1. Microevolutionary Processes
Chapter 21

2. Microevolutionary Processes
Microevolution is a change in allele frequencies in a population over generations
Drive a population away from genetic equilibrium
Small-scale changes in allele frequencies brought about by:
Natural selection
Gene flow
Genetic drift

3. Populations Evolve Biological evolution does not change individuals
It changes a population
Traits in a population vary among individuals
Evolution is change in frequency of traits

4. Gene Pools and Allele Frequencies
Population
localized group of individuals capable of interbreeding and producing fertile offspring
Gene Pool
consists of all the alleles in a population 4

5. Gene Mutations Infrequent but inevitable
Each gene has own mutation rate
The average is about one mutation in every 100,000 genes per generation
Mutation rates are often lower in prokaryotes and higher in viruses
Short generation times allow mutations to accumulate rapidly in prokaryotes and viruses
Lethal mutations
Neutral mutations
Advantageous mutations

6. Variation in Phenotype
Each kind of gene in gene pool may have two or more alleles
Individuals inherit different allele combinations
This leads to variation in phenotype
Offspring inherit genes, not phenotypes

7. Variation in Phenotype

8. Variation in Phenotype

9. Variation in Phenotype

10. What Determines Alleles in New Individuals?
Mutation
Crossing over at meiosis I
Independent assortment
Fertilization
Change in chromosome number or structure

11. The Hardy-Weinberg Principle
The Hardy-Weinberg principle describes a population that is not evolving
If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving 11

12. Hardy-Weinberg Equilibrium
The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation
In a given population where gametes contribute to the next generation randomly, allele frequencies will not change
Mendelian inheritance preserves genetic variation in a population 12

13. Hardy-Weinberg Rule
At genetic equilibrium, proportions of genotypes at a locus with two alleles are given by the equation:
p2 + 2pq + q2 = 1
Frequency of allele A = p
Frequency of allele a = q

14. Conditions for Hardy-Weinberg Equilibrium
The Hardy-Weinberg theorem describes a hypothetical population that is not evolving
In real populations, allele and genotype frequencies do change over time 14

15. Conditions for Hardy-Weinberg Equilibrium
The five conditions for non-evolving populations are rarely met in nature
No mutations
Random mating
No natural selection
Extremely large population size
No gene flow 15

16. Applying the Hardy-Weinberg Principle
We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that
The PKU gene mutation rate is low
Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele
Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions
The population is large
Migration has no effect, as many other populations have similar allele frequencies 16

17. Applying the Hardy-Weinberg Principle
The occurrence of PKU is 1 per 10,000 births
q2 
q  0.01
The frequency of normal alleles is
p  1 – q  1 – 0.01  0.99
The frequency of carriers is
2pq  2  0.99  0.01 
or approximately 2% of the U.S. population 17

18. Altering Allele Frequencies in a Population
Three major factors alter allele frequencies and bring about most evolutionary change
Natural selection
Genetic drift
Gene flow 18

19. 1.) Natural Selection
Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
Evolution by natural selection involves both chance and “sorting”
New genetic variations arise by chance
Beneficial alleles are “sorted” and favored by natural selection
Only natural selection consistently results in adaptive evolution, an increase in the frequency of alleles that improve fitness 19

20. Results of Natural Selection
Three possible outcomes:
A shift in the range of values for a given trait in some direction
Stabilization of an existing range of values
Disruption of an existing range of values

21. Directional Selection
Number of individuals
in the population
Favors individuals at one end of the phenotypic range
Allele frequencies shift in one direction
Range of values for the trait at time 1
Number of individuals
in the population
Range of values for the trait at time 2
Number of individuals
in the population
Range of values for the trait at time 3

22. Peppered Moths
Prior to industrial revolution, most common phenotype was light colored
After industrial revolution, dark phenotype became more common
PEPPERED MOTHS - KETTLEWELL

23. Adaptation
Darwin noticed that every species seemed well adapted to the life it leads
Adaptations
physical and behavioral characteristics combine to help the organism catch food, handle harsh conditions and reproduce

24. Examples of Adaptations
Camouflage:
a structural adaptation enabling an organism to blend in with its environment
Octopus
Cuttlefish
Mimicry:
a structural adaptation that provides protection for an organism by copying the appearance of another species

26. black on yellow kill fellow
red on black friend of jack

27. Pesticide Resistance Pesticides kill susceptible insects
Resistant insects survive and reproduce
If resistance has heritable basis, it becomes more common with each generation

28. Antibiotic Resistance
First came into use in the 1940s
Overuse has led to increase in resistant forms
Most susceptible cells died out and were replaced by resistant forms

29. Stabilizing Selection
Number of individuals
in the population
Intermediate forms are favored and extremes are eliminated
Range of values for the trait at time 1
Range of values for the trait at time 2
Range of values for the trait at time 3

30. Stabilizing Selection
Heterozygote advantage occurs when heterozygotes have a higher fitness than do both homozygotes
Natural selection will tend to maintain two or more alleles at that locus
For example, the sickle-cell allele causes deleterious mutations in hemoglobin but also confers malaria resistance 30

31. Plasmodium falciparum (a parasitic unicellular eukaryote) 7.5–10.0%
Figure 21.17
Key
Frequencies of the
sickle-cell allele
0–2.5%
Figure Mapping malaria and the sickle-cell allele
2.5–5.0%
5.0–7.5%
Distribution of
malaria caused by
Plasmodium falciparum
(a parasitic unicellular eukaryote)
7.5–10.0%
10.0–12.5%
12.5% 31

32. Disruptive Selection
Number of individuals
in the population
Forms at both ends of the range of variation are favored
Intermediate forms are selected against
Range of values for the trait at time 1
Number of individuals
in the population
Range of values for the trait at time 2
Number of individuals
in the population
Range of values for the trait at time 3

33. Sexual Selection Sexual selection natural selection for mating success
can result in sexual dimorphism
marked differences between the sexes in secondary sexual characteristics 33

34. Figure 21.15 Sexual dimorphism and sexual selection34

35. 2.) Genetic Drift Genetic Drift
how allele frequencies fluctuate unpredictably from one generation to the next
tends to reduce genetic variation through losses of alleles, especially in small populations 35

36. Effects of Genetic Drift
Genetic drift is significant in small populations
Genetic drift can cause allele frequencies to change at random
Genetic drift can lead to a loss of genetic variation within populations
Genetic drift can cause harmful alleles to become fixed 36

37. Founder Effect Founder effect
occurs when a few individuals become isolated from a larger population
allele frequencies in the small founder population can be different from those in the larger parent population due to chance

38. Bottleneck Bottleneck effect
results from a drastic reduction in population size due to a sudden environmental change
if the population remains small, it may be further affected by genetic drift
understanding the bottleneck effect can increase understanding of how human activity affects other species

39. 3.) Gene Flow Gene flow movement of alleles among populations
alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen)
tends to reduce genetic variation among populations over time 39

40. Barriers to Gene Flow
Whether or not a physical barrier deters gene flow depends upon:
Organism’s mode of dispersal or locomotion
Duration of time organism can move

41. Inbreeding Nonrandom mating between related individuals
Leads to increased homozygosity
Can lower fitness when deleterious recessive alleles are expressed
Amish, cheetahs