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Chapter 3

BIO220H1 Chapter Notes - Chapter 3: Bighorn Sheep, Heritability

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John Stinchcombe

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Reading Article 3 – Unnatural Selection (Nils Chr. Stenseth and Erin S. Dunlop)
Meta-analysis of previously published data
oAuthors compare rates of phenotypic changes in 40 populations subject to human
Fish (salmon)
Ungulates (bighorn sheep)
Invertebrates (marine snails)
Plants (ginseng)
High rates of change in these harvested populations
oHigher than changes from natural agents by 300%
Changes in;
oMorphological traits
oLife-history traits (reproductive age)
Can have phenotypic changes in 2 ways:
oGenetic (evolutionary)
oPlastic (ecological)
Population recovers more slowly from genetic than from plastic change
oGenetic change might be irreversible
Harvesting pressure on larger + older individuals:
oJuvenescence of populations
oIncreased variability in abundance
oReduced genetic variability
Harvesting-induced changes more significant than changes from other sources:
oR = h2S
h2 = heritability of trait
S = selection differential (strength of selection)
oIn harvesting selection S is higher than from other sources (i.e. natural selection)
Because mortality rates from hunting/fishing > natural mortality rate
Harvest-induced selective pressure acts in opposite direction to natural sources of
oUsually natural mortality rates higher in young/small individuals than large old
oHumans prefer to hunt larger prey for sport, catch bigger fish
Fishing also causing evolution of sexual maturation at younger ages and smaller size
oIf no fishing, good to be big:
Escape predation
Positive relationship between body size + fecundity
oSmall female that reproduce early in life still has reproductive advantage but not
as strong as females that are large and very fecund
oBut if have fishing, large fish targeted
= females that delay maturation until they are larger could have total
lifetime reproduction fitness of zero
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