Populations 2: Move, adapt, die Flashcards
What are trait values and phenotypic optimum
Trait values describe specific phenotypic traits and they vary around an optimum.
There is stabilizing selection towards the current phenotypic optimum for that trait value.
Fitness is greatest at the phenotypic optimum and it varies around a normal distribution
What effects the rate that fitness declines around the optimum trait value?
Ecological tolerance
Larger values means that fitness declines less rapidly around the optima and selection is weaker.
What happens to the optimum when the environment changes
The phenotypic optimum shifts and there is directional selection, selecting the species that have the more suitable trait values.
Breeders equation
R=h^2 S
R = response to selection
h^2 = heritable trait variation (how much of the variation is due to genetics and therefore heritable)
S = strength of selection
This equation is mainly used for traits coded for by one gene.
Quantitative genetics version of the breeder’s equation
∆x ̅ = G x β
∆x ̅ = change in mean trait value
G = additive genetic variance (G quantifies the genetic variation resulting from the additive effects of alleles at different loci across the genome)
β = selection gradient
This equation is used when traits are coded for by the additive effect of many genes so used additive genetic variance instead of heritable trait variation.
Genetic variance is worked out using the average effects of individual loci contributing to a trait.
What contributes to phenotypic variation?
Phenotypic variation = genetic variation + environmental variation + (Genotype x Environment)
Additive genetic variance
- It is the average effects of individual loci contributing to the trait.
- Excludes genetic interactions and just concentrates on the additive effects:
-> dominance (allele at one copy of diploid locus changes effect of allele at other copy)
-> epistasis (when allele at one locus changes effect of allele at another locus).
What effects evolutionary response?
The amount of additive genetic varaince
- High mutation rate
- Large population size
- Equal sex ratio
- Out breeding
The selection gradient
- The size of the environmental change
- The size of the ecological tolerance (w)
Example of Genetic change due to changing environment: change in flowering of Mustard flower and drosophila
Example 1:
Study found that after 7 years of drought, mustard flowered considerable earlier.
Evidence that it was a genetic change: when crossing pre and post-drought plants, there was an intermediate flowering time.
Flowering was not a early as predicted by breeder’s equation due factors like correlated traits and phenotypic plasticity (may reduce selection pressure)
Example 2:
STUDY SHOWED that Drosophila subobscura genetic changes are tracking climate change
Study over 24 years
I’m 22 of 26 populations genotypes associated with warm temperatures (low latitudes) increased in frequency
Quantitative traits
Traits that are coded for by many genes
What other factors effect evolutionary response to changing environments
1) Moving optima
2) correlated traits
3) phenotypic plasticity
Moving optima and evolutionary rate
There is a threshold rate of environmental change, relative to potential evolutionary rate, above which population cannot keep up and goes extinct.
what is ‘evolutionary rescue’ and example
As populations evolve to the new optima the population size may reduce, reducing genetic variation and making extinction more likely. “evolutionary rescue” can occur due to novel mutations.
A study grew yeast at increasing salinity. There was an initial decline in population size until novel mutation allowing higher tolerance emerged and spread through the population.
Example: anti Vitamin K resistance in rodents
- exposure to anti vitamin K pesticides lead to death
- Large drop in population
- selection on standing variation and 6 new mutations
Correlated traits
When some of the same genes influence variation in multiple traits. This is represented as genetic covariance.
Genetic covariance (Positive/ negative)
Positive genetic covariance: Genes involved in both traits are positively related and trend in the same directions
Negative genetic covariance: Gene involved in both traits are negatively related and trend in opposite directions
Covariance can lead to reinforced selection
If there is positive covariance and selection on both traits is in the same direction
If there is negative covariance and selection on both traits is in the opposite direction
Covariance can constrain selection
If there is positive covariance and genes selection is in opposite directions
If there is negative covariance and selection is in the same direction
Example of covariance between traits: Partridge pea
Study looked at covariance between reprodutive stage/ leaf number and leaf thickness/ lead number and the effect this has on adaptations to drought
Reproductive stage/ leaf number
- Traits have negative covariance, but selection is in the same direction.
leaf thickness/ lead number
- Traits have positive covariance but selection is in opposite direction
Phenotypic plasticity
Mechanisms that allow for changes in phenotypic without changes in the genotype
- e.g. gene expression changes or behavioral changes
Short term environmental changes select for phenotypic plasticity
Phenotypic plasticity example: Egg laying of great tits in Wytham woods and water flee
-example 1
As temperatures have increased the time of egg laying on the Great tits has become earlier
- The breeder’s equation showed that this was not possible through adaptation at this level of heritability and selection due to the rapid rate of change of the environment
Example 2
- genetically identical waterflees differed in morphology depending on whether they were grown in presence or absence of predator
- if common difference -> selects for plasticity
Effect of placticity on adaptations
Inhibit adaptations: Reduce selection pressure by increasing ecological tolerance
Enhance adaptations: Phenotypic changes can become genetic changes.
Possible responces to environmental Change
1) Nothing
2) Move
3) Adapt (phenotypic change, genetic adaptation)
4) die
Overview
Organisms have traits that are optimally adapted to their environment, and when their environment changes, natural selection causes these trait optimums to change.
The change in mean trait value can be quantified by the breeders equation
R= H x s
0r
Delta X= G x Beta
Increased genetic variation and steeper selection gradients lead to increased rate of adaptations.
Organisms may not be able to keep up with this moving optima and therefore go extinct.
The above model assumes that traits are independent and that an environment will select on one trait. This is not the case: Trait correlation
Phenotypic plasticity is an alternative to genotypic adapation.
If organisms cannot adapt they will either move or die.
Bird examples
Birds have to alter their behavior with increasing noise
Example: white-crowned sparrows
- Study found that their song varied predictably according to background noise.
- Birds holding breeding territories in areas with higher noise levels sing higher amplitude songs
- Birds in cities also sing with higher frequencys
- Uncertain whether evolutionary change or behvairal change -> could compare genomes
Move
A 2011 study found that plants and wildlife have moved to higher elevations at a median rate of 36 feet per decade throughout the last century
moose, coffee plants, mangrove trees, and mosquitos have all been found to be moving away from the tropics or to higher elevations
