Lecture 2: Evolution of Populations
Campbell Biology chapters:
Chapter 23
Microevolution – evolution at the population level
= change in allele frequencies over generations
Genetics
= science dealing with inheritance or heredity,
the transmission of acquired traits
Ultimate source of heritable variation is
change in DNA
• Change in DNA caused by:
1) Mutation
2) Genetic Recombination
1) Mutations
= change in genotype other than by
recombination.
Three types:
1) Point Mutations
2) Chromosome Mutations
3) Change in Chromosome Number
1) Point Mutation

Change in a single DNA Nucleotide.
Point mutation rate per gene =
~1 in 100,000 gametes. In humans:
=
1 mutation/gene x (~25,000 genes)
100,000 gametes
=
~0.25 point mutations/gamete
E.g., human hemoglobin:
2 alpha chains (141 amino acids)
2 beta chains (146 amino acids)
1973 sampling of population (thousands): 169 mutation types
recorded:
62 substitutions in alpha
99 substitutions in beta
1 deletion in alpha
7 deletions in beta
1 in 2,000 people have mutant
hemoglobin gene.
hemoglobin
2) Chromosome Mutations
Rearrangements (including losses and gains)
of large pieces of DNA. E.g., inversion:
A B C
Re-attaches here
D E F G
and here
A B F E D C G
[3% of pop. of Edinburgh, Scotland have inversion in Chromosome #1]
[Humans differ from chimps by 6 inversions, from gorillas by 8 (also
difference in chromosome number)]
3) Change in Chromosome No.
• a) Aneuploidy - change in chromosome
number of less than an entire genome.
Horse (2n = 64) versus donkey (2n = 62)
Humans (2n = 46) versus chimp or gorilla (2n = 48)
Some Genetic Diseases
Trisomy (addition of a chromosome to the original diploid pair) of
chromosome 21 in humans = Down's syndrome.
Extra or one sex chromosomes ( e. g., XYY, XXY, X).
b) Polyploidy
Evolution of chromosome number which is
a multiple of some ancestral set.
Has been a major mechanism of evolution in plants.
Two ways polyploidy can occur:
Polyploid evolution of wheat
2) Genetic Recombination
(in sexual reproduction)
• = Natural, shuffling of existing genes,
occurring with meiosis and sexual
reproduction
• Two types:
– Independent Assortment
– Crossing over
Independent assortment
• Sorting of homologous chromosomes
independently of one another during meiosis
• E. g., (where A,B,&C genes are unlinked)
AaBBcc X AabbCC ---> AaBbCc
(one of many possibilities)
Independent assortment
Results in great variation of gametes, and
therefore progeny.
[E. g., one human:
223 = 8,388,608 possible types of gametes
(each with different combination of alleles).]
Crossing over
Exchange of chromatid segments of two
adjacent homologous chromosomes during
meiosis (prophase).
Greatly increases variability of gametes and,
therefore, of progeny.
Genetic Variation
• Genetic recombination - source of most
variation (in sexual organisms), via new allele
combinations.
• Mutation - ultimate source of variation, source
of new alleles and genes.
Fitness
• = measure of the relative contribution of a
given genotype to the next generation
• Can measure for individual or population.
Fitness
=
E. g.,
allele/genotype freq. in future generation
allele/genotype freq. in prev. generation
1st gen. 25%AA : 50%Aa : 25%aa
[freq. A = 25% + .5(50%) = 50%]
2nd gen.: 36%AA : 48%Aa : 16%aa
[freq. A = 36% + .5(48%) = 60%]
Fitness of A allele is 60/50 = 1.2; a is 40/50 = 0.8
Fitness of AA genotype is 36/25 = 1.44 , etc.
Hardy-Weinberg Equilibrium (1908)
• The frequency of a gene / allele does not change
over time (given certain conditions).
A,a = alleles of one gene, combine as AA, Aa, or aa
Generation 1: p = freq. A
pA
qa
pA
p2AA
pqAa
qa
pqAa
q2aa
q = freq. a
}
p2AA + 2pqAa + q2aa = 1
p + q = 1 (100%)
=gene frequencies
in generation 1
Hardy-Weinberg Equilibrium (1908)
Example:
Generation 1: p = 0.4 q = 0.6
0.4A
0.4A 0.16AA
0.6a 0.24Aa
0.6a
0.24Aa
0.36aa
}
p2AA + 2pqAa + q2aa =
0.16 + 0.48 + 0.36 = 1
p + q = 1 (100%)
=gene frequencies
in generation 1
Hardy-Weinberg Equilibrium (1908)
• The frequency of a gene / allele does not change
over time (given certain conditions).
