Chapter 5 Notes
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Date Created: 09/28/15
32110 Notes Chapter 5 Evolution and Gene Frequencies Section 51 Populations and Gene Pools Evolution requires genetic changes that are passed from individuals of the same species within large populations Population can be defined as a group of individuals of the same species that occupies a given area at the same time and share common genes such as color of a coat or type of beak The sum of all of the alleles for all traits in a sexually reproducing population is known all the gene pool The variety of genes in the gene pool is where the differences among individuals of the same species come from such as coloration variance Variety within the population results from having combinations of alleles at each locus Sources of variation can come from o the independent assortment of chromosomes which creates a random distribution of chromosomes into gametes o crossingover which shuffles around alleles between homologous chromosomes 0 chance fertilization of an egg by a sperm 0 rearrangements in the number and structure of chromosomes 0 mutations of existing alleles The potential for genetic variation within a population is practically unlimited Alleles are constantly shuffling around which either changes the phenotypes or it does not If it does change the phenotypes then evolution has occurred Evolution occurs within groups of individuals of the same species over many generations because the changes occur during sexual reproduction Section 52 lust Evolution Happen Population genetics is the study of the genetic events in the gene pools THE HARDYWEINBERG THEOREM Godfrey Hardy and Wilhelm Weinberg created a mathematic formula describing what happens to the relative frequency of alleles in a sexually reproducing population over time This theorem states that evolution will not occur if certain assumptions are met since the relative allelic frequencies will not change from generation to generation even though the specific mixes of alleles in individuals will vary The assumptions are 32110 Notes 0 The population size must be large large sizes ensure that gene frequency will not change by chance 0 Individuals cannot migrate into or out of the population since that could introduce or delete alleles o Mutations must not occur 0 Sexual reproduction within the population must be random every individual must have an equal chance of mating any other individual in the population Otherwise natural selection may occur because some individuals might reproduce with specific others Since these assumptions are so restrictive very few populations if any actually meet them which means that nearly every population is evolving in some way However this theorem provides important framework for examining changes in allelic frequencies Section 53 Evolutionary Mechanisms Evolution is simply a matter of which individuals survive to the age of reproduction which changes the relative allelic frequencies in the gene pool There is no order or logic behind it POPULATION SIZE GENETIC DRIFT AND NEUTRAL EVOLUTION Chance plays an important role in the perpetuation of genes in a population and the smaller the population the more of a role chance is going to play Chance events that influence at the frequency of genes in population result in genetic drift Since gene frequencies are changing independently of natural selection genetic drift is more commonly known as neutral evolution The process of genetic drift is similar to flipping a coin There is about a 50 chance of getting either heads of tails just as there is an equal chance of inheriting a particular gene It is possible to get all of one gene but it is highly unlikely and will probably never happen in a normal population Gamete sampling in a small population may show unusual proportions of alleles in a generation since it is a random process Chance events like it may result in a particular allele increasing or decreasing in a population Inbreeding is also something that happens that can reduce genetic variation in a population Mutation could introduce a new allele into a population If that allele is more or less adaptive than existing alleles genetic drift may permit the new allele to become established within the population It is also possible for genetic drift to influence the genetic makeup of some population One such form is known as the founder effect which is when a parental population colonizes a new habitat 32110 Notes and introduces a very distinct genetic makeup into the larger population Another form is the bottleneck effect where if a population is drastically reduced even if they eventually replenish their numbers their genetic diversity will be significantly impacted due to the loss of genetic differences GENE FLow Immigration and emigration changes the relative allelic frequency which is known as gene flow Not all natural populations have gene flow but most do Gene flow can change the genetic makeup of a population It can also make the two populations more similar MUTATION Mutations are changes in the structure of genes and chromosomes Mutations are the origin of all new alleles and a source of variation that may be adaptive for an individual Mutations make extinctions less likely Mutations are random events and the likelihood of a mutation is not affected by the mutation s usefulness The effects of mutations vary greatly There are neutral mutations mutations that do no harm nor good as well as mutations that can cause harm or create something beneficial Mutation pressure is a measure of the tendency for gene frequencies to change through mutation NATURAL SELECTION REEXAMINED The tendency for natural selection to occur and disrupt the HardyWeinberg equilibrium is selective pressure Natural selection may affect a range of phenotypes in three ways 1 Directional selection occurs when individuals at one phenotypic extreme are at a disadvantage compared to all other individuals in the population These genes will reduce in frequency since those animals will die off faster than the rest of the population and all other genes will increase in frequency 2 Disruptive selection also known as diversifying selection produces distinct subpopulations Sexual selection can occur because of this which is when individuals have varying success obtaining mates This usually creates structures used in combat for males fighting for mates such as antlers for bucks or ornamentation that attracts the opposite sex such as the tail feathers of peacocks 3 Stabilizing selection occurs when both phenotype extremes are deleterious which narrows the phenotype range 32110 Notes BALANCED POLYMORPHISM AND HETEROZYGOTE SUPERIORITY Balanced polymorphism occurs when different phenotypes are maintained at relatively stable frequencies in the population and may resemble a population in which disruptive selection occurs An example is sicklecell anemia which results from a change in the structure of the hemoglobin molecule Some of the red blood cells of a person with sicklecell are misshapen reducing their ability to carry oxygen Section 54 Species and Speculation According to the biological specifies concept a species is a group of populations in which genes are actually or potentially exchanged through interbreeding There are problems with this definition such as some organisms not reproducing sexually and the inability to tell how creatures that are extinct reproduced Speciation is the formation of a new species A requirement of speciation is that subpopulations are prevented from interbreeding Gene flow among populations or subpopulations does not occur which is known as reproductive isolation When populations are reproductively isolated it can cause natural selection and genetic drift to take a different course in each subpopulation ALLOPATRIC SPECIATION Allopatric speciation occurs when subpopulations become geographically isolated from one another A mountain range or a river may permanently separate members of a population Adaptions may occur to deal with different climates or genetic drift may cause these subpopulations to not be able to mate with each other anymore PARAPATRIC SPECIATION Parapatric speciation occurs in small local populations called demes All of the frogs in a particular pond would make up a deme Individuals within a deme are more likely to reproduce together because they experience the same environment However since the deme is so small it could experience different selection pressures than other members of the population 32110 Notes SYM PATRIC SPECIATION Sympatric speciation occurs within a single population In order to demonstrate sympatric speciation researches must demonstrate that the two species arose without any form of geographic isolation Section 55 Rates of Evolution Charles Darwin perceived evolutionary change as occurring gradually over millions of years This concept is called phyletic gradualism and has been the traditional interpretation of the rate of evolution However some changes happen very rapidly When a group encounters an ecological crisis such as a change in climate or a major geological event evolution can occur quickly A variation that was previously considered neutral might now prove a huge advantage Long periods of stasis interrupted by brief periods of change characterize the punctuated equilibrium model of evolution One advantage of this model is that it explains why fossil records do not always show transitional stages between related organisms Section 56 Molecular Evolution Not all types of evolution can be seen with the naked eye Some evolution occurs at the molecular level Changes in the base sequence of DNA can be used to study relationships among organisms GENE DUPLICATION Most mutations are selected against However sometimes an extra copy of a gene is present One copy may be modified but as long as the second copy furnishes the protein the organism is likely to survive The accidental duplication of a gene is one way that extra genetic material can arise Section 57 Mosaic Evolution A species is a mosaic of different molecules and structures that have evolved at different rates Some evolve quickly while others are more conservative Birds for example are recognizable due to distinct traits feathers beaks and certain body forms but have adapted in many different ways This is an example of mosaic evolution
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