5.3 The Fruit Fly Literature
5.3.1 Drosophila paulistorum Dobzhansky and Pavlovsky (1971) reported a speciation event that occurred in a laboratory culture of Drosophila
paulistorum sometime between 1958 and 1963. The culture was descended from a single inseminated female that was captured in the Llanos of
Colombia. In 1958 this strain produced fertile hybrids when crossed with conspecifics of different strains from Orinocan. From 1963 onward crosses
with Orinocan strains produced only sterile males. Initially no assortative mating or behavioral isolation was seen between the Llanos strain and the
Orinocan strains. Later on Dobzhansky produced assortative mating (Dobzhansky 1972).
Such an occurence is common in the breeding and reproduction of canines and everyday street dogs. Are these spectacular examples of evolution? I think not.
In fact, it is, as I've been crying out all this time, simply a combination of genes and not the alteration or mutaton there-of.
5.3.2 Disruptive Selection on Drosophila melanogaster Thoday and Gibson (1962) established a population of Drosophila melanogaster from four gravid
females. They applied selection on this population for flies with the highest and lowest numbers of sternoplural chaetae (hairs). In each generation, eight
flies with high numbers of chaetae were allowed to interbreed and eight flies with low numbers of chaetae were allowed to interbreed. Periodically they
performed mate choice experiments on the two lines. They found that they had produced a high degree of positive assortative mating between the two
groups. In the decade or so following this, eighteen labs attempted unsuccessfully to reproduce these results. References are given in Thoday and
The flies that were used to breeding with hairy flies wanted to breed with hairy flies and those who were used to hairless flies wanted to breed with hairless flies.
This is not evolution. This is fly psychology. I might also ask why no other labs were able to accomplish this.
5.3.3 Selection on Courtship Behavior in Drosophila melanogaster Crossley (1974) was able to produce changes in mating behavior in two mutant strains of
D. melanogaster. Four treatments were used. In each treatment, 55 virgin males and 55 virgin females of both ebony body mutant flies and vestigial wing
mutant flies (220 flies total) were put into a jar and allowed to mate for 20 hours. The females were collected and each was put into a separate vial. The
phenotypes of the offspring were recorded. Wild type offspring were hybrids between the mutants. In two of the four treatments, mating was carried out
in the light. In one of these treatments all hybrid offspring were destroyed. This was repeated for 40 generations. Mating was carried out in the dark in
the other two treatments. Again, in one of these all hybrids were destroyed. This was repeated for 49 generations. Crossley ran mate choice tests and
observed mating behavior. Positive assortative mating was found in the treatment which had mated in the light and had been subject to strong selection
against hybridization. The basis of this was changes in the courtship behaviors of both sexes. Similar experiments, without observation of mating
behavior, were performed by Knight, et al. (1956).
This appears to have absolutely nothing to do with evolution.
5.3.4 Sexual Isolation as a Byproduct of Adaptation to Environmental Conditions in Drosophila melanogaster Kilias, et al. (1980) exposed D. melanogaster
populations to different temperature and humidity regimes for several years. They performed mating tests to check for reproductive isolation. They
found some sterility in crosses among populations raised under different conditions. They also showed some positive assortative mating. These things
were not observed in populations which were separated but raised under the same conditions. They concluded that sexual isolation was produced as a
byproduct of selection.
I have no idea what is being said here so I can't very well comment on it. The author is talking about reproduction that occured between two populations
raised in different environments. Then he turns around and says that this did not occur in populations from different environments, but in populations from similar
environments. I wish that the author would clarify these statements. If he was saying that the resulting population from the original two showed positive assortative mating,
then it was not conveyed clearly at all. If this is the case, then I would have to wonder why he chose the word "some." Instead of using "majority", "all", "most", or "many,
he uses the word some. This seems to say that, not a majority, but a minority (most likely small since he did not say many) showed assortive mating-that is-showed preferences.
