1. Chapter 23~
Chapter 23~ The Evolution of Populations

2. Population genetics
Population: a localized group of individuals belonging to the same species
Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring
Gene pool: the total aggregate of genes in a population at any one time
Population genetics: the study of genetic changes in populations
Modern synthesis/neo-Darwinism
“Individuals are selected, but populations evolve.”

3. Hardy-Weinberg Theorem
Serves as a model for the genetic structure of a nonevolving population (equilibrium)
Evolution = change in allele frequencies in a population
hypothetical: what conditions not would cause allele frequencies to change?
non-evolving population
REMOVE all agents of evolutionary change
very large population size (no genetic drift)
no migration (no gene flow in or out)
no mutation (no genetic change)
random mating (no sexual selection)
no natural selection (everyone is equally fit)

4. Hardy-Weinberg Equation
p=frequency of one allele (A); q=frequency of the other allele (a); p+q= (p=1-q & q=1-p)
p2=frequency of AA genotype; 2pq=frequency of Aa genotype; q2=frequency of aa genotype;
frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
G.H. Hardy
mathematician
W. Weinberg
physician

5. Using Hardy-Weinberg equation
population: 100 cats
84 black, 16 white
How many of each genotype?
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): = 0.6
p2=.36
2pq=.48
q2=.16 BB Bb bb
Must assume population is in H-W equilibrium!
What are the genotype frequencies?

6. 5 Agents of evolutionary change
Gene Flow
Non-random mating
Genetic Drift
Selection
Mutation

7. Microevolution, I
A change in the gene pool of a population over a succession of generations
1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

8. Microevolution, II: type of genetic drift
The Bottleneck Effect: type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

9. Conservation issues
Peregrine Falcon
Bottlenecking is an important concept in conservation biology of endangered species
loss of alleles from gene pool
reduces variation
reduces adaptability
Breeding programs must consciously outcross
Golden Lion Tamarin

10. Microevolution, III type of genetic drift
Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population
just by chance some rare alleles may be at high frequency; others may be missing
skew the gene pool of new population
human populations that started from small group of colonists
example: colonization of New World

11. Microevolution, IV
2- Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)
seed & pollen distribution by wind & insect
migration of animals

12. Microevolution, V 3- Mutations: Mutation creates variation
a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

13. Microevolution, VI 4- Nonrandom mating: Sexual selection
inbreeding and assortive mating (both shift frequencies of different genotypes)

14. Sexual selection
It’s FEMALE CHOICE, baby!

15. Microevolution, VII 5.NaturalSelection
differential success in reproduction;
climate change
food source availability
predators, parasites, diseases
toxins
only form of microevolution that adapts a population to its environment
combinations of alleles that provide “fitness” increase in the population

16. Natural Selection
Selection acts on any trait that affects survival or reproduction
predation selection
physiological selection
sexual selection

17. Variation & natural selection
Variation is the raw material for natural selection
there have to be differences within population
some individuals must be more fit than others

18. Where does Variation come from?
Mean beak depth of parents (mm)
Medium ground finch 8 9 10 11
1977
1980
1982
1984
Dry year
Wet year
Beak depth
Beak depth of
offspring (mm)
Mutation
random changes to DNA
errors in mitosis & meiosis
environmental damage
Sex
mixing of alleles
recombination of alleles
new arrangements in every offspring
new combinations = new phenotypes
spreads variation
offspring inherit traits from parent

19. Population variation
Polymorphism: coexistence of 2 or more distinct forms of individuals (morphs) within the same population
Geographical variation: differences in genetic structure between populations (cline)

20. Variation preservation
Prevention of natural selection’s reduction of variation
Diploidy nd set of chromosomes hides variation in the heterozygote
Balanced polymorphism heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia); frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

21. Natural selection
Fitness: contribution an individual makes to the gene pool of the next generation
3 types:
A. Directional
B. Diversifying
C. Stabilizing

22. Effects of Selection Changes in the average trait of a population
DIRECTIONAL SELECTION
STABILIZING SELECTION
DISRUPTIVE SELECTION
giraffe neck
horse size
human birth weight
rock pocket mice

23. Sexual selection
Sexual dimorphism: secondary sex characteristic distinction
Sexual selection: selection towards secondary sex characteristics that leads to sexual dimorphism

24. Any Questions??