Lecture 21 (Population genetics and natural selection) Flashcards
Evolution
Genetic changes in a population over time
Populations evolve, individuals don’t
Population
Localised group of individuals of the same species
Gene pool
Total aggregate of genes (and their alleles) in the population at one time
Why might we need to estimate frequencies of genotypes in a population?
To predict how many individuals will inherit a genetic disease
To estimate the proportion of individuals who are ‘carriers’ of a genetic disease
How do we predict genotypes in a population?
Hardy-Weinburg equation
Allele frequency
p+q=1
p^2+2pq+q^2 =1 (this equation shows that there are only three genotypes)
Allele frequencies change via…
Non-random mating (no choice) - assortive mating or inbreeding Random genetic drift Bottleneck effect Founder effect Natural selection Gene flow or migration Mutation
Assortative mating and interbreeding
Assortative mating - a mating pattern and form of sexual selection in which individuals with similar phenotypes mate with one another more frequently than would be expected under a random mating pattern.
Inbreeding - production of offspring from the mating or breeding of individuals or organisms that are closely related genetically.
Random genetic drift
A random change in allele frequencies due to sampling errors over generations
the change in allele frequencies due to accumulated effects of chance (random) over successive generations
Non random mating vs random mating
Nonrandom mating occurs when the probability that two individuals in a population will mate is not the same for all possible pairs of individuals. When the probability is the same, then individuals are just as likely to mate with distant relatives as with close relatives – this is random mating.
The bottleneck effect
where a large population suddenly reduces in size (catastrophe or human impact)
Only a few individuals survive so some alleles may become underrepresented or removed all together (allele frequencies change)
The founder effect
the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population.
Tends to be reduced genetic variation in the new, founding population (it is a type of boatneck event almost) (therefore changes allele frequencies)
Three types of selection
Same species but selecting for different phenotypes
Stabilising, directional and disruptive selection
Stabilising selection
Extreme phenotypes are selected against // Favours the average, tends to prevent evolutionary change
Reduces variation but does not change the mean
Example of stabilising selection
Babies - medium sized baby is good because you want them to be big enough to be healthy and small enough to be delivered
Directional selection
A single end of distribution curve is selected for and allele frequency shifts in one direction, results in evolutionary change
This type of selection changes the mean value towards one extreme
Directional selection example
Giraffes - being taller is more favourable as taller giraffes can eat things that are higher up
Disruptive selection
Both extremes are selected for, average phenotype selected against
This selection favours the two extremes producing two peaks
Sexual selection
Sexual selection is a mode of natural selection in which members of one biological sex choose mates of the other sex to mate with, and compete with members of the same sex for access to members of the opposite sex.
Frequency dependent selection
Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population.
This type of selection is based on whether something is rare or common in a population
You get fluctuations between one type of the species being more common to the other becoming more rare. You get an advantage by being rare but the better you do the less rare you become ( a constant cycle between the frequency of the different phenotypes in the same species)
Cline
a gradient of variation, often because a species extends across a geographical range
The gradual geographic change in genotypic/phenotypic composition is termed a cline
Mutation
permanent changes in the base sequence of DNA.
Mutations create new alleles and therefore new phenotypes. They may become more common in a population if they provide a survival or reproductive advantage in the current environment.
Very slow to act and usually disadvantageous, its role is usually of macroevolutionary proportions
Migration
An individual from another population successfully mates (contributes gametes) to the gene pool
Brings new alleles
Changes proportions of existing alleles
Changes population size
Makes two populations more similar
Natural selection
Where some phenotypes have the advantage in a certain environment, so individuals with that phenotype are more likely to survive, reproduce and pass on their alleles the next generation.
Gene flow (migration)
The ability of alleles to enter or leave a population.
