lecture 2: evolution of populations campbell & reece chapters: chapter 23 microevolution –...
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Lecture 2: Evolution of Populations
Campbell & Reece chapters:
Chapter 23
Microevolution – evolution at the population level
= change in allele frequencies over generations
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Genetics
= science dealing with inheritance or heredity, the transmission of acquired traits
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Ultimate source of heritable variation is change in DNA
• Change in DNA caused by:
1) Mutation
2) Genetic Recombination
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Mutations= change in genotype other than by
recombination.
Three types:
1) Point Mutations
2) Chromosome Mutations
3) Change in Chromosome Number
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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
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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
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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)]
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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).
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b) PolyploidyEvolution of chromosome number which is
a multiple of some ancestral set.
Has been a major mechanism of evolution in plants.
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Two ways polyploidy can occur:
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Polyploid evolution of wheat
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Genetic Recombination(in sexual reproduction)
• = Natural, shuffling of existing genes, occurring with meiosis and sexual reproduction
• Two types:
– Independent Assortment
– Crossing over
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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)
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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
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Crossing over
Exchange of chromatid segments of two adjacent homologous chromosomes during meiosis (prophase).
Greatly increases variability of gametes and, therefore, of progeny.
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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.
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Fitness
• = measure of the relative contribution of a given genotype to the next generation
• Can measure for individual or population.
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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.
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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
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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
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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
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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)
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• 2) No migration (no gene flow into or out of population)
But, can occur . . .
Hardy-Weinberg Equilibrium
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• 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
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• 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
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• 5) All genotypes equally adaptive
(i.e., no selection)
• But, selection does occur . . .
Hardy-Weinberg Equilibrium
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If any conditions of Hardy-Weinberg not met:
• Genotype frequencies change
• Evolution occurs!
• Evolution = change in gene frequency of a population over time.
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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
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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
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Fig. 23-9
Originalpopulation
Bottleneckingevent
Survivingpopulation
Population Bottleneck
Fig. 23-10
Rangeof greaterprairiechicken
Pre-bottleneck(Illinois, 1820)
Post-bottleneck(Illinois, 1993)
(a)
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Summary: Evolution can occur by two major mechanisms:
• Natural Selection (non-random)
• Genetic Drift (random)
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Pepper Moth: Biston betulariaSelective pressure=predation by birds
Single gene:
AA/Aa = dark
aa = light
Camoflague selected for!
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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
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• Homozygotes for sickle mutation (HsHs):
lethal
• Sickle Cell Anemia
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• Sickle Cell Anemia• Heterozygotes (HsHn):
resistant to malaria,
• selected for in malaria-infested regions,
• selected against where malaria not present.
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General Principle:
• Selection dependent on the environment!
• If environmental conditions change, selective pressure can change!!
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Stabilizing selection - selection against the two extremes in a population (e.g., birth weight in humans,
clutch size in birds)
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Directional selection - selection for one extreme in a population, against the other extreme (e.g., pesticide resistance in insects
antibiotic resistance in bacteria)
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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)
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Sexual Selection
• - selection resulting in greater reproductive fitness in certain individuals of one sex
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Sexual SelectionIntrasexual selection – within one sex;
competition between members of one sex (usually males)
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Sexual SelectionIntersexual selection – between two sexes;
preference by one sex for features of the other sex. Usu. female choice.
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Sexual Selection
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Sexual Selection
• Balance between
survivorship (decreased)
reproductive potential (increased)
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Sexual Selection:
decreased survivorship