What will be the frequency of alleles in the second
generation?
p2AA + 2pqAa + q2aa = 1
}
=gene frequencies
in generation 1
freq. A (generation 2) = (p2 + pq) / (p2 + 2pq + q2)
= p(p + q) / (p + q)2 = p / (p + q) = p
Therefore, freq. A = p; freq. a = q, same as in generation 1.
Hardy-Weinberg Equilibrium
• Maintained only if:
• 1) No mutation
Mutations rare, but do occur
(1 new mutation in 10,000 - 1,000,000 genes per
individual per generation)
Hardy-Weinberg Equilibrium
• 2) No migration (no gene flow into or out of
population)
But, can occur . . .
Hardy-Weinberg Equilibrium
• 3) Population size large
• Two things can disrupt:
– a) Population bottleneck (large pop. gets very small)
– b) Founder effect (one or a few individuals dispersed
from a large pop.)
Hardy-Weinberg Equilibrium
• 4) Mating is random
• But, most animals mate selectively, e.g.,
– 1) harem breeding (e. g., elephant seals);
– 2) assortative mating (like mates with like)
– 3) sexual selection
Hardy-Weinberg Equilibrium
• 5) All genotypes equally adaptive
(i.e., no selection)
• But, selection does occur . . .
If any conditions of Hardy-Weinberg not met:
• Genotype frequencies change
• Evolution occurs!
• Evolution = change in gene
frequency of a population over time.
Selective Pressure
• = agent or causative force that results in selection.
• E. g., for dark skin,
selective pressure = UV radiation
(UV increases sunburn and skin cancer in lighter
skinned individuals)
• E. g., for light skin,
selective pressure = Vitamin D synthesis
Genetic Drift
= change in genotype solely by chance effects
random!
promoted by:
Population Bottleneck -drastic reduction in
population size
Founder Effect - isolated colonies founded by
small no. individuals
Fig. 23-9
Population Bottleneck
Pre-bottleneck
(Illinois, 1820)
Post-bottleneck
(Illinois, 1993)
Range
of greater
prairie
chicken
(a)
Original
population
Bottlenecking
event
Fig. 23-10
Surviving
population
Founder Effect
Example: Silverswords in Hawaii
Tarweeds (Mainland)
Silverswords (Hawaiian Islands)
Summary: Evolution can occur
by two major mechanisms:
• Natural Selection (non-random)
• Genetic Drift (random)
Pepper Moth: Biston betularia
Selective pressure=predation by birds
Single gene:
AA/Aa = dark
aa = light
Camoflague
selected for!
Result: Balanced polymorphism
• E.g., Sickle Cell Anemia:
Mutation = single amino
acid subst. in beta chain of hemoglobin --> single a.a. difference.
• Sickle
blood cells
• Normal
blood cells
• Sickle Cell Anemia
• Homozygotes
for sickle
mutation
(HsHs):
lethal
• Sickle Cell Anemia
• Heterozygotes (HsHn):
resistant to malaria,
• selected for in malariainfested regions,
• selected against where
malaria not present.
General Principle:
• Selection dependent on the environment!
• If environmental conditions change,
selective pressure can change!!
Stabilizing selection
- selection against the two extremes in a
population (e.g., birth weight in humans,
clutch size in birds)
Directional selection
- selection for one extreme in a population,
against the other extreme
(e.g., pesticide resistance in insects
antibiotic resistance in bacteria)
Disruptive selection
- selection for the two extremes in a
population, against the average forms
(e.g., limpets w/ 2 color forms: light & dark
in mosaic environment; flies on two hosts:
apple & hawthorn)
Sexual Selection
• - selection resulting in greater reproductive
fitness in certain individuals of one sex
Sexual Selection
Intrasexual selection – within one sex;
competition between members of one sex
(usually males)
Sexual Selection
Intersexual selection – between two sexes;
preference by one sex for features of the
other sex. Usu. female choice.
Sexual Selection
Sexual Selection
• Balance between
survivorship (decreased)
reproductive potential (increased)
Sexual
Selection:
decreased
survivorship