably found in the kin you saved!

behavior

hawkish/doveish

aging

1. Natural selection becomes less powerful as organisms grow older and more of their reproduction is done.

2. There is a trade-off between early fitness and late fitness and between reproduction and health (e.g. allocating resources to reproduction versus repair).

maintenance

• The body constantly suffers small traumas, some of which are not fully repaired. As a result, damage builds up over time.

• DNA is one important target for damage from the environment. DNA repair may be very important in senescence rates

sex

the capacity of organisms to mix their genes together, resulting in novel genetic combinations

• can occur without reproduction

• reproduction can occur without it

sexual reproduction

1. Generally includes meiosis: the maternal and paternal chromosomes exchange genetic information. Then the recombined chromosomes are placed in different eggs or sperm.

2. When two parents mate, their genetic contribution to their offspring is diluted (daughters and mothers are 50% related).

sexual selection

• We see many traits in nature that seem to reduce fitness

  • eg. brightly colored birds, male-male combat

• Fitness is defined not as health but as reproductive success

  1. You have to survive to reproduce

  2. You have to find a mate (and a good one!)

• Sometimes survival and competition for mates are in conflict and a compromise is reached.

sexy son model

• In species where males compete for females, it is beneficial to a female to be choosy in selecting a mate and to have the same preferences as other females

• If she chooses a male most females find appealing, she is more likely to have “sexy sons” and so her genes will continue to be passed down when the next generation of males compete for females.

• Females who aren’t choosy or have unusual taste in males will have sons that have no mating advantage

• This causes a positive feedback loop!