1. HARDY WEINBERG CRITERIA & POPULATION EVOLUTION
CHAPTER 23
HARDY WEINBERG CRITERIA & POPULATION EVOLUTION
(PART 3)
Ms.Gaynor
AP Biology

2. Causes of Microevolution
A change in the gene pool of a population over a succession of generations
We use 5 criteria for non-evolution to determine causes of microevolution
The Hardy Weinberg equations are used to determine the degree of microevolution that is occurring for a given allele

3. Cause #1: Microevolution
Genetic drift: changes in the gene pool of a SMALL population due to chance (usually reduces genetic variability)
2nd to last video

4. Genetic Drift Example #1
The Bottleneck Effect
Results from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

5. Ex: NORTHERN ELEPHANT SEAL

6. Genetic Drift Example #2
Founder Effect: a cause of genetic drift due to the colonization by a limited number of individuals from a parent population

7. Founder Effect in Amish
Causes dwarfism and polydactyly

8. Cause #2: Microevolution
Gene Flow:
genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

9. Cause #3: Microevolution
Mutations:
a change in an organism’s DNA
original source of genetic variation (raw material for natural selection)
Mutations can immediately alter p and q in a population
Individual mutations are rare in a population, but there may be cumulative mutations that have an effect  cause evolution

10. Cause #4: Microevolution
Nonrandom mating
“picky” mating
Unequal chances of each egg getting fertilized
Assortative mating - choosing individuals more like self

11. Sexual Selection
Sexual selection is natural selection for mating success
It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics

12. Sexual Dimorphism Males are usually more colorful and larger
Does NOT help cope with environment
DOES lead to reproductive success
Can be termed “Female Selection”

13. Cause #5: Microevolution
Natural Selection
differential reproductive success in reproduction due to variation
only form of microevolution that adapts a population to its environment
Phenotypes are selected FOR or AGAINST (not genotypes)
In a population at equilibrium, no phenotypes (therefore, alleles) are selected over other alleles

14. Animations to Review Hardy Weinberg and its Criteria

15. 3 Modes of Natural selection
Directional selection favors individuals at one end of the phenotypic range
Disruptive selection favors individuals at both extremes of the phenotypic range
Stabilizing selection favors intermediate variants and acts against extreme phenotypes

16. LIGHT/DARK ROCK HABITAT MEDIUM COLOR ROCK HABITAT

17. Sources of Variations Mutations
At first the mutation may not be beneficial to the organism (resistance to antibiotics)
Once antibiotics are introduced the mutation is beneficial
2. Sexual recombination increases variation

18. 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)
SPRING
SUMMER

19. Evolutionary Fitness
“Struggle for existence”/“Survival of the fittest”
commonly used but can be misleading
Reproductive success is more subtle and depends on many factors
Fitness is determined by REPRODUCTIVE AND SURVIVAL success of individual
Variations & natural selection affect fitness
Sterile individuals even with a relatively fit allele will not be considered fit overall (=0)
Fitness is not determined by one trait but by the totality of traits in the organism

20. A 4th Type of Natural Selection: Neutral Variation
Neutral variation is genetic variation that appears to favor NO selective advantage
Frequencies can change but no advantage/ disadvantage for population

21. Asexual vs. Sexual Reproduction
Sexual reproduction produces FEWER reproductive offspring than asexual reproduction
a so-called “reproductive handicap”
Sexual reproduction produces genetic variation

22. Asexual reproduction Sexual reproduction Female Generation 1 Female