Lecture 5a: Population Biology Theory Flashcards
Population Dynamics
change in size and structure over time
what influences population dynamics
- deterministic processes (predictable)
- stochastic processes (chance)
stochastic processes
- demographic uncertainty
- environment uncertainty
- genetic uncertainty (drift)
Demographic uncertainty
random variation in reproduction and mortality
- birth rate
- death rate
- sex ratio
Environmental uncertainty
random variation in the biological and physical environment
- habitat and resources
- predation and disease
- competitive interactions
- invasive species
- catastrophes
Small Population Paradigm
population viability increases with population size
Extinction vortex
tendency for small populations to go extinct over time
Reason for extinction vortex
- environmental and demographic uncertainity
- positive population regulation
- genetic factors (drift, inbreeding)
- interactive effects
Genetic factors affecting small populations
short-term impacts: inbreeding depression
long-term impacts: loss of genetic diversity > loss of ability to adapt in future
Four mechanisms of evolution
- mutation
- natural selection
- Genetic drift
- Gene flow
Mutation
introduces new alleles
most mutations are bad or neutral, however
Natural selection
- 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
Darwin’s theory of evolution through natural selection
- individuals vary in their traits
- some of that variation is heritable
- some of that variation affects their fitness
Types of selection
directional and purifying selection
Directional selection
acts on positive alleles (beneficial alleles), causing them to increase over time
Purifying selection
Acts against negative alleles (deleterious alleles) over time, causing them to decline over time
Example of directional selection
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)
Example of purifying selection
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.
Fixation
- 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
Selection coefficients
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
Genetic Drift
changes in allele frequency due to random chance.
Random with respect to fitness
Always occurring in all populations
Most pronounced in very small populations
Consequences of drift in small populations
decrease in heterozygosity
loss of alleles
Effective population size
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
Ne
~ # breeding individuals in population
Ne can be reduced by
- Past bottleneck events / drift
- Inbreeding, especially across successive generations
- Variations in sex ratio or other factors that influence individual mating success
Gene Flow
Alleles move between populations
Effects of loss of gene flow
- inbreeding depression
- outbreeding depression
Negative density-dependent regulation
As population size increases, individual fitness decreases. This is due to intraspecific competition for resources, predation, and parasitism and pathogens.
Positive density-dependent regulation
As population size increases, so does individual fitness. Ex. Allee effects.
Evolution
Changes in Allele frequency over time.
Relative Fitness
Number of offspring an individual produces, or length of time it lives, relative to other individuals in the population (scaled from 0 to 1).
Genetic lethal mutations
Cause of death of individuals before they can reproduce (strong NS against these alleles).
Inbreeding depression
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.
Outbreeding depression
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.
Genetic load
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
Genetic purging
Inbreeding can ‘unmask’ deleterious recessive alleles as the frequency of homozygous genotypes increases.
Allee effects
Individual fitness decreases when population density decreases, occurs in small populations
Why does allee effects occur?
- mating
- predator avoidance
- thermoregulation
- other social behaviours
