1. Evolution

2. Evolutionary mechanisms
Forces that change the genetic structure of a population
Mutation
Gene flow
Genetic drift
Nonrandom mating
Natural selection

3. Evolutionary mechanisms
Mutations generate genetic variation
Origin of genetic variation; Any change in the DNA
Rate is typically one in a million locus
Can create considerable genetic variation because:
Large number of genes may mutate
Chromosomal rearrangements may change genes
Populations contain large numbers of individuals
Each human gamete contains 3 x 109; average of 3 point mutations. So, each zygote has ~6 mutations.
Current population is 6.5 billion people; would expect ~40 billion mutations that were not in the previous generation.

4. Evolutionary mechanisms
Gene flow may change allele frequencies
Migration of individuals; movements of gametes between populations (gene flow) are common

5. Evolutionary mechanisms
Genetic drift can cause large changes in small populations
Random events that change the gene pool
Population bottlenecks
Founder effects
The loss of genetic variation when
a new colony is formed by a very small
number of individuals from a larger
population.

6. Bottleneck
Northern elephant seals have reduced genetic variation probably because of a population bottleneck humans inflicted on them in the 1890s.
Hunting reduced their population size to as few as 20 individuals at the end of the 19th century.
Their population has since rebounded to over 30,000—but their genes still carry the marks of this bottleneck
they have much less genetic variation than a population of southern elephant seals that was not so intensely hunted.

7. Founder Effect
Polydactyl -- extra fingers or sometimes toes -- is one symptom of Ellis-van Creveld syndrome.
Commonly found among the Old Order Amish of Pennsylvania, a population that experiences the "founder effect."
Genetically inherited diseases like Ellis-van Creveld are more concentrated among the Amish because they marry within their own community
Which prevents new genetic variation from entering the population.
Children are therefore more likely to inherit two copies of the particular recessive genes that lead to genetic disease.

8. Evolutionary mechanisms
Nonrandom mating changes genotype frequencies
Individuals of choose particular genotypes as mates
If they mate with the same genotypes then homozygous genotypes will be overrepresented
Self-fertilization (type of nonrandom mating)
Sexual selection
Changes allele frequencies

9. Evolutionary mechanisms resulting in adaptation
Allele frequencies in the population change in a way that adapts individuals to the environment that influenced such reproductive success
Fitness
capability of an individual of certain genotype to reproduce
manifested through its phenotype
Determined by the average rates of survival & reproduction

10. Natural selection ~Produces variable results
Stabilizing selection
Preserves the average characteristics of a population (favors the average individual)
Reduces variation in populations; does not change the mean
Example:
In humans the
incidence of infant
mortality is higher
for very heavy as
well as for very light
babies

11. Natural Selection Directional Selection
Favors individuals that vary in one direction from the mean of the population
Evolutionary trend toward that extreme
For example, breeding of the greyhound dog
Early breeders were interested in dog with the greatest speed. They carefully selected from a group of hounds those who ran the fastest. From their offspring, the greyhound breeders again selected those dogs who ran the fastest. By continuing this selection for those dogs who ran faster than most of the hound dog population, they gradually produced a dog who could run up to 40mph.

12. Natural Selection
Directional Selection

13. Natural Selection Disruptive Selection
Favors individuals that vary in opposite directions from the mean
Individuals from both extremes have high fitness
Example
Smaller-billed birds feed on soft seeds
Large-billed birds crack hard seeds
Birds with intermediate sizes cannot use either kind of seed efficiently & survive poorly

16. Natural Selection Sexual selection
Natural selection that acts on characteristics that determine reproductive success.
Intrasexual selection – improve ability to compete for access to mates
Intersexual selection – more attractive to members of the opposite sex