Ornithology, Evolution, and Philosophy 123

A Modern Unified Theory of Evolution 229
Dobzhansky, the experiments were mostly conducted by Mayr. The method used
was multiple choice: one kind of males was placed in a container with two kinds of
suitably marked females, and it was tested whether and to what extent insemination
was random. The authors decided for male choice, although they realized that in
animals, female choice prevails (but is more difficult to test).
Innate species recognition is the rule in most of the lower vertebrates and invertebrates,
might be influenced by conditioning. Mayr and Dobzhansky’s (1945d)
findings with Drosophila pseudoobscura and D. persimilis indicated that males inseminate
a higher percentage of females of their own than of the other species
and that the degree of preference can be altered slightly through conditioning,
by previous association with the respective females. Since there is little doubt
that males’ preference of their own species is mainly controlled by genetic factors,
Mayr (1946d) tested the hybrids in regard to their position on the mating
preference scale: Males of D. persimilis inseminate more hybrids than females of
their own species. Males of D. pseudoobscura show a slight preference for their
own as compared to hybrid females. Mayr (1946g) also analyzed their courtship
behavior (wing vibration, circling the female), described copulation and discussed
the role of sense organs in species discrimination. In multiple choice experiments
mating is not random, but highly discriminative; conspecific matings are much
more frequent than heterogamic matings.
What Mayr labeled his “best experimental work” concerns the role of antennae
in the mating behavior of female Drosophila (Mayr 1950c). After many difficulties,
he mastered the operation to remove the antennae from these very small
flies. The readiness for mating was then drastically lowered. However, since the
antennae are not the only organ by which the females receive stimuli from males,
even antennae-less females will eventually copulate. The removal of the antennae
also decreased the females’ ability to discriminate between males of the sibling
species D. pseudoobscura and D. persimilis. Inretrospectitmustbenotedthat
these experiments were indecisive because, in nature, females choose their mates.
Dobzhansky had persuaded Mayr to base his experiments on male choice for
technical reasons.
“Dobzhansky was an amazingly hard worker. Part of our work was to determine
what percentage of the females had been inseminated. This meant anaesthetizing
these females (killing them) and then dissecting their abdomen to see whether
there was sperm in the genital tract. Each examination took a certain amount of
time, as did collecting the material, killing the females, spreading them out on
a glass slide, etc. Dobzhansky succeeded to do 250 such dissections in a single
day. I, who sat opposite him and tried to work just as hard and as fast, never got
above 180 females. The only consolation I had was that I said to myself that my
examination was more thorough than Dobzhansky’s, and I sometimes discovered
spermatozoa in females which Dobzhansky had recorded as uninseminated. This
illustrates one of his weaknesses. Dobzhansky was a very impatient person, and
the painstaking analysis of something was very difficult for him.”