Topic 6 - Natural Selection (Genetic Perspective) Flashcards
What is the definition of Natural Selection using a genetic perspective.
- it is the change of allele frequencies within a population resulting from fitness differences among individuals
What is one way of assessing natural selections.
- look at how it affects fitness
What are the 5 main components of fitness from which it could be measured from. - two stand outs as most important?
- survival ability
- mating success (sexual selection of a partner)
- Fecundity (producing enough offspring)
- Quality of Gametes
- Time to maturity
Survival and Fecundity
What does R stand for?
the absolute fitness of a genotype
What does R - absolute fitness mean? what is a good R value?
- the absolute fitness of a genotype
- meaning R is the MEAN number of progeny per parent (or per capita growth rate)
- if R equals 1, then that genotype is replacing exactly itself, and remains at constant numbers
What does w stand for?
- the relative fitness of a genotype
What does w - relative fitness mean? what does it vary between?
- the relative fitness is defined as “its absolute fitness RELATIVE to the maximum absolute fitness”
- varies between 0 and 1
- so w will indicate the increase or decrease in a population, relative to other genotypes
Define fitness (via the textbook)
- is the average per capita lifetime contribution of individuals of that genotype to the population after one or more genereation
- reproductive success
How do genetic models typically describe evolution? how does this answer relate to fitness levels?
- through allele frequencies, their abundance in a population
- higher fitness genotypes in a population contribute a greater proportion of their alleles to the next generation
What do allele frequencies and these population genetic models not tell us, how do we solve this through absolute and relative fitness?
- they give no information about population increases or decreases
- this is because the genotype with the highest relative fitness (w) in the population can still have an absolute fitness (R) below 0 thus have a declining population in overall numbers
How does natural selection affect populations? (2 ways)
- it will change relative frequencies of alleles (changing the TRAIT MEANS in a population)
- it will change allelic composition of genotypes (changes in VARIANCE of trait means)
What are the 3 modes of natural selection - in SINGLE-LOCUS traits?
- directional selection (which increases an advantageous allele)
- overdominance
- underdominance
Explain the homozygote advantage.
- occurs in directional selection
- the homozygote has the highest fitness
Explain the heterozygote advantage.
- this occurs in over-dominance selection
- when the heterozygote has the highest fitness (and the homozygote may be lethal)
Explain the heterozygote disadvantage. what could this lead to in extreme cases?
- this occurs in under dominance selection
- when the heterozygote has the lowest fitness
- may lead to two populations not mating with one another
What are the 3 modes of natural selection - in QUANTITATIVE, continuously varying traits?
- Directional selection (which increases an advantageous allele)
- stabilizing selection
- diversifying selection
Define each of the modes of selection when there is a quantitative continuously varying trait.
- directional selection will INCREASE a proportion of a genotype with the more extreme value of a favoured trait
- stabilizing selection does NOT alter the mean, but will REDUCE Variance
- Diversifying selection will shift the mean
What was Darwins basis for selection? and how did he describe new traits?
- he used his observations and writings focused on DIRECTIONAL selection
- through the appearance, spreading, and gradual change in populations and species over time
What does “s” denote? and what type of selection does it denote?
- ”s” is denoted as the selection coefficient
- this measures the rate of spread of an advantageous allele over time (a form of directional selection)
When “s” is high, what would you expect to observe? vs a low “s”
- there would be strong selection towards the favoured allele, thus those without would expect faster disappearance of individuals lacking this allele
- a low selection value will tend to keep unfavourable alleles around longer as more time is required to allow for disappearance
What rate of allele frequency do dominant, co-dominant, and recessive alleles take?
- dominant alleles would be expressing in both heterozygous and homozygous conditions
- recessive alleles would only be expressed under homozygous conditions
Why do deleterious alleles tend to hang around in populations at low frequencies?
- if the trait is recessive, it may take a while to become extremely prevalent in the population
What don’t stabilizing and diversifying selections NOT do? what do they do instead?
- DO NOT change the mean value of a trait in a population
- they will INCREASE OR DECREASE the amount of variation in that trait
What type of selection maintains a polymorphism, an example?
- the sickle-cell hemoglobin mutation
- the heterozygote advantage (aka overdominance)
- these heterozygotes (carrier) have the highest fitness while the homozygotes (carriers of the disease) still persist in the population at LOW frequency