PSYC 385 Lecture Notes - Lecture 8: Nikolaas Tinbergen, Resource Depletion, Ethology
25 Jun 2018
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Class 8 - Feb 28
●General Forms of Natural Selection
○Stabilizing selection: Acts against both extremes in a distribution to keep pop.
average
○Directional selection: Acts against one extreme of a phenotype (i.e., shifts toward
other)
○ “Repr
oduction” and Height in (a) British Men vs. (b) British Women
○Disruptive (“diversifying”) selection: Acts against intermediate phenotypes (i.e.,
split in distributions toward extreme values)
○
○ Recall: Purifying selection: conserves fixed traits and weeds out mutations at key
loci
■ Fixation = no genetic variation
● Also sometimes called negative selection
○ Adaptationist stance
■ If trying to explain a phenotypic effect ie. snout shape of foxes when you
trying to domestic them
■ Phenotypic effects can relate to each other and when you select for one
effect others can be drag along with it
■ Limitation of AS
●Two Specific Types of Selection Can Maintain Genetic Variation
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○ Inconsistent selection
■Sexually antagonistic selection occurs when the optimal phenotype is not
the same for male and female offspring
● E.g., as noted, British height under directional selection for men
but stabilizing for women
● Think about the effect on genes associated with height – they are
reproduction enhancing only when in a male body
■ So it is inconsistent selection as a function of type of internal environment
● Whether genes are in a male or female…
○ Frequency Dependent
■Negative frequency-dependent selection can maintain genetic variation
● Phenotype has high relative fitness if rare
■ Heterozygote Advantage and Frequency
● Normal form of haemoglobin beta allele S
● Allele causing sickle-cell trait s
○ One “abnormal” copy of allele (Ss) has higher fitness than
2 abnormal copies abnormal (ss) or even 2 normal copies
(SS)
● Why? Ss has little effect on individual, but offers resistance to
malaria SS does not
○ So, advantage in Ss heterozygous form
■ BUT, ss causes sickle-cell disease
○ I.e., negative frequency-dependent selection
● Adaptationist stance
■ Recall: Phenotypes are “adaptively constructed” so we can hypothesize
about how they increased ancestral fitness
● I.e., if something common in species then it is probably “designed”
that way because solved an “adaptive problem” in an ancestral
context
○ If not, it (and potential alleles supporting it) presumably
would have been outcompeted by some other design
solution (and set of alleles)
■Relies on the “phenotypic gambit”-- the idea that we can model complex
traits as if they are controlled by single, distinct alleles
● Thus, ignores proximate causes (inc. ontogeny)
○Strengths
■Can test different adaptationist hypotheses of adaptive origins
● Ie. find out which hypothesis is best supported by the available
evidence
● To reject all adaptationist hypotheses that you test, you can then
test non-adaptationist hypotheses such as e.g., genetic drift
■Fairly straightforward to test, whereas non-adaptive ones (e.g., genetic
drift) are often very difficult to test
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● Good starting point
○How to test?
■ E.g., adaptive function of melanin layer is to regulate amount of UV light
into body vs. say enabling camouflage against predators
■So, reverse engineer some hypotheses…
● A. Skin colour should be a function of sunlight
○ Why? The amount of UV light available will depend on
local climatic conditions
■ Should be darker when more sunlight, lighter when
less
● B. Women should be lighter skinned than men
○ Why? Women need more vitamin D than men because of
its role in pregnancy and lactation
■“natural experiments”
● C. Very pale skin should impair survival
○ E.g., albinism has higher rate skin cancer
● D. Lighter skin should have increased problems of folate
destruction
○ This is the case relative to darker skinned individuals in
neural tube defects (next slide)
● E. Darker skin should have increased problems of vitamin D
insufficiency
○ And indeed that is the case relative to lighter skinned
individuals
○
■“comparative evidence”
● F. Skin colour as function of UV light where population originates
(n = 86)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Stabilizing selection: acts against both extremes in a distribution to keep pop. average. Directional selection: acts against one extreme of a phenotype (i. e. , shifts toward other) Repr oduction and height in (a) british men vs. (b) british women. Disruptive ( diversifying ) selection: acts against intermediate phenotypes (i. e. , split in distributions toward extreme values) Recall: purifying selection: conserves fixed traits and weeds out mutations at key loci. If trying to explain a phenotypic effect ie. snout shape of foxes when you trying to domestic them. Phenotypic effects can relate to each other and when you select for one effect others can be drag along with it. Two specific types of selection can maintain genetic variation. Sexually antagonistic selection occurs when the optimal phenotype is not the same for male and female offspring. E. g. , as noted, british height under directional selection for men but stabilizing for women.
