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Chapter 15
Evolution on a Small Scale
15.1 Natural Selection
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Process resulting in adaptation of a population to the biotic (living) and abiotic (nonliving)
environments
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Darwin’s mechanism for evolution
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Most fit individuals become more prevalent in a population
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Leads to change in population over time
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Most fit individuals reproduce more than others because they are better adapted.
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Types of selection
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Most traits are polygenic and are controlled by more than one pair of alleles located at
different gene loci.
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Such traits have a range of phenotypes resembling a bell-shaped curve.
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Directional selection
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Stabilizing selection
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Disruptive selection
Directional selection
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Occurs when an extreme phenotype is favored
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Distribution curve shifts in that direction
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Can occur when a population is adapting to a changing environment
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Industrial melanism
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Drug resistance in bacteria
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Pesticide resistance in insects
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Malaria—Plasmodium becoming resistant to chloroquine and mosquitoes resistant to
DDT
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Equus adapting from forest conditions to grassland conditions
Stabilizing selection
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Occurs when an intermediate phenotype is favored
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Extreme phenotypes selected against
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Individuals near the average are favored
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Most common form of selection because the average individual is well adapted to its
environment
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Swiss starlings lay 4–5 eggs because this has the highest survival rate for young
Disruptive selection
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2 or more extreme phenotypes are favored over any intermediate phenotype
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British land snails are found in fields and forests
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In fields, thrushes eat the snails with dark shells that lack light bands
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In forests, thrushes feeds mainly on snails with light-banded shells
Adaptations are not perfect
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Natural selection does not produce perfectly adapted organisms.
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Evolution is constrained by the available variations.
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Imperfections are common because of necessary compromises.
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Success of humans attributed to freeing hands but walking upright puts stress
on the spine
Maintenance of variations
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A population always shows some genotypic variations.
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Population with limited variation may not be able to adapt to changing environmental
conditions
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Forces promoting variation constantly at work
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Mutations, recombination, independent assortment, and fertilization create
new combinations
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Gene flow
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Natural selection favors certain phenotypes but other remain
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Diploidy and the heterozygote
The heterozygote advantage
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Only alleles that are expressed are subject to natural selection.
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Expressed = cause phenotypic differences
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Heterozygotes can protect recessive alleles.
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Recessive allele might have greater fitness in a changing environment
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Balanced polymorphism—when natural selection favors the ratio of two or more
phenotypes in generation after generation
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Sickle-cell disease
Sickle-cell disease
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Individuals with sickle cell disease HbSHBS
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Tend to die early
Heterozygotes carry the trait HbAHBS
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Red blood cells only sickle at low oxygen concentrations
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Ordinarily the normal genotype is most fit HbAHBA
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Recessive allele HbS has a higher frequency in regions in Africa where malaria is present
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Malaria is caused by parasite that invades and destroys normal red blood cells
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Parasite unable to live in heterozygote red blood cells
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Each of the homozygotes is selected against but is maintained because the heterozygote
is favored in those parts of Africa
15.2 Microevolution
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Individuals do not evolve.
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As evolution occurs, genetic and phenotypic changes occur within a population.
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A population is all the members of a single species occupying a particular area at the same time
and reproducing with one another.
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Microevolution—small measurable evolutionary changes within a population from generation to
generation
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Darwin stressed that members of a population vary.
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Each gene in sexually reproducing organisms has many alleles.
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Reshuffling of alleles during sexual reproduction can result in a range of phenotypes.
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Evolution in a genetic context
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Gene pool—the various alleles at all the gene loci in all individuals of a population
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Described in terms of genotype and allele frequency
Peppered moth color example
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D = dark color d = light color
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From the genotype frequencies, you can calculate the allele frequencies of a
population.
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Assuming random mating, we can use allele frequencies (gamete frequencies) to
calculate the ratio of genotypes in the next generation using a Punnett square.
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Allele frequencies remain the same—sexual reproduction alone does not bring about a
change in allele frequencies
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Dominance does not cause an allele to become a common allele.
G. H. Hardy and W. Weinberg used the binomial equation to calculate the genotypic and allele
frequencies of a population.
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p = frequency of dominant allele
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q = frequency of recessive allele
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p2 + 2pq + q2 = 1
Hardy-Weinberg principle states that an equilibrium of allele frequencies in a gene pool will
remain in equilibrium as long as 5 conditions are met
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No mutations
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No gene flow
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Random mating
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No genetic drift
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No selection
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These conditions are rarely if ever met.
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Allele frequencies do change from one generation to the next.
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Therefore microevolution occurs
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Hardy-Weinberg equation is significant because it tells us what factors cause evolution
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Evolution can be detected and measured by noting the amount of deviation from a HardyWeinberg equilibrium of allele frequencies in the gene pool of a population.
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Industrial melanism example
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Increase in the frequency of a dark phenotype due to pollution
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Before soot darkened tree trunks, light moths escaped detection of birds and were
more common.
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After the advent of industry, dark-colored moths became more common as light moths
were detected and eaten.
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Natural selection can occur within a short time frame.
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Change in gene pool frequencies occurs as microevolution occurs
Causes of microevolution
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Any condition that deviates from the list of conditions for allelelic equilibrium causes
evolutionary change
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Genetic mutation
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Gene flow
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Nonrandom mating
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Genetic drift
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Natural selection
Genetic mutations
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Ultimate source for allele differences
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Without mutation there would be no new variations among members of a population
for natural selection to act on
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Adaptive value of mutation depends on current conditions
2. Gene flow
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Also called gene migration
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Movement of alleles among populations by migration of breeding individuals
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Can increase variation within a population by introducing novel alleles from another
population
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Continued gene flow reduces differences among populations—can prevent speciation
3. Nonrandom mating
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Selection of mate according to genotype or phenotype (not chance)
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Assortative mating—tend to mate with individuals with the same phenotype
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Homozygotes increase in frequency
Sexual selection—favors characteristics that increase the likelihood of obtaining mates
4. Genetic drift
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Refers to changes in the allele frequencies of a gene pool due to chance
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Allele frequencies “drift” over time depending on which members die, survive, or
reproduce
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More likely in small populations
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More likely to lose rare alleles
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Two types
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Bottleneck effect
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Founder effect
Bottleneck effect
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Species suffers a near extinction and only a few survivors go on to produce the next
generation
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Cheetahs—extreme similarity
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Infertility due to inbreeding
Founder effect
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Rare alleles occur at a higher frequency in a population isolated from the general
population.
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Alleles carried by founders are dictated by chance alone.
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Amish—1 in 14 carries recessive allele for unusual form of dwarfism compared to 1 in
1000 in most populations