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Chapter 6

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BIOL 2400
Cortland Griswold

Chapter 6: The Ways of Change 6.1 The Genetics of Populations  Because diploid organisms carry two copies of each autosomal chromosome, they can have as many as two alleles for each gene or locus. Individuals carrying two copies of the same allele are homozygous at that locus, whereas individuals carrying two different alleles are heterozygous for the locus  Genetic locus – the specific location of a gene or piece of DNA sequence on a chromosome. When mutations modify the sequence at a locus, they generate new alleles – variants of a particular gene or DNA region. Alleles are mutually exclusive alternative states for a genetic locus.  Populations contain a mixture of individuals, each with a unique genotype reflecting the alleles that they carry at all of their genetic loci. At any time, some alleles will be more common than others. Evolution is defined as “any change in the frequency of alleles from one generation to the next.”  Population genetics – the study of the distribution of alleles within populations and the mechanisms that can cause allele frequencies to change over time 6.2 Change over Time – or Not  The Hardy-Weinberg theorem proves that in the absence of drift, selection, migration, and mutation, allele frequencies at a genetic locus will not change from one generation to the next  Theorem – a mathematical statement that has been proven based on previously established theorems and axioms. Theorems use deductive reasoning and show that a statement necessarily follows from a series of statements or hypotheses – the proof. Theorems are not the same as theories. Theories are explanations supported by substantial empirical evidence – the explanations are necessarily tentative but weighted by the quantity of evidence that supports them  For any measured set of allele frequency, the Hardy-Weinberg theorem predicts the genotype frequencies expected for a population that is not evolving  Mechanisms of evolution are processes that can change allele frequencies in a population from one generation to the next 6.3 Evolution’s “Null Model”  Because it describes the conditions in which evolution will not occur, the Hardy-Weinberg theorem serves as the fundamental null model of population genetics 6.4 A Random Sample  biologist Peter Buri (1950s) bred flies with 2 different alleles bw and bw^75:  bw/bw^75 - light orange eyes  bw/bw - white eyes  bw^75/bw^75 - bright red eyes  he established 107 populations of bw/bw^75 flies, then randomly selected 8 flies from each (4 male, 4 female) and let them breed randomly for 19 generations (each generation starting w/ 8 randomly selected flies)  all assumptions for a population in Hardy-Weinberg equilibrium had been met:  no selection  no migration  probability of mutations occurring at that locus during the short duration of the experiment was small  the alleles should have stayed even; half bw, half bw^75  instead, bw became extremely rare until it disappeared completely; only bw^75/bw^75 flies were present; in other populations bw^75 disappeared completely leaving only flies with white eyes (bw/bw)  these populations of flies evolved through genetic drift (results purely down to chance)  genetic drift occurs when a random, non representative sample from a population produces the next generation  it eliminates alleles faster in smaller populations than it does in larger ones  fixed allele: an allele becomes fixed in a population when all of the alternative alleles have disappeared. No genetic variation exists at a fixed locus within a population, because all individuals are genetically identical at that locus  genetic drift tends to rob populations of their genetic variation  genetic drift is a mechanism of evolution because it causes the allelic composition of a population to change from generation to generation 6.5 Bottlenecks and Founder Effects  northern elephant seals experienced a population bottleneck in the 1800s when their numbers shrank to about 30. They lost much of their genetic diversity, which has not increased much as their population has expanded.  genetic bottlenecks: events in which the number of individuals in a population is reduced drastically. Even if this dip in numbers is temporary, it can have lasting effects on the genetic variation of a population  alleles can be lost during bottlenecks  the rarer an allele is before a bottleneck, the less likely it is to make it through  new alleles only result from mutations(rare) or gene flow from another population  southern elephant seals did not experience huge population losses (they therefore have a higher amount of original allelic variation than northern populations)  special bottleneck type: a small number of individuals leave a larger population and colonize a new habitat  founder effect: a type of genetic drift describing the loss of allelic variation that accompanies founding a new population from a very small number of individuals (a small sample of a much larger source population). This effect can cause the new population to differ considerably from the source population 6.6 Definitions Fitness: The success of an organism at surviving and reproducing, and thus contributing offspring to future generations. Relative Fitness (of a genotype): The success of the genotype at producing new individuals (its fitness) standardized by the success of other genotypes in the population (for example, divided by the average fitness of the population). Average Excess of Fitness (of an allele): The difference between the average fitness of individuals bearing the allele and the average fitness of the population as a whole. Pleiotropy: The condition when a mutation in a single gene affects the expression of many different phenotypic traits. Pleiotropy is considered to antagonistic is a mutation with beneficial effects for one trait also causes detrimental effects on other traits. Negative Selection: Selection that decreases the frequency of alleles within a population. Negative selection occurs whenever the average excess for fitness of an allele is greater than zero. Positive Selection: Selection that increases the frequency of alleles within a population. Positive selection occurs whenever the average excess for fitness of an allele is greater than zero. Epistasis: Occurs when the effects of an allele at one genetic locus are modified by alleles at one or more loci. Additive Allele: An allele that yields twice the phenotypic effect when two copies are present at a given locus than when only a single copy is present. Additive alleles are not influenced by the presence of other alleles (e.g, there is no dominance). Will always increase in a population steadily from the moment they arise until they are fixed in the population. Negative Frequency-Dependent Selections: Rare genotypes have higher fitness than common genotypes. This process can maintain genetic variation within populations. Balancing Selection: Selection that favors more than one allele. It acts to maintain genetic diversity in a population by keeping alleles at frequencies higher than would be expected by chance or mutation alone. Antagonistic Pleiotropy: A form of pleiotropy in which the effects of a mutation have the opposite effects on fitness.  Natural selection arises whenever:  Individuals vary in the expression of their phenotype  This variation causes some individuals to perform better than others  Selection can drive large-scale evolutionary change, allowing new adaptations to arise.  Occurs when individuals vary in their fitness  The best way to measure fitness is by tallying the lifetime reproductive contribution of an individual, then noting how many of the offspring manage to survive to reproductive age themselves.  Hard to accomplish in practice  Scientists sometime measure probability of reaching reproduction age.  e.g.) Measure # of offsprings produced in a specific season  Measuring Selection compares fitness measures for many different individuals  Relating variation in fitness with variation in expression of a phenotype  The fitness of an organism is the product of its entire phenotype  Population geneticists study fitness by focusing on the evolution of alleles at a genetic locus, instead of entire phenotype  Distill all variable components, such as survival, mating success, and fecundity, into a single value, called w 
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