Evolution of Populations PDF

Evolution of Populations Chapter 23 Key Concepts 23.1 Genetic variation makes evolution possible 23.2 The Hardy-Weinberg Principle can be used to test whether a population is evolving 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population 23.4 Natural s...

Evolution of Populations Chapter 23 Key Concepts 23.1 Genetic variation makes evolution possible 23.2 The Hardy-Weinberg Principle can be used to test whether a population is evolving 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population 23.4 Natural selection is the only mechanism that consistently causes adaptive evolution CONCEPT 23.1: Genetic variation makes evolution possible Genetic variation, variation in heritable traits, is a prerequisite for evolution by natural selection refers to the differences in genes or other DNA sequences among individuals Gregor Mendel’s work on pea plants provided evidence of discrete heritable units (genes) Phenotype is the product of inherited genotype and environmental influences Natural selection can only act on variation with a genetic component  Phenotypic differences determined by a single gene can usually be classified on an “either- or” basis For example, pea flowers are either purple or white Phenotypic differences determined by two or more genes usually vary in gradations along a continuum For example, coat color in horses or height in humans Sources of Genetic Variation Genetic variation originates when new genes and alleles arise by mutation, gene duplication, or other processes Genetic variations are produced rapidly in organisms with short generation times (think about bacteria!) Sexual reproduction can produce genetic variation by recombining existing alleles Formation of New Alleles New alleles arise by mutation, change in the nucleotide sequence of DNA Mutations can be caused by replication errors or exposure to certain types of radiation or chemicals Even a point mutation, change in a single nucleotide, can have significant impact on phenotype Harmful, silent, or sometimes beneficial Harmful mutations that are recessive can be hidden from selection in heterozygous individuals “Heterozygote protection” maintains a pool of alleles that could be beneficial if the environment changes Sexual Reproduction Most genetic variation in sexually reproducing organisms results from recombination of alleles New combinations of existing alleles occur through three mechanisms: Crossing over (exchange of genetic material between homologous chromosomes during meiosis) Independent assortment (random distribution of chromosomes into gametes during meiosis) Fertilization (random combination of gametes) CONCEPT 23.2: The Hardy-Weinberg principle can be used to test whether a population is evolving Genetic variation is required for a population to evolve, but does not guarantee that it will One or more factors that cause evolution must be at work for a population to evolve Gene Pools and Allele Frequencies A population is a group of individuals of the same species that live in the same area and interbreed Geographically isolated populations rarely exchange genetic material The gene pool consists of all copies of every allele at every locus in all members of the 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, individuals may be homozygous or heterozygous Each genotype and each allele has a frequency in the population that can be calculated The Hardy-Weinberg Equation The Hardy-Weinberg equation describes the expected genetic makeup 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, then the population may be evolving Hardy-Weinberg Equilibrium If a population is not evolving, genotype and allele frequencies will be constant from generation to generation Mendelian segregation and recombination of alleles must also occur for frequencies to remain constant Such a population is in Hardy-Weinberg equilibrium MORE ON THIS TOPIC IN THE NEXT LECTURE! CONCEPT 23.3: Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Three major factors alter allele frequencies directly and bring about most evolutionary change: Natural selection Genetic drift Gene flow Mutations can create new alleles! Natural Selection Natural selection is based on differential success in survival and reproduction Individuals have variations in their heritable traits Those with traits better suited to the environment produce more offspring than others  Selection results in alleles being passed to the next generation in proportions that differ from those in the present generation For example, an allele for DDT resistance increased in frequency in wild fruit flies after 20 or more years of DDT use Natural selection can cause adaptive evolution, a process in which traits that enhance survival or reproduction increase in frequency over time Directional, Disruptive, and Stabilizing Selection There are three ways in which natural selection can alter the frequency distribution of heritable traits: 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 Figure 23.13 Genetic Drift The smaller the sample, the greater the chance of random deviation from a predicted result Genetic drift is a process in which chance events cause allele frequencies to fluctuate unpredictably from one generation to the next Genetic drift tends to reduce genetic variation through the random loss of alleles Figure 23.9 The Founder Effect The founder effect occurs when a few individuals become isolated from a larger population Allele frequencies in the smaller founder population are different from those in the parent population For example, genetic drift could occur if a few individuals are indiscriminately blown to a new island by a storm The Bottleneck Effect The bottleneck effect occurs when there is a drastic reduction in population size due to a sudden 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 Figure 23.10 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 chickens in Illinois The surviving birds had low levels of genetic variation, and only 50% of their eggs hatched Genetic drift during the bottleneck likely reduced genetic variation and increased the frequency of harmful alleles Effects of Genetic Drift: A Summary 1. Genetic drift is significant in small populations 2. Genetic drift can cause allele frequencies to change at random 3. Genetic drift can lead to a loss of genetic variation within populations 4. Genetic drift can cause harmful alleles to become fixed Gene Flow Gene flow consists 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 (not within!) over time  Gene flow affects adaptation to local environments For example, mainland and island populations of Lake Erie water snakes have different color patterns A strong banding pattern is favored on the mainland; unbanded snakes are better camouflaged on islands Ongoing migration of banded snakes from the mainland population maintains disadvantageous alleles for banding pattern on the islands  Gene flow can also increase a population’s fitness Consider the spread of alleles for resistance to insecticides Insecticides have been used to target mosquitoes that carry West Nile virus and other diseases Alleles have evolved in some populations that confer insecticide resistance to these mosquitoes The flow of resistance alleles into a new population can increase its fitness Bioflix: Mechanisms of Evolution Natural selection is the only mechanism that consistently causes adaptive evolution Only natural selection consistently increases the frequencies of alleles that provide reproductive advantage Genetic drift and gene flow do not consistently increase the frequency of alleles that enhance survival and reproduction Both processes may increase or decrease the frequency of beneficial alleles in a population Why Natural Selection Cannot Fashion Perfect Organisms 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