Lecture 2: Evolution of Populations

Campbell & Reece 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

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.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:E.g., human hemoglobin:E.g., human hemoglobin: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 MutationsRearrangements (including losses and gains)

of large pieces of DNA. E.g., inversion:

Re-attaches here and here

A B A B C D E F C D E F GG

A B A B F E D C F E D C GG

[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) PolyploidyEvolution 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

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)

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).]

Independent assortment

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= allele/genotype freq. in future generation

allele/genotype freq. in prev. generation

E. g., 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 q = freq. a p + q = 1 (100%)

pA qapA p2AA pqAaqa pqAa q2aa

} =gene frequenciesin generation 1

p2AA + 2pqAa + q2aa = 1

Hardy-Weinberg Equilibrium (1908)

Example:

Generation 1: p = 0.4 q = 0.6 p + q = 1 (100%)

0.4A 0.6a0.4A 0.16AA 0.24Aa0.6a 0.24Aa 0.36aa

} =gene frequenciesin generation 1

p2AA + 2pqAa + q2aa = 0.16 + 0.48 + 0.36 = 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

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.

} =gene frequenciesin 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)

• 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 . . .

Hardy-Weinberg Equilibrium

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 effectsrandom!

promoted by:

Population Bottleneck -drastic reduction in population size

Founder Effect - isolated colonies founded by small no. individuals

Fig. 23-9

Originalpopulation

Bottleneckingevent

Survivingpopulation

Population Bottleneck

Fig. 23-10

Rangeof greaterprairiechicken

Pre-bottleneck(Illinois, 1820)

Post-bottleneck(Illinois, 1993)

(a)

Summary: Evolution can occur by two major mechanisms:

• Natural Selection (non-random)

• Genetic Drift (random)

Pepper Moth: Biston betulariaSelective 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

• Homozygotes for sickle mutation (HsHs):

lethal

• Sickle Cell Anemia

• Sickle Cell Anemia• Heterozygotes (HsHn):

resistant to malaria,

• selected for in malaria-infested 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 SelectionIntrasexual selection – within one sex;

competition between members of one sex (usually males)

Sexual SelectionIntersexual 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