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Posted by OneGlove [OneGlove] on November 26, 1999 at 09:04:31 {2lTH7z2znE4DjahjRee6VOGhhkwG5g}:

In Reply to: *WOW! posted by DaJahVeu on November 26, 1999 at 08:11:39:

I have offered no theories that I no longer accept as valid.

As for instances of speciation, I have put quite a few below. (I've had to break this up into two posts. This is taken from the speciation FAQ at www.talkorigins.org)

Would you care to eat your words?

5.0 Observed Instances of Speciation The following are several examples of observations
of speciation.

5.1 Speciations Involving Polyploidy, Hybridization or Hybridization Followed by
Polyploidization.

5.1.1 Plants (See also the discussion in de Wet 1971).

5.1.1.1 Evening Primrose (Oenothera gigas)

While studying the genetics of the evening primrose, Oenothera lamarckiana, de Vries (1905) found
an unusual variant among his plants. O. lamarckiana has a chromosome number of 2N = 14. The
variant had a chromosome number of 2N = 28. He found that he was unable to breed this variant
with O. lamarckiana. He named this new species O. gigas.

5.1.1.2 Kew Primrose (Primula kewensis)

Digby (1912) crossed the primrose species Primula verticillata and P. floribunda to produce a sterile
hybrid. Polyploidization occurred in a few of these plants to produce fertile offspring. The new
species was named P. kewensis. Newton and Pellew (1929) note that spontaneous hybrids of P.
verticillata and P. floribunda set tetraploid seed on at least three occasions. These happened in
1905, 1923 and 1926.

5.1.1.3 Trapopogonan

Owenby (1950) demonstrated that two species in this genus were produced by polyploidization
from hybrids. He showed that Tragopogon miscellus found in a colony in Moscow, Idaho was
produced by hybridization of T. dubius and T. pratensis. He also showed that T. mirus found in a
colony near Pullman, Washington was produced by hybridization of T. dubius and T. porrifolius.
Evidence from chloroplast DNA suggests that T. mirus has originated independently by hybridization
in eastern Washington and western Idaho at least three times (Soltis and Soltis 1989). The same
study also shows multiple origins for T. micellus.

5.1.1.4 Raphanobrassica

The Russian cytologist Karpchenko (1927, 1928) crossed the radish, Raphanus sativus, with the
cabbage, Brassica oleracea. Despite the fact that the plants were in different genera, he got a sterile
hybrid. Some unreduced gametes were formed in the hybrids. This allowed for the production of
seed. Plants grown from the seeds were interfertile with each other. They were not interfertile with
either parental species. Unfortunately the new plant (genus Raphanobrassica) had the foliage of a
radish and the root of a cabbage.

5.1.1.5 Hemp Nettle (Galeopsis tetrahit)

A species of hemp nettle, Galeopsis tetrahit, was hypothesized to be the result of a natural
hybridization of two other species, G. pubescens and G. speciosa (Muntzing 1932). The two
species were crossed. The hybrids matched G. tetrahit in both visible features and chromosome
morphology.

5.1.1.6 Madia citrigracilis

Along similar lines, Clausen et al. (1945) hypothesized that Madia citrigracilis was a hexaploid
hybrid of M. gracilis and M. citriodora As evidence they noted that the species have gametic
chromosome numbers of n = 24, 16 and 8 respectively. Crossing M. gracilis and M. citriodora
resulted in a highly sterile triploid with n = 24. The chromosomes formed almost no bivalents during
meiosis. Artificially doubling the chromosome number using colchecine produced a hexaploid hybrid
which closely resembled M. citrigracilis and was fertile.

5.1.1.7 Brassica

Frandsen (1943, 1947) was able to do this same sort of recreation of species in the genus Brassica
(cabbage, etc.). His experiments showed that B. carinata (n = 17) may be recreated by hybridizing
B. nigra (n = 8) and B. oleracea, B. juncea (n = 18) may be recreated by hybridizing B. nigra and B.
campestris (n = 10), and B. napus (n = 19) may be recreated by hybridizing B. oleracea and B.
campestris.

5.1.1.8 Maidenhair Fern (Adiantum pedatum)

Rabe and Haufler (1992) found a naturally occurring diploid sporophyte of maidenhair fern which
produced unreduced (2N) spores. These spores resulted from a failure of the paired chromosomes
to dissociate during the first division of meiosis. The spores germinated normally and grew into
diploid gametophytes. These did not appear to produce antheridia. Nonetheless, a subsequent
generation of tetraploid sporophytes was produced. When grown in the lab, the tetraploid
sporophytes appear to be less vigorous than the normal diploid sporo- phytes. The 4N individuals
were found near Baldwin City, Kansas.

