Lecture 5a: Population Biology Theory

Question 1

Population Dynamics

Answer 1

change in size and structure over time

Question 2

what influences population dynamics

Answer 2

• deterministic processes (predictable)
• stochastic processes (chance)

Question 3

stochastic processes

Answer 3

• demographic uncertainty
• environment uncertainty
• genetic uncertainty (drift)

Question 4

Demographic uncertainty

Answer 4

random variation in reproduction and mortality
- birth rate
- death rate
- sex ratio

Question 5

Environmental uncertainty

Answer 5

random variation in the biological and physical environment
- habitat and resources
- predation and disease
- competitive interactions
- invasive species
- catastrophes

Question 6

Small Population Paradigm

Answer 6

population viability increases with population size

Question 7

Extinction vortex

Answer 7

tendency for small populations to go extinct over time

Question 8

Reason for extinction vortex

Answer 8

1. environmental and demographic uncertainity
2. positive population regulation
3. genetic factors (drift, inbreeding)
4. interactive effects

Question 9

Genetic factors affecting small populations

Answer 9

short-term impacts: inbreeding depression
long-term impacts: loss of genetic diversity > loss of ability to adapt in future

Question 10

Four mechanisms of evolution

Answer 10

1. mutation
2. natural selection
3. Genetic drift
4. Gene flow

Question 11

Mutation

Answer 11

introduces new alleles
most mutations are bad or neutral, however

Question 12

Natural selection

Answer 12

• populations go through adaptations due to natural selection
• variants that have a fitness advantage would increase, disadvantage would decrease, and those that are neutral would not be affected

Question 13

Darwin’s theory of evolution through natural selection

Answer 13

• individuals vary in their traits
• some of that variation is heritable
• some of that variation affects their fitness

Question 14

Types of selection

Answer 14

directional and purifying selection

Question 15

Directional selection

Answer 15

acts on positive alleles (beneficial alleles), causing them to increase over time

Question 16

Purifying selection

Answer 16

Acts against negative alleles (deleterious alleles) over time, causing them to decline over time

Question 17

Example of directional selection

Answer 17

black variants of the eastern gray squirrel are common in colder climates, high elevation, and in urban environments because they have a higher fitness in these areas. (Can be easily seen by predators when they are white)

Question 18

Example of purifying selection

Answer 18

In southern populations, black squirrels may also be selected against because they are more sensitive to higher temperatures / are more visible to predators. White variants of the gray squirrel are rare natural areas because they cannot blend in the background and hide from predators.

Question 19

Fixation

Answer 19

• One allele becomes the only allele in the genepool for a specific locus. All individuals are now homozygous for that allele.
• Speed of fixation varies between dominant, additive, and recessive beneficial alleles
• Dominant alleles increase in frequency quickly but takes a longer time
• Recessive alleles increase slowly but once common, fixation is very fast
• Additive alleles are slower to increase than dominant but reaches fixation faster

Question 20

Selection coefficients

Answer 20

Numerical measure of degree of natural selection (NS) against a specific genotype (measured through relative fitness).
S = 0 : variant has an average lifespan or produces the average number of offspring in their lifetime
S = 1 : variant dies young (ex. Has a genetic lethal mutation)
S = 0.30 : variant lives 30% less time or produces 30% less offspring

Question 21

Genetic Drift

Answer 21

changes in allele frequency due to random chance.
Random with respect to fitness
Always occurring in all populations
Most pronounced in very small populations

Question 22

Consequences of drift in small populations

Answer 22

decrease in heterozygosity
loss of alleles

Question 23

Effective population size

Answer 23

the size of an ideal population (i.e., one that meets all the Hardy-Weinberg assumptions) that would lose heterozygosity at a rate equal to that of the observed population

Question 24

Ne

Answer 24

~ # breeding individuals in population

Question 25

Ne can be reduced by

Answer 25

- Past bottleneck events / drift - Inbreeding, especially across successive generations - Variations in sex ratio or other factors that influence individual mating success

Question 26

Gene Flow

Answer 26

Alleles move between populations

Question 27

Effects of loss of gene flow

Answer 27

- inbreeding depression - outbreeding depression

Question 28

Negative density-dependent regulation

Answer 28

As population size increases, individual fitness decreases. This is due to intraspecific competition for resources, predation, and parasitism and pathogens.

Question 29

Positive density-dependent regulation

Answer 29

As population size increases, so does individual fitness. Ex. Allee effects.

Question 30

Evolution

Answer 30

Changes in Allele frequency over time.

Question 31

Relative Fitness

Answer 31

Number of offspring an individual produces, or length of time it lives, relative to other individuals in the population (scaled from 0 to 1).

Question 32

Genetic lethal mutations

Answer 32

Cause of death of individuals before they can reproduce (strong NS against these alleles).

Question 33

Inbreeding depression

Answer 33

Breeding between close relatives in small populations leads to increased mortality of offspring, production of fewer offspring, unfit or sterile offspring, or offspring with reduced mating success.

Question 34

Outbreeding depression

Answer 34

Production of offspring that are unfit, sterile, lack of adaptations for local environment due to interbreeding of individuals who are genetically too different from one anothers.

Question 35

Genetic load

Answer 35

A number between 0 and 1 and it measures the extent to which the average individual in a population is inferior to the best possible kind of individual

Question 36

Genetic purging

Answer 36

Inbreeding can ‘unmask’ deleterious recessive alleles as the frequency of homozygous genotypes increases.

Question 37

Allee effects

Answer 37

Individual fitness decreases when population density decreases, occurs in small populations

Question 38

Why does allee effects occur?

Answer 38

- mating - predator avoidance - thermoregulation - other social behaviours