Name/Section __________________________

BIO. 140 QUIZ A

1. (MANDATORY QUESTION 25pts.) Explain how one mode of selection contributes to speciation and the evolution of species recognition mechanisms. Why do you think a behavioral isolating mechanism might evolve between two species? Is sterility enough to prevent species from mixing?

Disruptive selection can lead to the formation of a new type from an

existing type. (+4 POINTS)

Post-mating isolating mechanisms lead to hybrid unfitness (+3 PTS), which leads to selection on behaviors for mate discrimination mechanisms (+3 PTS): need male signal and female recognition (in most systems).

EXAMPLE (+2 PTS)

Behavioral isolating mechanisms evolve b/c of costs associated with mating with the wrong species. An organism that mates with a different "semispecies" may produce low viability offspring and this individual would have lower fitnesscompared to an organism that discriminates against such semispecies and mates with members of its own semispecies. (+6 PTS)

STERILITY BY ITSELF WILL NOT LEAD TO CHOOSIER FEMALES, NEED SOME ACTION ON THE FEMALE OR MALE CHOICE FRONT FOR SPECIATION BY BEHAVIOR TO BECOME EFFECTIVE (OR SELECTED FOR) -- SELECT FOR MORE MATE DISCRIMINATION ON THE PART OF FEMALES (OR MALES.) (+7 PTS)

2. (CHOOSE 1 OF THE FOLLOWING 2 QUESTIONS TO ANSWER 15 pts.)

a) Name and explain three primary ways (modes) selection affects fitness of individuals. Draw graphs to represent the changes each mode of selection would be expected to cause in the mean and the variance of a population

1.directional selection in which the trait is linearly related to fitness,

NAME/EXPLANATION (+2 PTS)

CORRECT GRAPH (+2 PTS)

CORRECT MENTION OF MEAN/VARIANCE (+1 PT)

2.stabilizing in which there is an optimal value for the trait of interest

NAME/EXPLANATION (+2 PTS)

CORRECT GRAPH (+2 PTS)

CORRECT MENTION OF MEAN/VARIANCE (+1 PT)

3.disruptive in which individuals with the smallest and largest values of the trait have the highest fitness and individuals with intermediate

values are at a fintess disadvantage.

NAME/EXPLANATION (+2 PTS)

CORRECT GRAPH (+2 PTS)

CORRECT MENTION OF MEAN/VARIANCE (+1 PT)

OR

b) In the t-allele example, how is the force of genic selection balanced by the force of individual selection? How does the process of group selection favor non-t-allele mice?

GENIC SELECTION: The case of the t-allele is useful for illustrating a selfish gene. The t-allele spreads and increases in frequency up to the point where a significant number of males in the population are in fact sterile. Thus, without concern for the fitness of the individual (e.g., sterility in some cases), the t-allele marches to very high frequencies and leads to a considerable depression in the mean fitness of males in the population.

INDIVIDUAL SELECTION balances genic selection by selecting against those individuals homozygous for the t-allele (i.e. sterility = genetic death)

[The t-allele cannot spread beyond a certain frequency or the whole population will end up sterile. The t-allele needs a few +-alleles around in order

to propagate. However, when two t-alleles end up in a single individual, the process of individual selection limits the spread of the allele.]

(+7.5 POINTS)

The spread of the t-allele is in fact limited by the group structure and behavior of mice. Mice live in small demes with usually 2 males, and a few

females. The t-allele should increase in frequency to well beyond 70% in the population. With only two males in each group there is a really good

possibility that some groups will have no fertile males and other groups will have fertile males. Groups that breed true for +/+ spawn off more groups that breed true and are more resistant to extinction relative to t-allele "infected" groups. The

groups that have a t-allele will produce progeny with a greater incidence of the t-allele. Genetic drift, the random sampling of alleles might actually

push some of the spawn of these groups over the top and they go to the "t/t and t/t" sterile state. Any sterile group goes extinct. Thus, t=alleles are

being continually removed by a group extinction process.

(+7.5 POINTS)