iv. variation in quantitative traits a. quantitative effects b. partitioning variance c. selection...

IV. Variation in Quantitative Traits A. Quantitative Effects B. Partitioning Variance C. Selection on Quantitative Traits

- Traits affected by many genes have a higher probability of including a pleiotrophic gene – a gene that affects more than one trait. So, we might expect complex, quantitative traits to be CORRELATED to other traits.

If selection is acting on both traits in different ways, neither will be “optimized”. Adaptations will be a compromise, depending on the relative strengths of the selective pressures, the relative values of the adaptive traits, and their heritabilities (ease with which they can respond to selection).

Consider the Grant’s work on medium ground finches during the drought of ‘76-’77. Birds with deep and narrow beaks had the greatest fitness.

Consider the Grant’s work on medium ground finches during the drought of ‘76-’77. Birds with deep and narrow beaks had the greatest fitness. But beak depth and beak width are POSITIVELY CORRELATED (probably developmentally).

Consider the Grant’s work on medium ground finches during the drought of ‘76-’77. Birds with deep and narrow beaks had the greatest fitness. But beak depth and beak width are POSITIVELY CORRELATED (probably developmentally).

So, although selection should have pushed the pop along the blue line, it went along the green line, because there is greater benefit to a deep bill rather than a narrow bill.

IV. Variation in Quantitative Traits A. Quantitative Effects B. Partitioning Variance C. Selection on Quantitative Traits D. Selection and Genetic Variation

IV. Variation in Quantitative Traits A. Quantitative Effects B. Partitioning Variance C. Selection on Quantitative Traits D. Selection and Genetic Variation

- modes of selection and phenotypic variation:

-modes of selection:

- directional: changes the mean phenotype and tends to reduce variation.


- stabilizing: does not change the mean but reduces variation.


- disruptive: does not change the mean but increases variation.

Directional

Stabilizing:

Disruptive:

African seedcrackers; disruptive selection due to efficiencies on large or small seeds.

D. Selection and Genetic Variation


if most selection is directional and stabilizing, then variation is reduced; including genetic variation (these are quantitative traits, not single genes maintained by heterozygote advantage at one locus).



if most selection is directional and stabilizing, then variation is reduced; including genetic variation (these are quantitative traits, not single genes maintained by heterozygote advantage at one locus).

But, even for very adaptive traits, there is usually still phenotypic and genetic variation. Why?



- sources of variation

- new adaptive mutations are constantly produced and are increasing in frequency in the population.



- sources of variation:


- deleterious mutations are maintained at low frequency; especially for genes contributing to quantitative traits where the selective pressure on any one locus may be weak.



- sources of variation:


- deleterious mutations are maintained at low frequency; especially for genes contributing to quantitative traits where the selective pressure on any one locus may be weak, or recessive alleles.

- disruptive, frequency dependent, multiple niche polymorphisms, etc., in which the adaptive value of existing alleles changes through time or across space within the population.

IV. Variation in Quantitative Traits

V. Selection and Adaptation



A. Definitions



A. Definitions

- selection: differential reproductive success



A. Definitions


- fitness = reproductive success



A. Definitions



- adaptation = a trait or suite of traits that increases reproductive success.



A. Definitions



- adaptation = a trait or suite of traits that increases reproductive success.

- exaptation = an adaptation co-opted for a new function. (flight feathers are an exaptation of thermoregulatory feathers, which may be an exaptation of feathers initially adaptive as sexual ornaments).



A. Definitions

B. Issues



A. Definitions

B. Issues

- all traits are NOT ‘adaptations’ – “spandrels of San Marco” (Gould and Lewontin) – even if we can envision a function for them.



A. Definitions

B. Issues

- all traits are NOT ‘adaptations’ – “spandrels of San Marco” (Gould and Lewontin) – even if we can envision a function for them.

- some are due to drift in different populations

- some are correlated or linked to adaptive genes



A. Definitions

B. Issues

C. How do we identify adaptations?



A. Definitions

B. Issues


1. Experiment

Zonosemata flies (Family Tephritidae) wave their banded wings when threatened. Why?

