Evolution__3rd_Edition

422 PART 4 / Evolution and Diversity
Study and review questions
1 When two populations are kept experimentally in
different conditions for a number of generations,
reproductive isolation is found to have evolved between
them. What is the genetic reason for the evolution of
reproductive isolation? Give an example.
2 How can the amount of (a) postzygotic, and
(b) prezygotic isolation be expressed quantitatively? For
(b) imagine a mating experiment, where an individual
female of species 1 is given a choice between a male of
species 1 and a male of species 2. The experiment is
repeated with 100 females. The numbers of females who
mated with each kind of male are given below. Calculate
an index of prezygotic reproductive isolation (I) for the
three cases.
Species 1 male Species 2 male I
(i) 100 0
(ii) 75 25
(iii) 50 50
3 How does the phylogeny of the greenish warblers
around the Tibetan Plateau help us to understand the
evolution of ring species?
4 Why is postzygotic isolation theoretically unlikely to be
due genetically to a single locus?
5 (a) What is Haldane’s rule? (b) If humans split in the
future into two species, for instance following
colonization into the galaxy, do you expect the sons or
daughters of the hybrids bertween the two emerging
species to evolve sterility first? (c) How can the
Dobzhansky–Muller theory explain Haldane’s rule?
6 What is meant by “valley crossing” in the origin of
species? Is there valley crossing when: (a) prezygotic
isolation evolves by pleiotropy; (b) postzygotic isolation
evolves by the Dobzhansky–Muller process; and
(c) postzygotic isolation evolves by changes at a
single genetic locus?
7 Two species have partly overlapping ranges. Females
of species 1, taken from an area where only species 1
lives and given an experimental choice between males
of the two species, mate indiscriminately. Females of
species 1 taken from the area where both species live and
given the same choice mate preferentially with males of
species 1. What is the phenomenon called? And what are
the two main evolutionary explanations for it?
8 What reasons suggest that reinforcement may be a
weak evolutionary force in nature?
9 Explain why hybrid zones exist according to the theory
of (a) allopatric and (b) parapatric speciation.
10 How can we test between the allopatric and
sympatric theories of speciation, using phylogenetic
trees?
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