Cross posted from talk_origins:
From: firstname.lastname@example.org (Joseph E Boxhorn) Path:
Newsgroups: talk.origins Subject: FAQ: Observed Instances of
Organization: Computing Services Division, University of
Wisconsin - Milwaukee
Message-ID: <23tmqeINN598@uwm.edu> Date: 6 Aug 1993 13:37:18
Observed Instances of Speciation
1.0 Introduction and Acknowledgements
This FAQ presents descriptions of instances where speciation has been
observed. It also discusses several issues related to speciation. I have
divided it into several sections. Part 2 discusses several definitions of what
a species is. Part 3 explains the context in which observations of speciation
are made. Part 4 looks at the question, "How can we tell when a speciation
event has occurred?" Part 5 describes a number of observed speciation events
and several experiments which (IMO) failed to produce speciation. Part 6 is a
list of references. Part 7 is a table of contents. I place the table of
contents at the end rather than the beginning to aid readers in getting through
a post which is already longer than it has any right to be. I have divided
this into two files. The first contains parts 1 -5. The second contains parts
6 and 7. They will be consolidated when I send this to Brent's archive.
The descriptions of each observation come from the primary literature.
I went back to this literature for two reasons. First, many of these
observations are not discussed (or not discussed in much detail) in secondary
sources such as reviews, texts, etc. Second, it is difficult, if not
impossible, to evaluate what a piece of research actually established without
looking at the methods or data. Secondary sources rarely give this information
in any detail. Anyway, I have only included observations that I have been able
to find the original sources for.
I consider this FAQ incomplete. One reason for this is that I am still
chasing references (I still have a list of over 25 to find). More important is
the fact that observations of speciation are buried in papers on a number of
topics. If you know of observations that I should include, let me know and I
will modify the file. I ask that you try to give me as complete a reference as
possible to aid me in finding the original source.
Back in April, Rich Fox asked a series of questions related to species
and speciation events. These questions got me interested in the topic. I hope
that I have, at least, provided grist for the mill that will grind out an
answer to Rich's questions. In any case, Rich deserves the credit (or blame
:-)) for inspiring me to write this. My starting point was the references
contained in the old speciation FAQ. I wish to thank the authors of this,
Chris Stassen, James Meritt and Anneliese Lilje. Finally, Tom Scharle and
Simon Clippingdale sent a couple of references my way. Many thanks to all.
2.0 Species Definitions
A discussion of speciation requires a definition of what constitutes a
species. This is a topic of considerable debate within the biological
community. Three recent reviews in the Journal of Phycology give some idea of
the scope of the debate (Castenholz 1992, Manhart and McCourt 1992, Wood and
Leatham 1992). There are a variety of different species concept currently in
use by biologists. These include folk, biological, morphological, genetic,
paleontological, evolutionary, phylogenetic and biosystematic definitions. In
the interest of brevity, I'll only discuss four of these -- folk, biological,
morphological and phylogenetic. A good review of species definitions is given
in Stuessy 1990.
2.1 The Folk Concept of Species
Naturalists around the world have found that the individual plants and
animals they see can be mentally grouped into a number of taxa, in which the
individuals are basically alike. In societies that are close to nature, each
taxon is given a name. These sorts of folk taxonomies have two features in
common. One aspect is the idea of reproductive compatability and continuity
within a species. Dogs beget dogs, they never beget cats! This has a firm
grounding in folk knowledge. The second notion is that there is a
discontinuity of variation between species. In other words, you can tell
species apart by looking at them (Cronquist 1988).
2.2 The Biological Species Concept
Over the last few decades the theoretically preeminent species
definition has been the biological species concept (BSC). This concept defines
a species as a reproductive community.
2.2.1 History of the Biological Species Concept.
The BSC has undergone a number of changes over the years. The earliest
precursor that I could find was in Du Rietz 1930. Du Rietz defined a species
"... the smallest natural populations permanently separated
from each other by a distinct discontinuity in the series of
Barriers to interbreeding are implicit in this definition and explicit
in Du Rietz's dicussion of it.
A few years later, Dobzhansky defined a species as
"... that stage of evolutionary progress at which the once
actually or potentially interbreeding array of forms becomes
segregated into two or more separate arrays which are
physiologically incapable of interbreeding."
It is important to note that this is a highly restrictive definition of
species. It emphasizes experimental approaches and ignores what goes on in
nature. By the publication of the third edition of the book this appeared in,
Dobzhansky (1951) had relaxed this definition to the point that is
substantially agreed with Mayr's.
