1. The Genetic Basis for Evolution
MICROEVOLUTION
The Genetic Basis for Evolution

2. The Theory of Evolution by Natural Selection
Populations of organisms have the capacity to change over time, generally in response to changes in their supporting environment.

3. Microevolution We know…. Alleles Genotype Phenotype Form
Adaptation to Environment,
Evolution

4. Microevolution
So in order to REALLY figure out Evolutionary Change, we’ve got to back up…
Microevolution = Changes in allele (gene) frequencies over time (in populations).
Alleles
Genotype
Phenotype
Form

5. Microevolution GENE POOL:
All of the alleles at all gene loci in all individuals of a population.
The Alleles/Genes, not the Individuals.
The Genotypes, not the Phenotypes.

6. Microevolution p2 + 2pq +q2 = 1 Please read pp. 454-458…
The Hardy-Weinberg Theorem:
Where p = allele frequency 1
Where q = allele frequency 2
p2 + 2pq +q2 = 1

7. Microevolution REVIEW Frequency of alleles = AA Alleles
Diploid/Haploid
Homozygous
Heterozygous
Dominant/Recessive
Frequency of alleles =
total # of alleles of one type
total # of alleles of all types

8. Microevolution Frequency of alleles= Traditionally, in Hardy-Weinberg:AA Aa aa
Frequency of alleles=
A= 9/22 = 0.41 = 41%
a = 13/22 = 0.59 = 59%
Traditionally, in Hardy-Weinberg:
p represents the frequency of one allele &
q represents the frequency of the other allele.

9. Hardy-Weinberg Theorem
Says that frequencies of alleles in a population remain constant over time.
This depends on the inheritance of variation introduced during segregation & recombination (Mendelian Genetics).
Defines when a population is NOT evolving.

10. Hardy-Weinberg Theorem
Organisms donate gametes to the next generation randomly.
Organisms mate with each other randomly.
SO, we can predict the genotypes of offspring…

11. Hardy-Weinberg TheoremAA Aa aa
Frequency of alleles=
A= 9/22 = 0.41 = 41%
a = 13/22 = 0.59 = 59%
The chance of a gamete carrying each allele is the same.
So, the chance of having offspring =
For AA, p x p = p2 = .41x.41 = .168 = 16.8%
For aa, q x q = q2 = .59 x .59 = .348 = 34.8%
For Aa, p x q = .24 = 24.2% AND q x p = .24 = 24.2%

12. Hardy-Weinberg Equilibrium
This set of relationships (predictions) can be summarized as:
In any population, Hardy-Weinberg predicts the frequencies of alleles in each generation, which always = 1.
Sort of like a deck of cards…
p2 + 2pq +q2 = 1

13. Hardy-Weinberg Theorem
Assumptions for Hardy-Weinberg to hold (populations NOT evolving):
Large Populations
No Migration
No Mutation
Random Mating
No Natural Selection
BUT, in Natural Populations, these 5 assumptions are violated…

14. Agents of Microevolutionary Change
Processes that elicit changes in allele frequencies in populations:
Mutation
Gene Flow
Non-Random Mating
Genetic Drift
Natural Selection

15. Agents of Microevolutionary Change
Mutation: A random heritable change in the DNA.
Only source of NEW alleles in a population.
Actually rather common!
Can be:
Harmful (most – we’re complex), these do not remain in the gene pool and so are rare (selected against!).
Neutral (many), masked, occur in genes not essential for survival, often passed on.
Beneficial (rare – we’re complex), these are the key to better-adapting to changing environments.

16. Agents of Microevolutionary Change
Gene Flow: New individuals enter or exit a population (Migration).
Introduces the possibility of new allele combinations.
Often has a homogenizing (mixing) effect on the population – offering stability.

17. Agents of Microevolutionary Change
Non-Random Mating: Mating & offspring production by individuals with specific phenotypes (genotypes).
A.K.A. “Assortive Mating” or “Preferential Mating” (Not random).
Results in accumulation of certain traits in the population (sometimes reduction of heterozygotes).
Examples: Mate selection in white-tailed deer (“Sexual Selection”), tall Men & tall women, etc.

18. Agents of Microevolutionary Change
Genetic Drift: Changes in allele frequency due to chance.
Often due to natural disasters or major catastrophes.
Especially in small populations.
Bottleneck Effect: loss of certain alleles as a population dwindles, reduction in allelic diversity with a smaller and smaller population.
Founder Effect: small population with a specific and limited gene pool is separated from the main population.

19. Agents of Microevolutionary Change
Natural Selection: Differential survival & reproduction of individuals in a population.
Stabilizing (the extremes of the population are selected against in favor of the average/most common)
Disruptive (the average/most common individuals of the population are selected against in favor of the extremes)
Directional (one extreme or the other of the population are selected against)

20. Agents of Microevolutionary Change
Natural Selection
(Peppered Moths)

21. Natural Selection Hardy Weinberg & Natural Selection:

22. The “Modern Synthesis”
Incorporation of population genetics into natural selection.
Biochemistry
Genes
DNA
RNA
Amino Acids
T. Dobzhansky

23. Next?
What is a species?