The nonrandom differential survival and reproduction of certain phenotypes based on selective pressures that results in adaptive evolution
Does this graph indicate the phenotypic variation in the population?
The graph shows variation because there are bars that represent birds at almost every size
Selective Pressure
Some change in the environment that is potentially limiting
Heavy rain and "twice as cold"
Lasted 6 days
Flying insects not active
What components of the graphs would be important for gathering information about survival, reproduction, and whether differences in survival are based on phenotypic differences?
D) Two of the above would be important
Y-axes can be used to directly compare population sizes
X-axes can be used to compare phenotype distributions
Which of the following accurately describes the graphs?
C) The top graph shows the initial population and the bottom shows the survivors
Do these graphs depict evolution of the population?
A) Yes, these graphs present adequate evidence for natural selection. This population has likely evolved
B) No, we cannot state that this population has evolved
B) No, we cannot state that this population has evolved
The graphs show initial population and survivors. This is not evolution because it is not a change from generation to generation. It is the same generation.
Why can't we state that the next generation will share the same mean body size phenotype as survivors and thus the population evolved?
We don't know if the trait is heritable
Selection Differential (S)
Quantifies shift in mean phenotype of population before and after selection event
Indicates how strong the selective pressure is
How can you quantify the selection differential using these graphs?
B) Use X-axis to calculate change in body size
If you ignore the Y-axis then you can still see the mean shift
Selection Differential Equation
(Population trait after event) - (Population trait before event)
Heritability of Phenotypic Characteristic
Proportion of phenotypic variation attributable to genotypic variation
Also need information on offspring phenotype to determine heritability
Body sizes of offspring
What does each figure suggest about heritability?
C) Fig. A = Low heritability, Fig. B = high heritability
A- mean body size identical to initial population pre-selection event
B- mean body size close to post-selection event mean
If these data represent the offspring, has this population evolved by natural selection?
No, since there is no evidence of heritability, environmental effects drive phenotypic variation.
There is no heritability because the mean body size is identical to initial population pre-selection event
Response to Selection (R)
Compares mean phenotype values of post-selection offspring to hypothetical offspring to hypothetical offspring of pre-selection parental generation
Post-selection offspring mean?
Mean phenotype for the post-selection offspring mean
Which circled value would best represent the hypothetical pre-selection offspring mean?
A
What does this hypothetical pre-selection offspring mean?
Without the selective event, environment of parents and offspring is the same
Irrespective of heritability, offspring mean should mirror parent mean
How to know if the phenotypic variation is due to genotypic variation?
Compare the mean phenotype values of post-selection offspring to hypothetical offspring of pre-selection parental generation
This means that 94% of body size phenotypic variation is due to genotypic variation -- highly heritable
Characteristics of selection
Variation in the population for a given phenotypic characteristic
Selective pressure (i.e environmental pressure)
Differential survival/reproduction of phenotypes in response to selective pressure
Heritability of the phenotypic characteristic
Alleles for adaptive phenotype more common in next generation
Directional Selection
Selective pressure can push the mean to either side left or right
Disruptive Selection
When the pressure is against the mean and favor the extremes
Stabilizing Selection
Selection narrows the variation towards the middle
Components of differential reproductive success?
Does the allele impact...
Viability
Fecundity
Gamete Viability
Mating success
Fertilization success
Fitness
Viability
Probability that an individual bearing genotype will survive
Fecundity
Number of gametes per individual
i.e. spotted salamander spermatophores: more spermatophores = more sperm
Gamete Viability
Alleles that impact longevity or quality of sperm/egg
i.e. is one genotype's spermatophore more prone to drying out?
Mating Success
Number and quality of mates
Fertilization Success
Alleles impact the probability that fertilization will occur
Fitness
The average reproductive output (R bar) of all organisms bearing a given genotype: integrates differences in survival and reproduction
Individuals with higher fitness pass their alleles to the next generation at a greater rate than individuals with lower fitness
If individual fitness differences due to differences in alleles, alleles associated with higher fitness more frequent in the next generation
Environmental effects on Fitness
Individuals of same genotype differ in reproductive output because of their environment
# of reproductive females, food availability, net site availability, weather
Calculate fitness for this genotype from individual reproductive outputs
RTT = 9
Relative Fitness (W)
Relative measure of the survival and reproduction among known genotypes
Divide Rbar of each genotype by Rbar of fittest genotype
Which of the following is the best interpretation of these relative fitness data?
B) On average, individuals of the Tt genotype will produce twice as many offspring as the individuals of the tt genotype
If there were NO differences in relative fitness between genotypes, what is your prediction for changes in allele frequency in the next generation (assume no mutation, gene flow, drift)
C) Allele frequencies would not change from the parental to offspring generation
Because relative fitness is the same not one individual is reproducing better
If there were no differences in fitness in our population (each genotype equally likely)
Imagine a population with allele frequencies p= 0.5, q= 0.5.
How would the allele frequencies change?
After random mating, allele frequencies stay the same
Using our calculated relative fitness values, predict how allele frequencies will change in the next generation (assume no mutation, gene flow, drift)