The definition of a species that is accepted as the BSC was promugated
by Mayr (1942). He defined species as
"... groups of actually or potentially interbreeding
natural populations which are reproductively isolated from
other such groups."
Note that the emphasis in this definition is on what happens in nature.
Mayr later amended this definition to include an ecological component. In this
form of the definition a species is
"... a reproductive community of populations (reproductively
isolated from others) that occupies a specific niche in nature."
The BSC is most strongly accepted among vertebrate zoologists and
entomologists. Two facts account for this. First, these are the
groups that the authors of the BSC worked with :-). (Note: Mayr
is an ornithologist and Dobzhansky worked extensively with
Drosophila). More importantly, obligate sexuality is the
predominant for of reproduction in these groups. It is not
coincidental that the BSC is less widely accepted among botanists.
Terrestrial plants exhibit much greater diversity in their "mode of
reproduction" than do vertebrates and insects.
2.2.2 Criticisms of the Biological Species Concept
There has been considerable criticism of the theoretical validity
and practical utility of the BSC. (Cracraft 1989, Donoghue 1985,
Levin 1979, Mishler and Donoghue 1985, Sokal and Crovello 1970).
The application of the BSC to a number of groups, including land
plants, is problematical because of interspecific hybridization
between clearly delimited species (McCourt and Hoshaw 1990,
There is an abundance of asexual populations that this definition
just doesn't apply to (Budd and Mishler 1990). Examples of taxa
which are obligately asexual include bdelloid rotifers, euglenoid
flagellates, some members of the Oocystaceae (coccoid green algae),
chloromonad flagellates and some araphid pennate diatoms. Asexual
forms of normally sexual organisms are known. Obligately asexual
populations of Daphnia are found in some arctic lakes. The BSD
can be of no help in delimiting species in these groups. A similar
situation is found in the prokaryotes. Though genes can be exchanged
among bacteria by a number of mechanisms, sexuality, as defined in
eukaryotes, in unknown in the prokaryotes. One popular microbiology
text doesn't even mention the BSC (Brock and Madigan 1988).
The applicability of the BSC is also questionable in
those land plants that primarily self-pollinate (Cronquist 1988).
A more serious criticism is that the BSC is
inapplicable in practice. This charge asserts that, in most
cases, the BSC cannot be practically applied to delimit
species. The BSC suggests breeding experiments as the test
of species membership. But this is a test that is rarely
made. The number of crosses needed to delimit membership in
a species can be astronomical. The following example will
illustrate the problem.
Here in Wisconsin we have about 16,000 lakes and
ponds. A common (and tasty ;-)) inhabitant of many of these
bodies of water is the bluegill sunfish. Let's ask a
question -- do all these bluegill populations constitute one
species or several morphologically similar species? Assume
that only 1,000 of these lakes and ponds contain bluegills.
Assuming that each lake constitutes a population, an
investigator would have to perform 499,500 separate crosses
to determine whether the populations could interbreed. But
to do this right we should really do reciprocal crosses
(i.e. cross a male from population A with a female from
population B and a male from population B with a female from
population A). This brings the total crosses we need to
make up to 999,000. But don't we also need to make
replicates? Having three replicates brings the total to
2,997,000 crosses. In addition, you just can't put a pair
of bluegills into a bucket and expect them to mate. In
nature, male bluegills excavate and defend nests in large
mating colonies. After the nests are excavated the females
come in to the colony to spawn. Here the females choose
among potential mates. This means that we would need to
simulate a colony in our test. Assume that 20 fish would be
sufficient for a single test. We find that we would need
about 60,000,000 fish to test whether all these populations
are members of the same species! (We would also need a
large number of large aquaria to run these crosses in). But
bluegills are not restricted to Wisconsin...
I could go on, but I think I've belabored this point
enough. The fact of the matter is that the time, effort and
money needed to delimit species using the BSC is, to say the
Another reason why using the BSC to delimit species
is impractical is that breeding experiments can often be
inconclusive. Interbreeding in nature can be heavily
influenced by variable and unstable environmental factors.
(Any angler who has waited for the bluegills to get on to
the beds can confirm this one). If we can't duplicate
natural conditions of breeding, a failure to breed doesn't
mean that the critters can't (or don't) interbreed in the
wild. The difficulties that were encountered in breeding
pandas in captivity illustrate this. In addition,
experimentally showing that A doesn't interbreed with B
doesn't preclude both interbreeding with C. This gets even
more complicated in groups that don't have nice, straight-
forward sexes. Finally, breeding experiments can be
inconclusive because actual interbreeding and gene flow
among phenetically similar, geneticallly compatible local
populations is often more restricted than the BSC would
suggest (Cronquist 1988).
