31. Which of these should decline in hybrid zones where reinforcement is occurring?
(A) gene flow between distinct gene pools
(B) speciation
(C) the genetic distinctness of two gene pools
(D) mutation rate
(E) hybrid sterility

27. Trace elements are those required by organisms in only minute quantities. Which of the following is a trace element that is required by humans and other vertebrates? (A) nitrogen (B) calcium (C) iodine (D) sodium (E) phosphorus

28. Which of the following pieces of evidence most strongly supports the common origin of all life on Earth? (A) All organisms require energy. (B) All organisms show heritable variation. (C) All organisms reproduce. (D) All organisms use essentially the same genetic code. (E) All organisms have undergone evolution.

29. Which of these is the smallest unit upon which natural selection directly acts? (A) a species' gene frequency (B) a population's gene frequency (C) an individual's genome (D) an individual's genotype (E) an individual's phenotype

30. The higher the proportion of loci that are "fixed" in a population, the lower is that population's (A) nucleotide variability. (B) genetic polyploidy. (C) average heterozygosity. (D) A, B, and C (E) A and C only

32. The largest unit within which gene flow can readily occur is a (A) population. (B) species. (C) genus. (D) hybrid. (E) phylum.

33. Which statement represents the best explanation for the observation that the nuclear DNA of wolves and domestic dogs has a very high degree of homology? (A) Dogs and wolves have very similar morphologies. (B) Dogs and wolves belong to the same order. (C) Dogs and wolves are both members of the order Carnivora. (D) Dogs and wolves shared a common ancestor very recently. (E) Convergent evolution has occurred.

34. Which of these statements about prokaryotes is correct? (A) Bacterial cells conjugate to mutually exchange genetic material. (B) Their genetic material is confined within a nuclear envelope. (C) They divide by binary fission, without mitosis or meiosis. (D) The persistence of bacteria throughout evolutionary time is due to their genetic homogeneity (i.e., sameness). (E) Genetic variation in bacteria is not known to occur, nor should it occur, because of their asexual mode of reproduction.

35. If all prokaryotes on Earth suddenly vanished, which of the following would be the most likely and most direct result? (A) The number of organisms on Earth would decrease by 10—20%. (B) Human populations would thrive in the absence of disease. (C) Bacteriophage numbers would dramatically increase. (D) The recycling of nutrients would be greatly reduced, at least initially. (E) There would be no more pathogens on Earth.

36. Which of the following is an ongoing trend in the evolution of land plants? (A) decrease in the size of the leaf (B) reduction of the gametophyte phase of the life cycle (C) elimination of sperm cells or sperm nuclei (D) increasing reliance on water to bring sperm and egg together (E) replacement of roots by rhizoids

37. Cephalization is primarily associated with (A) adaptation to dark environments. (B) method of reproduction. (C) fate of the blastopore. (D) type of digestive system. (E) bilateral symmetry.