THE FORCES OF EVOLUTIONARY CHANGE MICROEVOLUTION

1
THE FORCES OF EVOLUTIONARY CHANGE -
MICROEVOLUTION

• Chapter 15

2

• Evolution occurs at the population level as
  allele frequencies change.

3

• A. Hardy-Weinberg Equilibrium
• A theoretical state in which allele frequencies
  of a population do not change from one generation
  to the next.
• H-W equilibrium is only possible if
• mating population is large
• mating is entirely random
• there is NO migration, mutation, or natural
  selection

4

• Hardy-Weinberg Equation
• p2 2pq q2 1
• p2 frequency of homozygous dominant
  individuals
• 2pq frequency of heterozygotes
• q2 frequency of homozygous recessive
  individuals
• p frequency of dominant allele
• q frequency of recessive allele
• NOTE p q 1

5

• H-W example 1
• In a certain population, 36 have sickle cell
  anemia. What is the frequency of the dominant
  allele?
• What do we know? (p2, 2pq, q2, p or q)
• q2 36 or 0.36
• What do we want to find?

p
Calculations ?q2 q ?0.36 0.6 q
0.6 p q 1 Thus, p 1 - 0.6 or 0.4
6

• H-W example 2
• In a certain population, the frequency of the
  dominant allele is 0.7. What is the frequency of
  heterozygous individuals?
• What do we know? (p2, 2pq, q2, p or q)
• p 0.7
• What do we want to find?

2pq
Calculations p q 1 Thus, q 1 - 0.7
or 0.3 2pq 2 x 0.7 x 0.3 or 0.42
7

• From the previous example we know
• p 0.7 q 0.3 2pq 0.42
• Calculate the frequency of homozygous dominant
  individuals.

0.49
Calculate the frequency of homozygous recessive
individuals.
0.09

• If there are 1000 individuals in this population,
  how many are
• heterozygous?

420

• homozygous dominant?

490

• homozygous recessive?

90
8

• H-W equilibrium provides a background against
  which microevolution can be detected.
• If allele genotype frequencies change from one
  generation to the next, then evolution is
  occurring with respect to that particular gene.
• If frequencies remain unchanged, then evolution
  is not occurring.

9

• B. Factors That Cause Microevolution in Natural
  Populations
• 1. Nonrandom Mating
• Nonrandom mating causes certain alleles to become
  more common in future generations (some
  individuals leave more offspring than others).
• Ex. Albinism among Arizonas Hopi Indians

10

• 2. Migration
• Individuals migrate between populations.
• Immigrating individuals introduce new alleles.
• Emigrating individuals remove alleles.
• Ex. New York Citys waves of immigration

11

• 3. Genetic Drift
• A change in the gene pool of a small population
  due to chance.

Genetic drift in human populations may be caused
by the founder effect or a population bottleneck.
12

• Founder effect genetic drift due to a few
  individuals leaving a large population to found a
  new group.
• Unlikely that gene pool of founding population is
  representative of original population.
• Ex. Ellis-van Creveld syndrome among Pennsylvania
  Almish.

13

• Population bottleneck genetic drift due to high
  mortality in a population.
• Unlikely that gene pool of the remaining
  population is representative of original
  population.
• Ex. Pingelapese blindness among Pingelapese
  people of the eastern Caroline islands.
• Decreased genetic diversity among Cheetahs.

14
(No Transcript)
15

• 4. Mutation
• A change in the DNA - introduces new alleles
  into the population.

Mutations can be beneficial, silent, or harmful.
16

• 5. Natural Selection
• The differential survival and reproduction of
  organisms whose genetic traits better adapt them
  to a particular environment.
• Considered to be the major driving force of
  evolution.

17

• Types of Natural Selection
• Directional Selection
• Environment selects against one phenotypic
  extreme, allowing the other to become more
  prevalent.

18

• Disruptive Selection
• Environment selects against intermediate
  phenotype, allowing both extremes to become more
  prevalent.

19

• Stabilizing Selection
• Environment selects against two extreme
  phenotypes, allowing the intermediates to become
  more prevalent.

20

• Balanced Polymorphism
• A form of stabilizing selection that maintains
  deleterious recessive alleles in a population
  because heterozygotes resist an infectious
  disease.
• Sickle cell anemia is maintained because
  heterozygotes are resistant to malaria.
• Cystic fibrosis is maintained because
  heterozygotes are resistant to cholera typhoid
  fever.