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AP® Biology Review: Genes Within Populations Hardy-Weinberg Equilibrium & Types of Selection AP® is a registered trademark of the College Board, which was not involved in the production of, and does not endorse, this product. Gene Variation is the Raw Material of Evolution • evolution—widely used to refer to how an entity changes through time • The first five editions of Darwin’s book, On the Origin of the Species, never used the term! • Rather, he used the phrase “descent with modification”. • It still captures the essence of evolution – all species arise from other, pre-existing species – however, through time they accumulate differences such that ancestral and descendant species are NOT identical Natural Selection is an Important Mechanism of Evolutionary Change • Darwin was not the first to propose a theory of evolution • Darwin proposed natural selection as the mechanism of evolution • Natural selection produces evolutionary change when, in a population, some individuals possessing certain inherited characteristics produce more surviving offspring than individuals lacking these characteristics Natural Selection is an Important Mechanism of Evolutionary Change • The end result—the population will gradually come to include more and more individuals with the advantageous characteristics • the population evolves and becomes better adapted to its local circumstances Lamarck and the Inheritance of Acquired Characteristics • A second proposed [albeit wrong] mechanism of evolution • Individuals pass on to offspring body and behavior changes acquired during their lives • ancestral giraffes with short necks tended to stretch their necks to feed on tree leaves • this extension of the neck was passed on to subsequent generations leading to long-necked giraffe Darwin’s take on giraffe • By contrast, Darwin’s theory used the idea that preexisting genetic differences among individuals made them more fit for their environment thus they thrive, reproduce and introduce more of their kind into the population. Speaking of freak mutants… • Natural selection is not the only process that can lead to the genetic makeup of populations • Allele frequencies can also change as the result of repeated mutations from one allele to another and from migrants bringing alleles into a population • In small populations, the frequencies of alleles can change randomly as the result of chance events Gene Variation in Nature • Evolution within a species may result from ANY process that causes a change in the genetic composition of a population • Start by looking at the genetic variation present among individuals within a species • this is the raw material available for the selective process Measuring Levels of Genetic Variation • Blood groups – 30 blood group genes exist in humans in addition to the ABO locus – 1/3 are routinely found in several alternative allelic forms – more than 45 variable genes encoding other proteins in human blood cells and plasma which are not considered blood groups – total: 75 genetically variable genes in this one system alone Measuring Levels of Genetic Variation • Enzymes – use electrophoresis to determine alternative alleles of genes specifying particular enzymes – About 5% of the enzyme loci of a typical human are heterozygous Enzyme Polymorphism • Many loci in a given population have more than one allele at frequencies significantly greater than would occur from mutation alone • polymorphic—many forms; refers to a locus with more variation than can be explained by mutation • most populations of insects and plants are polymorphic at more than ½ of their enzymecoding loci Enzyme Polymorphism • vertebrates are somewhat less polymorphic • heterozygocity—the probability that a randomly selected gene will be heterozygous for a randomly selected individual—about 15% in Drosophila and other invertebrates and between 5% and 8% in vertebrates and around 8% in outcrossing plants [don’t memorize this stuff!] • Point being—lots of raw material for evolution! Population Genetics • The study of the properties of genes in populations • Darwin had no knowledge of meiosis! • The theory of blending inheritance—where offspring were expected to be phenotypically intermediate relative to their parents—was widely accepted. • WRONG—the effect of any new genetic variant would quickly be diluted to the point of disappearance in subsequent generations. The Hardy-Weinberg Principle • Following the rediscovery of Mendel’s research, two people, independently [hence the hyphen] solved the puzzle of why genetic variation persists • G. H. Hardy, an English mathematician • G. Weinberg, a German physician • They pointed out that the original proportions of the genotypes in a population will remain