In practice, even strong adherents of the BSC use
phenetic similarities and discontinuities for delimiting
species. If the organisms are phenotypically similar, they
are considered conspecific until a reproductive barrier is
Another criticism of the BSC comes from the
cladistic school of taxonomy (e.g. Donoghue 1985). The
cladists argue that sexual compatibility is a primitive
trait. Organisms that are no longer closely related may
have retained the ability for genetic recombination with
each other through sex. This is not a derived
characteristic. Because of this it is invalid for defining
A final problem with the BSC is that groups that do
not occur together in time cannot be evaluated. We simply
cannot know whether two such groups would interbreed freely
if they came together under natural conditions.
Several alternatives to the biological species
concept have been suggested. I will discuss two.
2.3 The Phenetic (or Morphological) Species Concept
Cronquist (1988) proposed an alternative to the BSC
that he called a "renewed practical species definition". He
defines species as
"... the smallest groups that are consistently and persistently
distinct and distinguishable by ordinary means."
Three comments must be made about this definition.
First, "ordinary means" includes any techniques that are
widely available, cheap and relatively easy to apply. These
means will differ among different groups of organisms. For
example, to a botanist working with angiosperms ordinary
means might mean a hand lens; to an entomologist working
with beetles it might mean a dissecting microscope; to a
phycologist working with diatoms it might mean a scanning
electron microscope. What means are ordinary are determined
by what is needed to examine the organisms in question.
Second, the requirement that species be persistently
distinct implies a certain degree of reproductive
continuity. This is because phenetic discontinuity between
groups cannot persist in the absence of a barrier to
Third, this definition places a heavy, though not
exclusive, emphasis on morphological characters. It also
recognizes phenetic characters such as chromosome number,
chromosome morphology, cell ultrastructure, secondary
metabolites, habitats and other features.
2.4 Phylogenetic Species Concepts
There are several phylogenetic species definitions.
All of them assert that classifications should reflect the
best supported hypotheses of the phylogeny of the organisms.
Baum (1992) describes two types of phylogenetic species
(1) A species is the smallest cluster of organisms
that possesses at least one diagnostic character. This
character may be morphological, biochemical or molecular and
must be fixed in reproductively cohesive units. It is
important to realize that this reproductive continuity is
not used in the same way as in the BSC. Phylogenetic species
may be reproductive communities. Reproductively compatible
individuals need not have the diagnostic character of a
species. In this case, the individuals need not be
(2) A species must be monophyletic and share one or
more derived character. There are two meanings to
monophyletic (de Queiroz and Donoghue 1988, Nelson 1989).
The first defines a monophyletic group as all the
descendents of a common ancestor and the ancestor. The
second defines a monophyletic group as a group of organisms
that are more closely related to each other than to any
other organisms. These distinctions are discussed in Baum
1992 and de Queiroz and Donoghue 1990.
2.5 Why This is Included
What is all of this doing in a discussion of
observed instances of speciation? What a biologist will
consider as a speciation event is, in part, dependent on
which species definition that biologist accepts. The
biological species concept has been very successful as a
theoretical model for explaining species differences among
vertebrates and some groups of arthropods. This can lead us
to glibly assert its universal applicability, despite its
irrelevance to many groups. When we examine putative
speciation events, we need to ask the question, which
species definition is the most reasonable for this group of
organisms? In many cases it will be the biological
definition. In many other cases some other definition will
be more appropriate.
3.0 The Context of Reports of Observed Speciations
The literature on observed speciations events is not
well organized. I found only a few papers that had an
observation of a speciation event as the author's main point
(e.g. Weinberg, et. al. 1992). In addition, I found only one
review on the topic (Callaghan 1987). This review cited only
four examples of speciation events. Why is there such a
seeming lack of interest in reporting observations of
IMHO, four things account for this lack of interest.
First, it appears that the biological community considers
this a settled question. Many researchers feel that there
are already ample reports in the literature. Few of these
folks have actually looked closely. To test this idea, I
asked about two dozen graduate students and faculty members
in the department where I'm a student whether there were
examples where speciation had been observed in the
literature. Everyone said that they were sure that there
were. Next I asked them for citings or descriptions. Only
eight of the people I talked to could give an example, only
three could give more than one. But everyone was sure that
there were papers in the literature.
Second, most biologists accept the idea that
speciation takes a long time (relative to human life spans).
Because of this we would not expect to see many speciation
events actually occur. The literature has many more
examples where a speciation event has been inferred from
evidence than it has examples where the event is seen. This
is what we would expect if speciation takes a long time.
