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[Review]
Evolution by Natural Selection tends to make the
mean phenotype converge on the optimum.
Sexual reproduction continually generates
variation around the optimum.
Sexual selection builds adaptations (multiple
optima) that increase mating success:
1. more mates
2. better mates
Sexual selection is driven by sex differences in
reproductive rate.
Evidence in favor of evolution
1. Existence and pattern of the fossil record.
2. Homology.
3. Universality of genetic code.
4. Analogy with plant and animal breeding.
5. Direct observation.
[New topics]
Facultative adaptations
Heritability
Useful to recognize two types of gene
action:
1.Obligate: resist environmental
interference.
2. Facultative: monitor environmental
variation and adjust phenotype
accordingly.
Suntanning (example of facultative
adapatation)
UVb is both good and bad:
vitamin D synthesis, melanoma.
Melanin (skin pigment) block UVb.
Produce enough melanin to optimize risks.
OK, but UVb is seasonally variable.
Produce more melanin when there more
UVb.
Facultative adaptation!
Selection favored genes that produce the
right pattern of environmental
responsiveness:
Melanin synth.
right dependent variable.
right independent variable.
right norm of reaction.
Ambient UVb
What determines your skin color?
The way your genes translate your
experience into melanin synthesis.
“Natural selection wrote the rules for how
nurture works”
Facultative adaptations: The effects of
experience are pre-programmed by natural
selection.
Figure 03.14
Figure 03.15
Figure 03.16
So… does the nature/nurture
dichotomy have any use?
Not in relation to individuals.
1.Why is my skin this color?
Genes AND environment:
Interaction between genes and
environment that has been preprogrammed by natural selection.
Other nature/nurture questions?
1.Why is my skin this color?
causes of development
2. Why do people differ in skin color?
causes of variation
Differences between individuals can
arise because:
A. They have different genes,
B. They have different experiences.
We can “partition the variance”
How much of the phenotypic difference
between individuals:
Is due to the fact that they have
different genes?
And how much is due the fact that they
have different experiences?
Vp is the phenotypic variation in a trait.
frequency
e.g., How much variation is there in
height in some human population?
height
Vp (phenotypic variance) has two
components: Vg and Ve
Vg is the fraction of phenotypic
variance that is due to individuals
having different genes affecting the
trait.
Ve is the fraction of phenotypic
variance that is due to individuals
having different experiences affecting
the trait.
So…
Vp = Vg + Ve
It’s just a sentence!
Vp = Vg + Ve
Phenotypic variation is the sum of:
underlying genetic variation
plus
the effects of different environments
on the expression of those genes.
And so, if we want to know how
heritable a trait is…
h2 = Vg/Vp, thus
h2 = Vg/(Vg+Ve)
h2 varies from 0 to 1.
Selection is more effective when
heritability is significant.
The nature/nurture dichotomy is:
Meaningless for explaining the traits of
individuals,
But it makes sense when we want to
explain differences,
Heritability provides the necessary
“purchase” for natural selection; if
differences are heritable, selection can
spread advantageous traits.