Download Sympatric speciation: when is it possible

Biological Juurnal of the Linnean Suciep (1986), 27: 201-223. With 2 figures
Sympatric speciation: when is it possible?
ALEXEY S. KONDRASHOV
Research Computer Centre, Academy of Sciences of the U.S.S.R., Pushchino,
Moscow Region, 172291, U.S.S.R.
AND
MIKHAIL V. MINA
N . K. Koltzov Institute of Developmental Biology, Academy of Sciences of the U . S . S . R . ,
Vavilov Street 26, MOSCOW,
117808, U . S . S . R .
Received 13 August 1984, accepted for publication I1 July 1985
This paper is written to compare the results of theoretical investigations of sympatric speciation
with the relevant experimental data. We understand sympatric speciation as a formation of species
out of a population whose spatial structure is not important genetically. A necessary prerequisite for
speriation is an action of disruptive selection on sufficiently polymorphic traits. The present analysis
confirms the view that such a selection is ecologically realistic. The genetical part of speciation
begins with a development of reproductive isolation between those individuals that are opposed in
strme characters. I t is shown that selection for reproductive isolation may be quite strong.
Extinction of intermediate individuals, which completes speciation, proceeds under a wide range of
ronditions, including those when the newly formed species differ in quantitative characters, though
most of the genes arc likely to remain the same in both species. The whole process seems possible if
differences i n srveral (up to 10) loci are sufficient to adapt the forming species to different niches
and to establish reproductive isolation. It is shown that populations with bimodal distributions of
some genetically determined quantitative characters can have a considerable life-time. Such
distributions may he formed either as a transition stage of sympatric speciation or represent a
stationary state under conditions close to those necessary to complete speciation. They are very
important Tor experimental investigations. Sympatric speciation always follows the same principal
course; it does not contradict the idea of a genome coadaptedness. The occurrence of sympatric
speciation is different for different taxa depending rather on how frequently populations are
subjected to the appropriate kind of selection than on their ability to obey it.
KEY WORDS:- Sympatric speciation
roadaptedness - quantitative characters.
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reproductive isolation
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disruptive selection
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CONTENTS
Introduction . . . . . . . . . . . . . . .
Terminology . . . . . . . . . . . . . . .
'l'heoretiral studies of sympatric speciation . . . . . . . .
Ecological conditions . . . . . . . . . . . .
Development of reproductive isolation . . . . . . . .
Reduction in the number of the intermediate individuals and formation
Dumb-bell-shaped structures . . . . . . . . . .
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A. S. KONDRASHOV AND M. V. MINA
On the possibility of sympatric speciation in nature . . . .
Observation ofthe final stages ofsympatric speciation . .
Detection of the results of sympatric speciation . . . .
Qualitative evaluation of the factors of sympatric speciation.
Experiments on sympatric speciation . . . . . .
Sympatric speciation and genome coadaptedness . . . .
Conclusions . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . .
References. . . . . . . . . . . . . .
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INTRODUCTION
The mechanisms underlying speciation are a key problem of evolution
theory. Although there may sometimes be difficulties in defining species as such,
here we will simplify the problem by defining species as closed gene pools,
groups of individuals that can mate only with each other, and that have been
reproductively isolated from other groups in the same area for many
generations, i.e. we will consider non-dimensional species Senm Mayr ( 1970).
This definition suits our purpose, since problems connected with the
geographical structure of populations will not be considered here.
Sympatric speciation traditionally means that two species are formed out of a
single population. We may assume that some polymorphic character is subjected
to disruptive selection that gives advantage to marginal phenotypes. Since
progeny from matings between different marginals have intermediate genotypes,
reproductive isolation between them can develop because the genes precluding
their matings would be selected. If disruptive selection eliminates all the
intermediates, there will appear two distinct species. The very possibility of this
process has been debated until recently, although many modern authors
recognize sympatric as well as allopatric speciation. However, the lack of
theoretical and experimental evidence for sympatric speciation has made it a
subject of great controversy, for at least two reasons. First, sympatric speciation
(as much as any drastic change in a population) is difficult to observe in nature
and in experiments. Second, a population under sympatric speciation is affected
by opposing factors so that intuitive prediction of the outcome is next to
impossible. Such a situation calls for an investigation of mathematical models
together with field observations and experimental data. An analysis of this type
is attempted here.
TERMINOLOGY
The study of sympatric speciation is complicated by confusion in using the
word ‘sympatric’. As Greenwood (1984) has remarked “to a large degree the
debate about sympatric speciation was more semantic than biological, devolving
on the spatial limits implied by the prefixes sym- and allo-” (p. 15). However,
there are cases of speciation that are unambiguously sympatric. We shall call
speciation ‘sympatric’ if in its course the probability of mating between two
individuals depends on their genotypes only.
This definition needs a comment. What non-genetical factors could control
the probability of mating? First, individual birthplaces. It follows from o u r
definition of sympatry that genes are dispersed all over the range of the
SYMPATRIC SPECIATION
203
population, at least during the period of reproduction. The opposite case, when
the genes are separated in space by some physical barrier, is known as allopatric
speciation. An intermediate situation is referred to as parapatric speciation. In
this case speciation goes on in an area where there are no physical barriers, but
matings between individuals living far from each other are rare no matter what
their genctic difference may be (Bush, 1975). The terms ‘stasipatry’ (White,
1978) and ‘marginal sympatry’ (Grant, 1981) have similar meanings.
