The Evolution of Populations

Chapter 23 → The Evolution of Populations Student Guided Notes Overview: The Smallest Unit of Evolution • Natural selection does act on individuals. Each individual’s traits affect its survival and its reproductive success relative to other individuals in the population. • The evolutionary impact of natural selection is apparent only in the changes in a population of organisms over time and so it is the __________________________, not the ____________________________, which evolves. • Microevolution is defined as ___________________________________________________________ ___________________________________________________________________________________. • Three mechanisms can cause allele frequencies to change: ____________________________________, __________________________________________ (chance events that alter allele frequencies), and __________________________________ (the transfer of alleles between populations). Concept 23.1 Genetic variation makes evolution possible Genetic variation occurs within a population. • Individual variation occurs in all species and often reflects genetic variation, ____________________ __________________________________________________________________________________ ___________________________________________________________________________________. • Both quantitative and discrete characters contribute to variation within a population. • Discrete characters, such as flower color, are usually determined by a single locus with different alleles that produce distinct phenotypes. • Quantitative characters, such as plant height, vary along a continuum within a population. Genetic variation occurs between populations. • Species also exhibit geographic variation, differences in the genetic composition of geographically separate populations. • Geographic variation in the form of graded change in a trait along a geographic axis is called a cline (example: birds have increasing body fat as latitude increases.) 23-1 New genes and new alleles originate only by mutation. • The genetic variation on which evolution depends originates when _____________________________, ______________________________________, or other processes produce new __________________ and new _______________________________. • New alleles can arise by mutation, ______________________________________________________ ___________________________________________________________________________________. • Some genetic variation in populations represents neutral variation that does not __________________ ________________________________________________________________ (example: fingerprints.) • The tendency for natural selection to reduce variation is countered by mechanisms that _____________ ___________________________________________, including diploidy and balanced polymorphisms. • Diploidy, having two copies of each chromosome, in eukaryotes prevents the elimination of recessive alleles via selection because recessive alleles do not affect the phenotype in heterozygotes. o Even recessive alleles that are unfavorable can persist in a population by “hiding” in heterozygous individuals. • Heterozygote protection maintains a huge pool of alleles that may not be suitable under the present conditions but may become beneficial when the environment changes. (balanced polymorphism) • Sexual reproduction also increases variety via: _______________________, ____________________ ____________________________ and __________________________________. Concept 23.2 The Hardy-Weinberg equation can be used to test whether a population ________________ • For a population to evolve, individuals must differ genetically and one of the factors that causes evolution must be at work. A population’s gene pool is defined by its allele frequencies. • A population is _____________________________________________________________________ ___________________________________________________________________________________. • The total of all the alleles for all of the loci for all of the individuals in a population is called the population’s ____________________________. • Each allele has a frequency or proportion in the population’s gene pool. • For example, imagine a population of 500 wildflower plants with two alleles (CR and CW) at a locus that codes for flower pigment. 23-2 o Suppose that in the imaginary population of 500 plants, 20 (4%) are homozygous for the CW allele (CWCW) and have ______________________ flowers. o Of the remaining plants, 320 (64%) are homozygous for the CR allele (CRCR) and have __________ flowers. o These alleles show incomplete dominance, so 160 (32%) of the plants are heterozygous (CRCW) and produce ___________________ flowers. • Because these plants are diploid, the population of 500 plants has 1,000 copies of the gene for flower color. o The dominant allele (CR) accounts for 800 copies (320 × 2 for CRCR + 160 × 1 for CRCW) → 640 + 160 = 800. o The frequency of the CR allele in the gene pool of this population is 800/1,000 = 0.8, or ______%. o The CW allele must have a frequency of 1.0 − 0.8 = 0.2, or ______%. • When there are two alleles at a locus, the convention is to use p to represent the frequency of one allele and q to represent the frequency of the other. o Thus p, the frequency of the CR allele in this population, is _____. o The frequency of the CW allele, represented by q, is ______. • Allele and genotype frequencies can be used to test whether ____________ is occurring in a population. The Hardy-Weinberg principle describes a non-evolving population. • The Hardy-Weinberg principle states that the frequencies of alleles and genotypes in a population’s gene pool will remain constant over generations unless acted upon by agents other than Mendelian segregation and recombination of alleles. o The shuffling of alleles by meiosis and random fertilization has no effect on the overall gene pool of a population. o Such a gene pool is said to be in _____________________________________________________. • The Hardy-Weinberg principle states that the repeated shuffling of a population’s gene pool over generations does not increase the frequency of one allele over another. o Theoretically, the allele frequencies in our flower population should remain 0.8 for CR and 0.2 for CW forever. • In a population that has two alleles with frequencies p and q, the combined frequencies must add to 1, or 100%. o Therefore p + q = 1.0 23-3 o ____= frequency of dominant allele (B) o ____ = frequency of recessive allele (b) In addition, the genotype frequencies must add to 1.0: _____ + _____ + _____= ______. o _____ = frequency of homozygous dominant (BB) o _____ = frequency of heterozygous dominant (Bb) o _____ = frequency of homozygous recessive (bb) • This general formula is the Hardy-Weinberg equation. • Using this formula, we can calculate the frequencies of alleles in a gene pool if we know the frequencies of genotypes, or we can calculate the frequencies of genotypes if we know the frequencies of alleles. o TIP: ALWAYS SOLVE FOR q FIRST!!! Five conditions must be met for a population to remain in Hardy-Weinberg equilibrium. • Populations evolve because five conditions for non-evolving populations are rarely met for long in nature. A population must satisfy all five conditions to remain in Hardy-Weinberg equilibrium: 1. _______________________________________________________. The gene pool is modified if mutations alter alleles or if entire genes are deleted or duplicated. 2. _______________________________________________________. If individuals pick mates with certain genotypes, or if inbreeding is common, the mixing of gametes will not be random and genotype frequencies will change. 3. _______________________________________________________. Differential survival or reproductive success among genotypes will alter allele frequencies. 4. _______________________________________________________. In small populations, chance fluctuations in the gene pool will cause allele frequencies to change over time, a process called genetic drift. 