constant from generation as long as you meet the five assumptions Hardy-Weinberg Equilibrium Assumptions: The allele frequencies with a population will remain constant as long as… • The population size is very large • Random mating is occurring • No mutation takes place • No Immigration takes place • No selection occurs Hardy-Weinberg Equilibrium • • • • • Dominant alleles do NOT replace recessive ones The proportion of dominant to recessive alleles does NOT change in a H-W equilibrium p represents the frequency of the dominant allele q represents the frequency of the recessive allele frequency is the fraction of the allele’s abundance in a population [part/whole or a percentage expressed as a decimal value] Hardy-Weinberg Equilibrium • p+q=1 • p2 + 2pq + q2 = 1 • • • p2 = the number of homozygous dominant individuals q2 = the number of homozygous recessive individuals 2pq = the heterozygotes • the above equation is a binomial expansion of (p + q)2 which is equal to 12 since p + q = 1 A Feline Example • Consider a population of 100 cats • 84 black [frequency of black is then 84/100 = 84% or .84] • 16 white [frequency of white is then 16/100 = 16% or .16] • Assume the white cats are homozygous recessive, designated bb • The black cats are either BB or Bb A Feline Example • Start with the white cats • q2 = homozygous recessive = bb = .16 • that means that the frequency of b = q and is the square root of q2 .16 0.4 • Now, if q =0 .4 and the sum of p + q = 1, then 1 - 0.4 = p which equals 0.6 A Feline Example • If p = B = 0.6 and q = b = 0.4, then p2 = the number of homozygous dominant = 0.62 = 0.36 of the population = 36 homozygous, BB, black cats 2pq = the number of heterozygotes = 2(0.6)(0.4) = .48 of the population = 48 heterozygote, Bb, black cats This population was easy since there were 100 individuals! A Punnet Square May Prove Useful A Human Example • Cystic fibrosis patients are homozygous for the recessive allele that causes the disease. • In North American Caucasians, the allele is present at a frequency of about 22 per 1,000 individuals • Therefore, q = .022 What proportion of North American Caucasians is expected to express this trait? • If q = 0.022, then q2 represents the number of afflicted individuals • 0.0222 = 0.000484 of the population • 0.000484 = which is about .0005 or 5/10,000 which reduces to 1/2,000 • So one of every 2,000 individuals in that population is expected to have Cystic Fibrosis What proportion is expected to be heterozygous carriers? • This is where the real risk lies! • If q = 0.022, the 1-0.022 = p = 0.978 • 2pq = the heterozygous carriers = 2(0.022)(0.978) = 0.043032 • Which is about 0.043 which is about 43/1,000 or 1 of every 23 people. • If two carriers mate [think Punnet square] there is a 25% chance the child will express Cystic Fibrosis. Why Do Allele Frequencies Change? • They won’t change from generation to generation as long as H-W conditions are observed – no mutation – no gene flow [immigration & emigration] – no selection – NO CHANCE!! Why Do Allele Frequencies Change? 5 reasons • mutation—the ultimate source of variation! Individual mutations occur so rarely that mutation alone does not change allele frequency much • gene flow—a very potent agent of change. Populations exchange members • nonrandom mating—Inbreeding is the most common form. Artificial selection—It does not alter allele frequency but decreases the proportion of heterozygotes. Why Do Allele Frequencies Change? 5 reasons • genetic drift—statistical accidents; usually occurs only in very small populations • selection—the only form that produced adaptive evolutionary changes • Selection is the only agent that depends on the nature of the environment. The other 4 are independent of the environment. A bit more about genetic drift • In small populations the allele frequencies may change drastically by chance alone • Since these changes occur randomly, as if the frequencies were drifting, we call it genetic drift • There are two related causes of decreases in a population’s size – founder effects – bottleneck effect Founder Effects • Sometimes one or a few individuals disperse and become the founders of a new, isolated population at some distance from their place of origin • Since these individuals may not represent all of the alleles present in the original population, alleles are lost and frequencies of alleles changed • Darwin’s finches • the blue people of Kentucky The Bottleneck Effect • Even without movement, populations can be drastically reduced in size • Flooding, drought, epidemic disease, other natural forces or progressive changes in the environment are all causes • the few survivors represent a random genetic sample of the original population • the resultant loss of genetic variability has been termed