There is another non-genetic factor that can prevent mating: various types of
conditioning. T o illustrate this we consider host-races of membracids (Wood,
1980; Wood & Guttman, 1982). It was shown that individuals that had grown
up on plants of one species prefer to mate and to lay eggs on the same plant
species, choosing a partner of the same origin. This might arise either from
individuals hatching on one plant possessing genes in common, which makes
them choose a particular plant and mates; or in the absence of genetic
differences between the races if all the individuals are inclined to choose a host
which is known to them. The first casc is an example of sympatric races, since
nothing prevents individuals of one race from rejoining their race in
neighbouring plants, if they had been accidentally transferred to a new plant. In
the second case the races should be called allopatric or parapatric; genetically
speaking, they are essentially the same as geographically isolated populations,
the role of migrants being played by individuals that have abandoned h e i r
home plant accidentally.
There is no doubt about the existence of genetic distinctions of the races of
membracids (Guttman, Wood & Karlin, 1981). However, more work should be
done on conditioning, which is known to be powerful, e.g. in some insects such
as Drosophila melanogaster during oviposition (Jaenicke, 1982), to reveal the extent
of parapatry involved in speciation.
For a population to be considered as a n evolving unit, neglecting its spatial
structure, i t is sufficient that only zygotes or only gametes should disseminate
freely over the entire area. Hence, although one stage of the life cycle may be
conditioned, sympatry will still prevail. For example, the females of the parasitic
wasp Nasonia vilripennis often lay their eggs on the same insect species that has
been their host, but their mating is known to be random (Smith & Cornell,
1979). The same may be said about wind-pollinated plants whose pollen is
dispersed over a vast territory, their seeds being comparatively immobile, so that
matings occur without restriction.
In the opposite case (which is found in some plants) seeds are freely dispersed
whilc pollen is mainly distributed among neighbouring plants. Suppose that the
environment is heterogeneous, so that individuals with some genotypes would
survive in one kind of region and those with other genotypes can survive in some
other regions. ‘l’he observer would find plants with differing genotypes growing
in different places and could consider speciation to be para- or even allopatric in
this case. In fact, however, spatial structure only provides a basis for assortative
mating, as the individuals’ location does not depend on its parents’ locations.
Such assortativeness determined by birth-place may exist in sedentary benthic
invertebrates whose floating larvae select their habitat.
Naturalists would generally define sympatric speciation in a wider and less
precise sense as, say, occurring within the dispersion range of the offspring from
one deme.
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Sympatry, as we define it, is a deliberate abstraction. A natural situation will
always comprise one or another parapatric feature, particularly among
immobile organisms. However, a study of sympatric speciation in our definition,
as well as other local models, is desirable, first, because of its simplicity, and,
second, because under strong migration and sufficiently homogeneous habitat
the role of non-sympatric features is likely to be unimportant. Spatial barriers of
any kind facilitate speciation even more (Caisse & Antonovics, 1978; Lande,
1982; Stam, 1983), so that the fact that speciation has proved possible in our
models suggest that the same will hold in natural populations provided all the
other conditions are the same.
Models for allopatric speciation often assume that the establishment of
reproductive isolation is completed in the zone of secondary contact, i.e. in
sympatry (Sawyer & Hartl, 1981). The contrasting situation, when speciation
begins as sympatric and ends as allopatric, must be considered. Suppose that a
group of individuals invades a new region and their progeny increase their
numbers, founding new demes. When the size of these demes is still small, the
individuals are not particularly attached to their birth-places, but once the
whole region has been colonized and the demes are growing in size, the
attachment increases, which means that the dispersion range of the offspring
from one deme is reduced. In other words, at the beginning of the process the
situation can be qualified as sympatric, the population being undivided all over
the region, while at the last-considered stage the region is inhabited by several
allopatric populations. This is likely to be the case with many animals when
they colonize a new habitat, e.g. salmonids are known to display strong homing
but at the same time are able to occupy new areas. Pink salmon (Oncorhynhus
gorbuscha), for one, has been exploding in numbers since it was introduced into
water bodies in the Kolsky Peninsula (Kamyshnaya & Smirnov, 1981); the
ranges of dispersion of the progeny from single demes are many times greater
than those typical of this species in its native region.
For some African cichlids it was shown that they can easily colonize newly
arisen microhabitats suitable for them establishing new populations (McKaye &
Gray, 1984). Of course it is always difficult to say to what degree and for how
long the newly formed populations retain connections with mother stocks.
THEORETICAL STUDIES OF SYMPATRIC SPECIATION
Ecological conditions
What kind of selection can lead to sympatric speciation and what conditions
allow of such selection? We shall attempt to answer these questions using the
results of Maynard Smith (1966), Rosenzweig (1978), Udovi; (1980) and Pimm
(1979).
