Chapter 23 Notes
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Date Created: 02/12/16
Biology Chapter 23 Notes The Smallest Unit of Evolution o A common misconception is that organisms evolve during their lifetimes o Natural selection acts on individuals, but only populations evolve For ample, a population of medium ground finches of Daphne Major Island During a drought, large-beaked birds were more likely to crack large seeds and survive The finch population evolved by natural selection Microevolution is a change in allele frequencies in a population over generations o Three main mechanisms cause allele frequency change: Natural selection Genetic drift Gene flow o Only natural selection consistently causes adaptive evolution Concept 23.1: Genetic variation makes evolution possible o Variation in heritable traits is a prerequisite for evolution Mendel’s work on pea plants provided evidence of discrete heritable units (genes) o Individuals within a species vary in their specific characteristics Phenotypic variation (blood type, etc.) o Phenotypic variations reflect genetic variation, differences among individuals in the composition of their genes or other DNA sequences Example: plant color determined by single gene locus, with different alleles producing different o Genetic variation among individuals is caused by differences in genes or other DNA segments Phenotype is the product of inherited genotype and environmental influences Natural selection can only act on variation with a genetic component Some phenotypic differences are determined by a single gene and can be classified on an either-or- basis Other phenotypic differences are determined by the influence of two or more genes and vary long a continuum with a population Genetic variation can be measured as gene variability or nucleotide variability Genetic variation at the whole-gene (gene variability) can be quantified as the average percentage of loci that are heterozygous Nucleotide variability is measured by comparing the DNA sequences of pairs of individuals Nucleotide variation rarely results in phenotypic variation Some phenotypic variation does not result from genetic differences among individuals, but rather from environmental influences Only genetically determined variation can have evolutionary consequences o Sources of Genetic Variation Formation of new alleles New alleles can arise by mutation, a change in the nucleotide sequence of an organism’s DNA Only mutations in cells that produce gametes can be passed to offspring A little a change as a point mutation (one base in a gene) can have a significant impact on a phenotype (sickle-cell disease) o Mutations that result in a change in protein production are often harmful Harmful mutations can be hidden from selection in recessive alleles Mutations that result in a change in protein production can sometimes be beneficial o Point mutations in noncoding regions generally result in neutral variation, difference in DNA sequence that do not confer a selective advantage or disadvantage Mutations to genes can be neutral because of the redundancy in the genetic code Altering gene number or position Chromosomal changes that delete, disrupt, or rearrange many loci are usually harmful Duplication of small pieces of DNA increases genome size and is usually less harmful A key potential source of variation is the duplication of genes due to errors in meiosis (unequal crossing over) Rapid reproduction Mutation rates are low in animals and plants Mutation rates are often lower in prokaryotes and higher in viruses o Mutations accumulate quickly in prokaryotes and viruses because they have short generation times Sexual reproduction Most of the genetic variation in a population results from the unique combination of alleles that each individual receives from its parents In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving o Gene Pools and Allele Frequencies A population is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring A gene pool consists of all the alleles for all loci in a population A locus is fixed if all individuals in a population are homozygous for the same allele If there are two or more alleles for a locus, diploid individuals may be either homozygous or heterozygous o The Hardy-Weinberg Equation The Hardy-Weinberg equation describes the genetic makeup we expect for a population that is NOT evolving at a particular locus If the observed genetic makeup of the population differs from expectations under Hardy-Weinberg, it suggests that this locus may be undergoing evolutionary change in the population Change in allele frequencies from one generation to the next If a population exists where gamete contribute to the next generation randomly and Mendelian inheritance occurs, and allele an genotype frequencies at a locus remained constant from generation to generation, such a population is in Hardy-Weinberg Equilibrium A population is said to be in Hardy-Weinberg Equilibrium when the population is not evolving and allele and genotype frequencies will remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work 2 2 P + 2pq + q o If p and q represent the relative frequencies of the only two possible alleles in a population at aparticular l2cus, then: 2 2 2 o P + 2pq + q = 1 where p and q represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype Conditions for Hardy-Weinberg Equilibrium The Hardy-Weinberg theorem describes a hypothetical population that is not evolving at a particular locus In real populations, allele and genotype frequencies do change over time