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Lecture Notes
Lecture 7: Levels and Types of Selection
Outline Types of selection Stabilizing Directional Disruptive Levels of Selection Gene Individual Group Problems with ‘good of the group’ arguments Kin Measuring relatedness Hamilton’s rule Inclusive fitness
Types of Selection (Fig. 7.1 Pianka)
Stabilizing - under constant conditions, once population has reached local adaptive peak, then selection maintains mean trait value at optimum. Selection against phenotypes that differ from current mean in either direction. Reduces variability but does not change allele frequencies.
force of selection
frequency
phenotype Example: call frequency (pitch) in many frogs and toads force of selection
Number of females Mated
Call frequency
before selection
after selection
Other species' calls overlap the high and low extremes of the pitch produced by toads, so females avoid both extremes to avoid infertile matings with wrong species
Lecture Notes
Directional - under changing conditions, when current mean trait value differs from optimum. Selection favors phenotypes that differ from current mean in direction of optimum. Changes allele frequencies.
frequency
phenotype
Example: sexual selection by female choice on tail length in widowbirds, experiments by Malte Andersson
Male widowbirds have spectacularly long tails that obviously impair flight performance, thus survival ↓. Andersson hypothesized ↑ in reproductive success through female choice. Tested in experiment w/4 groups
- cut tail to 14 cm (experimental short)
- banded only (procedural control)
- cut tail off and reattached it (procedural control)
- lengthened tail by 25 cm
Before manipulation: reproductive success
After manipulation: reproductive success
shorten tail - RS ↓ lengthen tail - RS ↑ banding ←→ tail cut and restore, small ↓, but does not affect comparison of short/long
So female choice imposes directional sexual selection on tails, in favor of longer tails.
All 4 types equally fit (random)
before selection
after selection
force of selection
Lecture Notes
Individual Selection
Most common unit in evolutionary analyses: trait spreads if it increases bearer’s fitness (survival and reproduction). Survival and reproduction combine to determine Lifetime Reproductive Success, a property of individuals. Individuals express phenotypes, ie. a gene is selected +/- because of effects on phenotype. Individuals express phenotypes, so alleles at one locus are selected for or against on the basis of the complete genetic “background” formed by all the other loci in that individual. Emphasis on bearer , in the combination of gene and its bearer.
Gene Selection
Very similar to individual-level analysis, but views individual simply as a carrier for gene, which is the self replicating entity that persists through time. Genes go unchanged through generations, but individuals are unique - they die and their total phenotype, due to many loci, does not pass on exactly to their offspring. Gene & individual level selection are two ways of describing the same process, namely that an allele spreads if it raises the lifetime reproductive success of the individual that bears it.
Kin Selection
Haldane (1932) “ I would gladly lay down my life for 8 of my cousins”.
Kin selection is an extension of gene/individual selection that takes into account that selection can act through effects of an allele (trait) on relatives (kin), who are also likely to carry the allele. Takes into account that one’s relatives are likely to carry the same genes as oneself. Wright’s coefficient of relationship measures proportion of genes that an individual carries that are identical by descent (IBD).
r = Σ 0.5L
where L = # of steps between two individuals in a genealogy. (At each step is a meiosis, so genes in common cut in half at each step).
A x B one link in each case
C
One link between offspring and parent (A & C or B & C), r = 0.5^1 = 0.5. Parent and offspring have 50% of genes IBD.
Lecture Notes
A x B
C D
Siblings (C & D): one link up and one link down so r = 0.5 2 = 0.25 for each parent. Sum across two parents, 0.25 + 0.25 = 0.50.
B x A x C
D E
Half siblings (D & E): r = 0.5^2 = 0.
GM x GF
A x M MS x D
Haldane Haldane’s cousin
r = 2(0.5^4 ) = 2 * ( 1 / 16 ) = 1 / (^8)
If r is 1 / 8 , then r * 8 = 1. In genetic terms, saving eight cousins from death is equivalent to surviving yourself.
Thus, kin selection can explain the evolution of traits that are harmful to the bearer such as sterility in worker hymenoptera (wasps, bees, termites).
Hamilton’s rule (1963, 1964) for spread of a rare ‘altruism allele’ for trait harmful to donor and helpful to recipient. g = nonaltruist , G =altruist
rb - c(1) > 0
r = coefficient of relationship (probability that recipient carries the G allele) b = benefit of help to recipient c = cost of help to donor
In the reading assignment (Hamilton 1963), this was written k > 1 / (^) r, where k = b^ / (^) c
b (^) / c >^
r