' A necessary condition for speciation is a sufficient genetic heterogeneity
within the initial population. As an advantage of heterozygotes must impede
speciation, it is only the frequency-dependent selection that can be a factor
maintaining variability. Selection of this kind occurs when individuals of similar
genotypes compete more intensely with each other than with unlike individuals.
This may be when individuals of different genotypes use different environmental
resources, so that those individuals who have more rare genotypes enjoy an
SYMPATRIC SPECIATION
205
advantage. Under weakening competition the frequency-dependent selection
disappears (Nassar, 1979). The requirement that the available resources for a
population should be heterogeneous and that individuals of different genotypes
should utilize them differently is valid also for the final stage of speciation, since,
according to the Gause principle, the newly originating species cannot occupy
the same niche (see McMurtrie, 1976).
In addition, to make speciation possible i t is necessary that under the
equilibrium genotype frequencies the fitness of the ‘marginal’ individuals be
higher than that of intermediate ones, e.g. when the intermediates can compete
with the marginals for neither of the resources. Thus, to ensure sympatric
speciation selection must be both frequency-dependent and disruptive.
These requirements are to some extent contradictory. Assume a population
consisting of small individuals, where large individuals (if appearing) enjoy an
advantage, and intermediates are maladapted. Then, if size depends on several
genes, the population will produce mainly the maladapted intermediates, giving
no opportunity for appearance of a polymorphism, while in a population
initially consisting of both small and large individuals it could be stable. T o
overcome this apparent contradiction, we have to assume that in a population
of small individuals both large and intermediate animals enjoy a n advantage,
but when an equilibrium is established selection turns against the intermediates.
This problem can be solved mathematically for a one-locus case (Udovic, 1980)
by separating the frequency-dependent and disruptive selection components,
which allows the estimation of the conditions of stable polymorphism. We shall
consider a simple case of two resources to illustrate the possibility of such
selection.
Assume that all the individuals are able to utilize both resources, but small
size gives an advantage in a competition for one of them while large size is
advantageous in a competition for the other. Then in a population of small
individuals the intermediates as well as the large ones will have an advantage in
a competition for an unutilized resource which depends on increase in size for its
availability. However, if the frequencies of small and large individuals are at
equilibrium, the intermediates would be maladapted, as is shown in fig. 1 of
Rosenzweig’s paper (1978). The above consideration seems to be a
reformulation of his conclusions.
Thus, resources for small and large individuals should not differ greatly,
otherwise the rare intermediates in a population of small ones would not be able
to use another resource and, consequently, would not be at an advantage.
However, these resources should not be too similar, as in this case disruptive
selection would not have a chance to operate.
Situations may be possible when development of polymorphism precedes
disruptive selection. A population initially may have utilized two similar
resources, e.g. two similar food species, and could have become genetically
variable due to frequency-dependent selection without disruptive selection.
Various morphs in such a population feed on different species. If these food
species should diverge, disruptive populations would develop among the
consumers. No problem in connection with a polymorphism established in an
initially monomorphic population would arise here.
When different individuals utilize different resources disruptive selection is
likely to involve many characters, selection for coadapted phenotypes also
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A. S. KONDRASHOV AND M. V. MINA
occurring. For instance, if some resource is best used by ‘small white’ and
another by ‘large black’ individuals, then ‘small blacks’ and ‘large whites’
should be able to use neither of the resources and their fitness would be no
better than that of ‘intermediate greys’.
There are two possible cases for disruptive selection. The first is when an
environment is homogeneous and intermediates are equally maladapted
everywhere. The other may be when a population occupies spatially separated
subniches which offer different advantages to different marginal genotypes.
Then if individuals are allowed to choose a niche, selection in such a population
will also be disruptive, as intermediate individuals will have no place and
opportunity to adapt. Selection for coadaptedness may be present, for example,
when one of the characters determines the choice of the niche, and the other
controls viability of the individuals which occupy it (Bush & Diehl, 1982).
When individuals cannot choose a niche, as in the case of passive dispersion, all
genotypes will have equal fitnesses if selection is additive (Felsenstein, 1981),
and therefore disruptive selection might not be expected (Bush & Diehl, 1982).
Data on some insects, i.e. Megarhyssa (Hymenoptera) (Gibbons, 1979), Drosophila
(Parsons, 1981) and various Diptera families (Nartchuk, 1983) as well as on
Puccinia graminis and some other fungi (see Dyakov & Lekomtseva, 1984), also
suggest that stipulation of disruptive selection is by no means improbable. The
question is whether such selection can lead to the establishment of reproductive
isolation.
Development of reproductive isolation
It is clear that selecting a partner instead of mating at random is
advantageous if the fitness of the offspring depends on who its other parent is.
Otherwise assortativeness would lead only to a reduction in the chance of an
individual’s mating at all. Three possible types of natural selection might then
operate in the population.
( 1 ) Selection may be directional and, consequently, the most fit individuals
are those belonging to one of the marginal genotypes, which makes the latter the
best mating partners for all the rest. This may lead to sexual selection, in other
words (Lewontin, Kirk & Crow, 1966) selective mating that enhances the
directional component of selection.
