1. The Mechanisms of Evolution

2. Charles Darwin H.M.S. Beagle (1831) Evolutionary Change
Survey voyage around the world
Noticed the strikingly difference between species in S. America from those of Europe
Galapagos Island
Most animal species were found nowhere else
Similar to those on the mainland of S. America
Evolutionary Change
Species are not immutable; they change over time
The process that produces these changes is Natural Selection

3. Facts of Evolution Natural Selection Artificial Selection
The differential contribution of offspring to the next generation by various genetic types belonging to the same population.
Slight variations among individuals
Artificial Selection
Individuals with certain desirable traits by animal & plant breeders
Ex: pigeons have more than 300
varieties that have been artificially
selected for characteristics such
as color, size, & feather distribution

4. Facts of Evolution Adaptation
Processes by which characteristics appear that are useful
Characteristics adjust to the conditions of the environment
A phenotypic characteristic that has helped an organism adjust to conditions in its environment.
Mimicry of leaves by insects is an adaptation for evading predators. This example is a katydid from Costa Rica.

5. Population Genetics
Three main goals for the field of Population Genetics
To explain the origin & maintenance of genetic variation
To explain the patterns and organization of genetic variation
To understand the mechanisms that cause changes in allele frequencies in populations

6. Population Genetics Gene Pool
The sum of all copies of all alleles at all loci found in a population

7. Population Genetics Most populations are genetically variable
Single European species of wild mustard produced many important crop plants

8. Evolutionary change can be measured by allele & genotype frequencies
Population Genetics
Evolutionary change can be measured by allele & genotype frequencies
Mendelian populations
Allele frequencies are estimated in locally interbreeding groups within a geographic population
Frequency = proportion
Frequency of an allele or genotype is the proportion of the gene pool at that locus
P = number of copies of the allele in the population
sum of alleles in the population

9. Population Genetics Two alleles (A & a) are in a diploid population
Combine in three different forms
AA, Aa, aa
Calculate the relative frequencies of the alleles in a population of N individuals
NAA is the # that are homozygous dominant
NAa – heterozygous
Naa – homozygous recessive
NAA + NAa + Naa = N
Total # is 2N because population is diploid

10. Population Genetics p represents the frequency of A
q represents the frequency of a
p = 2NAA + NAa 2N
q = 2Naa + NAa

11. Calculate Population 1 Population 2 AA = 90; Aa = 40; aa = 70

13. Population Genetics Calculations demonstrate 2 important points
For each population p + q = 1
Population 1 (mostly homozygotes) & Population 2 (mostly heterozygotes) have the same allele frequencies for A & a
Have the same gene pool
BUT alleles are distributed differently
Genotype frequencies of the populations differ
If an allele is not advantageous, its frequency remains constant from generation to generation (even if the allele is dominant)
Hardy-Weinberg equilibrium
Equation that shows that the allele frequencies do not change

14. Hardy-Weinberg equilibrium
Conditions that must be met:
Mating is random
Individuals do not prefer certain genotypes
Population size is infinite
Larger a population, the smaller the effect of genetic drift
No Gene Flow
No migration either into or out of the population
No Mutation
No change to alleles; no new alleles added to the gene pool
Natural selection does not affect the survival of particular genotypes
No differential survival of individuals with different genotypes

15. Hardy-Weinberg equilibrium
If these ideal conditions hold:
Frequencies of alleles at a locus remain constant from generation to generation
Following one generation of random mating, genotype frequencies occur in the following proportions
Genotype: AA Aa aa
Frequency: p2 2pq q2
Probability that a particular sperm or egg will bear an A allele is .55 (or 55 out of 100)
a allele is .45
Probability of 2 A-bearing gametes
p x p = p2 (.55)2 =
.3025 or 30.25% of the next generation will have the AA genotype
Hardy-Weinberg Equation: p2 + 2pq + q2 = 1

18. Why study the H-W equilibrium
Equation is useful for predicating the approximate genotype frequencies of a population from its allele frequencies
Describes conditions that would result if there were no evolution in a population
Shows that allele frequencies will remain the same from generation to generation unless some mechanism acts to change them
Model conditions are never met; allele frequencies deviate from the H-W equilibrium
There are mechanisms acting to change allele frequencies, and these mechanism drive evolution
Patterns of deviation help identify specific mechanisms of evolutionary change