5.1.1.9 Woodsia Fern (Woodsia abbeae)

Woodsia abbeae was described as a hybrid of W. cathcariana and W. ilvensis (Butters 1941).
Plants of this hybrid normally produce abortive sporangia containing inviable spores. In 1944 Butters
found a W. abbeae plant near Grand Portage, Minn. that had one fertile frond (Butters and Tryon
1948). The apical portion of this frond had fertile sporangia. Spores from this frond germinated and
grew into prothallia. About six months after germination sporophytes were produced. They survived
for about one year. Based on cytological evidence, Butters and Tryon concluded that the frond that
produced the viable spores had gone tetraploid. They made no statement as to whether the
sporophytes grown produced viable spores.

5.1.2 Animals Speciation through hybridization and/or polyploidy has long been considered much less
important in animals than in plants [[[refs.]]]. A number of reviews suggest that this view may be
mistaken. (Lokki and Saura 1980; Bullini and Nascetti 1990; Vrijenhoek 1994). Bullini and Nasceti
(1990) review chromosomal and genetic evidence that suggest that speciation through hybridization
may occur in a number of insect species, including walking sticks, grasshoppers, blackflies and
cucurlionid beetles. Lokki and Saura (1980) discuss the role of polyploidy in insect evolution.
Vrijenhoek (1994) reviews the literature on parthenogenesis and hybridogenesis in fish. I will tackle
this topic in greater depth in the next version of this document.

5.2 Speciations in Plant Species not Involving Hybridization or Polyploidy

5.2.1 Stephanomeira malheurensis Gottlieb (1973) documented the speciation of Stephanomeira
malheurensis. He found a single small population (< 250 plants) among a much larger population (>
25,000 plants) of S. exigua in Harney Co., Oregon. Both species are diploid and have the same
number of chromosomes (N = 8). S. exigua is an obligate outcrosser exhibiting sporophytic
self-incompatibility. S. malheurensis exhibits no self- incompatibility and self-pollinates. Though the
two species look very similar, Gottlieb was able to document morphological differences in five
characters plus chromosomal differences. F1 hybrids between the species produces only 50% of the
seeds and 24% of the pollen that conspecific crosses produced. F2 hybrids showed various
developmental abnormalities.

5.2.2 Maize (Zea mays) Pasterniani (1969) produced almost complete reproductive isolation between
two varieties of maize. The varieties were distinguishable by seed color, white versus yellow. Other
genetic markers allowed him to identify hybrids. The two varieties were planted in a common field.
Any plant's nearest neighbors were always plants of the other strain. Selection was applied against
hybridization by using only those ears of corn that showed a low degree of hybridi- zation as the
source of the next years seed. Only parental type kernels from these ears were planted. The strength
of selection was increased each year. In the first year, only ears with less than 30% intercrossed
seed were used. In the fifth year, only ears with less than 1% intercrossed seed were used. After five
years the average percentage of intercrossed matings dropped from 35.8% to 4.9% in the white
strain and from 46.7% to 3.4% in the yellow strain.

5.2.3 Speciation as a Result of Selection for Tolerance to a Toxin: Yellow Monkey Flower (Mimulus guttatus)
At reasonably low concentrations, copper is toxic to many plant species. Several plants have been
seen to develop a tolerance to this metal (Macnair 1981). Macnair and Christie (1983) used this to
examine the genetic basis of a postmating isolating mechanism in yellow monkey flower. When they
crossed plants from the copper tolerant "Copperopolis" population with plants from the nontolerant
"Cerig" population, they found that many of the hybrids were inviable. During early growth, just after
the four leaf stage, the leaves of many of the hybrids turned yellow and became necrotic. Death
followed this. This was seen only in hybrids between the two populations. Through mapping studies,
the authors were able to show that the copper tolerance gene and the gene responsible for hybrid
inviability were either the same gene or were very tightly linked. These results suggest that
reproductive isolation may require changes in only a small number of genes.

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).

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 Gibson
1970.

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).

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.

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
speciation.

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.

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 discussed below.

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.

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.

5.4 Housefly Speciation Experiments

5.4.1 A Test of the Founder-flush Hypothesis Using Houseflies Meffert and Bryant (1991) used houseflies
to test whether bottlenecks in populations can cause permanent alterations in courtship behavior that
lead to premating isolation. They collected over 100 flies of each sex from a landfill near Alvin,
Texas. These were used to initiate an ancestral population. From this ancestral population they
established six lines. Two of these lines were started with one pair of flies, two lines were started
with four pairs of flies and two lines were started with sixteen pairs of flies. These populations were
flushed to about 2,000 flies each. They then went through five bottlenecks followed by flushes. This
took 35 generations. Mate choice tests were performed. One case of positive assortative mating
was found. One case of negative assortative mating was also found.