(2) Selection may be stabilizing. I n this case it is advantageous for
marginal individuals to choose from the phenotypes opposite themselves, in
other words negative assortativeness is preferable.
(3) Selection may be disruptive, in which case-as was predicted by Wallace
(see Grant, 1981)-positive assortativeness gains the advantage which may well
lead to reproductive isolation between phenotypes that differ greatly. Disruptive
selection here may result from unbalanced genotypes of the hybrids produced in
a zone of secondary contact between two independent populations (Sawyer &
Hartl, 1981), as well as from purely ecological factors in a genuinely sympatric
situation.
Disruptive selection also involves three possibilities. ( 1) There always exists an
assortativeness for some special character and it cannot be intensified. This
might refer to characters responsible for temporal or spatial isolation of different
phenotypes during the mating period. (2) There may be no assortativeness and
SYMPATRIC SPECIATION
207
no selection for assortativeness. (3) Assortativeness for some special character
may be either manifested to a certain extent or be absent initially to be later
increased (or to appear) under selection.
No quantitative estimates of the required intensity of such selection were
available until recently when they have been obtained analytically for the initial
stage of sympatric speciation (Kondrashov, 1984). A computer simulation
(Kondrashov, 1985) provides the relevant data at every stage of the process.
This work shows that at an early stage of speciation the advantage of positive
assortativeness is considerable (of the order of 1 %) if the intensity of disruptive
selection is of the order of loyo,and if the number of genes which determine this
character is not large (up to 10). Assortative mating that can most effectively
prevent matings of different marginal phenotypes would be most advantageous
here. The maximal advantage of assortativeness is reached when the evolving
species are about equal in numbers. Low heritability of a character leads to a
dramatic decrease in the intensity of selection needed for the establishment of
reproductive isolation. An unpredictable environment is known to favour a
decrease in heritability (Cooper & Kaplan, 1982), so that a population existing
in such conditions might be prevented from developing reproductive isolation.
In the course of sympatric speciation, which implies an increase of phenotypic
variance in the evolving population, the advantage of assortativeness rapidly
increases (Kondrashov, 1985). In other words, assortative mating in the original
population must transform into reproductive isolation between the new species.
Since assortativeness always lowers the probability of mating, selection for
reproductive isolation, and hence sympatric speciation, will be facilitated in
flourishing populations where individuals have many opportunities to select
their mates while small effective size of a population is in itself a factor
preventing sympatric speciation (Kondrashov, 1984). Thus, selection for
assortative mating in a character subject to disruptive selection can be
sufficiently intensive under a variety of conditions.
Maynard Smith (1966) regards a situation in which the same locus
determines both selection and assortativeness as highly improbable and
considers another situation (see also Udovic, 1980; Felsenstein, 1981) with
reproductive isolation determined by some loci while disruptive selection is
acting upon some other. This situation requires linkage disequilibrium between
the selected and the isolating loci for speciation to run its course. Our model
indicates, however, that linkage disequilibrium is possible only when
reproductive isolation is already pronounced. A question here arises: how can
reproductive isolation be determined by a character unaffected by disruptive
selection? Such a process seems possible only as a result of preference of different
subniches by different genotypes (Bush & Diehl, 1982). While the case of strong
assortativeness and selection in one locus could not be very frequent, the case of a
single pobgenic character is quite different. Kilias, Alahiotis & Pelecanos (1980)
found that individuals from cage Drosophila populations living for 50 generations
under different conditions (in temperature, humidity, feeding) developed a
reproductive isolation to a certain degree, while those living under the same
conditions did not. Therefore, assortativeness can develop as a by-product of not
allopatry as such, but of adaptation to different environments. Consequently, it
may be due to differences in selective characters depending on a number of
genes (Kilias & Alahiotis, 1983). Assortativeness might have been even stronger
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if it were specially selected for, i.e. under disruptive selection in sympatry.
However, in the experiments with disruptive selection for geotaxis in the housefly (Soans, Pimentel & Soans, 1974; Hurd & Eisenberg, 1975) reproductive
isolation was equally in progress in allopatric and sympatric conditions. This
result may also be due to the fact that selection for isolation in a sympatric
situation will to some extent be compensated for by the exchange of genes that
can prevent divergence. There are also data indicating a possibility of rapid
development of reproductive isolation between both allopatric (Levin, 1976)
and sympatric (MacNair & Christie, 1983) plant populations existing under
different selection pressures.
We therefore conclude that the principal part in sympatric speciation is
played by reproductive isolation determined by selectable characters, such as
size, coloration, habitat preferences, time of the day and season of mating, etc.,
while special mechanisms such as pistil-pollen incompatibility in plants (de
Nettancourt, 1977: ch. 5) or hyphal incompatibility in fungi (Dyakov &
Lekomtseva, 1984) develop subsequently to complete isolation. This conclusion
is confirmed by the complications encountered in the efforts of developing
reproductive isolation between subspecies of Drosophila in a cage population,
when the individuals are evolving under artificially simplified conditions (see a
review by Sved, 1981a). The model of this (Sved, 1981a, b) is similar to the
models mentioned above (Udovic, 1980; Felsenstein, 1981) in that selection in
all of them acts on those characters that do not determine isolation. However, i t
could be very difficult to make some forms reproductively isolated in a cage
though they are completely isolated in nature (see Sved, 1981b).
