Evolution of Populations microevolution- changes in alleles frequencies Genetic variation makes evolution possible. Differences in genes or DNA sequences among individuals. Is the raw material for natural selection. Sources of Genetics Variation Gene Mutations In germ line cells, not somatic cells Chromosomal Mutations

Evolution of Populations microevolution- changes in alleles frequencies In prokaryotes and viruses, mutations produce genetic variation very rapidly due to their short generation span. In sexually reproducing populations, most of the genetic variation comes from crossing over and independent assortment in meiosis and the random combination of gametes in fertilization.

Hardy-Weinberg equations p + q = 1 p2 + q2 = 1

Hardy-Weinberg #1 In a population of 200 mice, 98 are homozygous dominant for brown fur (BB), 84 are heterozygous (Bb), and 18 are homozygous recessive for white fur (bb). A) the genotype frequencies of this population are _____BB _____Bb ______bb B) the allele frequencies of this population are ______B allele _______b allele

Hardy-Weinberg Principle How do allele and genotype frequencies change from generation to generation? If only Mendelian segregation and recombination of alleles in sexual reproduction are involved, the frequencies of alleles and genotypes in a population will remain constant, as described by the Hardy-Weinberg principle. Such a nonevolving gene pool is said to be in Hardy-Weinberg equilibrium.

Hardy-Weinberg Principle #2 Use the allele frequencies you determined in #1 to predict the genotype frequencies of the next generation. Frequencies of B (p)=______ b (q) = _____ BB = p2 =_______; Bb = 2pq = ______; bb = q2 = ______

Hardy-Weinberg Principle Hardy-Weinberg equilibrium is maintained only if all of the following five conditions are met: No mutations Random mating (because nonrandom mating changes genotype frequencies) No natural selection (no unequal survive and reproductive success) Large population size No gene flow or movement of alleles into or out of the population (immigration or emigration)

Hardy-Weinberg Problems If the frequency of homozygous recessive individuals is known (q2), then the frequency of q may be estimated as the square root of q2. #3a The allele frequencies in a population are A = 0.6 and a = 0.4. Predict the genotype frequencies for the next generation. AA______ Aa_______ aa_______ #3b What would the allele frequencies be for the generation you predicted in part a? A______ a_______ #3c Suppose you are able to determine the actual genotype frequencies in the population and find that these frequencies differ significantly from what you predicted in part a. What would such results indicate?

Example: Phenylketonuria (PKU) In the US, one out of approximately 10,000 babies are born with PKU. Caused by a recessive allele, thus the frequency of individuals in the US population born with PKU corresponds to q2 in the Hardy-Weinberg equation. What is the frequency of heterozygous individuals?

Example: Sickle-cell disease Sickle-cell anemia is caused by a recessive allele. Roughly one out of every 500 African Americans is afflicted with sickle-cell disease. Use Hardy-Weinberg equation to calculate the percentage of African Americans who are carriers of the sickle-cell allele.

Hardy-Weinberg violations Natural Selection- individuals with traits that are better suited to their environment tend to be more successful in producing viable, fertile offspring, and pass their alleles to the next generation in disproportionate numbers, resulting in adaptive evolution. Genetics drift- changes in allele frequencies due to chance events. Has a larger effect on small populations, causes gene frequencies to randomly fluctuate over time, can lead to the loss genetic variation within populations. Founder effect – genetic drift that occurs when only a few individuals colonize a new area. Bottleneck effect- occurs when some disaster or other factor reduces the population size dramatically, and the few surviving individuals are unlikely to represent the genetic makeup of the original population.

Hardy-Weinberg violations Gene flow- the migration of individuals or transfer of gametes between populations, may change allele frequencies. Non-random mating- selection factors Mutations

Causes of Microevolution Genetic Mutations Natural populations contain high levels of allele variations. a. Analysis of Drosphilia enzymes indicates have at least 30% of gene loci with multiple alleles. b. Similar results with other species indicates that allele variation is the rule in natural populations. Gene mutations provide new alleles, and therefore are the ultimate source of variation. a. A gene mutation is an alteration in the DNA nucleotide sequence of an allele. b. Mutations may not immediately affect the phenotype. c. Mutations can be beneficial, neutral, or harmful; a seemingly harmful mutation that requires Daphnia to live at higher temperatures becomes advantageous when the environment changes. d. Specific recombinations of alleles may be more adaptive than other combinations.