Third, the literature contains many instances where
a speciation event has been inferred. The number and
quality of these cases may be evidence enough to convince
most workers that speciation does occur.
Finally, most of the current interest in speciation
concerns theoretical issues. One recent book on speciation
(Otte and Endler 1989) has few example of observed
speciation, but a lot of discussion of theory.
Most of the reports, especially the recent reports,
can be found in papers that describe experimental tests of
hypotheses related to speciation. Usually these experiments
focus on questions related to mechanisms of speciation.
Examples of these questions include:
1) Does speciation precede or follow adaptation to local
2) Is speciation a by-product of genetic divergence among
populations or does it occur directly by natural selection
through lower fitness of hybrids?
3) How quickly does speciation occur?
4) What is the role of genetic drift in speciation?
5) Can speciation occur sympatrically (i.e. can two or more
lineages diverge while they are intermingled in the same
place) or must the populations be separated in space or
4.0 Telling Whether a Speciation Event Has Occurred
What evidence is necessary to show that a change
produced in a population of organisms constitutes a
speciation event? The answer to this question will depend
on which species definition applies to the organisms
4.1 Cases Where the Biological Species Concept Applies
One advantage of the BSC is that it provides a
reasonably unambiguous test that can be applied to possible
speciation events. Recall that under the BSC species are
defined as being reproductively isolated from other species.
Demonstrating that a population is reproductively isolated
(in a nontrivial way) from populations that it was formerly
able to interbreed with shows that speciation has occurred.
In practice, it is also necessary to show that at least one
isolating mechanism with a hereditary basis is present.
After all, just because a pair of critters don't breed
during an experiment doesn't mean they can't breed or even
that they won't breed. Debates about whether a speciation
event has occurred often turn on whether isolating
mechanisms have been produced.
4.1.1 Isolating Mechanisms
Mechanisms which produce reproductive isolation fall
into two broad categories -- premating mechanisms and
Premating isolating mechanisms operate to keep
species separate before mating occurs. Often they act to
prevent mating altogether. Examples of premating mechanisms
include ecological, temporal, behavioral and mechanical
Ecological isolation occurs when species occupy or
breed in different habitats. It is important to be careful
when claiming ecological isolation. For example, I have a
population of Dinbryon cylindricum (a colonial algal
flagellate) growing in a culture tube in an environmental
chamber. It's been there for three years (which is a lot of
time in flagellate years! :-)). Even though there is no
possibility that they will mate with the D. cylindricum in
Lake Michigan, it would be silly to assert that they
constitute a separate species. Physical isolation alone does
not constitute an isolating mechanism with an hereditary
Temporal isolation occurs when species breed at
different times. This may be different times of the year or
different times of day.
Behavioral isolating mechanisms rely on organisms
making a choice of whether to mate and a choice of who to
mate with. Differences in courtship behavior, for instance,
may be sufficient to prevent mating from occurring. A
behavioral isolating mechanism should result in some sort of
positive assortative mating. Simply put, positive
assortative mating occurs when organisms that differ in some
way tend to mate with organism that are like themselves.
For example, if blonds mate exclusively with blonds,
brunettes mate exclusively with brunettes, redheads mate
exclusively with redheads (and those of us without much hair
don't get to mate :-() the human population would exhibit a
high degree of positive assortative mating. In most
examples in the literature when positive assortative mating
is seen it is not this strong. Positive assortative mating
is especially important in discussions of sympatric
Mechanical isolating mechanisms occur when
morphological or physiological differences prevent normal
Postmating isolating mechanisms prevent hybrid
offspring from developing or breeding when mating does
occur. There are also several examples of postmating
Mechanical postmating isolating mechanisms occur in
those cases where mating is possible, but the gametes are
unable to reach each other or to fuse. Mortality acts as an
isolating mechanism when the hybrid dies prior to maturity.
Sterility of hybrids can act as an isolating mechanism.
Finally a reduction in the fitness of the hybrid offspring
can isolate two populations. This happens when the F1
hybrid is fertile but the F2 hybrid has lower fitness than
either of the parental species.
4.2 Cases Where the Biological Species Concept Does Not
There is no unambiguous criterion for determining
that a speciation event has occurred in those cases where
the BSC does not apply. This is especially true for
obligately asexual organisms. Usually phenetic (e.g.
phenotypic and genetic) differences between populations are
used to justify a claim of speciation. A few caveats are
germane to this. It is not obvious how much change is
necessary to claim that a population has speciated. IMHO,
the difference between the "new species" and its "ancestor"
should be at least as great as the differences among
recognized species in the group (i.e. genus, family)
involved. The investigator should show that the change is
persistent. Finally, many organisms have life cycles/life
histories that involve alternative morphologies and/or an
ability to adjust their phenotypes in response to short term
changes in ecological conditions. The investigator should
be sure to rule these things out before claiming that a
phenetic change constitutes a speciation event.