One might wonder whether sympatric speciation is at all possible if
reproductive isolation requires differences at more than one locus. We discuss
this in the following section.
Reduction of the number of the intermediate individuals
and formation of species
Reproductive isolation between marginals is evidently a necessary but not
sufficient condition for speciation, which can be completed only with the
disappearance of intermediate individuals. This process is influenced by at least
three factors: segregation and recombination, assortative mating of some kind,
and disruptive selection. It is next to impossible to predict the result of such a
complicated interaction without either substantial experimental data (which are
lacking) or mathematical modelling. It is not surprising that statements made
on the possibility of termination of sympatric speciation at its critical stage have
been quite contradictory.
Our model was aimed at investigating the final stage of speciation when the
intermediate individuals are much fewer than the marginals. The reproductively
isolated marginal phenotypes differ either in one quantitative character
(Kondrashov, 1983a) or in two characters (Kondrashov, 198313) controlled by
up to eight loci.
The intensity of disruptive selection required to complete speciation was
found to be no less than 10%. It tends to grow with the number of loci
responsible for reproductive isolation and depends strongly on the type of
selection. Of no less consequence is the nature of assortative mating: when it can
SYMPArRIC SPECIAI'ION
209
prevent distant mating it facilitates selection for isolation and the intensity of the
selection required will be smaller. In any case, selection of a reasonable intensity
such as that often found in nature (see for example Allard, Kahler & Clegg,
1977; Nadeau & Baccus, 1981) could lead to completion of speciation in a wide
variety of conditions. In some cases an active role of assortative mating,
predicted by Breese (1956), is great enough for the elimination of intermediates
to be completed without disruptive selection (Kulagina & Lyapunov, 1966;
Kamenshikov, 1972; Markova & Shapiro, 1974; Kondrashov & Molchanov,
1980; Kondrashov, 1980). Unfortunately, this is often overlooked in works on
sympatric speciation.
Speciation is also possible if reproductive isolation requires a difference in two
characters. Another possibility is that selection may act on one character while
reproductive isolation is determined by the other (Kondrashov, 1983b). The
latter case proved to require only a slightly stronger selection for completion of
speciation than in the case when both isolation and selection depend on the
same character (Kondrashov, 198313: fig. 2c). It can be accounted for by the fact
that by the end of speciation one of the intermediate's parents would be a
marginal, so that distributions of different characters in a population are closely
correlated (Kondrashov, 1983b: table 2); consequently, the number of
characters is of little importance. Speciation can be facilitated by selection for
coadaptation operating together with disruptive selection.
Computer simulations allow us to trace the whole process of extinction of
intermediates beginning from the population with binomial phenotype
distribution (Kondrashov, 1985). Two different situations were observed. When
opposite marginals differ in a small number of loci (up to 10) the early stage of
speciation proceeds under a weaker selection than that required to complete the
extinction of intermediates. This may be because the matings between
intermediates, who are frequent in a population with unimodal phenotype
distribution, produce some proportion of marginals while at the final stage of
speciation rare intermediates always mate with marginals, which leads to an
increased proportion of intermediates due to recombination.
I n contrast, with a larger number of loci involved, selection sufficient to
complete speciation may be insufficient to change the initial population
markedly, probably because of a small phenotype distribution variance in
Hardy-Weinberg multilocus population. Considered together with the results
on selection for reproductive isolation, these findings suggest that sympatric
speciation is possible when essential differences (including isolating mechanisms)
between the formed species depend on up to 10 loci. This makes the studies of
interspecies genetical differences important.
Data on morphological differences between Hawaiian Drosophila silvestis and
D . heleroneura (Val, 1977) and between species of stem borer Ostrinia (Frolov,
1981), as well as on seasonality of reproduction in Chrysopa carnea and
C. duuinesii (Tauber & Tauber, 1977a, b), demonstrate that the number of loci
involved in speciation can be surprisingly small. A single dominant gene causing
incompatibility can protect Hordeum vulgare from pollination with Hordeum
bulbusurn (Pickering, 1983). Evidently intraspecies polymorphism involves many
more loci than do some interspecies differences (for reviews see Lande, 1981;
Maynard Smith, 1983). However, more data on various plants and animals are
desirable. There is also some evidence that routine methods of studying the
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A. S. KONDRASHOV AND M. V. MINA
number of loci may lead to considerable overestimations (Ginzburg, 1982,
1984). Note that the number of differences between two species, even closely
related, is usually larger than that required for speciation, because of subsequent
divergence.
It has also been shown that when the loci responsible for a selective character
and those providing assortativeness are linked, speciation is facilitated
(Kondrashov, 1985). Disruptive selection is likely to be able to produce a
linkage (Mayr, 1970). However, discrete phenotypes may appear within a
panmictic population if selection operates on characters determined by only a
few loci, an appearance of linkage between many loci responsible for different
characters being rather unrealistic. O n the other hand, disruptive selection
springing from utilization of different resources must involve several characters.
