12.4 Warning coloration in a western coral snake (Micrurus euryxanthus)

Adaptation and Natural Selection

The Outcome of Natural Selection Depends Upon:

(1) Relationship between phenotype and fitness.

(2) Relationship between phenotype and genotype.

Determine the relationship between fitness and genotype.

Outcome determines if there is evolution

12.2 Growth of 2 genotypes in an asexually reproducing population w/ nonoverlapping generations

% survival to reproduction:

A = 0.05B = 0.10

Fecundity (eggs produced):

A = 60B = 40

Fitness A = 0.05 x 60 = 3Fitness A = 0.05 x 60 = 4

R = Per Capita Growth Rate = Represents Absolute Fitness

The rate of genetic change in a populations depends upon relative fitness:

Relative Fitness of A = Absolute fitness AHighest Absolute Fitness

WA = 3/4 = 0.75

Often by convention, fitness is expressed relative to the genotype with highest absolute fitness.

Thus,WB = 4/4 = 1.0

The fitness of a genotype is the average lifetimecontribution of individuals of that genotype to thepopulation after one or more generations, measuredat the same stage in the life history.

12.3 Components of natural selection that may affect the fitness of a sexually reproducing organism

12.1(1) Modes of selection on a heritable quantitative character

12.1(2) Modes of selection on a polymorphism consisting of two alleles at one locus

Genotype A1A1 A1A2 A2A2

Frequency p2 2pq q2

Fitness w11 w12 w22

Individuals may differ in fitness because of their underlying genotype

Incorporating Selection

Average fitness of the whole population:

p2w11 + 2pqw12 + q2w22w =

Given variable fitness, frequencies after selection:

Genotype A1A1 A1A2 A2A2

Freq p2 w11 2pq w12 q2 w22

w w w

New Frequency of A1

New allele frequencies after mating:

New Frequency of A2

p2 w11

w

pq w12 pq w12

w

q2w22+ +

Fitness: Probability that one’s genes will be represented in future generations.

Hard to measure. Often, fitness is indirectly measured:(e.g. survival probability given a particular genotype)

WAA WAa Waa

1 1 1 + s

Selection coefficientFitness is often stated in relative terms gives the selection differential

Persistent Selection Changes Allele Frequencies(i.e. Evolution by Natural Selection)

Strength of selection is given by themagnitude of the selection differential

Selection Experiments Show Changes in Allele Frequencies

Cavener and Clegg (1981)

Food spiked with ethanol

HW

Selection can drive genotype frequenciesaway from Hardy Weinberg Expectations

Predicted change in allele

frequencies at CCR5

High frequency (Europe)High selection/transmisson (Africa)

High frequency (Europe)Low selection/transmisson (Europe)

Low frequency (Africa)High selection/transmisson (Africa)