the bottleneck effect The Bottleneck Effect Examples • Northern elephant seal, breeds on the western coast of North America and nearby islands • Hunted to near extinction in the 19th century • Reduced to a single population of 20 individuals on the Island of Guadalupe off the coast of Baja, California • 10s of thousands now, but very little genetic variation Examples • Cheetahs—almost extinct • Today’s cheetahs are so genetically identical that skin taken from one cheetah and grafted onto another is not rejected Selection • natural—nature decides! The environmental conditions determine which individuals in a population produce the most offspring • artificial—a breeder decides! The breeder makes purposeful crosses between animals or plants to produce a set of desired characteristics Natural Selection’s 3 Conditions • Variation must exist among individuals in a population. Some traits are favored over others—no variation, no natural selection • Variation results in differences in number of offspring surviving in the next generation. The essence of natural selection! Because of phenotype or behavior, some individuals are more successful in producing offspring, thus passing on their genes • Variation must be genetically inherited. Necessary for natural selection to result in evolutionary change Natural Selection ≠ Evolution Natural selection is a process. Evolution is the historical record of change through time. It is an outcome, not a process. Only if variation is genetically based will natural selection, the process, lead to evolution, the outcome. Selection to Avoid Predators • Camouflage—green caterpillars blend into the leaves upon which they feed making it more difficult for birds to see them. • Industrial Melanism [where darker individuals predominate over light] and the peppered moth • The trees in England are covered in light bark and lichens. During the industrial revolution, soot covered the trees making them dark. There was a complete swap between the frequencies of light, peppered moths and their dark, near black, cousins. • Birds are quite adept at picking out individuals that are not adapted to their backgrounds. The Peppered Moth • Before 1850 the trees were light and although black coloration is dominant, it was rare • After the industrial revolution that situation reversed • Sometimes, it pays to blend in! Pesticide resistance in Insects • Rapid evolution of more than 400 resistant species • Some mutants were born with fewer receptor sites on their cell membranes • This decreases the binding ability of the insecticide • Other alleles in other mutants enhance the ability of the insects’ enzymes to identify and detoxify insecticide molecules. Heterozygote Advantage • Selections that favor individuals with copies of both alleles give heterozygotes an advantage • Sickle Cell Anemia is a striking example • Abnormal red blood cells are irregular, or sickle shaped • Particularly common among African Americans 3/1,000 • Often fatal—before treatments developed, all affected individuals died as children Heterozygote Advantage • So why didn’t the allele become “extinct”? • In Africa, where the frequencies are MUCH higher than here in the states, malaria plays a role. • The plasmodium parasite that causes malaria invades blood cells. When it invades a red blood cell of a heterozygote it causes it to sickle. These sickled cells are then quickly swept up by the spleen thus eliminating the parasite. • The spleen’s filtering effect is what leads to anemia in homozygotes with the disease Heterozygote Advantage • This means that being a heterozygote carrier for sickle cell has an added bonus for a natural immunity to malaria • These carriers have an advantage over the homozygous dominant individuals that have no protection against Malaria. • Selection is acting to eliminate the allele in the African American population where Malaria is no threat, thus the heterozygotes have no advantage. Forms of Selection • Disruptive Selection—intermediate types are eliminated • African fire-bellied seedcracker finch – Populations of these birds contain individuals with large and small beaks while very few individuals have intermediate-sized beaks. – Why? The seeds they feed upon come in two sizes, small and large – Intermediate beaks are clumsy with small seeds and not strong enough to crack large seeds. Forms of Selection • Directional selection—acts to eliminate one extreme from an array of phenotypes • Selection has changed the population in the direction of lower light attraction read this Forms of Selection • Stabilizing selection—selection acts to eliminate both extremes; it favors an optimum condition and increases the frequency of the already common intermediate type, making it even more common by eliminating the extremes.