Populations 2: Move, adapt, die

Question 1

What are trait values and phenotypic optimum

Answer 1

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

Question 2

What effects the rate that fitness declines around the optimum trait value?

Answer 2

Ecological tolerance

Larger values means that fitness declines less rapidly around the optima and selection is weaker.

Question 3

What happens to the optimum when the environment changes

Answer 3

The phenotypic optimum shifts and there is directional selection, selecting the species that have the more suitable trait values.

Question 4

Breeders equation

Answer 4

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.

Question 5

Quantitative genetics version of the breeder’s equation

Answer 5

∆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.

Question 6

What contributes to phenotypic variation?

Answer 6

Phenotypic variation = genetic variation + environmental variation + (Genotype x Environment)

Question 7

Additive genetic variance

Answer 7

• 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).

Question 8

What effects evolutionary response?

Answer 8

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)

Question 9

Example of Genetic change due to changing environment: change in flowering of Mustard flower and drosophila

Answer 9

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

Question 10

Quantitative traits

Answer 10

Traits that are coded for by many genes

Question 11

What other factors effect evolutionary response to changing environments

Answer 11

1) Moving optima

2) correlated traits

3) phenotypic plasticity

Question 12

Moving optima and evolutionary rate

Answer 12

There is a threshold rate of environmental change, relative to potential evolutionary rate, above which population cannot keep up and goes extinct.

Question 13

what is ‘evolutionary rescue’ and example

Answer 13

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

Question 14

Correlated traits

Answer 14

When some of the same genes influence variation in multiple traits. This is represented as genetic covariance.

Question 15

Genetic covariance (Positive/ negative)

Answer 15

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

Question 16

Covariance can lead to reinforced selection

Answer 16

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

Question 17

Covariance can constrain selection

Answer 17

If there is positive covariance and genes selection is in opposite directions

If there is negative covariance and selection is in the same direction

Question 18

Example of covariance between traits: Partridge pea

Answer 18

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

Question 19

Phenotypic plasticity

Answer 19

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

Question 20

Phenotypic plasticity example: Egg laying of great tits in Wytham woods and water flee

Answer 20

-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

Question 21

Effect of placticity on adaptations

Answer 21

Inhibit adaptations: Reduce selection pressure by increasing ecological tolerance

Enhance adaptations: Phenotypic changes can become genetic changes.

Question 22

Possible responces to environmental Change

Answer 22

1) Nothing

2) Move

3) Adapt (phenotypic change, genetic adaptation)

4) die

Question 23

Overview

Answer 23

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.

Question 24

Bird examples

Answer 24

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

Question 25

Move

Answer 25

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