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
What is the African finch an example of?
- birds with intermediary sized beaks are less likely to survive to adulthood than birds with wide or narrow beaks (diversifying selection)
- superior fitness of difference genotypes on different resources to maintain polymorphism
What are two forms that dictate selective pressures? what does this statement mean as a whole in the direction of selection.
- selection pressures can vary over space and time - there may be multiple variables acting applying selection pressures, however there will be a net effect pushing the population in one direction
- temporal variation
- spatial variation
Temporal variation can present its selective pressure in two ways? what are they?
- short-term
- long-term
Define short-term temporal variation in selection pressures, what about their predictability?
- INDIVIDUAL GENOTYPES are exposed to different selection pressures in their lifetime
- where individuals are able to favour the conditions the parents were under, ex fur colours with changing seasons
- temporal variations from selection pressures may be predictable (in observed PHENOTYPIC PLASTICITY) or unpredictable
Define long-term temporal variation selection pressures
- UNIFORM selection pressures on individual genotypes over a SINGLE LIFESPAN - but we observe CHANGE over MULTIPLE generations
- so the population will show genetic change over TIME
- long term changes in the environment may dictate changes in the genetic make-up of the population and multiple generations due to LONG TERM CHANGES
What are the two types of selection pressures in spatial variation - and define “grain”
- fine-grained
- course-grained
- grain refers to how large the habitat patch is in relation to the organism
Explain fine-grained selection pressures during spatial variation
- INDIVIDUAL GENOTYPES encounter DIFFERENT patches of habitat
- in each DIFFERENT habitat they are exposed to different selection pressures
- thus the fittest genotype will be the generalist (more prevalent in the habitat favouring it)
Explain coarse-grained selection pressures during spatial variation
- INDIVIDUAL GENOTYPES more likely to stay in a SINGLE PATCH of habitat
- local adaptation is favoured (fittest genotypes are more likely to be specialized)
What does the blue mussel case tell use about selection pressures
- there is a spatial gradient (fine or coarse grained) where the blue mussel demonstrate changing allele frequencies of the aminopeptidase I based on changing salinity levels
- changing environment favoured brackish waters to open ocean
What is the third mechanism for variable selection pressures? and what two forms arise?
- Frequency-dependent selection - the frequency of selection is dependent on how prevalent a species is (as a result of competition between individuals)
- INVERSE frequency-dependent selection
- POSITIVE frequency-dependent selection
What are the variables for selection pressures, and their acting mechanisms.
- Temporal: short and long term
- Spatial: fine and coarse grained
- Frequency dependent: inverse or positive
Define the two forms of Frequency-Dependent selection pressures.
- Inverse frequency dependent selection: RARER genotype has the HIGHER fitness
- Positive frequency-dependent selection: COMMON genotype has the HIGHER fitness
What example orchestrates INVERSE frequency dependent selection - explain how these selection works and what form of natural balance is struck?
- the colour morph of the elderflower orchid
- the rarer morph of the flower colour will have higher reproductive success because of more pollinator attention
- THUS THE RARER GENOTYPE HAS HIGHER FITNESS
- an equilibrium will be reached where the RARE morph will increase in abundance leading to the once abundant morph becoming rare and bees favouring this morph now
What are 3 other examples of INVERSE FREQUENCY DEPENDENT selection
- Mating Strains: individuals cannot mate with individuals with identical set of alleles thus mating favours the RARER genotype
- 1 to 1 Sex ratios???
- or competition for food by similar genotypes
What form of selection does Mullerian mimicry in butterflies fall under?
- this is a form POSITIVE frequency-dependent selection
- the more common colour morph has a higher fitness since predators associate the common colour with unpalatability and toxicity
Explain further how the two species of Heliconius butterflies
- the butterflies eat a passion flower vine containing alkaloids
- they have bright aposematic (warning) colouring within multiple races of the species
- 2 differing species of this butterfly have corresponding pairs of colour races which occur together in the same geographic region
- thus with the convergence of colour patterns (and prevalence of one colour pattern over another) is driven by bird predation
What MOLECULAR signatures are linked with natural selection, what acts in its place if NS does not?
- Natural selection CAN act at the molecular level, in its absence EVOLUTION is driven by MUTATION and GENETIC DRIFT
What are 3 examples of molecular signatures on DNA differences
- NS on DNA differences can cause a reduction of genetic variation (Background & Positive Selection) OR an increase in genetic variation (Balancing selection)
1. Background (purifying) - decreases deleterious mutations (mutation has a negative effect on the organisms)
2. Positive (directional) - increases favourable mutations
3. Balancing - favours heterozygotes for a given locus (the combination of an allele is more favourable than a single allele on its own)