Causes of Microevolution Gene Flow Gene flow moves alleles among populations by migration of breeding individuals. Gene flow can increase variation within a population by introducing novel alleles produced by mutation in another population. Continued gene flow decreases diversity among populations, causing gene pools to become similar. Gene flow among populations can prevent speciation from occurring.

Causes of Microevolution Nonrandom Mating Random mating involves individuals pairing by chance, not according to genotype or phenotype. Nonrandom mating involves individuals inbreeding and assortative mating. Inbreeding is mating between relatives to a greater extent than by chance. a. Inbreeding decreases the proportion of heterozygotes. b. Inbreeding increases the proportions of both homozygous at all gene loci. c. In human populations, inbreeding increases the frequency of recessive abnormalities.        

Causes of Microevolution Genetic Drift Genetic drift refers to changes in allele frequencies of a gene pool due to chance. Genetic drift occurs in both large and small populations; large populations suffer less sampling error. Genetic drift causes isolated gene pools to become dissimilar; some alleles are lost and others are fixed. a. Separate cypress groves in California show patchy variation. b. Because there is no apparent adaptive advantage to the variation, this is due to genetic drift.

Causes of Microevolution Genetic Drift 4. Genetic drift occurs when founders start new population, or after a genetic bottleneck with interbreeding. a. The bottleneck effect prevents most genotypes from participating in production of the next generation.           1) Bottleneck effect is caused by a severe reduction in population size due to natural disaster, predation, or habitat reduction.           2) Bottleneck effect causes severe reduction in total genetic diversity of the original gene pool.           3) The cheetah bottleneck causes relative infertility because of the intense interbreeding when populations were reduced in earlier times.         

Causes of Microevolution Genetic Drift    b. Founder effect is genetic drift where rare alleles or combinations occur in higher frequency in a population isolated from the general population.       1) This is due to founding individuals containing a fraction of total genetic diversity of original population.        2) Which particular alleles are carried by the founders is dictated by chance alone.        3) Dwarfism is much higher in a Pennsylvania Amish community due to a few German founders.

Natural Selection Types of Selection 1. Directional selection occurs when extreme phenotype is favored; the distribution curve shifts that direction.  a. A shift of dark-colored peppered moths from light-colored correlated with increasing pollution. b. Increases in insecticide-resistant mosquitoes and resistance of malaria agent to medications are examples of directional selection.  c. The gradual increase in the size of the modern horse, Equus, correlates with a change in the environment from forest-like conditions to grassland conditions. 2. Stabilizing selection occurs when extreme phenotypes are eliminated and the phenotype is favored. a. The average human birth weight is near optimum birth weight for survival. b. The death rate is highest for infants at the extremes of the ranges of birth weights.

Natural Selection Types of Selection 3. Disruptive selection occurs when extreme phenotypes are favored and can lead to more than one distinct form. a. British snails (Cepaea nemoralis) vary because a wide range causes natural selection to vary. b. In forest areas, thrushes feed on snails with light bands. c. In low-vegetation areas, thrushes feed on snails with dark shells that lack light bands.

Natural Selection Maintenance of Variations Populations that lack variation may not be able to adapt to new conditions. How is variation maintained in the face of constant selection pressure? The following forces promote genetic variation.  a. Mutation and genetic recombination still occur.  b. Gene flow among small populations introduces new alleles.  c. Natural selection, such as disruptive selection, itself sometimes results in variations.

Natural Selection Sickle-Cell Disease In sickle-cell disease, heterozygotes are more fit in malaria areas because the sickle-cell trait does not express unless the oxygen content of the environment is low; but the malaria agent causes red blood cells to die when it infects them (loss of potassium). Some homozygous dominants are maintained in the population but they die at an early age from sickle-cell disease. Some homozygotes are maintained in the population for normal red blood cells, but they are vulnerable to malaria.