Ch 23 Evolution Of Populations

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Microevolution

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change in allele frequencies in a population over generations
organisms are provided with traits to adapt and survive--continue species

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What are the two processes that make evolution possible?

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Mutation and sexual reproduction produce the variation in gene pools that contribute to differences among individuals

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Variation. Discrete characters

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Produced by a single gene locus with different alleles that produce distinct phenotypes
Mendel's purple or white flowers in pea plants

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locus

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location of alleles of a gene on chromosomes

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Variation. Quantitative characters

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very within population. Skin color, height.
Due to the additive effect( two or more genes on a single phenotypic character)

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polymorphisms

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genetic variation

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How do population geneticists measure polymorphisms

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determine the amount of heterozygosity at the gene and molecular levels
Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals

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Average heterozygosity

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measures the average percent of loci that are heterozygous in a population

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Geographic variation
ex. cline

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differences between gene pools of separate populations or population subgroups
cline- a graded changed in a trait along a geographic axis

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Mutations. what? cause? what kind can be passed off to offspring?

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changes in the nucleotide sequence of DNA
Cause new genes and alleles to arise
Only mutations in cells that produce gametes can be passed to offspring

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Point Mutations
what? effect?

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Change in one base in a gene
In noncoding regions of DNA are often harmless. May not affect protein production because of redundancy in the genetic code

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Point Mutations effect

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Point mutations that result in change in protein production are often harmful, or can increase the fit between organism and environment

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Chromosomal mutations

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delete, disrupt, or rearrange many loci are typically harmful

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Mutations: Duplication

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Duplication of large chromosome segments are typically harmful
Duplication of small pieces of DNA sometimes less harmful and increases genome size
Duplicated genes can take on new functions by further mutations

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Why are mutation rates lower in prokaryotes and higher in viruses?

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Some viruses have an RNA genome(higher mutation rate of a typical DNA)
BECAUSE
the lack of RNA repair mechanisms in host cells.

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Why are mutation rates higher in prokaryotes and viruses than in other organisms?

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Because both of them have short generation spans so mutations can quickly generate genetic variation

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Sexual Reproduction

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can shuffle existing alleles into new combinations

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for organisms that produce sexually, whats more important for making adaption possible?

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recombination of alleles is more important than mutation in producing genetic differences that make adaption possible

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The Hardy-Weinberg Equation

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used to test whether a population is evolving

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Population

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localized group of individuals capable of interbreeding producing fertile offspring

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Gene Pool

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consists of all the alleles for all loci(locations of alleles of a gene on chromosomes) in a population

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Allele(or locus) is fixed in a gene pool if

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If only 1 allele exists for a particular locus in a population, that allele is said to be fixed in the gene pool and all individuals are homozygous for that allele

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If there are 2 or more alleles for a particular locus in a population

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individuals may be either homozygous or heterozygous

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The Hardy-Weinberg Principle
worksheet

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Describes a population that is not evolving. Frequencies of alleles and genotypes in a population remain constant from generation to generation
IF
1. Meiotic crossing over & independent assortment cause recombination of alleles.
2. Random gametes recombination at fertilization is at work. 80%(Cr allele) 20% (Cw allele)

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The Hardy-Weinberg Equilibrium
worksheet

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describes the constant frequency of alleles in such a gene pool. If p and q represent the relative frequencies of the only two possible alleles in population at a particular locus then
p2+2pq+q2=1
p2 and q2 represent frequencies of homozygous genotypes and 2pq is heterozygous genotype

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5 Conditions for Hardy-Weinberg Equilibrium, nonevolving populations

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rarely met in nature
1. no mutations
2. random mating
3. no natural selection
4. extremely large population size
5. No gene flow

Natural populations can evolve at some loci, while being in hardy-weinberg equilibrium at other loci

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How can we assume the locus that causes PKU is in hardy-weinberg equilibrium?

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PKU- metabolic disorder that results from homozygosity at a recessive allele.
-PKU gene mutation rate is low. --Mate selection is random wrt whether or not an individual is a carrier for the PKU allele
-Natural Selection can only act on rare homozygous individuals who no not follow dietary restrictions
-The population is large
-Migration has no effect as many other populations have similar allele frequencies

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Three major factors that alter allele frequencies and bring about most evolutionary change

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1. Natural Selection
2. Genetic Drift
3. Gene Flow

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Natural Selection

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differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

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Genetic Drift

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describes how allele frequencies fluctuate unpredictably from one generation to the next
-the smaller the sample the greater the chance of deviation from a prediction result

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Does genetic drift reduce or increase genetic variation? why?

