Topic 2 - Natural Selection & Adaptive Evolution Flashcards
Adaptive evolution of a trait requires that the trait fulfils which three conditions in Darwin’s theory of natural selection?
1) Variation in the trait among individuals
2) correlation between the trait and survival or reproductive potential
3) heritable trait variation between parent and offspring.
What is the Hardy–Weinberg equation for a locus at which there are two alleles?
p2 + 2pq + q2 = 1, where p and q are the frequencies of the two different alleles in the population.
Explain the term ‘fitness’ in natural selection.
The success of an individual at surviving and reproducing and therefore contributing offspring to future generations.
Explain relative fitness value
Fitness can be quantified when it is assigned to particular genotypes, an approach adopted by population geneticists. To do this, all the different fitness components (survival, mating success and fecundity) are included in a single relative fitness value, w.
Explain selection coefficient.
A measure of the intensity of selection against a genotype; the difference between the mean relative fitness of individuals of a given genotype and that of a reference genotype.
For each genotype: s = 1−w, where w is relative fitness.
Define ‘directional’ in terms of relative fitness
Describes selection that favours one extreme phenotype over others, causing allele frequencies to shift over time in the direction of that phenotype.
Explain ‘over dominant’ in terms of relative fitness
Describes selection in which, at a given locus, heterozygous individuals have a higher fitness than homozygous individuals; can also be described as heterozygote advantage.
Define pleiotropic.
Describes a cellular process, signal or gene that has more than one effect, or more than one phenotypic outcome. At the genetic level, describes a situation where a single mutation affects two or more apparently unrelated phenotypic traits.
Explain antagonistic pleiotropy.
Refers to a situation in which a single gene creates multiple opposing effects, such that beneficial effects of a trait created by the gene are offset by ‘losses’ in other traits.
Explain balancing selection
A form of natural selection that maintains polymorphism at a locus within a population.
Explain frequency dependent selection.
A form of balancing selection that occurs when the relative fitness (survival and reproductive success) of a phenotype (and hence its underlying genotype) differs according to whether it is common or rare in a population.
Why would a founder population have an increased likelihood of homozygous offspring?
Members of a founder population carry only a fraction of the different alleles of the original population.
Explain inbreeding depression.
A reduction in the fitness of offspring that results from breeding between close relatives; caused by the expression of deleterious recessive genes that were present in the parents as heterozygotes.
What is heterozygosity?
The proportion of a population that has a heterozygous genotype.
