BIOL - Lecture 3

Question 1

Key ingredients for evolution

Answer 1

• Organisms come from different organisms
• Organisms inherit at least some trait from their parents
• Organisms vary in traits within species, and this variation can be heritable

Question 2

NS also requires one more ingredient

Answer 2

Organisms with some traits produce more offspring or survive longer than those with other traits

of offspring = its fitness

Question 3

Relative fitness

Answer 3

The time it takes for a trait to increase relative to another depends on the relative fitness of each trait

Question 4

Frequency-dependent selection

Answer 4

The fitness of a phenotype depends on how common it is in the population.

EX: Rock paper-scissors
- A rare strategy can win when others are common
- If everybody plays rock, it’s best to play paper
- But if everybody switches to paper, it’s best to play
scissors

Question 5

Does evolution make the perfect organism?

Answer 5

NO

1. Historical constraints
   - NS uses existing structures
   - Evolution does not build new structures from scratch
   - Panda’s thumb - a large wrist bone from a meat eating ancestry, evolved to eat bamboo
2. Adaptations are compromises
   - ex: peacocks tail attracts female and predators
3. Not all evolution is adaptive
   - 4 other mechanisms (mutation, gene flow, genetic drift, non random mating)
4. NS works with available variation
   - best rather than ideal
   - slight imperfections are a sign of NS

Question 6

5 Mechanisms of evolution

Answer 6

NS
Mutation
Gene flow
Genetic drift
Non-random mating

These mechanisms interact in all populations to drive evolution.

Population size is a key variable for determining which force is strongest.

Question 7

Mutation

Answer 7

Happens constantly but at a very low rate.

Is a result from genes changing, due to mistakes in genetic copying

It is the ultimate source of all genetic variation but generally happens ar a low rate for each allele/gene

Question 8

Gene flow (ie migration)

Answer 8

Movement of an organism from one existing population to another

Transfer of alleles into or out of a population

Often reduces genetic variation between the two groups. And can sometimes lead to two populations becoming one. Mixing of individuals between pops tends to reduce differences between populations over time

May cause population to lose alleles - aka decrease genetic variation - and another population to gain alleles - aka increase genetic variability

Question 9

Genetic drift

Answer 9

Result of sampling error
Strongest in small populations
It’s not about selecting for adaptive traits, it’s about RANDOMNESS. (ie by chance, only three white flowers leave offspring)

Heavily impacts SMALL populations

Question 10

Two types of genetic drift

Answer 10

Founders effect - if a small # of individuals colonize a new area. (ie a bird carries flower seeds to a new area)
It is starting a new population rather than happening between 2 existing populations.

Bottleneck effect - follows chance events such as storms, fires, clear-cutting etc.. can also be farming/hunting
The population is extremely reduced.

Question 11

Effects of genetic drift

Answer 11

Significant in small populations
Allele frequency can change at random over time
Loss of genetic variation within a population
Harmful alleles can become fixed

Question 12

Conservation implications of genetic drift

Answer 12

Heavily impacts SMALL populations
Endangered species are the “narrow” portion of a genetic bottleneck
Reduced genetic variation
Less able to adapt to change (due to less genetic variation)

Question 13

Genetic Drift vs NS

Answer 13

The change in allele frequencies is not related to the alleles’ influence on reproductive success.

Question 14

Non-random mating

Answer 14

Not all individuals have an equal chance to mate with one another
ie: bees tend to visit flowers of the same colour

Many species show assortative mating - non-random mating systems where mates are chosen according to their phenotype.
- positive - mates are chosen based on similarity of phenotype
- negative - mates are chosen based on difference of phenotype
Plays an important role in speciation (sympatric speculation)

Question 15

Size of population affects the strength of each mechanism

Answer 15

small populations are more affected by random events (genetic drift) and gene flow

Question 16

Only consistent mechanism of adaptive evolution

Answer 16

NS = chance + selection

Question 17

Does NS create new phenotypes

Answer 17

No, it edits existing phenotypes

BUT over time it can make new phenotypes

Question 18

Evolutionary mechanisms can interact

Answer 18

Mutation is the ultimate source of new variation

All traits have been shaped by 1 or more forces - New traits come first from mutations, secondarily from recombination and gene flow

Other forces may act against NS
- some fitter traits may not be selected in the long-term, because of drift (e.g. the ability to make vitamin C)
- mutation can keep rare non-adaptive alleles around
- gene flow can prevent adaption to local

Question 19

Interaction between gene flow and selection

Answer 19

• Gene flow can slow adaptation down
• New individuals may be less adapted than those already in the population
• But movement can also lead to more adapted individuals finding the habitat and gene flow can bring new alleles, increasing variation

Question 20

Hardy-Weinberg

Answer 20

HW equilibrium - a population is NOT evolving - the allele frequency remains constant

The frequencies of alleles and genotypes in a population will remain constant unless acted upon by outside agents or forces (such as NS, mutation, gene flow, genetic drift, non-random mating)

Question 21

Assumptions for HW to hold

Answer 21

No NS
No mutation
No gene flow (isolated population)
No genetic drift (large population)
Random mating

THIS NEVER HAPPENS

Question 22

HW Equation

Answer 22

genotype frequency
p^2 2pq q^2

allele frequency
p q

1 = p^2 + 2pq + q^2
or
1 = p + q

Question 23

Why do we care about HW Theorem

Answer 23

It states that variation alone is not enough for evolution, frequencies of alleles and genotypes in a population will remain constant unless acted upon by outside forces

If a population is not in HW equilibrium, at least 1 of 5 evolutionary mechanisms are at work

Question 24

HW conclusions

Answer 24

It predicts expected frequencies of genotypes given allele frequency

If observed frequencies match expected frequencies - in equilibrium - it is not evolving

If observed frequencies do not match expected frequencies - not in equilibrium - at least 1 evolutionary mechanism is at play

Question 25

HW and Genes

Answer 25

All the genes satisfying HW?

• Very unlikely that a population will be in HW equilibrium (i.e., not evolving at all). But it’s possible that the assumptions of HW could be met for particular genes

Just one gene satisfying HW?

• Are there mutations in that gene? Did organisms mate randomly with respect to their genotype for that gene?