Topic 6 Flashcards
what is Natural selection
Differences is survival and reproduction of phenotypes, leading to differences in their contribution to the next generation. This results in a change in the frequency of HERITABLE phenotypic variations (alleles) in population over time.
3 requirements for Natural selection
- phenotypic variation
- the variation must cause differential survival and reproduction
- the variations must be heritable
Adaptation
- an adaptation is a characteristic or trait that increases an individual’s fitness relative to individuals that do not possess it
- Natural selection is the primary means of adaptive evolution
fitness
an individual’s contribution to the next generation, and depends on:
- survival (viability sel’n)
- fecundity (# of gametes)
- ability to obtain a mate
Absolute fitness
the average number of offspring (zygotes) contributed to the next generation by an individual (zygote) in this generation
e.g. Waa, Wab, Wbb
Relative fitness
Measure of a genotype’s contribution to the next generation RELATIVE to that of other genotypes in the pop’n
(normally scaled to the genotype with the largest absolute fitness)
-eg. Waa=3, Wab=2, Wbb= 0.5
-waa= 1 (3/3), wab= 0.66 (2/3), wbb= 0.17 (0.5/3)
Population fitness
- the mean absolute fitness of a population
- denoted: W (with bar on top)
- if the population fitness is greater than 1, the population will increase in size (and vice versa)
Relative population fitness
- the average fitness of the entire population relative to those individuals within it that have the highest fitness
- denoted: w (with bar on top)
4 components of fitness
- viability: prob of surviving until reproductive maturity (viability selection)
- Fecundity: The number of gametes produced (fecundity selection)
- Mating ability: the ability to obtain a mate (sexual selection)
- gamete competition: normally sperm competition (some gametes have a higher prob of forming a zygote than others)
Antagonistic selection
when any of the following 3 oppose each other:
-viability sel’n, fecundity sel’n, sexual sel’n
Single locus model of natural selection assumptions
- Fitness differences are only due to differences in survival (viability)*****
- Fitness differences are attributed to a single locus
- Mating is random
- No mutation, drift or gene flow occurs
- Fitnesses are constant and independent of both allele frequencies and pop’n size
Equilibrium point
a point where allele frequencies will remain constant
2 major types:
a. stable eq’m
b. unstable eq’m
Stable eq’m
- a point of attraction
- allele frequencies approach a point of stable eq’m from a short distance away, and stay in vicinity of the eq’m
- *stable eq’m is GLOBALLY STABLE if it is always approached, regardless of starting conditions
Unstable eq’m
a point of repulsion
-allele freq’s move away from the point of unstable eq’m UNLESS there precisely on it
adaptive radiation
- the diversification of multiple descendent lineages from a single ancestral lineage
- typically result in ecological specialization and morphological innovation
- driven by nat. sel’n, often by sexual sel’n too
punctuated equilibrium
- adaptive radiation typically involve rapid changes (ecological and morphological innovation)
- followed by long periods of stasis
sexual selection
differential reproductive success resulting from differential abilities to find a mate
Inverse frequency dependent selection
- the LOWER the frequency of a phenotype (genotype), the GREATER its relative fitness
- alleles will always move to a pt of fixation (move away from either A or B)–>ex. of global ..fixation
Balancing selection
- any type of selection that acts to maintain genetic variation
a) heterozygote superiority
b) inverse frequency dependent selection
hitchhiking
when one genotype is dragged to fixation with another genotype because they are linked
-e.g. AABB, aabb–> bb has low fitness hence eliminated by nat. sel’n and aa also eliminated b’x they are linked (even though it has a high fitness)
epistasis
- an effect of the interaction of two or more gene loci on phenotypes and/or fitness
- joint effect of the loci is different than the sum of the individual loci taken separately
pleiotropy
- a phenotypic effect of a single gene on more than one character
- e.g. drosophila single gene involved in both larval development rate and body size
antagonistic pleiotropy
- specific type of pleiotropy
- e.g. nat. sel’n favours rapid larval development rate
1. this increases survival
2. but also results in smaller body size & lower fecundity
sexually antagonistic genes
sometimes alleles at an autosomal locus can have opposing fitness effects in males and females
-e.g. AA is high fitness in males and low fitness in females
adaptive topography
- sewall wright developed a model of “adaptive landscapes”
- there are multiple equilibria and outcomes of selection are far more complex (when traits are controlled by multiple loci)