If there is a character which brings about no assortativeness and different
resources impose equal requirements on it, then newly formed species will not
differ in this character (Kondrashov, 1983b, 1985). This theoretical prediction is
confirmed by the data (Carson, 1976; Nevo & Cleve, 1978), indicating that the
new species retain many common features, which we consider very important
(see p. 219).
There is a view (see Grant, 1981) that a population cannot bear the genetic
load that necessarily appears under selection strong enough to create sympatric
speciation. In simulation models of sympatric speciation (Menshutkin, 1977:
Kondrashov, 1985), however, individuals produced not more than 10
offspring. Evidently, natural populations with their much greater fecundity can
bear even stronger selection.
While species are being formed it is interesting to observe the shape of the
residual band that links them in the genotype space, i.e. the relative frequencies
of various intermediate phenotypes. This shape remains practically invariant
when all intermediates make u p less than 20-30% of the population and
characterizes some concrete factors of speciation process, which are often
difficult to measure directly. For example, appearance of a local maximum in
the middle of the band suggests that reproductive isolation is due to more than
two loci and that assortativeness of intermediates’ matings is not very strong
(Kondrashov, 1983a, b) .
Dumb- bell-shaped structures
A population at the last stage of sympatric speciation includes more marginal
than intermediate individuals and can be depicted as a dumb-bell structure.
This term will be used to denote two aggregates of individuals in a space of
characters with a zone between them where the individuals are present but
scarce. We shall consider only those cases when a bimodal distribution is formed
by individuals belonging to one local population and is not a result of a partial
overlap of character distributions in two different local populations. We do not
consider hybrid zones either (Endler, 1977), because in this intermediate case
mating of marginals is prevented by their spatial separation (see Caisse &
Antonovics, 1978). A dumb-bell structure may tend to split altogether having
formed new species, but may well continue as a stable form for the life of the
population. This can occur in at least three circumstances.
( 1) There is no complete reproductive isolation between the marginal
SYMPA'IRIC SPECIA'I'ION
21 I
genotypes, so that new intermediates appear repeatedly from mating of the
marginals, but are rare and maladapted.
(2) Reproductive isolation between the marginals is complete, but the
disruptive selection is not sufficient to ensure the completion of speciation.
( 3 ) In previous cases i t was supposed that there were two resources, each of
them efficiently utilized by one of the marginal groups. Now assume that there is
another resource that suits intermediates better than marginals. If this resource
is not abundant, disruptive selection in a population not yet approaching the
completion of speciation will change towards stabilizing selection as the number
of the intermediate individuals becomes small. It is in this particular case that a
disadvantage of complete splitting into species does occur (Mayr, 1970). In
other cases, during the course of speciation disruptive selection may even
increase because of the growing number of competable marginals.
These three possibilities have been predicted analytically and confirmed by
computer simulation (Kondrashov, 1985). Cases ( 1 ) and (2) produce the dumbbell structure as a result of counter-balance of selection and gene exchange.
Selection here works to create a complete reproductive isolation of the opposite
marginals (the first case) and to increase the assortativeness of the intermediates
(the second case), leading eventually to a termination of speciation. Therefore,
we will call structures of types (1) and (2) evolutionarily unstable. However,
their division by speciation will take longer than that under invariant
assortativeness. The longest speciation process for case ( 1 ) may occur if a
violation of reproductive isolation is determined not by a genotype subjected to
a strong negative selection but results from a probability of error equal for all
individuals and caused by a n imperfection of the isolating mechanism. T h e
direction of selection in case (3) will depend on the nature of assortativeness. If
the latter is weak, the offspring would comprise more intermediates compared to
the parent population. A very strong assortativeness producing a smaller
proportion of intermediates would result in the opposite situation. Selection in
the former case is disruptive, and in the latter case it is stabilizing. T h e
population, eventually, acquires a type of assortativeness under which
distribution of the parents will be the same as that of their offspring, when
frequency-dependent selection is the only selection acting. We have in such a
situation an evolutionarily stable dumb-bell which allows the population to
continue without change, if the environment is stable. One could show an
absence of disruptive selection by measuring the ratio of the frequencies of
young and adult intermediate individuals, and use this to verify experimentally
the evolutionary stability of natural dumb-bell structures. Cases ( 1 ) and (2)
may be experimentally discriminated, since in the first case most of the
intermediates are hybrids between different marginals in the first generation.
Thus, dumb-bell structures are formed under quite marked reproductive
isolation and disruptive selection, i.e. under conditions leading to sympatric
speciation or at least resembling these.
The initial stage of speciation, i.e. a n accumulation of polymorphisms and an
establishment of reproductive isolation, may be very prolonged. However, it is
difficult to recognize it in nature as the population at this stage has undergone
n o pronounced changes. T h e next stage, i.e. a n elimination of the intermediate
individuals, is likely to be rapid, being determined by the pre-existing
polymorphism and strong selection. Hence, dumb-bells that are more stable
212
A. S. KONDRASHOV AND M. V. MINA
than a transitory phase of speciation, and particularly those that are
evolutionary stable, are the most likely to be found in nature. I n addition, if the
conditions are changing so as to eliminate the third resource, the evolutionarily
stable dumb-bell would slowly undergo sympatric speciation.
