18 P RC) ESS cF EVC) IL LAT I 0 NI Microevolution refers to the evolutionary changes within a population. The Hardy-Weinberg principle states that the gene pool remains constant as long as there are no mutations, no gene flow, random mating, no genetic drift, and no selection. The reverse of these conditions causes evolution to occur. Gene flow causes the gene pool of two populations to become similar, and genetic drift causes them to become dissimilar. Today it is possible to see that natural selection occurs when certain alleles become more frequent in a gene pool. Directional selection occurs when an extreme phenotype is favored and the distribution curve shifts in one direction; stabilizing selection occurs when an intermediate phenotype is favored; S -r ui c•Ir and disruptive selection occurs when more than one extreme phenotype is favored over any intermediate phenotype. Mutation and gene flow maintain variation within a population despite natural selection. Balanced polymorphism exists due to a heterozygote that hides the recessive allele from selection. Species remain reproductively isolated due to prezygotic and postzygotic isolating mechanisms. Speciation occurs when populations become isolated from one another, most often due to a geographic barrier. Allopatric speciation requires a geographic barrier; sympatric speciation does not. EXEIRCASES Study the text section by section as you answer the following questions. PPRININIPIMMISMIMP 11111111 HIONIFMINivmm 3 0 6) • The Hardy-Weinberg principle defines evolution in terms of allele frequency changes in a population over time. The raw material for evolutionary change is mutations, both genetic and chromosomal. Recombination of genes is another source in sexually reproducing organisms. An investigator determines, by inspection, that 4% of a population is albino. Answer the following questions about this population: a. q 2 b. This represents the percentage of the population that is c. What is the frequency of the recessive allele in this population? q = d. Considering the frequency of the recessive allele, what is the frequency of the dominant allele? p= e. If p = this value, then p 2 = f. This is the frequency of the population that is g. The value of 2pq = h. This represents the frequency of the population that is i. What percentage of the population has normal pigmentation? 145 2. Forty-nine percent of a population cannot taste a chemical called PTC. The presence of a dominant allele is necessary to taste this substance. Complete the following information about the gene pool of the population: a. q2 • b. q = c. p = d. p2 = e. 2pq = 3. a. Using this Punnett square, show that the next generation of the population in question 2 will have exactly the same composition, assuming a Hardy-Weinberg equilibrium. I ( ( )T )t ( ( )TT )Tt ( ( )Tt )tt b. The frequency of T = c. The frequency of t= d. Describe the gene pool of the next generation. e. What does this prove? f. How do we know when evolution occurs? 4. Label each statement as one of the following agents of evolutionary change: gene flow genetic drift mutations natural selection nonrandom mating a. Investigators have discovered that multiple alleles are common in a population. b. Populations are subject to new alleles entering due to the migration of organisms between populations. c. Female birds of paradise choose mates with the most splendid feathers. d. Investigators discovered that if they randomly picked out a few flies from each generation to start the next generation, gene pool frequency changes appeared. e. Giraffes with longer necks get a larger share of resources and tend to have more offspring. 5. Match these descriptions to one of the agents of evolutionary change listed in question 4. (Some agents are used more than once.) 146 a. Dwarfism is common among the Amish of Lancaster County, Pennsylvania. b. Cheetahs are homozygous for a larger proportion of their genes. c. This agent of change tends to make the members of a population dissimilar to one another. d. This agent of change tends to make the members of a population similar to one another. e. Certain members of a population are more fit than other members. f. Bacteria and insects become resistant to agents that formerly killed them. 6. Indicate whether these statements, related to sources of variation among diploid members in a population, are true (T) or false (F). a. Genetic mutations occur .at random. b. The only mutations that occur are those that make organisms more fit. c. Some chromosomal mutations are simply a change in chromosomal number. d. An offspring receives recombined genes because of the events of gametogenesis and fertilization. e. Recombination is a significant source of variation because many traits are polygenic. • Natural selection occurs when the allele frequencies in a population change due to the differential ability of certain phenotypes to reproduce. • Natural selection results in adaptation to the environment. The three types of natural selection are directional selection, stabilizing selection, and disruptive selection. 7. Natural selection can now be understood in terms of genetics. Many of the variations that exist between members of a population are due to differences in ab. to a(n) C. . Some of these genotypes result in better adapted to the environment. Individuals better adapted to the environment reproduce extent, and therefore these genotypes and phenotypes become more prevalent in the population. 8 . Match the observations to the correct type of natural selection at work: directional disruptive stabilizing a. Trees in a windy area tend to remain the same size each year. b. The brain size of hominids steadily increases. c. Individuals from the same moth species tend to have blue stripes in open areas and orange stripes in forested areas. 9. Match the types of natural selection listed in question 8 with the following diagrams: 10. Variation is maintained in a population despite directional and stabilizing selection when members have a. alleles for every trait. That's because the 13. allele is hidden by the C. allele. In instances such as sickle cell disease, in which the d. genotype is more fit, the two e. genotypes are maintained due to the reproductive process, which involves meiosis and fertilization. • New species come about when populations are reproductively isolated from other, similar populations. • Adaptive radiation is the rapid development of several species from a single species; each species is adapted in a unique way. 11. Bush babies (a type of primate) that live higher in the tropical canopy are a different species from those living lower in the canopy. Answer the following yes or no: a. Would the two species of bush babies reproduce with each other? b. Would a premating mechanism separate the two species of bush babies? c. Could the two species of bush babies eat the same food? d. Would the two species of bush babies have to look dissimilar enough to be distinguishable by the naked eye? 12. Label each of these statements with pre for prezygotic isolating mechanism or post for postzygotic isolating mechanism. Zygote mortality exists. a. Species reproduce at different times. b. Species have genitalia unsuitable to each other. c. 13. Label the statements with the modes of speciation. Some statements involve both modes of speciation. SS—sympatric speciation AS—allopatric speciation Geographic isolation occurs. a. Does not require geographic isolation to develop. b. Each population continues on its own evolutionary path. c. Two species now exist. d. A population develops into two or more reproductively isolated groups. e. f. g. 148 Postzygotic and prezygotic reproductive isolation develop. The occurrence of polyploidy in plants is an example.