Lecture 3- Mechanisms of evolution III

Question 1

What are the three major forces that maintain genetic variation within populations?

Answer 1

Neutral mutations, sexual recombination, frequency dependent selection

Question 2

What are neutral alleles?

Answer 2

An allele that does not affect the fitness of an organism as a result of mutations that do not effect the function of the resulting proteins.

These are unaffected by natural selection, and tend to accumulate over time to provide significant genetic variation.

Question 3

Much phenotypic variation that can be observed is not neutral. How is neutral variation measured?

Answer 3

Molecular techniques allow neutral alleles to be identified. These can be used to estimate rates of evolution.

Question 4

How does sexual recombination increase genetic variation?

Answer 4

It amplifies the number of possible genotypes by crossing over and independent assortment of chromosomes during meiosis, as well as combination of different gametes.

Question 5

What are the short-term disadvantages of sexual reproduction?

Answer 5

• Recombination breaks up adaptive combinations of genes
• Sex reduces the rate at which a female passes on her genes to their offspring
• Dividing the population into genders greatly reduces the overall reproductive rate.

Question 6

What are the advantages of sexual reproduction that possibly led to this type of reproduction arising?

Answer 6

• Facilitates the repair of damaged DNA
• Permits elimination of deletrious mutations
• Defense against pathogens and parasites

Question 7

What is Muller’s Ratchet?

Answer 7

The accumulation of deletrious mutations in asexual lineages- each mutation is passed on to its offspring, the number of such mutations can only increase

Question 8

Why does sexual recombination defend organisms from pathogens and parasites?

Answer 8

Parasites and pathogens have much shorter life cycles than their hosts so can evolve counter-adaptations against their host’s defense mechanisms. Sexual recombination gives the host a chance to keep up.

Question 9

What effect does sexual recombination have on allele frequency?

Answer 9

Allele frequency does not change. Sexual recombination generates new combinations of alleles on which natural selection can act.

Question 10

What is frequency dependent selection?

Answer 10

The maintenance of polymorphism when the fitness of a genotype or phenotype depends on its frequency in a population.
This maintains genetic variation within populations.

Question 11

Give an example of frequency-dependent selection.

Answer 11

The mouth of Perissodus microlepis- a small fish in Lake Tanganyika, opens on either the right side or left side due to an asymmetrical jaw joint.
The scale eater approaches its prey from behind and notes off several scales.
Vigilance by prey favors an equal number of left and right mouthed scale-eaters- if attacks from one side were more common, prey would pay more attention to potential attacks from that side.

Question 12

Environmental variation favors genetic variation because no single genotype is likely to perform well under all conditions. Explain how genetic variation is maintained by environmental variation in Colias butterflies in the Rocky Mountains.

Answer 12

Dawn temperatures are cold, afternoon temperatures are hot. The Colias butterfly is polymorphic for the enzyme phosphoglycose isomerase which influences how well a buttefly flies at different temperatures.
At different temperatures, heat or cold tolerate genotypes are favoured.
Heterozygous Colias butterflies fly over a greater temperature range- giving them a foraging and mating advantage.

Question 13

Why do subpopulations vary genetically?

Answer 13

They are subjected to different selective pressures in different environments.

Question 14

Temperature and soil moisture vary between north and south facing mountain slopes. How does this effect the Pinus ponderosa in the rocky mountains?

Answer 14

On south facing slopes, there are more heterozygous Pinus pnderosa for peroxidase enzyme where temperatures fluctuate dramatically.
On north facing slopes, temperatures are cooler and less fluctuating, peroxidase homozygote is more frequent (this has a lower optimum temperature)

Question 15

Explain the geographic variation in white clovers across Europe.

Answer 15

Subpopulations of Trifolium repens produce cyanide depending on the winter temperature of their environment. Poisonous clover plants are less appealing to herbivores, but are more likely to be killed by frost (freezing damages the cell membrane and releases cyanide into the plants own tissue).

Question 16

How does a lack of genetic variation constrain evolution?

Answer 16

If an allele for a given trait does not exist in a population, that trait cannot evolve, even if it would be favored by natural selection.

Question 17

Developmental processes constrain evolution. Evolutionary innovations are modifications of previously existing structures. This can be seen in the evolution of bottom dwelling fish. What are these?

Answer 17

Bottom-dwelling skates and rays evolved from a common ancestor with sharks, whose bodies were ventrally flattened with flexible cartilage.
Plaice, sole and flounder are descendants of deep-bellied, laterally flattened ancestors with bony skeletons. To become laterally flattened, they flopped to one side and during development, their eyes twist to one side of their body.

Question 18

What universal constraints must evolution work within?

Answer 18

Cell-size to volume ratio
Protein folding (constrained by bonding that can occur)
Laws of thermodynamics (constrains energy transfers)

Question 19

Why do trade-off’s constrain evolution?

Answer 19

Adaptations involve fitness costs and benefits. Benefits must outweigh the costs if adaptation is to evolve.

Question 20

What trade-off’s do Garter snakes face when they develop resistance to TTX?

Answer 20

They are unable to move rapidly having eaten a snake. Benefits outweigh the cost only in areas where the newts are common and major parts of the snakes diet.

Question 21

What is sexual dimorphism?

Answer 21

Dramatic differences between sexes

Question 22

Why are patterns of evolutionary change unlikely to be observed during short-term studies?

Answer 22

They occur infrequently, such as meteorite, or are very slow, such as continental drift. Other types of evidence are used to measure and demonstrate effects of rare and unusual events. (fossil records). Short-term evolutionary outcomes may differ from long-term evolutionary outcomes.

Question 23

Human’s influence evolution. For example, by decreasing pests, and increasing ‘desirable’ animals. This can have undesirable consequences, such as…

Answer 23

Antibiotic resistance by pathogens

Resistance to pesticides by pests

Question 24

How do sport’s hunters change the population of rams?

Answer 24

• Seek large trophy animals
• Preferentially shoot male mammals with large ornaments
• At Ram Mountain in Alberta, large individuals with large horns have been hunted before the reach reproductive success
• Trophy animals have become more scarce.