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It tends to reduce genetic variation through losses of alleles
refer to slide pictures

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The Founder Effect

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Example of Genetic Drift.
Occurs when a few individuals become isolated from a larger population
Allele frequencies in the small founder population can be different from those in larger parent population

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The Bottleneck Effect

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Example of Genetic Drift
A sudden reduction in population size due to a change in the environment.
-the resulting gene pool may no longer be reflective of the original population's gene pool
-if the population remains small it may be further affected by the genetic drift.
-understanding of how human activity affects other species

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Impact of Genetic Drift on the Greater Prairie Chicken

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Loss of prairie habitat caused a severe reduction in the population of greater prairie chickens in IL
Surviving birds had low levels of genetic variation, and only 50% of their eggs hatched
compared DNA to compare genetic variation in the population before and after the bottleneck. Loss of alleles at several loci. Introduced new alleles and increased egg hatch rate to 90%

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4 Effects of Genetic Drift

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1. Significant in small populations
2. causes allele frequencies to change at random
3. lead to a loss of genetic variation within populations
4. cause harmful alleles to become fixed

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Gene Flow
how can alleles be transferred?
what does this reduce?

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consists of the movement of alleles among populations
- alleles can be transferred through movement of fertile individuals or gametes(pollen)
-tends to reduce differences between populations over time

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Is gene flow or mutation more likely to alter allele frequencies directly?

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Gene Flow

The migration of people throughout the world has increased gene flow between populations that were once isolated from each other.
-can homogenize the gene pools of such populations thereby reducing geographic variation in appearance. TIME magazine

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How can gene flow decrease the fitness of a population?

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- In grass, alleles for copper tolerance are beneficial in populations near copper ines, but harmful to populations in other soils
-windblown pollen moves these alleles between populations
-Movement of unfavorable alleles into a populations results in a decrease in fit between organism and environment

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How can gene flow increase the fitness of a population?

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-Insecticides have been used to target mozzies that carry west nile virus and malaria
-alleles have evolved in some populations that confer insecticide resistance to these mosquitoes
-The flow of insecticide resistance alleles into a population can cause an increase in fitness

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What is the only mechanism that consistently causes adaptive evolution?

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Natural Selection
-brings about adaptive evolution by acting on an organism's phenotype

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Relative Fitness

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-Reproductive success
-measured in terms of the ability of an individual to leave more viable offspring than others of its species
-selection favors certain genotypes by acting on the phenotypes of certain organisms

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Directional Selection

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favors individuals at one end of the phenotypic range

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Disruptive Selection

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favors individuals at both extremes of phenotypic range

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Stabilizing Selection

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favors intermediate variants and acts against extreme phenotypes

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What is the key role of Natural Selection in Adaptive Evolution?

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Natural selection increases the frequencies of alleles that enhance survival and reproduction
Adaptive evolution occurs as the match between organisms and its environment increases

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Why is adaptive evolution a continuous process?
Why don't gene flow and genetic drift consistently lead to adaptive evolution?

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because the environment can change
Gene flow and genetic drift do not consistently lead to adaptive evolution as they can increase or decrease the match between organism and its environment

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Sexual Selection

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is natural selection for mating success. Can result in sexual dimorphism(marked differences between the sexes in secondary sexual characteristics)

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sexual dimorphism

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marked differences between the sexes in secondary sexual characteristics(features that distinguish the two sexes of a species, but that are not directly part of the reproductive system)

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secondary sexual characteristics

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features that distinguish the two sexes of a species, but that are not directly part of the reproductive system

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Intrasexual selection

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competition among individuals of one sex(often males) for mates of the opposite sex

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Intersexual selection
-male showiness

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often called mate choice
occurs when individuals of one sex(usually females) are choosy in selecting their mates
-male showiness due to mate choice can increase a male's chance of attracting a female, while decreasing his chances of survival

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Diploidy

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maintains genetic variation in the form of hidden recessive alleles

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Balancing selection

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occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population

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Heterozygote Advantage

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-occurs when heterozygotes have a higher fitness than do both homozygotes
-natural selection will tend to maintain two or more alleles at that locus when heterozygotes have survival & reproductive advantage
-The sickle-cell allele causes mutations in hemoglobin but also confers malaria resistance

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Frequency-Dependent Selection

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the fitness of a phenotype declines if it becomes too common in the population
Selection can favor whichever phenotype is less common in a population

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Neutral Variation

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Genetic variation that appears to conger no selective advantage or disadvantage
-variation in noncoding regions of DNA
-Variation in proteins that have little effect on protein function or reproductive fitness

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Why can't Natural Selection Fashion Perfect Organisms? (4)

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1. Selection can act only on existing variations
2. Evolution is limited by historical constraints
3. Adaptations are often compromises
4. Chance, natural selection, and the environment interact