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Evolution in Large Populations I: Natural Selection & Adaptation

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1 Evolution in Large Populations I: Natural Selection & Adaptation
Chapter 6 Evolution in Large Populations I: Natural Selection & Adaptation Species have to cope with a plethora of Environmental changes. Red Queen Hypothesis -- Adaptations in competitors, parasites, and pests are so common that species have to continually evolve to avoid falling behind competing organisms.

2 There are limits to physiological adaptations.
If environmental changes are greater than any individual can cope with then the species becomes extinct. Evolutionary change through natural selection is an 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 over long periods of time, they may allow a population to cope with conditions more extreme than any individual could originally tolerate. Adaptive evolution is observed when large genetically variable populations are subjected to altered biotic or physical environments.

4 Conservation Importance of Adaptive Evolution
Preservation of ability of species to evolve in response to new environments. Loss of adaptive evolutionary potential in small populations. Most endangered species exist on the periphery of their historic range so they must adapt to what was previously marginal habitat.

5 Genetic adaptation to captivity and its deleterious
effects on reintroductions. Adaptation of translocated populations to their new environment.

6 Conservation biology is concerned with preserving
species as dynamic entities that can evolve to cope with environmental change. Retaining the ability to evolve requires the preservation of genetic diversity. Consequently, we must understand the factors that influence the evolution of natural populations.

7 An evolving population is a complex system
influenced by mutation, migration, selection, and chance operating within the context of the breeding system. To understand this complexity, we use modelling with no factors, one factor, two factors etc. In its simplest form, evolution involves a change in gene frequency and its importance can be summarized as:

8 Mutation is the source of all genetic diversity
but is a weak evolutionary force over the short-term. Selection is the only force causing adaptive evolutionary change. Migration reduces differences between populations generated by mutations, selection, and chance.

9 Chance effects in small populations lead to loss
of genetic diversity and reduced adaptive evolutionary potential. Fragmentation and reduced migration lead to random differentiation among subpopulations derived from the same original source population.

10 Selection arises because different genotypes have
different rates of reproduction and survival (reproductive fitness) and such selection changes allele frequencies. Selection operates at all stages of life-cycle. In animals this involves mating ability and fertility of males and females, fertilizing ability of sperm, number of offspring per female, survival or offspring 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 for Chondrodystrophic dwarfism (dwdw) in endangered California condors die around time of hatching. Modeling impact of selection against Chondrodystrophy in California condors.

12 Genotype ++ +dw dwdw total Zygotic p2 2pq q2 1.0 Frequency Relative (lethal) Fitness After p2X1 2pqX1 q2X0 1 -q2 Selection Adjusted p2 (1-q2) 2pq

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 to reduce the frequency of the deleterious recessive allele as its frequency declines!

14 In conservation genetics we are concerned with
both selection against deleterious mutations and selection favoring alleles that improve the ability of a population to adapt to changing environments.

15 Prior to industrial revolution, its peppered wings
provided camouflage as it rested on lichen-covered tree trunks. Sulfur pollution killed most lichen and soot darkened Previously rare dark variants (melanics) were now better camouflaged. Melanic form was first reported in 1848 but by 1900 they represented 99% of all moths in this part 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 population no migration no mutation selection occurs on adults but before reproduction mm individuals have a relative fitness of 1 - s where s is the selection coefficient.

17 MM Mm mm total Zygotic Freq. p2 2pq p2 1.0 Relative Fitness After Selection p2 2pq q2(1 - s) 1 - sq2 Adjusted Freq. 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 allele increases in frequency. Rate of increase depends upon the selection coefficient (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 6.5 In each case, the selection coefficient (s) represents the reduction in relative fitness of the genotype compared to that in the most fit genotype (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 a fitness equal to the A1A1 homozygote. Partial Dominance Case -- heterozygote has a fitness nearer one of the homozygotes than the other with its position on the scale depending on the value of h. Overdominant Case -- heterozygote has higher fitness than either homozygote.

22 The length of time it takes for an allele frequency
to change by a given amount of selection depends upon the intensity of selection and on the mode of inheritance. For a recessive lethal allele we can determine the number 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 decreased fitness when returned to the wild. Fitness Freq. Before Selection Freq. After Phenotype

24 Stabilizing Selection: Favors intermediate phenotype.
Expected to reduce genetic variation. May cause phenotypic stabilizing selection, leading to retention of genetic diversity. Fitness Freq. Before Selection Freq. After Phenotype

25 Disruptive Selection: Favors both phenotypic extremes
May lead to increased variation In future. In fragmented habitats this leads To adaptation in each local Environment. Speciation is possible. Fitness Freq. Before Selection Freq. After Phenotype

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