Chapter 23

CHAPTER 23 THE EVOLUTION OF POPULATIONS
Lecture Outline
Population genetics
Integration of Darwin’s and Mendel’s views: allele frequencies within a population
Hardy-Weinberg’s theorem
Causes of microevolution
Genetic variation, the substrate for natural selection

Fig. 23.0 Patterns of coloration in a population of marine shells

Population genetics
Little was known about inheritance during Darwin’s time
Mendel not rediscovered until 1900
Population genetics emerged in the 1930s
Emphasized genetic variation within populations
Reconciled Mendel and Darwin
Importance of quantitative characters

The Modern Synthesis
1940s
integrated paleontology, taxonomy, biogeography, and population genetics

Major points:
populations as the units of evolution
natural selection as primary mechanism of evolution
gradualism
Populations
Population: localized group of individuals that belong to the same species.
Species: group of populations whose individuals interbreed and produce fertile offspring.

populations may be isolated
favor members of same population
closely related near center

Fig. 23.2 Examples for population distribution

The gene pool of a population
Gene pool
= total gene aggregate in a population at any time

consists of all alleles at all gene loci
allel is fixed, if all individuals are homozygous
relative allele fequency can be determined, if several alleles are present for a gene

The Hardy-Weinberg Theorem
describes the gene pool of a non-evolving population:
- frequencies of alleles and genotypes in a population’s gene pool will remain constant unless acted on by other agents.
Generalities:
only for NON-evolving populations
assumes mating is random
population is in state of equilibrium

Fig. 23.3a The Hardy-Weinberg theorem

Fig. 23.3b The Hardy-Weinberg theorem

The Hardy-Weinberg Equation
Allows calculation of frequencies in gene pool:
p=0.8
q=0.2
p + q = 1
1-p =q or 1-q = p
Frequency of genotypes:
--> p2 + 2 pq + q2 = 1

Phenylketonuria
1/10000 babies are born with PKU
= 0.0001

Phenylketonuria
1/10000 babies with PKU
= 0.0001
Recessive babies are q2
q2=0.0001 --> then q =0.01
p=1-q --> p= 1-0.01=0.99
Frequency of carriers = heterozygotes
2pq
2 x 0.99 x 0.01=0.0198 ~ 0.02
~ 2% are carriers of PKU

Assumptions of the Hardy-Weinberg Theorem
Very large population sizes: in a small population, genetic drift (chance) can cause genotype frequencies to shift
No immigration
No net mutations
Random mating
No natural selection

- Evolution usually results when any of these five conditions are not met

Microevolution
Microevolution:
generation-to-generation change in a population’s frequencies of alleles

Causes of microevolution:
genetic drift
gene flow
mutation
natural selection

Genetic drift
Genetic drift:
the gene pool of a population changes due to chance

Happens when:
Population size is small
Often caused by catastrophes
--> Large populations less vulnerable

Fig. 23.4 Genetic drift in a small plant population can completely eliminate some alleles:

What causes genetic drift ?
often occurs as result of
bottleneck effect or founder effect

Bottleneck effect:
survivors have only a subset of the genetic characteristics

Example for bottleneck effect:
cheetahs

alleles lost from gene pool
low genetic variation
reduces variation & adaptability

-Bottlenecking is an important concept in conservation biology of endangered species

Founder effect:
genetic drift occurs as a result of colonization of a new, isolated environment by only a few individuals

random drift occurs until the population grows
Extreme examples: single seed or pregnant female
Examples:
- Galapagos Islands & Hawai
- Human populations established
by small number of colonists

Gene flow:
genetic exchange due to migration of fertile individuals or gametes between populations.

reduces differences
between populations

Mutations
Mutation alone does not have much quantitative effect on a large population in a single generation.

mutations are rare
cumulative effect of all mutations may be significant
increase in mutant allele frequency by natural selection
mutation is the original source of genetic variation that serves as the raw material for natural selection

Natural selection
Natural selection will lead some individuals to leave more offspring than others.
Disproportionate passing of certain alleles that enhance success.

Natural selection accumulates and maintains favorable genotypes in a population.

Genetic variation
Combination of inheritable and non-heritable traits.

Quantitative and discrete characters contribute to variation within a population.
Quantitative characters vary along a continuum within a population (e.g. plant height)
- polygenic
Discrete characters (e.g. flower color)
- single locus with different alleles

Genetic variation

1. within a population: polymorphic

Genetic variation
2. between populations of the same species

Geographic variation
Cline
may reflect direct environmental
effects on phenotype
e.g. mammal body size

may also involve
genetic differences
e.g. plant size

Genetic variation
Isolated populations

Differences in karyotype
through genetic drift
e.g. house mice on Madeira

Genetic variation
3. through mutation and sexual recombination

Mutations are changes in nucleotide sequence of DNA
MUST occur in sex cells
Rarely beneficial
ocasionally increased fitness
e.g. HIV resistance to antiviral drugs

Diploidy and balanced polymorphism
preserve and restore genetic variation
Diploidy preserves recessive alleles:
heterozygotes hide recessive traits
Balanced polymorphism maintains genetic diversity in a population via natural selection
heterozygote advantage e.g. sickle cell anemia

Sickle cell anemia & heterozygote advantage:
caused by recessive allele for one chain of hemoglobin
Homozygous recessive
= harmed by disease
Homozygous dominant
= vulnerable to malaria
Heterozygous
= resistant to malaria

Neutral variation
--> No selective advantage or disadvantage occurs as result of a variation in a trait
e.g. human fingerprints

No consensus if ‘neutral’ traits are really neutral

Natural selection as the mechanism of adaptive evolution
‘chance and sorting’ - better adapted organisms
natural selection acts on phenotype
increased reproductive success
Darwinian fitness
contribution of individual to gene pool of next generation
Favored by:
long life span
early maturity

Effects of selection
directional, diversifying or stabilizing

Stabilizing selection
- birth weight in humans

directional selection
- beak size in ground finch
- European black bear

diversifying selection
- distinct size differences
within a population

Natural selection & sexual reproduction
Disadvantages:
- less offspring

Advantages:
- genetic variation
- resistance to disease

Sexual selection
Sexual dimorphism is a product of sexual selection
distinction between sex characteristics of male & female
- size, manes, antlers

Intrasexual selection
combat rare
show off & mental game

Intersexual selection
‘choosy’ females
showy = healthy

Natural selection & adaptation
Natural selection cannot produce perfect organisms:
Limited by historic constraints
modification of ancestral traits
Adaptations are often compromises
flexible human limbs - prone to injury
Not all evolution is adaptive
by chance - bottleneck & founder effect
Selection can only edit variations
new alleles do not arise on demand

Key terms 23
population genetics
polymorphic
modern synthesis
gene diversity
population
geographic variation
species
cline
gene pool
balanced polymorphism
Hardy-Weinberg theorem
heterozygote advantage
Hardy-Weinberg equation
neutral variation
microevolution
Darwinian fitness
genetic drift
directional selection
bottleneck effect
diversifying selection
founder effect
stabilizing selection
natural selection
sexual dimorphism
gene flow
intrasexual selection
mutation
intersexual selection