Zonosemata flies (Family Tephritidae) wave their banded wings when threatened. Are they mimicking spiders to deter other predators, mimicking spiders to deter spider predators, or does it have nothing to do with predation? (Waving for courtship?)

Housefly..no waving

Responses of other preds….

ALL EATEN!!!



A. Definitions

B. Issues


1. Experiment

2. Observational Studies

- Do desert lizards thermoregulate behaviorally?

can define the physiological relationships between temp and metabolism and activity in the lab, but do they choose areas that maintain their temp in this range? Go look in an environment with variable temps, and see if choice meets the adaptive expectation.

Natural distribution



A. Definitions

B. Issues


1. Experiment

2. Observational Studies

3. Comparative Method

In some species of bats, males have disproportionately large testis.

And some evolutionary biologists are interested in knowing why.

Is it related to sperm competition and social group size?

- Females in larger groups would have the chance to mate with more males, so there would be greater benefit to producing more sperm…

Looks good, but..!

Data points need to be independent, and these are NOT phylogenetically independent… if we make them so, the data set decays to just two points... Not too conclusive.

Compare sister taxa;When divergence occurs, does the one with a bigger social group have big testes?

Compare sister taxa;When diverge occurs, does the one with a bigger social group have big testes?

Then, slide each relationship to the origin, standardizing the divergence to “0”. Are the endpoints correlated? This controls for phylogenetic correlations.

Hosken 1998



A. Definitions

B. Issues


D. Constraints on the POWER of selection

- physical constraints:

why do flying fish return to water?



A. Definitions

B. Issues




- contradictory selective pressures

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A. Definitions

B. Issues





- historical constraints (extant genome, physiology, anatomy, behavior)



A. Definitions

B. Issues





- historical constraints (extant genome, physiology, anatomy, behavior)

- lack of genetic variation



VI. Levels of Selection


Selection can occur wherever there is differential reproduction among variable

entities.



entities.

A. Gene Selection



entities.

A. Gene Selection1. Meiotic Drive:



entities.


In some organisms, the heterozygote produces a preponderance of one gamete type - this is called "segregation distortion". This gene is at a selective advantage over other genes at this locus. Of course, as it increases in frequency and more organisms are homozygous for it, the differential reproduction drops. However, this can be balanced by the reduced number of gametes these organisms produce.



entities.


In some organisms, the heterozygote produces a preponderance of one gamete type - this is called "segregation distortion". This gene is at a selective advantage over other genes at this locus. Of course, as it increases in frequency and more organisms are homozygous for it, the differential reproduction drops. However, this can be balanced by the reduced number of gametes these organisms produce. An example is the t-allele in mice. Heterozygotes only produce gametes with the 't' allele - no 'T' gametes.



entities.


In some organisms, the heterozygote produces a preponderance of one gamete type - this is called "segregation distortion". This gene is at a selective advantage over other genes at this locus. Of course, as it increases in frequency and more organisms are homozygous for it, the differential reproduction drops. However, this can be balanced by the reduced number of gametes these organisms produce. An example is the t-allele in mice. Heterozygotes only produce gametes with the 't' allele - no 'T' gametes. However, the rise in frequency of the 't' allele is balanced at the organismal level by selection against the homozygote - 'tt' is lethal. So, the allele can not increase in frequency and is dependent upon other alleles in the population.



entities.


- Stalk-eyed flies, Cyrtodiopsis dalmanni(Presgraves, et al.1997). • X(d) meiotic drive element on the X chromosome causes female-biased sex ratios in natural populations of both species.



entities.


- Stalk-eyed flies, Cyrtodiopsis dalmanni(Presgraves, et al.1997). • X(d) meiotic drive element on the X chromosome causes female-biased sex ratios in natural populations of both species. • spermatid degeneration in male carriers of X(d).



entities.


- Stalk-eyed flies, Cyrtodiopsis dalmanni(Presgraves, et al.1997). • X(d) meiotic drive element on the X chromosome causes female-biased sex ratios in natural populations of both species. • spermatid degeneration in male carriers of X(d). • balanced by Y-linked and autosomal factors that decrease the intensity of meiotic drive.



entities.


