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 = 0.05 B = 0.10 Fecundity (eggs produced): A = 60 B = 40 Fitness A = 0.05 x 60 = 3 Fitness 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 A Highest Absolute Fitness W A = 3/4 = 0.75 Often by convention, fitness is expressed relative to the genotype with highest absolute fitness. Thus, W B = 4/4 = 1.0

The fitness of a genotype is the average lifetime contribution of individuals of that genotype to the population after one or more generations, measured at 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

GenotypeA 1 A 1 A 1 A 2 A 2 A 2 Frequencyp 2 2pqq 2 Fitnessw 11 w 12 w 22 Individuals may differ in fitness because of their underlying genotype Incorporating Selection Average fitness of the whole population: p 2 w pqw 12 + q 2 w 22 w =

Given variable fitness, frequencies after selection: GenotypeA 1 A 1 A 1 A 2 A 2 A 2 Freq p 2 w 11 2pq w 12 q 2 w 22 www New Frequency of A1 New allele frequencies after mating: New Frequency of A2 p 2 w 11 w pq w 12 w q 2 w

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) W AA W Aa W aa 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 the magnitude of the selection differential

Selection Experiments Show Changes in Allele Frequencies Cavener and Clegg (1981) Food spiked with ethanol HW

Selection can drive genotype frequencies away 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)