Bsci110b Chapter 23 Concept Check

Explain why genetic variation within a population is a prerequisite for evolution. Within a population, genetic differences among individuals provide the raw material on which  and other mechanisms can act.  Without such differences,  could not change over time, and hence the population could not .What factors can produce genetic variation among populations.Genetic variation among populations can arise by natural selection if selection favors different  in different ; this might occur, for example, if the different populations experienced different .  Genetic variation among populations can also arise by  when the genetic differences between populations are selectively .

 Of all the mutations that occur in a population, why do only a small fraction become widespread among the population’s members? Many mutations occur in  cells that do not produce gametes and so are lost when the organism dies.  Of mutations that do occur in cell lines that produce gametes, many do not have a phenotypic effect on which  can act.  Others have a harmful effect and are thus unlikely to increase in frequency because they decrease the reproductive success of their bearers.

 If a population stopped reproducing sexually (but still reproduced asexually), how would its genetic variation be affected over time?  Its genetic variation, whether measured at the level of the gene or at the level of nucleotide sequences, would probably  over time.  During meiosis,  and  of chromosomes produce many new combinations of alleles.  In addition, a population contains a vast number of possible mating combinations, and fertilization brings together the gametes of individuals with different genetic backgrounds.  Thus, via crossing over, independent assortment of chromosomes, and fertilization, sexual reproduction reshuffles alleles into fresh combinations each generation.  Without , new sources of genetic variation would be reduced, causing the overall amount of genetic variation to drop.

What are the three sources of alleles to produce fresh combinations each generation.

 In what sense is natural selection more “predictable” than genetic drift? Natural selection is more “predictable” in that it alters allele frequencies in a  way: It tends to  the frequency of alleles that increase the organism’s reproductive success in its environment and  the frequency of alleles that decrease the organism’s reproductive success.  Alleles subject to genetic drift increase or decrease in frequency by  alone, whether or not they are advantageous.

Distinguish genetic drift from gene flow in terms of (a) how they occur and (b) their implications for future genetic variation in a population.  results from chance events that cause allele frequencies to fluctuate at random from generation to generation; within a population, this process tends to  genetic variation over time.   is the exchange of alleles between populations; a process that can introduce new alleles to a population and hence may  its genetic variation.

Explain why natural selection is the only evolutionary mechanism that consistently leads to adaptive evolution.Although both  and  can increase the frequency of advantageous alleles in a population, they can also  the frequency of advantageous alleles or the frequency of harmful alleles.  Only  consistently results in an increase in the frequency of alleles that enhance survival or reproduction.  Thus, is the only mechanism that consistently causes adaptive evolution.