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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.
-modes of selection:
- stabilizing: does not change the mean but
reduces variation.
-modes of selection:
- 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
- modes of selection and phenotypic 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).
D. Selection and Genetic Variation
- modes of selection and phenotypic 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).
But, even for very adaptive traits, there is usually still
phenotypic and genetic variation. Why?
D. Selection and Genetic Variation
- modes of selection and phenotypic variation:
- sources of variation
- new adaptive mutations are constantly produced
and are increasing in frequency in the population.
D. Selection and Genetic Variation
- modes of selection and phenotypic variation:
- sources of variation:
- new adaptive mutations are constantly produced
and are increasing in frequency in the population.
- 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.
D. Selection and Genetic Variation
- modes of selection and phenotypic variation:
- sources of variation:
- new adaptive mutations are constantly produced
and are increasing in frequency in the population.
- 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
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
- selection: differential reproductive success
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
- selection: differential reproductive success
- fitness = reproductive success
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
- selection: differential reproductive success
- fitness = reproductive success
- adaptation = a trait or suite of traits that increases
reproductive success.
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
- selection: differential reproductive success
- fitness = reproductive success
- 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).
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
IV. Variation in Quantitative Traits
V. Selection and Adaptation
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.
IV. Variation in Quantitative Traits
V. Selection and Adaptation
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
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
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!!!
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
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
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
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
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
D. Constraints on the POWER of selection
- physical constraints:
why do flying fish return to water?
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
D. Constraints on the POWER of selection
- physical constraints:
- contradictory selective pressures
Leaf size
Photosynthetic Potential
Water retention
Leaf Size
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
D. Constraints on the POWER of selection
- physical constraints:
- contradictory selective pressures
- historical constraints (extant genome, physiology,
anatomy, behavior)
IV. Variation in Quantitative Traits
V. Selection and Adaptation
A. Definitions
B. Issues
C. How do we identify adaptations?
D. Constraints on the POWER of selection
- physical constraints:
- contradictory selective pressures
- historical constraints (extant genome, physiology,
anatomy, behavior)
- lack of genetic variation
IV. Variation in Quantitative Traits
V. Selection and Adaptation
VI. Levels of Selection
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
- 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
- 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).
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
- 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
- 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
- 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 Ybearing sperm, and on average, production of 63%
male progeny.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
2. Transposable Elements
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
2. Transposable Elements
3. 'Selfish' Genes (Richard Dawkins)
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
2. Transposable Elements
3. 'Selfish' Genes (Richard Dawkins)
- genes are the fundamental replicators
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
2. Transposable Elements
3. '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')
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
1. Meiotic Drive:
2. Transposable Elements
3. '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
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle Selection
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle 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.
- 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle 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.
- 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.
- 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle 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.
- 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.
- 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.
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle Selection
C. Cell Selection
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle Selection
C. Cell Selection
- Cancerous Tumour - cell division increases, and the effects may be
balanced at a higher level (organism).
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle Selection
C. Cell Selection
D. Organism Selection (Darwinian)
VI. Levels of Selection
Selection can occur wherever there is differential reproduction among variable
entities.
A. Gene Selection
B. Organelle Selection
C. Cell Selection
D. Organism Selection (Darwinian)
E. Group Selection
Pseudomonas flourescens
Colonies with high concentration
of mat-builders (expensive
proteins) float; if cheaters
increase in number, colony sinks
and dies.
Nowak, M. A. 2006. Five rules for the evolution of
cooperation. Science 314:1560-1563.