Genetics for Human Evolution Genome The total genetic endowment of a species The alleles you posses define your genotype or hereditary makeup The way they are expressed physically is your phenotype the sum of observable traits EVOLUTION This is the definition of evolution we will use A change in the allele frequency of a population over time Where does the variation in populations come from 1 Individuals vary as a result of sexual reproduction 2 Populations vary through time due to a number of factors we ll cover today REVIEW Variation in individuals due to sexual reproduction In all but the sex cells genes come in pairs Any particular gene may display variants called alleles The ultimate source of alleles variants is mutation a random change in the coding of a gene that is a change in the base pairs in codons in addition to mutation other sources of variation in individuals may also arise during sexual reproduction Segregation also called independent sorting Linked genes Crossing over and the resultant Recombination Segregation independent sorting During the production of gametes when allele pairs split into haploid gametes during meiosis Each allele in a pair sorts independently from the other pairs You don t just get your grandmother s alleles for example you get some of your grandfather s GRANDPARENTS PARENTS YOU Linked genes However instead of sorting independently sometimes a segment of a chromosome will stay together Crossing over and Recombination And sometimes during meiosis paired chromosomes may exchange material crossing over The result is a new mixture a recombination When one chromosome from the mother with over 8 million possibilities combines with one chromosome from the father ditto and this happens with all 23 chromosomes that is 2 to the 23rd times 2 to the 23rd or about 70 trillion possible zygotes But evolution is a process that involves change in POPULATIONS not individuals over time Population a group of individuals that interbreeds We can genetically characterize populations by referring to the gene pool the total genes of a population One way to think about gene pool is to think of allele or gene frequency The process of evolution is genetic change in populations through time Evolution is a change in allele frequency in a population over time In theory the gene pool or allele frequency of a population could remain stable over time no evolution This is expressed algebraically by the Hardy Weinberg Equilibrium Principle p2 2pq q2 1 This principle states that a population s allele frequency gene pool will remain the same from one generation to the next IF p q p pp pq q pq qq Punnett square Hardy Weinberg Equilibrium Principle holds true IF Mating is entirely random The population is large enough No new variants are introduced into the gene pool All individuals are equally successful at surviving and reproducing However in life these conditions are rarely met What are the sources of change in populations over time At the population level the following processes contribute to evolution change in allele frequency over time These are the mechanisms of genetic evolution 1 Mutation 2 Natural selection 3 Genetic drift 4 Gene flow 1 Mutations random changes in DNA molecules Mutations are frequent Most are thought to be neutral The only mutations that are passed on are those that take place in the sex cells 2 Natural selection variation at the population level is already present The environment exerts pressure that selects for some individuals and against others So not all individuals are equally successful at surviving and reproducing against the Hardy Weinberg Equilibrium Principle Whether there is change or not in the allele frequency of the population depends on whether or not there is a change in the environmental forces through time Directional selection Natural selection promotes change in the allele frequency of the population Stabilizing selection Natural selection promotes stability because there is no environmental change Oscillating selection Adaptive variation around a norm in response to environmental variation Later on we ll talk about the rate of evolution or how we picture evolution taking place The traditional view of the rate was Gradualism in which changes gradually accumulate to the point that we call something a new species We expect to see intermediate fossil forms According to the Theory of Punctuated Equilibrium long periods of stabilizing or oscillating selection are punctuated by bursts of change We expect to see sudden new species 3 Genetic drift change in allele frequency of a population by random fluctuations Genetic drift occurs with small populations when they are isolated from the rest of the gene pool Gene frequencies fluctuate from generation to generation the smaller the population the wider the fluctuations When a few members of a population split off from the original larger group fission you get what is called the Founder Effect That small new group cannot contain all the genetic diversity gene pool of the original larger group Fission the population splits and the new subpopulations will differ in allele frequency from each other and the parent population A new small population has a founder effect 50 50 17 83 67 33 Example Old Order Amish Founder effect migration from Europe to America of a small religious group of about 200 people originally Do not allow outsiders to marry in Today a high incidence of Ellis van Creveld EvC syndrome dwarfism with polydactyly strange fingernails and heart abnormalities Researchers charted the genealogies of all cases of this disease and found they can all trace back to one couple of the original founders Samuel King and wife One of them evidently carried EvC 4 Gene flow the introduction of new genetic material alleles into a population Occurs when two formerly separated populations begin to interbreed How does all this relate to evolution Natural selection takes place on the level of populations not on individuals The basis of evolution is the transmission of traits with variation from one generation to the next The study of this process is called genetics Species a population or group of populations that is capable of interbreeding and is reproductively isolated from other populations Note that this definition is relative rather than absolute A species will maintain gene flow within its gene pool Populations within species that are capable of interbreeding but that do not regularly do so are
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