ON THE POSSIBILITY OF SYMPATRIC SPECIATION IN NATURE
T o investigate the problem of the existence of a true sympatric speciation in
nature, two related approaches are possible. The first is to observe a population
directly (or a pair of species formed as a result of speciation) and to try to
understand which factors are involved in speciation. The second is to evaluate
quantitatively evolutionary factors such as selection and assortativeness and to
make conclusions about a possible course of the evolution in the population. We
begin with the first approach. As it is difficult to recognize the early stage of
speciation we shall only deal with investigations of dumb-bells.
Observation of the Jinal stages of sympatric speciation
Dumb-bell structures have been observed by many authors. A good example
is provided by a salmonid fish, Brachymystax lenok (Mina & Vasilyeva, 1979;
Borisovets, Alexeev & Mina, 1983; Alexeev, 1983). This species is represented
by two sympatric forms (‘sharp-nosed’ and ‘blunt-nosed’) in some lakes and
rivers of the Uda, Amur and Lena basins. Dumb-bell structures of various
dimensions are shown in Fig. 1. They belong to type (1) of our classification
because the intermediates are hybrids of the marginal forms having, it seems, a
somewhat decreased viability. A similar situation has been described in a
goodeid genus Ilyodon (Turner & Grosse, 1980; Turner, Grudzien, Adkisson &
Withe, 1983) where two sympatric morphs differing in mouth size, teeth
structure, number of gill-rakers and coloration of males are presumed to be in
genetic contact, with disruptive selection eliminating intermediates.
Sympatric host-races in insects may also be considered as dumb-bell
structures. Besides the well-known data on Rhagoletis (Bush, 1975), sympatric
host races were observed in Enchenopa binotata (Wood, 1980; Wood & Guttman,
1982), Liriomyza brassica (Tavormina, 1982), Yponomenta padellus (Menken, 1981) ,
and Lochmaea capreae (Mikheev & Kreslavsky, 1980). Two sympatric races of
Lochmaea capreae in the Moscow region live on birch (Betula pubescenus) and
willow (Salix capreae). Beetles of each race strongly prefer their host-plant.
Larvae of the willow race cannot develop on birch, while the birch-race larvae
can feed on both host species. The ability of larvae to develop on birch is
inherited as a dominant character during interracial matings. In nature and in
experiment such matings are rare, though the hybrids are viable and fertile
(Kreslavsky, Mikheev, Solomatin & Gritsenko, 1981). Strong assortativeness
leads to a low level of interrace gene flow in nature. Further investigations
demonstrated some degree of subdivision within both birch and willow races
(Solomatin, Mikheev & Kreslavsky, 1984; Mikheev, Kreslavsky, Solomatin &
Gritsenko, 1984).
In some insect populations dumb-bell structures are not connected with
formation of host races, e.g. in the soldier beetle Changliognathus pensylvannicus
(McLain, 1982).
213
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Figure 1. Dumb-bell structures revealed in studies of Brac$mystax lenok. A, One-dimensional dumbbell structure. Number of gill-rakers in specimens from the Duki River (Amur River Basin) (from
Alexeev, 1983). B, Two-dimensional dumb-bell structure. Relative width of supraethmoideum and
number of gill rakers in specimens from the Duki River; open circles, hybrids, and filled circles,
‘sharp-nosed’ and ‘blunt-nosed’ forms (from Alexeev, 1983). C, Multidimensional dumb-bell
structure (10 characiers). Distribution of specimens from the Kuanda River and Leprindokan Lake
(Lena River Basin) in the plane of the first two principal components (from Borisovets et al., 1983).
Strictly speaking, sympatric host races can be considered as dumb-bell rather
than a hybrid zone between parapatric population only when the role of
conditioning is not essential, as shown for Lochmaea caprene (Mikheev &
Kreslavsky, 1980) and Liriomyra brassicae (Tavormina, 1982). Without such
observations and data on genetic differences, bimodal phenotype distribution
could be supposed to result solely from different conditions of ontogeneses of
opposite marginals (Futuyama & Mayer, 1980).
The chief argument against dumb-bell structures as indicators of sympatric
speciation is that they may be formed as a result of secondary contact. But we
can expect that if this contact is not of long standing, different marginal
214
A. S. KONDRASHOV AND M. V. MlNA
individuals must differ in many characters, as they represent allopatrically
evolved populations. Meanwhile the species being formed under sympatric
speciation will differ in only a small number of characters relevant to the
speciation process (Kondrashov, 198313).
However, a dumb-bell structure after sufficiently long stable existence ‘forgets’
its history and carries no features indicating the way it has been developed
(Kondrashov, 1983b). The same is true for hybrid zones between parapatric
populations and for clines (Endler, 1977: ch. 6). This is manifested in the
levelling out of all differences between marginal individuals except for those
maintained by selection or assortativeness, which is achieved by the gene
exchange between different marginals. This process is most rapid if
intermediates comprise a considerable part of the population (Turner & Grosse,
1980; McLain, 1982), and are not just rare hybrids of the first generation as is
the case with Drosophila silvestris and D. heteroneura (Kaneshiro & Val, 1977).