- Stalk-eyed flies, Cyrtodiopsis dalmanni(Presgraves, et al.1997). • X(d) meiotic drive element on the X chromosome causes female-biased sex ratios in natural populations of both species. • spermatid degeneration in male carriers of X(d). • balanced by Y-linked and autosomal factors that decrease the intensity of meiotic drive. • Even a Y-linked polymorphism for resistance to drive which reduces the intensity and reverses the direction of meiotic drive.



entities.


- Stalk-eyed flies, Cyrtodiopsis dalmanni(Presgraves, et al.1997). • X(d) meiotic drive element on the X chromosome causes female-biased sex ratios in natural populations of both species. • spermatid degeneration in male carriers of X(d). • balanced by Y-linked and autosomal factors that decrease the intensity of meiotic drive. • Even a Y-linked polymorphism for resistance to drive which reduces the intensity and reverses the direction of meiotic drive. • When paired with X(d), modifying Y chromosomes (Y(m)) cause the transmission of predominantly Y-bearing sperm, and on average, production of 63% male progeny.



entities.

A. Gene Selection1. Meiotic Drive: 2. Transposable Elements



entities.

A. Gene Selection1. Meiotic Drive: 2. Transposable Elements

these genes replicate themselves independently of cell division... they are gene parasites that make nothing for the cell. yet they increase in frequency relative to other genes in the genome.



entities.

A. Gene Selection1. Meiotic Drive: 2. Transposable Elements3. 'Selfish' Genes (Richard Dawkins)



entities.

A. Gene Selection1. Meiotic Drive: 2. Transposable Elements3. 'Selfish' Genes (Richard Dawkins)- genes are the fundamental replicators



entities.

A. Gene Selection1. Meiotic Drive: 2. Transposable Elements3. 'Selfish' Genes (Richard Dawkins)- genes are the fundamental replicators - genes which confer an advantage, when averaged across other genetic

backgrounds, will be selected for. (Analogy of 'crews')



entities.

A. Gene Selection1. Meiotic Drive: 2. Transposable Elements3. 'Selfish' Genes (Richard Dawkins)- genes are the fundamental replicators - genes which confer an advantage, when averaged across other genetic

backgrounds, will be selected for. Analogy of 'crews')- co-adaptive assemblages and non-additive effects are not explained



entities.

A. Gene Selection

B. Organelle Selection



entities.

A. Gene Selection


- some mitochondria in yeast are non-respiring parasites - they survive but don't produce much energy for the cell. They reproduce fast in a cell.



entities.

A. Gene Selection



- In small populations of yeast, where selection at the organismal level is weak, there is no cost to the cell to reproducing slowly and the parasitic mitochondria dominate within cells.



entities.

A. Gene Selection




- In large populations, where aerobic respiration is advantageous at a cellular level, cells with parasites are selected against and the frequency of parasitic mitochondria is reduced.



entities.

A. Gene Selection




- In large populations, where aerobic respiration is advantageous at a cellular level, cells with parasites are selected against and the frequency of parasitic mitochondria is reduced.

- There is a balance of selection at different levels that must be understood to explain the different frequency of parasitic mitochondria.



entities.

A. Gene Selection


C. Cell Selection



entities.

A. Gene Selection


C. Cell Selection

- Cancerous Tumour - cell division increases, and the effects may be balanced at a higher level (organism).



entities.

A. Gene Selection


C. Cell Selection

D. Organism Selection (Darwinian)



entities.

A. Gene Selection


C. Cell Selection

D. Organism Selection (Darwinian)

E. Group Selection

Nowak, M. A. 2006. Five rules for the evolution of cooperation. Science 314:1560-1563.

Pseudomonas flourescens

Colonies with high concentration of mat-builders (expensive proteins) float; if cheaters increase in number, colony sinks and dies.

iv. variation in quantitative traits a. quantitative effects b. partitioning variance c. selection...

Documents

genetic variation slide

modes of selection

phenotypic variation

genetic variation modes

quantitative traits

sources of variation

directional slide

adaptive traits