5.3.5 Sympatric Speciation in Drosophila melanogaster In a series of papers (Rice 1985, Rice and Salt 1988 and Rice and Salt 1990) Rice and Salt presented
experimental evidence for the possibility of sympatric speciation. They started from the premise that whenever organisms sort themselves into the
environment first and then mate locally, individuals with the same habitat preferences will necessarily mate assortatively. They established a stock
population of D. melanogaster with flies collected in an orchard near Davis, California. Pupae from the culture were placed into a habitat maze. Newly
emerged flies had to negotiate the maze to find food. The maze simulated several environmental gradients simultaneously. The flies had to make three
choices of which way to go. The first was between light and dark (phototaxis). The second was between up and down (geotaxis). The last was between
the scent of acetaldehyde and the scent of ethanol (chemotaxis). This divided the flies among eight habitats. The flies were further divided by the time of
day of emergence. In total the flies were divided among 24 spatio-temporal habitats.
They next cultured two strains of flies that had chosen opposite habitats. One strain emerged early, flew upward and was attracted to dark and
acetaldehyde. The other emerged late, flew downward and was attracted to light and ethanol. Pupae from these two strains were placed together in the
maze. They were allowed to mate at the food site and were collected. Eye color differences between the strains allowed Rice and Salt to distinguish
between the two strains. A selective penalty was imposed on flies that switched habitats. Females that switched habitats were destroyed. None of their
gametes passed into the next generation. Males that switched habitats received no penalty. After 25 generations of this mating tests showed
reproductive isolation between the two strains. Habitat specialization was also produced.
They next repeated the experiment without the penalty against habitat switching. The result was the same -- reproductive isolation was produced.
They argued that a switching penalty is not necessary to produce reproductive isolation. Their results, they stated, show the possibility of sympatric
I always considered this to be a known fact, but apparently I was wrong. I have always been under the impression that the things humans (and apparently other
animals) liked were controlled, to some degree, by genetics. This experiment merely proved this and also proved that it can be passed down through multiple generations.
5.3.6 Isolation Produced as an Incidental Effect of Selection on several Drosophila species In a series of experiments, del Solar (1966) derived positively and
negatively geotactic and phototactic strains of D. pseudoobscura from the same population by running the flies through mazes. Flies from different
strains were then introduced into mating chambers (10 males and 10 females from each strain). Matings were recorded. Statistically significant positive
assortative mating was found.
In a separate series of experiments Dodd (1989) raised eight populations derived from a single population of D. Pseudoobscura on stressful media. Four
populations were raised on a starch based medium, the other four were raised on a maltose based medium. The fly populations in both treatments took
several months to get established, implying that they were under strong selection. Dodd found some evidence of genetic divergence between flies in the
two treatments. He performed mate choice tests among experimental populations. He found statistically significant assortative mating between
populations raised on different media, but no assortative mating among populations raised within the same medium regime. He argued that since there
was no direct selection for reproductive isolation, the behavioral isolation results from a pleiotropic by-product to adaptation to the two media. Schluter
and Nagel (1995) have argued that these results provide experimental support for the hypothesis of parallel speciation.
Less dramatic results were obtained by growing D. willistoni on media of different pH levels (de Oliveira and Cordeiro 1980). Mate choice tests after 26,
32, 52 and 69 generations of growth showed statistically significant assortative mating between some populations grown in different pH treatments. This
ethological isolation did not always persist over time. They also found that some crosses made after 106 and 122 generations showed significant hybrid
inferiority, but only when grown in acid medium.
First of all, I would like to know just what "statistically significant" means. This can be interpreted several ways. Because of word choice, I am inclined
to believe that this means a low occurence, but high enough to be noticeable. I would also like to know just what that genetic divergence was that Dodd found. I would
also like to know just what this has to do with evolution. I would also point out the final statement above.
5.3.7 Selection for Reinforcement in Drosophila melanogaster Some proposed models of speciation rely on a process called reinforcement to complete the
speciation process. Reinforcement occurs when to partially isolated allopatric populations come into contact. Lower relative fitness of hybrids between
the two populations results in increased selection for isolating mechanisms. I should note that a recent review (Rice and Hostert 1993) argues that there
is little experimental evidence to support reinforcement models. Two experiments in which the authors argue that their results provide support are
Ehrman (1971) established strains of wild-type and mutant (black body) D. melanogaster. These flies were derived from compound autosome strains
such that heterotypic matings would produce no progeny. The two strains were reared together in common fly cages. After two years, the isolation
index generated from mate choice experiments had increased from 0.04 to 0.43, indicating the appearance of considerable assortative mating. After four
years this index had risen to 0.64 (Ehrman 1973).
