Transcript Chapter 23 (OLD)

Chapter 23
Diversity of Individuals and
Evolution of Populations
I. Evolution occurs with the gradual change in the
allele frequency within a population over time
a
population of one species has a variety of individuals
for any feature, there can be many phenotypes
 phenotypes are determined by alleles
 must examine change in allele frequency of a
population over time

II. Examining population’s allele and genotype
frequency to study evolution
 population
- localized group of individuals of the same
species
 species
 gene
- a group of populations that can interbreed
pool - all alleles for all genes present in a population
 genetic
structure of a population - population frequency
of alleles & genotypes
III. Hardy-Weiberg theorem for a non-evolving
population - a look at population structure
 Assume
a gene with 2 alleles: A and a
 allele frequency is: A (0.8 = 80%) and a (0.2 = 20%)
 chance of AA offspring = 0.8 x 0.8 = 0.64**
 chance of aa offspring = 0.2 x 0.2 = 0.04**
 chance of Aa offspring = (0.8 x 0.2) x 2 = 0.32 **
 **
this would be expected genotype structure at
equilibrium (Hardy-Weinberg equlibrium)
 Assume
a gene with 2 alleles: A and
a
 frequency
of allele #1 (eg. A = 0.8) = p
 frequency of allele #2 (eg. a = 0.2) = q
of resulting in AA = p x p = p2
 chance of resulting in aa = q x q = q2
 chance of resulting in Aa = 2 (p x q) = 2 p q
 chance
 p2
+ 2pq
 allows
+ q2 = 1
us to calculate population structure
IV. Evolution is - a generation-to-generation change in
a population’s frequency of alleles and genotypes
 change
in the Hardy-Weinberg frequency over time is a
sign that some alleles can result in a phenotype which
gives a “selective” advantage over other alleles
 for
Hardy-Weinberg equilibrium to be maintained:
 large population size must exist
 no gene flow from other populations can occur
 no net mutations can occur
 random mating must be happening
 no natural selection can be occurring
V. Deviations from Hardy-Weinberg can result from:
 genetic
drift (population size)
 gene flow
 mutation results in new alleles
 non-random mating (assortive mating, inbreeding)
 natural selection (differential success of offspring)
VI. Natural selection is dependent upon variation alleles that give rise to a variety of phenotypes
 most
phenotypes are the result of multiple genes
 polymorphism - variation of a certain item for discussion
 sources
of polymorphism:
genetic mutation - results in new and different alleles
 recombination - results new combinations of alleles

VII. Fitness - the relative contribution of an individual
to the population gene pool
 Think
 gene,
about what is meant by the following:
allele, genotype, phenotype
 species
 population
 natural selection
 allele and genotype structure of a population
 evolution