Evolution__3rd_Edition

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Figure 14.2
Prezygotic isolation emerges
between populations that have
adapted to different conditions.
(a) The experimental design:
four populations of fruitflies
were kept on starch medium
and four others on maltose
medium. After a number of
generations the tendency of
the flies to mate with others
like themselves was measured.
In the experimental series,
12 females from a maltose
population and 12 more from
a starch population were put
in a cage with 12 males from
a starch and 12 males from a
maltose population; the
numbers of the four kinds of
mating couple that formed were
counted. One such experiment
was done for each of the four
starch populations with each of
the four maltose populations,
making 16 experiments in all.
(b) An example of the results.
(c) The average isolation for all
16 experiments. In the control
series, 12 females and 12 males
from one of the four starch
populations were put with
another 12 males and 12
females from another of the
four starch populations; the
same controls were done with
the maltose populations too.
Again, one example is given
in (b): a pair may be formed
between a male and a female
from the same starch
population, or a male from one
of the four starch populations
may pair with a female from
a different starch population.
Notice the higher value of
the isolation index for the
experimental crosses than the
control ones in (c). Prezygotic
isolation had evolved between
the populations that had
experienced different media,
but not among populations
that were isolated but on the
same media. Drawn from
data in Dodd (1989).
(a)
Mating
experiments
CHAPTER 14 / Speciation 385
Initial sample of fruitflies
Starch medium Maltose medium
Lines evolve separately
for several generations
No preference
(c) Average isolation indexes for all 16 crosses
Preference for mate
from maltose line
(b) Examples of results
Experimental cage Control cage
Female Female
Starch Maltose
Male
Starch
22
9
Maltose
8
20
Isolation index = ( ) = 0.42
42 – 17
Same Different
Male
Same
18
15
Different 12
15
––––––
59
Isolation index = ( ) = 0.1
33 – 27
––––––
60
Starch × maltose population crosses
Control crosses
Average isolation index
0.33
0.014
The isolation index is calculated as (number of matings to same type – number of matings to different type)/total number
of matings. It varies from 1 (for complete reproductive isolation) to 0 (for random mating, or zero isolation) through to the
theoretical extreme of –1 if matings were exclusively between opposite typed flies.