Agents of Evolution

Question 1

Hardy- Weinberg Equilibrium

Answer 1

models a gene pool that would not evolve

Question 2

most powerful agent of evolution

Answer 2

• Natural Selection
• favoured alleles tend to become more common in the 
population
• if unfavourable alleles were recessive, they might persist at low levels in a population for long periods of time without being exposed frequently in 
recessive (homozygous) phenotypes.

Question 3

Genetic Drift

Answer 3

• second most powerful agent of evolution
• results from the sampling errors that occur when a population is small
• In large populations, the next generation will usually be pretty representative of the alleles in the previous generation.
• In small populations one allele may get over represented or under represented by chance alone. This makes a big difference in a small population vs large population.
• genetic drift might be responsible 
for “resetting” the gene pool and allowing adaptive natural selection to 
take a population off in a new direction.

Question 4

Non-adaptive evolution

Answer 4

• when the allelic frequency changes but it does not make population more adapted to environment, basically, wehn environment does not do the selecting

Question 5

bottleneck effect

Answer 5

• When an existing population is decimated by natural or man-made factors, the few surviving members may be quite unrepresentative of the original population.
• Populations often lose much of their genetic variation, but it is also possible for rare alleles to increase suddenly in frequency.
• after this happens, populations still stay small and the genetic drift continues

Question 6

founder effect

Answer 6

• type of bottleneck effect
• The ultimate founder effect occurs when a new population is formed by one gravid (fertilized) female.
• Her alleles, even the rare ones, each achieve a frequency of at least 0.25 instantly (each allele is in 0.5 of her offspring, and 
paired with one allele from her mate).

Question 7

Genetic Drift and carrying capacity

Answer 7

• populations often stay small for many generations, allowing genetic drift to 
continue.
• Note also that while populations are small the conditions of 
competition required for Darwinian selection are relaxed, and genetic 
drift may become the dominant agent of evolution.
• Therefore, in small populations, genetic drift is the great force of 
evolution.

Question 8

mutations

Answer 8

• Mutations cause changes in the gene pool by creating new alleles.
• mutation rates are not rapid enough to drive rapid change in a population but mutation does provide the variation on which other agents can act to change allelic frequencies.
• In some asexual organisms, especially bacteria and viruses, mutation 
may be a fairly important agent of evolution, creating new strains that 
then multiply into large populations.
• Mutations are generally non-adaptive.

Question 9

gene flow

Answer 9

• through immigration, you cam breed with another population which is not adapted to the native environment.
• this means the population will never adapt.
• Gene flow between populations (a result of immigration) will generally tend to make the two populations more homogeneous.
• Since gene flow can change allelic structure, it is an agent of evolution, but it is non-adaptive and may even act to oppose adaptation.

Question 10

non-random mating

Answer 10

• does not change allelic frequency but it changes the distribution of those alleles in genotypes
• assortive mating where like attract like results in homozygotes
• disassortive mating tends to increase heterozygotes
• While these forms of non-random mating do not lead to adaptive evolution 
themselves, they may expose alleles to natural selection in different ways.

Question 11

handicap principle

Answer 11

• type of non-random mating (sexual selection)
• suggests that males with elaborate displays actually show off the value of their genes, because they can demonstrate an 
ability to survive despite the handicap of their displays
• males are effectively saying “I’m healthy enough to survive 
even with my immune system at 80%. Therefore your offspring will have 
excellent immune systems.”

Question 12

Sexy Son Hypothesis

Answer 12

• another type of non random mating (sexual selection)
  acknowledges that males could cheat by false advertising, but that false advertising can work as long as it produces another generation of females willing to fall 
for false advertising!
• Clearly, use of unreliable signals would not be stable 
in the long run, but of course natural selection does not consider the long 
term consequence

Question 13

sexual selection

Answer 13

• the most limiting 
resource in a sexual population will be access to potential mates
• females produce less gametes than males
• selection should favour males with traits that allow them to win in real or ritualized combat with other males.(bigger antlers)
• If female choice is a bigger factor in mating, selection should favour males 
who emphasize whatever traits females use to choose. feathers, red balloon bellies, (weird stuff)
• non random mating (non adaptive)

Question 14

non-adaptive agents of evolution

Answer 14

• all except for natural selection

- genetic drift, mutation, gene flow, non-random mating (sexual selection: sexy son, handicap)

Question 15

model of a gene pool that would not evolve

Answer 15

Very large population - sampling errors could change the population if it were small

No immigration or emigration - alleles cannot enter or leave

No mutation - alleles do not change spontaneously

No natural selection - no allele is favoured relative to any other allele

Random mating - individuals have equal chances of mating with each 
other, so alleles get fully mixed