Biology 1201A Study Guide - Final Guide: Decomposer, Parallel Evolution, Soltyrei

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13 Apr 2012
Biology 1201A 1
Biology 1201A- Final Exam
Upsetting the Equilibrium
Population Genetics
Gene Pool: the sum of all alleles at all gene loci in all individuals in a population
o Genotype frequencies: percentages of people having each genotype
o Knowing that each diploid organism has 2 alleles at each gene locus, they can then
calculate allele frequencies, the relative abundances of the different alleles
Hardy-Weinberg Principle: specifies the conditions under which a population of diploid
organisms achieves genetic equilibrium (the point at which neither allele frequencies nor
genotype frequencies change in succeeding generations
According to the model, genetic equilibrium is possible only if all the following conditions are
1. No mutations are occurring
2. The population is closed to migration from other populations
3. The population is infinite in size
4. All the genotypes in the population survive and reproduce equally well
5. Individuals in the population mate randomly with respect to genotypes
Genetic variation can occur:
o Within individuals: proportion of heterozygous loci within an individual
o Within populations: number of polymorphic loci in the gene pool of a population
o Between populations: degree of difference in alleles, allele frequencies
The Agents of Microevolution
Mutation: a heritable change in DNA
o So much variation is possible even though mutations are rare because some genes
(homeotic) regulate the expression of other genes (on/off switch)
Mutation is a major source of heritable variation, and have been accumulating in biological
lineages for billions of years (slow mechanism of evolution)
o Deleterious: alter structure, function or behaviour
o Lethal: cause death in organism
o Neutral: neither harmful nor helpful
o Advantageous: natural selection may preserve the allele
Modes of selection:
o Directional selection: individuals near one end of the phenotypic spectrum have the
highest relative fitness (shifts a trait away from the existing mean to an extreme)
o Stabilizing selection: individuals having intermediate phenotypes have the highest
fitness, eliminating extremes (reduces genotypic and phenotypic variation)
o Disruptive selection: extreme phenotypes have higher fitness, promoting
Selection and genetic variation
o Negative frequency-dependent selection: being the rare phenotype is an advantage
o Positive frequency-dependent selection: being the common phenotype is an
o Overdominance: heterozygotes have higher fitness than either homozygote
o Underdominance: both homozygotes have higher fitness than the heterozygote
Tends to erode genetic variation (an allele is lost while the other goes to
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Biology 1201A 2
o Neutral variations: remain in the population because there is no fitness impact
Gene flow: organisms sometimes migrate, reproduce, and may introduce novel alleles into the
population they have joined
o Migration: movement of alleles from one population to another tends to equalize
allele frequencies among populations, maintains genetic variation within populations
o Non-random mating: individuals select mates based on phenotype. Disrupts genotype
frequencies, but not allele frequencies
Assortative mating: like mates with like inbreeding, homozygousity
Disassortative mating: opposites attract, greater heterozygousity
Genetic drift: chance events sometimes cause allele frequencies in a population to change
unpredictably More pronounced in smaller populations than in larger ones
o Population bottleneck: genetic drift, a stressful factor such as disease kills a great deal
of the population, and eliminates some alleles from the population
o Founder effect: a change in a gene pool, a few individuals colonize elsewhere, carrying
only some genes from the parent population. By chance, some alleles may be totally
missing, or the genes that were rare back home could be present
Conservation implications: endangered species experience severe bottlenecks, also loss of
genetic variability, so no matter how large the population size, will still be susceptible to
Relative fitness: the number of surviving offspring that an individual produces compared with
the number left by others in the population
Maintaining Genetic and Phenotypic Variation
Diploidy can hide recessive alleles from the action of natural selection (eliminating harmful
recessive alleles)
Balanced polymorphism: two or more phenotypes are maintained in fairly stable proportions
over many generations. Preserved when:
o Heterozygotes have higher fitness
o Different alleles are favoured in different environments
o Rarity of a phenotype provides an advantage: frequency-dependent selection
Neutral variation hypothesis: some of the genetic variation at loci coding for enzymes and
other soluble proteins is selectively neutral
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Biology 1201A 3
Sexual Selection
Sexual selection: fostered the evolution of showy structures
o Intersexual selection: males produce otherwise useless structures because females
found them attractive (leads to elaborate courtship displays, ornamentation)
o Intrasexual selection: males use their large body size to compete against rival males
(such as through direct control of females, infanticide, control of a resource important
to the female)
Advantages of sexual selection:
o Reversing Muller’s ratchet harmful mutations occur over time, recombination
generates offspring with more, same, fewer mutations than parent
o The Lottery Model sexual reproduction increases the likelihood that some offspring
will be appropriately equipped to survive
Disadvantages of sexual selection:
o Can’t reproduce without a male
o Energy/time wasted on mating rituals
o Only pass on half of genes (meiotic cost)
o Room for error
Sexual dimorphism: differences in size/appearance of males and females pushes phenotype
towards an extreme
Gender differences- why are females choosier?
o Mothers make a larger parental investment in offspring than fathers (male fitness
limited by mates, females are motivated by quality, not quantity)
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