Along the same lines, Koopman (1950) was able to increase the degree of reproductive isolation between two partially isolated species, D.
pseudoobscura and D. persimilis.
I'm still waiting to see how this is related to evolution. That's not an evolutionary change! That's a reproductive pattern!
5.3.8 Tests of the Founder-flush Speciation Hypothesis Using Drosophila The founder-flush (a.k.a. flush-crash) hypothesis posits that genetic drift and
founder effects play a major role in speciation (Powell 1978). During a founder-flush cycle a new habitat is colonized by a small number of individuals
(e.g. one inseminated female). The population rapidly expands (the flush phase). This is followed by the population crashing. During this crash period the
population experiences strong genetic drift. The population undergoes another rapid expansion followed by another crash. This cycle repeats several
times. Reproductive isolation is produced as a byproduct of genetic drift.
Dodd and Powell (1985) tested this hypothesis using D. pseudoobscura. A large, heterogeneous population was allowed to grow rapidly in a very large
population cage. Twelve experimental populations were derived from this population from single pair matings. These populations were allowed to flush.
Fourteen months later, mating tests were performed among the twelve populations. No postmating isolation was seen. One cross showed strong
behavioral isolation. The populations underwent three more flush-crash cycles. Forty-four months after the start of the experiment (and fifteen months
after the last flush) the populations were again tested. Once again, no postmating isolation was seen. Three populations showed behavioral isolation in
the form of positive assortative mating. Later tests between 1980 and 1984 showed that the isolation persisted, though it was weaker in some cases.
Galina, et al. (1993) performed similar experiments with D. pseudoobscura. Mating tests between populations that underwent flush-crash cycles and
their ancestral populations showed 8 cases of positive assortative mating out of 118 crosses. They also showed 5 cases of negative assortative mating
(i.e. the flies preferred to mate with flies of the other strain). Tests among the founder-flush populations showed 36 cases of positive assortative mating
out of 370 crosses. These tests also found 4 cases of negative assortative mating. Most of these mating preferences did not persist over time. Galina, et
al. concluded that the founder-flush protocol yields reproductive isolation only as a rare and erratic event.
Ahearn (1980) applied the founder-flush protocol to D. silvestris. Flies from a line of this species underwent several flush-crash cycles. They were
tested in mate choice experiments against flies from a continuously large population. Female flies from both strains preferred to mate with males from
the large population. Females from the large population would not mate with males from the founder flush population. An asymmetric reproductive
isolation was produced.
In a three year experiment, Ringo, et al. (1985) compared the effects of a founder-flush protocol to the effects of selection on various traits. A large
population of D. simulans was created from flies from 69 wild caught stocks from several locations. Founder-flush lines and selection lines were derived
from this population. The founder-flush lines went through six flush-crash cycles. The selection lines experienced equal intensities of selection for
various traits. Mating test were performed between strains within a treatment and between treatment strains and the source population. Crosses were
also checked for postmating isolation. In the selection lines, 10 out of 216 crosses showed positive assortative mating (2 crosses showed negative
assortative mating). They also found that 25 out of 216 crosses showed postmating isolation. Of these, 9 cases involved crosses with the source
population. In the founder-flush lines 12 out of 216 crosses showed positive assortative mating (3 crosses showed negative assortative mating).
Postmating isolation was found in 15 out of 216 crosses, 11 involving the source population. They concluded that only weak isolation was found and that
there was little difference between the effects of natural selection and the effects of genetic drift.
A final test of the founder-flush hypothesis will be described with the housefly cases below.
Well, I hate to sound smug, but there you have it! The second paragraph describes how the first half of the theory worked, but not the second. Even the
results of the first experiment trailed off it says. The third paragraph describes an experiment designed to prove the theory, but in fact worked against it. Not
only did it produce low results, but opposite results also. The fourth paragraph describes an experiment that actually works. I would like to know, however, just
how the female flies knew which male flies came from the large population. The fifth paragraph explains another experiment that turned out to work against
the theory and the results closely followed those of the second experiment.
Go to Part 3