Microevolution

1
Microevolution

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 18

2
Introduction

• Microevolution means that changes have occurred
  in a populations allele frequencies.
• Allele frequencies can change through mutations,
  gene flow, genetic drift, and natural selection.
• Natural selection relies on there being variation
  in populations.

3
Individuals Dont Evolve, Populations Do

• Population - Group of individuals of the same
  species occupying a given area
• Polymorphism - traits come in two or more
  distinct forms
• Gene Pool - pool of genetic resources that, in
  theory, is shared by all members of the
  population
• Mutation change in DNA sequences

4
Base Pair Mutations

• Point mutations
• Substitution
• AATTGC ? AATAGC
• Insertion
• AATTGC ? AATTGGC
• Deletion
• AATTGC ? AATGC
• Result in frame shifts (codon changes)
• Editing errors can also occur

5
Chromosome Mutations

• Translocation
• Inversion
• Deletion
• Duplication
• Aneuploidy and Polyploidy

6
Other sources of variation

• Aside from mutation, variation can also come from
  sexual reproduction.
• Crossing over
• Random alignment of chromosomes
• Random fertilization

7
Applying what you know

• Explain how the following can change the gene
  pool
• Natural selection
• Genetic drift
• Gene flow

8
Variation
9
Variation Lab Discussion

• Shape of histogram?
• What does the shape tell you about selection in
  terms of pinto bean length?
• What other traits show variability?
• What determines variation?
• Advantages for being
• Long pinto
• Short pinto

10
Types of Selection Directional

• Directional Selection
• Shift in allele frequency in a consistent
  direction
• Phenotypic Variation in a population of
  butterflies

11
The Case of the Peppered Moths

• Industrial revolution
• Pollution darkened tree trunks
• Camouflage of moths increases survival from
  predators
• Directional selection caused a shift away from
  light-gray towards dark-gray moths

12
Directional Selection Other Examples

• Pesticide Resistance
• Pest resurgence
• Antibiotic Resistance
• With directional selection, allele frequencies
  tend to shift in response to directional changes
  in the environment

13
Types of Selection Stabilizing

• Stabilizing Selection
• Intermediate forms of a trait are favored
• Alleles that specify extreme forms are eliminated
  from a population
• Ex. Gall size in
• E. solidaginis

14
Flies and Gall Sizes
Fig. 18.7, p. 288
15
Types of Selection Disruptive

• Disruptive Selection
• Both forms at extreme ends are favored
• Intermediate forms are eliminated
• Bill size in African finches

16
Special Types of SelectionSexual and Balancing
Distribution of Malaria

• Sexual selection
• Sexual dimorphism
• Balancing selection
• Balanced polymorphism
• Sickle-Cell Anemia
• Malaria

Sickle Cell Trait
17
Gene Flow and Genetic Drift

• Gene Flow
• Flow of alleles
• Emigration and immigration of individuals
• Genetic Drift
• Random change in allele frequencies over
  generations brought about by chance
• In the absence of other forces, drift leads to
  loss of genetic diversity

18
Genetic Drift

• Magnitude of drift is greatest in small
  populations

19
Genetic Drift and Inbred Populations

• Inbreeding
• Increase in homozygous conditions
• Elimination of variations in alleles
• Increase is susceptibility to environmental
  influences
• Examples
• Amish
• Cheetahs

20
Effects of Inbreeding

• Increased infant mortality
• Increased morbidity (disease)
• Why?
• Emergence of recessive phenotypes
• Examples Hemophilia and Tay Sachs

21
Hardy-Weinberg Equilibrium
22
Hardy-Weinberg Equilibrium

• Genetic equilibrium is a theoretical state in
  which a population is not evolving
• Genetic equilibrium/ H-W conditions only apply
  if
• The population is very large
• There is random mating
• There is no mutation
• There is no selection
• There is no immigration or emigration.

23
Hardy-Weinberg Equations

• There are two equations
• p q 1
• p2 2pq q2 1
• p and q refer to allele frequencies
• p2, 2pq, and q2 refer to genotype frequencies

24
Hardy-Weinberg Equations

• p frequency of dominant allele
• q frequency of recessive allele
• p2 freq. of homozygous dominants
• 2pq freq. of heterozygotes
• q2 freq. of homozygous recessives

25
Hardy-Weinberg Problems
Starting generation
490 AA butterflies (dark-blue wings)
420 Aa butterflies (medium-blue wings)

• For the starting generation, calculate p and q.
• What are p2 , 2pq, and q2 ?
• If there is no change in this population over
  several generations, is it evolving?

90 aa butterflies (white wings)
The next generation
490 AA butterflies
420 Aa butterflies
90 aa butterflies
(NO CHANGE)
The next generation
490 AA butterflies
420 Aa butterflies
90 aa butterflies
(NO CHANGE)
Fig. 18.3, p. 285
26
Hardy-Weinberg Problems

• Total population 1000
• q2 freq. of recessive genotype 90/1000 .09
• if q2 .09, q square root of .09, q .3
• if q .3 then p 1-.3 .7
• if p .7, then p2 .49
• 2pq .42
• What do these numbers mean?

27
Hardy-Weinberg Problems

• Total population 100,000
• Agouti is dominant to white.
• q2 25,000/100,000 .25
• q square root of .25 .5
• p 1-.5 .5
• p2 .25
• 2pq .5
• What do these numbers mean?

28
Hardy-Weinberg Problems

• Total population 500
• Red flowers are dominant to white.
• q2 20/500 .04
• q square root of .04 .2
• p 1-q 1-.2 .8
• p2 .64
• 2pq .32
• What do these numbers mean?