Presentation on theme: "Evolution in Large Populations I: Natural Selection & Adaptation"— Presentation transcript:
1 Evolution in Large Populations I: Natural Selection & Adaptation Chapter 6Evolution in Large Populations I:Natural Selection & AdaptationSpecies have to cope with a plethora ofEnvironmental changes.Red Queen Hypothesis -- Adaptations incompetitors, parasites, and pests are so commonthat species have to continually evolve to avoidfalling behind competing organisms.
2 There are limits to physiological adaptations. If environmental changes are greater than anyindividual can cope with then the speciesbecomes extinct.Evolutionary change through natural selection isan alternative means (vs. physiological adaptation)for adjusting to environmental change.This is Adaptive Evolution!
3 Natural Selection -- differential reproduction & survival of different genotypes.When adaptive evolutionary changes occur overlong periods of time, they may allow a populationto cope with conditions more extreme than anyindividual could originally tolerate.Adaptive evolution is observed when largegenetically variable populations are subjected toaltered biotic or physical environments.
4 Conservation Importance of Adaptive Evolution Preservation of ability of species to evolve inresponse to new environments.Loss of adaptive evolutionary potential in smallpopulations.Most endangered species exist on the periphery oftheir historic range so they must adapt to whatwas previously marginal habitat.
5 Genetic adaptation to captivity and its deleterious effects on reintroductions.Adaptation of translocated populations to theirnew environment.
6 Conservation biology is concerned with preserving species as dynamic entities that can evolve tocope with environmental change.Retaining the ability to evolve requires thepreservation of genetic diversity.Consequently, we must understand the factorsthat influence the evolution of natural populations.
7 An evolving population is a complex system influenced by mutation, migration, selection, andchance operating within the context of thebreeding system.To understand this complexity, we use modellingwith no factors, one factor, two factors etc.In its simplest form, evolution involves a changein gene frequency and its importance can besummarized as:
8 Mutation is the source of all genetic diversity but is a weak evolutionary force over theshort-term.Selection is the only force causing adaptiveevolutionary change.Migration reduces differences between populationsgenerated by mutations, selection, and chance.
9 Chance effects in small populations lead to loss of genetic diversity and reduced adaptiveevolutionary potential.Fragmentation and reduced migration lead torandom differentiation among subpopulationsderived from the same original source population.
10 Selection arises because different genotypes have different rates of reproduction and survival(reproductive fitness) and such selection changesallele frequencies.Selection operates at all stages of life-cycle.In animals this involves mating ability and fertilityof males and females, fertilizing ability of sperm,number of offspring per female, survival oroffspring to reproductive age and longevity.
11 The most intensive selection that can apply against a recessive allele is when all homozygotes die(lethal).For example, all individuals homozygous forChondrodystrophic dwarfism (dwdw) in endangeredCalifornia condors die around time of hatching.Modeling impact of selection againstChondrodystrophy in California condors.
13 The frequency of the dw allele in the next Generation (q1) is:q1 = q/(1 + q)The change in frequency (q) = -q2/(1 + q)Thus, the lethal allele always declines in frequency.Importance: it becomes progressively harder toreduce the frequency of the deleterious recessiveallele as its frequency declines!
14 In conservation genetics we are concerned with both selection against deleterious mutations andselection favoring alleles that improve the abilityof a population to adapt to changing environments.
15 Prior to industrial revolution, its peppered wings provided camouflage as it rested on lichen-coveredtree trunks.Sulfur pollution killed most lichen and soot darkenedPreviously rare dark variants (melanics) were nowbetter camouflaged.Melanic form was first reported in 1848 but by1900 they represented 99% of all moths in thispart of England.
16 Simple model for this type of selection: Beginning frequencies of melanic (M) and typical (m)Alleles with frequencies of p and q, respectively.Assumptions:Large random mating populationno migrationno mutationselection occurs on adults but beforereproductionmm individuals have a relative fitness of 1 - swhere s is the selection coefficient.
17 MM Mm mm totalZygoticFreq. p2 2pq p2 1.0RelativeFitnessAfterSelection p2 2pq q2(1 - s) 1 - sq2AdjustedFreq.p2(1 - sq2)2pq(q2 - sq2)
18 Frequency of M after selection (p1): Change of M (p):Since the sign of p is positive, melanic alleleincreases in frequency.Rate of increase depends upon the selectioncoefficient (s) and allele frequencies.p(1 - sq2)(spq2)
19 1848, frequency of M = p = 0.005 and typicals had Only 70% survival of melanics (s = 0.3) then:p1 = p/(1 - sq2) = 0.005/(1 - (0.3 X )] =Change in frequency (p) =p1 - p = =
20 Models of 4 different degrees of dominance are given in Figure 6.5In each case, the selection coefficient (s)represents the reduction in relative fitness of thegenotype compared to that in the most fitgenotype (Fitness = 1.0).Values of s range from 0 to 1.
21 Additive Case -- heterozygote has a fitness intermediate between the two homozygotes.Completely Dominant Case -- heterozygote has afitness equal to the A1A1 homozygote.Partial Dominance Case -- heterozygote has afitness nearer one of the homozygotes than theother with its position on the scale depending onthe value of h.Overdominant Case -- heterozygote has higherfitness than either homozygote.
22 The length of time it takes for an allele frequency to change by a given amount of selection dependsupon the intensity of selection and on the modeof inheritance.For a recessive lethal allele we can determine thenumber of generations to change an allele freq.from q0 to qt as:t = 1/qt - 1/q0.
23 Directional Selection: Captive populations are likely to show adaptation to captivity.Typically results in decreasedfitness when returned to thewild.FitnessFreq. BeforeSelectionFreq. AfterPhenotype
24 Stabilizing Selection: Favors intermediate phenotype. Expected to reduce geneticvariation.May cause phenotypic stabilizingselection, leading to retentionof genetic diversity.FitnessFreq. BeforeSelectionFreq. AfterPhenotype
25 Disruptive Selection: Favors both phenotypic extremes May lead to increased variationIn future.In fragmented habitats this leadsTo adaptation in each localEnvironment.Speciation is possible.FitnessFreq. BeforeSelectionFreq. AfterPhenotype
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