5. ____________________________________________. Gene flow, the transfer of alleles due to the migration of individuals or gametes between populations, will change the frequencies of alleles. NOTE: Hardy-Weinberg explains NON-EVOLVING populations! If one or more of these 5 conditions ARE NOT met, then that means the population is EVOLVING. We can apply the Hardy-Weinberg principle to a human population. 23-4 • We can use the Hardy-Weinberg principle to estimate the percent of the human population that carries the allele for the inherited disease ___________________________________ (PKU). (refer to Genetic Disorder Chart CH 14/15) • From the epidemiologic data, we know that the frequency of homozygous recessive individuals (q2 in the Hardy-Weinberg principle) is one in _____________________, or 0.0001. o The frequency of the ___________________________ allele (q) is the square root of 0.0001 = 0.01. o The frequency of the ___________________________ allele (p) is p = 1 − q, or 1 − 0.01 = 0.99. o The frequency of carriers (heterozygous individuals) is 2pq = 2 × 0.99 × 0.01 = 0.0198, or about __%. • Thus, about 2% of the U.S. population carries the PKU allele. Concept 23.3 Natural selection, genetic drift, and gene flow can alter allele frequencies in a population Natural selection is based on differential survival and reproductive success. • Individuals with variations better suited _______________________________________________ tend to produce more ______________________________ than those with variations that are less well suited. • As a result of selection, alleles are passed on to the next generation in frequencies different from their relative frequencies in the present population. • For example, the fruit fly Drosophila melanogaster has an allele that confers resistance ______________ ____________________________, including ______________. o In 20 years after the first use of DDT, the allele frequency shifted from 0% to _____%. o DDT was a strong ______________________ force, favoring alleles that conferred resistance. o Note: If DDT had eliminated ALL pests in the early application (no survivors) it would have been effective. • By _________________________________________________________________________ can cause adaptive evolution (evolution that results in a better match between organisms and their environment). Genetic drift results from chance fluctuations in allele frequencies in small populations. • __________________________________ occurs when changes in gene frequencies from one generation to another occur because of ________________ events (sampling errors) that occur in small populations. • Genetic drift in _______________________ populations may occur as a result of two situations: the bottleneck effect or the founder effect. 23-5 o The __________________________ effect occurs when a new population is started by a small number of individuals who do not represent the gene pool of the larger source population. o The bottleneck effect occurs when the numbers of individuals in a large population are drastically ____________________________ by a disaster. • FOUR key points about Genetic Drift: o _____________________________________________________________________________________ _____________________________________________________________________________________. o _____________________________________________________________________________________ _____________________________________________________________________________________. o _____________________________________________________________________________________ _____________________________________________________________________________________. o _____________________________________________________________________________________ _____________________________________________________________________________________. A population may lose or gain alleles by gene flow. • ___________________________________ is the transfer of alleles among populations due to the migration of ________________________ individuals or ______________________. • Gene flow tends to reduce _____________________________________________________________. • Gene flow is an increasingly important agent of evolutionary change in _________________ populations. o With increased human mobility, mating is more common between previously isolated populations, leading to an exchange of alleles and reducing _____________________________________ between human populations. Concept 23.4 Natural selection is the only mechanism that consistently causes adaptive evolution • Evolution by natural selection is _________________________________________________________ _____________________________________________________________________________________ _____________________________________________________________________________________. o • Because of this favoring process, the outcome of natural selection is __________________________. Adaptive advantage can lead to greater relative fitness (aka “Darwinian Fitness”): ________________ _____________________________________________________________________________________ _____________________________________________________________________________________. 23-6 There are three modes of selection: ___________________________, ____________________________, and ________________________________. • Natural selection can alter the frequency distribution of heritable traits in three ways, depending on _____________________________________________________________________________________. o Directional selection is most common during periods of environmental change or migration and favors _________________________________________________________________________. o Disruptive (aka “diversifying”) selection occurs when environmental conditions favor individuals ____________________________________________________________________ ____________________________________________ over those with intermediate phenotypes. o Stabilizing selection favors ________________________________________________________ ____________________________________ (eg. human _______________________________.) Sexual selection may lead to pronounced secondary differences between the sexes. • Sexual selection results in sexual dimorphism, _________________________________________________ _____________________________________________________ such as size, coloration, and ornamentation. • _______________________________ selection is direct competition among individuals of one sex for mates of the opposite sex. • Intersexual selection, or _________________________, occurs when members of one sex (usually females) ________________________________________________________________________________________. Balancing selection preserve genetic variation. • _______________________________ selection occurs when natural selection maintains stable frequencies of two or more phenotypes in a population. • One mechanism that produces balanced polymorphism is _________________________________________. o In some situations, individuals who are heterozygous at a particular locus have greater fitness than homozygotes. (Ex. Individuals who are heterozygous for ______________________________________ are also resistant to ___________________________) • A second mechanism that promotes balanced polymorphism is ____________________________________. o Frequency-dependent selection occurs when the fitness of any one morph declines if it becomes too _______________________________ in the population. o Frequency-dependent selection has been observed in a number of predator-prey interactions in the wild. 23-7 Natural selection cannot fashion perfect organisms. • There are at least four reasons natural selection cannot produce perfection. 1. ___________________________________________________________________________________. 2. ___________________________________________________________________________________. 3. ___________________________________________________________________________________. 4. ___________________________________________________________________________________. • With these constraints, evolution does not tend to craft perfect organisms. 23-8