5.4.2 Selection for Geotaxis with and without Gene Flow Soans, et al. (1974) used houseflies to test
Pimentel's model of speciation. This model posits that speciation requires two steps. The first is the
formation of races in subpopulations. This is followed by the establishment of reproductive isolation.
Houseflies were subjected to intense divergent selection on the basis of positive and negative
geotaxis. In some treatments no gene flow was allowed, while in others there was 30% gene flow.
Selection was imposed by placing 1000 flies into the center of a 108 cm vertical tube. The first 50
flies that reached the top and the first 50 flies that reached the bottom were used to found positively
and negatively geotactic populations. Four populations were established:

Pop A + geotaxis, no gene flow
Pop B - geotaxis, no gene flow
Pop C + geotaxis, 30% gene flow
Pop D - geotaxis, 30% gene flow

Selection was repeated within these populations each generations. After 38 generations the time to
collect 50 flies had dropped from 6 hours to 2 hours in Pop A, from 4 hours to 4 minutes in Pop B,
from 6 hours to 2 hours in Pop C and from 4 hours to 45 minutes in Pop D. Mate choice tests were
performed. Positive assortative mating was found in all crosses. They concluded that reproductive
isolation occurred under both allopatric and sympatric conditions when very strong selection was
present.

Hurd and Eisenberg (1975) performed a similar experiment on houseflies using 50% gene flow and
got the same results.

5.5 Speciation Through Host Race Differentiation Recently there has been a lot of interest in
whether the dif- ferentiation of an herbivorous or parasitic species into races living on different hosts
can lead to sympatric speciation. It has been argued that in animals that mate on (or in) their
preferred hosts, positive assortative mating is an inevitable byproduct of habitat selection (Rice
1985; Barton, et al. 1988). This would suggest that differentiated host races may represent incipient
species.

5.5.1 Apple Maggot Fly (Rhagoletis pomonella) Rhagoletis pomonella is a fly that is native to North
America. Its normal host is the hawthorn tree. Sometime during the nineteenth century it began to
infest apple trees. Since then it has begun to infest cherries, roses, pears and possibly other members
of the rosaceae. Quite a bit of work has been done on the differences between flies infesting
hawthorn and flies infesting apple. There appear to be differences in host preferences among
populations. Offspring of females collected from on of these two hosts are more likely to select that
host for oviposition (Prokopy et al. 1988). Genetic differences between flies on these two hosts
have been found at 6 out of 13 allozyme loci (Feder et al. 1988, see also McPheron et al. 1988).
Laboratory studies have shown an asynchrony in emergence time of adults between these two host
races (Smith 1988). Flies from apple trees take about 40 days to mature, whereas flies from
hawthorn trees take 54-60 days to mature. This makes sense when we consider that hawthorn fruit
tends to mature later in the season that apples. Hybridization studies show that host preferences are
inherited, but give no evidence of barriers to mating. This is a very exciting case. It may represent the
early stages of a sympatric speciation event (considering the dispersal of R. pomonella to other
plants it may even represent the beginning of an adaptive radiation). It is important to note that some
of the leading researchers on this question are urging caution in inter- preting it. Feder and Bush
(1989) stated:

Hawthorn and apple "host races" of R. pomonella may therefore represent incipient
species. However, it remains to be seen whether host-associated traits can evolve into
effective enough barriers to gene flow to result eventually in the complete reproductive
isolation of R. pomonella populations.

5.5.2 Gall Former Fly (Eurosta solidaginis) Eurosta solidaginis is a gall forming fly that is associated with
goldenrod plants. It has two hosts: over most of its range it lays its eggs in Solidago altissima, but in
some areas it uses S. gigantea as its host. Recent electrophoretic work has shown that the genetic
distances among flies from different sympatric hosts species are greater than the distances among
flies on the same host in different geographic areas (Waring et al. 1990). This same study also found
reduced variability in flies on S. gigantea. This suggests that some E. solidaginis have recently shifted
hosts to this species. A recent study has compared reproductive behavior of the flies associated with
the two hosts (Craig et al. 1993). They found that flies associated with S. gigantea emerge earlier in
the season than flies associated with S. altissima. In host choice experiments, each fly strain
ovipunctured its own host much more frequently than the other host. Craig et al. (1993) also
performed several mating experiments. When no host was present and females mated with males
from either strain, if males from only one strain were present. When males of both strains were
present, statistically significant positive assortative mating was seen. In the presence of a host,
assortative mating was also seen. When both hosts and flies from both populations were present,
females waited on the buds of the host that they are normally associated with. The males fly to the
host to mate. Like the Rhagoletis case above, this may represent the beginning of a sympatric
speciation.

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