Chapter 20 Genes Within Populations

20.1 Genetic Variation and Evolution All species arise from other pre-existing species. Darwin proposed that natural selection is the mechanism of evolution. 1.Natural selection produces evolutionary change when in a population some individuals possess certain inherited characteristics and produce more surviving offspring than those individuals lacking those characteristics. 2.Evolution within species may result from any process that causes a change in the genetic composition of a population.

20.1 Ex: blood groups- chemical analysis has revealed the existence of more than 30 blood group genes in humans (in addition to the ABO locus). At least one-third is found in several alternative allelic forms in humans. There are 45 variable genes encoding other proteins in human blood cells.

20.2 Changes in Allele Frequency The study of properties of genes in a population. Hardy-Weinberg- stated that original proportion of the genotypes in a population will remain constant from generation to generation as long as the following assumptions are met: 1.The population size is very large 2.Random mating is occurring (individuals do not choose mates based on their phenotype or genotype) 3.No mutations take place 4.No genes are input from other sources (no immigration or emigration) 5.No selection occurs Dominant alleles do not, in fact, replace recessive one because their proportions do not change; their genotypes are said to be in a Hardy-Weinberg equilibrium.

20.2 Hardy-Weinberg Equation Enables us to calculate frequencies of alleles in a gene pool if we know the frequencies of genotypes and vice versa. p= one allele in a population (dominant) q= other allele in a population (recessive) If we know the frequency of one allele in the population, the frequency of the other can be calculated. (As long as there are only two alleles) p + q = 1; so p = 1 – q and q = 1 – p

20.2 Swimmers in a “pool” (f) A = 12/30=.40 (f) a = 18/30=.60 p + q = 1 (allele frequencies) = 1 f(AA) = p 2 =.40 x.40 =.16 f(aa) = q 2 =.60x.60 =.36 f (Aa) = 2pq= 2 x.40 x.60 =.48 p 2 + 2pq + q 2 = 1 (genotype frequencies) = 1 Now four more swimmers (a) dive into pool….what are the new frequencies? A a A a A a A a a a a a A a A a A a A A A a A a A a a a

20.3 Five agents of evolutionary change 1.Mutation- generally so low that they have little effect on the Hardy- Weinberg proportions of common alleles. Nonetheless, mutation is the ultimate source of genetic variation and thus makes evolution possible. 2.Gene flow- movement of alleles from one population to another. Powerful agent of change because members of two different populations can exchange genetic material. 3.Nonrandom mating- inbreeding- causes the frequencies of particular genotypes to differ greatly from those predicted by the H.W.P. Does not change the frequency of alleles but increases proportion of homozygotes because relatives are likely to be genetically similar. 4.Genetic drift- frequencies of particular alleles may change drastically by chance alone. Such changes occur randomly, as if frequencies were drifting.

20.3 Founder effect- one or a few individuals disperse and become founders of a new, isolated population at some distance from their place of origin. The pioneers are not likely to have all the alleles present in the source population. Bottleneck effect- population drastically decrease; individuals that survive may constitute a random genetic sampling of original population 5. Selection (natural)- environmental conditions determine which individuals in a population produce the most offspring. (three conditions must be met) 1.Variation must exist among individuals- N.S. works by favoring individuals with some traits over others with alternative traits. 2.Variation among individuals result in differences in number of offspring surviving in the next generation. 3.Phenotypic variation must have genetic basis.

20.3 Natural selection is a process, whereas, evolution is a historical record of change over time. N.S. is a process that can lead to evolution. The result of evolution driven by natural selection is that populations become better adapted to their environment. 1.Selection to avoid predators- involve genetic changes that decrease the probability of capture by a predator. Ex: pocket mice from the Tularosa Basin in New Mexico. Color matches the background. Differences are due to small differences in the DNA of alleles of a single gene. 2.Selection to match climate conditions- some enzyme allele frequencies vary with latitude. Ex: fish (mummichog) range along North America and possess the enzyme lactate dehydrogenase. Northern fish posses the form of the enzyme that breaks pyruvate into lactate much more efficiently at cooler temperatures.

Selection for pesticide and microbial resistance- widespread use of pesticides has lead to the rapid evolution of resistance in more than 500 pest species. Selection imposed by humans has led to the evolution of resistance to antibiotics in many disease-causing pathogens. Ex: Staphylococcus aureus used to be killed by penicillin. Because of mass production of the antibiotic, S. aureus modified an enzyme that renders penicillin inactive.

20.5 Natural Selection’s Role in Maintaining Variation In some circumstances, selection can actually maintain population variation. Frequency-dependent selection- favors certain phenotypes depending on how common or uncommon they occur. 1.Negative frequency-dependent selection-rare phenotypes are favored by selection. Ex: fish preying upon water boatman insect; insect occurs in three colors and research shows they are preyed upon disproportionately when it is the most common one- meaning fish eat more of the common color bugs. 2.Positive frequency-dependent selection-favors common forms and tends to eliminate variation from a population. 3.Oscillating selection-selection favors one phenotype at one time and another phenotype an another time. This effect will maintain genetic variation. 4.Heterozygote advantage- favors individuals with copies of both alleles and works to maintain both in the population. Ex: sickle cell anemia SS= normal red blood cells Ss= some normal/some sickled ss= sickle cell anemia (resistant to malaria)

20.6 Selection Acting on Traits Affected by Multiple Genes Forms of Selection 1.Disruptive selection- selection acts to eliminate rather than to favor intermediate types. Ex: African fire-bellied seed cracker have either large or small beaks due to the seeds they eat. 2.Directional selection- selection acts to eliminate one extreme from an array of phenotypes; the genes promoting this extreme become less frequent. Ex: Drosophila population- elimination of flies that move toward light causes the population to contain fewer individuals with alleles promoting such behavior. 3.Stabilizing selection- acts to eliminate both extremes from an array of phenotypes; the result is to increase the frequency of the already common intermediate type.