The five conditions for nonevolving populations are rarely met in nature o No mutations o Random mating o No natural selection o Extremely large population size o No gene flow H-W equilibrium only is about a particular locus Applying the Hardy-Weinberg Equation We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that o The PKU gene mutation rate is low o Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele o Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions o The population is large o Migration has no effect as many other populations have similar allele frequencies Concept 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in population o Three major factors alter allele frequencies and bring about most evolutionary change in populations: Natural selection Genetic drift Gene flow (Mutation itself can also cause microevolutionary change in a population be introducing new alleles) o Natural selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions For example, an allele that confers resistance to DDT in fruit flies increased in frequency after DDT was used widely in agriculture Natural selection can cause adaptive evolution, an improvement in the match between organisms and their environment o Genetic drift is the evolutionary change due to sampling bias The smaller the sample, the greater the chance of random deviation from a predicted result Genetic drift describes how allele frequencies fluctuate unpredictably from one generation to the other Genetic drift tends to reduce genetic variation through losses of alleles The Founder Effect – an example of genetic drift The founder effects occurs when a few individuals become isolated from a larger population Allele frequencies in a small founder population can be different from those in the larger parent population The Bottleneck Effect – another type of genetic drift The bottleneck effect is 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 genetic drift Understanding the bottleneck effect can increase understanding of how human activity affects other species Case Study: Impact of Genetic Drift on the Greater Prairie Chicken Loss of prairie habitat caused a severe reduction in the population of greater prairie chicken in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Researchers used DNA from museum specimens to compare genetic variation in the population before and after the bottleneck The results showed a loss of alleles at several loci Researchers introduced greater prairie chickens from populations in other states and were successful in introducing new alleles and increasing the egg hatch rate to 90% o Gene flow consist of the movement of alleles among populations Alleles can be transferred through the movement of fertile individuals or gametes (for example, pollen) Gene flow tends to reduce variation among populations over time Gene flow can decrease the fitness of a population Consider, for example, the great tit (Parus major) on the Dutch island of Vlieland Mating causes gene flow between the central and eastern populations Immigration from the mainland introduces alleles that decrease fitness on the island Natural selection removes alleles that decrease fitness Birds born in the central region with high immigration have a lower fitness; birds born in the east with low immigration have a higher fitness Gene flow can increase the fitness of a population Consider, for example, the spread of alleles for resistance to insecticides Insecticides have been used to target mosquitoes that carry West Nile virus and malaria Alleles have evolved in some populations that confer insecticide resistance to those mosquitos The flow of insecticide resistance alleles into a population can cause an increase in fitness Gene flow is an important agent of evolutionary change in modern human populations Concept 23.4: Natural selection is the only mechanism that consistently causes adaptive evolution o Evolution by natural selection involves both chance and “sorting” New genetic variations arise by chance Beneficial alleles are “sorted” and favored by natural selection o Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage o Natural selection brings about adaptive evolution by acting on an organism’s phenotype o Relative fitness is the contribution an individual makes to the gene pool of the next generation, relative to the contributions of other individuals Selection favors certain genotypes by acting on the phenotypes of individuals There are three main modes of selection on phenotypes Directional selection favors individuals at one extreme end of the phenotypic range Disruptive selection favors individuals at both extremes of the phenotypic range Stabilizing selection favors intermediate variants and acts against extreme phenotypes o Natural selection increases the frequencies of alleles that enhance survival and reproduction Adaptive evolution occurs as the match between a species and its environment increases Because the environment, adaptive evolution is a continuous process o Genetic drift and gene flow do not consistently lead to adaptive evolution as they can increase or decrease the match between an organism and its environment o Sexual selection is natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Intrasexual selection is direct competition among individuals of one sex (often males) for mates of the opposite sex Intersexual selection, 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 chances of attracting a female, while decreasing his chances of survival
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