5.0 Observed Instances of Speciation
The following are several examples of observations
5.1 Plant Speciations Involving Polyploidy or Hybridization
Followed by Polyploidization.
(See also the discussion in de Wet 1971).
5.1.1 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.
5.1.2 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.
Owenby (1950) demonstrated that two species in this
genus were produced by polyploidization from hybrids. She
showed that Trapopogonan miscellus found in a colony in
Moscow, Idaho was produced by hybridization of T. dubius and
T. pratensis. She also showed that T. mirus found in a
colony near Pullman, Washington was produced by
hybridization of T. dubius and T. porrifolius.
The Russian cytologist Karpchenko (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.
5.1.5 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 morplology.
5.2 Speciations in Plant Species not Involving Hybridization or
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
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
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
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
5.3.3 Selection on Courtship Behavior in Drosophila
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 possiblility 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 Drosophila pseudoobscura
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. Significant
positive assortative mating was found.
5.3.7 Tests of the Founder-flush Speciation Hypothesis Using
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.
Dood and Powell (1985) tested this hypothesis using
D. pseudoobscura. A large, heterogenous 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
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
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 thes 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
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
5.5 Flour Beetles (Tribolium castaneum)
Halliburton and Gall (1981) established a population
of flour beetles collected in Davis, California. In each
generation they selected the 8 lightest and the 8 heaviest
pupae of each sex. When these 32 beetles had emerged, they
were placed together and allowed to mate for 24 hours. Eggs
were collected for 48 hours. The pupae that developed from
these eggs were weighed at 19 days. This was repeated for
15 generations. The results of mate choice tests between
heavy and light beetles was compared to tests among control
lines derived from randomly chosen pupae. Positive
assortative mating on the basis of size wat found in 2 out
of 4 experimental lines.
5.6 Speciation in a Lab Rat Worm, Nereis acuminata
In 1964 five or six individuals of the polychaete
worm, Nereis acuminata, were collected in Long Beach Harbor,
California. These were allowed to grow into a population of
thousands of individuals. Four pairs from this population
were transferred to the Woods Hole Oceanographic Institute.
For over 20 years these worms were used as test organisms in
environmental toxicology. From 1986 to 1991 the Long Beach
area was searched for populations of the worm. Two
populations, P1 and P2, were found. Weinberg, et. al.
(1992) performed tests on these two populations and the
Woods Hole population (WH) for both postmating and premating
isolation. To test for postmating isolation, they looked at
whether broods from crosses were successfully reared. The
results below give the percentage of successful rearings for
each group of crosses.
WH X WH 75% P1 X P2 77%
P1 X P1 95% WH X P1 0%
P2 X P2 80% WH X P2 0%
They also found statistically significant premating
isolation between the WH population and the field
populations. Finally, the Woods Hole population showed
slightly different karyotypes from the field populations.
5.7 A Couple of Ambiguous Cases
So far the BSC has applied to all of the experiments
discussed. The following are a couple of major morphological
changes produced in asexual species. Do these represent
speciation events? The answer depends on how species is
5.7.1 Coloniality in Chlorella vulgaris
Boraas (1984) reported the induction of
multicellularity in a strain of Chlorella pyrenoidosa (since
reclassified as C. vulgaris) by predation. He was growing
the unicellular green alga in the first stage of a two stage
continuous culture system as for food for a flagellate
predator, Ochromonas sp., that was growing in the second
stage. Due to the failure of a pump, flagellates washed
back into the first stage. Within five days a colonial form
of the Chlorella appeared. It rapidly came to dominate the
culture. The colony size ranged from 4 cells to 32 cells.
Eventually it stabilized at 8 cells. This colonial form has
persisted in culture for about a decade. The new form has
been keyed out using a number of algal taxonomic keys. They
key out now as being in the genus Coelosphaerium, which is
in a different family from Chlorella.
5.7.2 Morphological Changes in Bacteria
Shikano, et. al. (1990) reported that an
unidentified bacterium underwent a major morphological
change when grown in the presence of a ciliate predator.
This bacterium's normal morphology is a short (1.5 um) rod.
After 8 - 10 weeks of growing with the predator it assumed
the form of long (20 um) cells. These cells have no cross
Joseph Boxhorn (email@example.com)/|\ Same boring
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Who I am, where I am. and Center for Great Lake Studies
/|\ Feel free to insert what I am: University of Wisconsin--
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