Hence, the coincidence in the frequencies of several isozyme loci in different
marginals (e.g. in trophotypes of Ilyodon, Turner et al., 1983) proves only the
absence of disruptive selection acting on these loci and not a total absence of
genetic differences as Futuyama & Mayer (1980) suggest.
It is often impossible to know whether a dumb-bell structure has evolved
through an allopatric phase without investigating its history. This is true, for
example, for the Brachymystax lenok mentioned above. Whatever their origin,
dumb-bell structures with viable and fertile intermediates comprising 5-20°/, of
the whole population and with marginals differing in but a small number of
characters can only be stable under conditions close to those necessary to
complete sympatric speciation in the same situation. Therefore, we see no
grounds for denying the possibility of sympatric development of a dumb-bell
structure, in which opposite marginals differ in several (up to 10) loci once they
can stably exist under such conditions or even split up into new species. Thus,
although it may be difficult to detect a sympatric origin of a dumb-bell structure
(reasonable explanation though it might be: Wood, 1980; Turner & Grosse,
1980; Tavormina, 1982; Gritsenko, Kreslavsky, Mikheev, Severtsov &
Solomatin, 1983), its mere existence confirms the plausibility of conditions
necessary to complete sympatric speciation.
We believe that further progress in the study of sympatric speciation should
be primarily connected with detailed genetical and ecological investigations of
dumb-bell structures. These have been too often overlooked by naturalists.
Detection of the results of sympatric speciation
Two presently parapatric or allopatric species may have diverged
sympatrically. Sympatric speciation may reasonably be presumed on one of two
bases. The first is that the new species had no room to be formed in allopatry.
For instance, sympatric speciation is highly plausible if a ‘species flock’ (i.e. a
number of closely related species) are living in an isolated habitat (which can be
considered small taking into account dispersion abilities of the species) : e.g. bugs
of Rapa Island (White, 1978), Cottocomephoridue of Baikal lake (Mednikov, 1963),
Barbinae of Lanao lake (Myers, 1960; Kornfield & Carpenter, 1984), cichlids in
the Great Lakes of Africa (Fryer, 1977; Greenwood, 1984; Witte, 1984; McKaye
& Gray, 1984), etc. It is also plausible if the habitat of one species is large and
SYMPA'I'RIC SPECIATION
215
that of the other species is small and incorporated within it (Tauber & Tauber,
1977a, b). Our model allows us to explain these data without what seems to us
artificial hypotheses on pre-existing barriers which later disappeared. However,
in all the cases mentioned i t is imposible to distinguish sympatric speciation as
we define it from speciation in which conditioning plays the most important
part.
The second basis may be the sympatric existence of a pair of species whose
artificial hybrids are viable and fertile with no traces of 'gene imbalance'. Such
a case is analogous to dumb-bell structures in that even if an allopatric origin of
the forms in question cannot be excluded secondary contact would bring about
the same problems as sympatric speciation does; selection against intermediates
could not be related to their 'hybrid inferiority', but would be only due to
circumstances of their ecology (Anderson, 1949). The number of such pairs of
species seems to be large, e.g. two species of Drosophila athabasca complex
(Johnson, 1978), Chrysopa carnea and C. downesii (Tauber & Tauber, 1977a, b).
If both criteria apply (e.g. Tauber & Tauber, 1977a,b), sympatric speciation
is the most likely scenario (see the discussions of Hendrickson, 1978; Tauber &
Tauber, 1978, 1982), although the exact genetic mechanism of the process can
only be guessed at.
Qualitative evaluation of the factors of sympatric speciation
Dumb-bell structures represent a relatively brief phase of evolution and
therefore may be rare at a given time, although many species have probably
passed or will be passing through this phase.
The factors of sympatric speciation can be reasonably investigated at this
stage, because such structures can only appear under sufficient disruptive
selection and reproductive isolation. Conversely, sufficient selection and
isolation will lead to formation of a dumb-bell. Without disruptive selection,
assortative mating (e.g. in coloration of the snow goose and Arctic skua (see the
review of Mayr, 1970) in size of Rana temporaria (Mina, 1974), Chrysomelid
beetles of various subfamilies (Solomatin, Kreslavsky, Mikheev & Gritsenko,
1977), and Littorina obtusala (Sergievsky, 1983)) is merely a side-effect (probably
detrimental) of variability. Such assortativeness is usually weak and does not
affect the population considerably. This does not imply, however, that
assortative mating does not play an important role in evolution (Mayr, 1970).
Assortativeness is important when it is advantageous, i.e. when disruptive
selection acts.
?'here are a number of cases when assortativeness is known to be strong
(McKaye, 1980; Wood, 1980; Kreslavsky et al., 1981; McLain, 1982) but these
data are not abundant.
Quantitative measurements of disruptive selection in natural populations are
not available, though observations on selection of the intensity required for
speciation should not be very difficult to obtain.
'The best available indirect data on disruptive selection are given in a classical
work by Zinger ( 1928). The author showed Alectrolophus (Rhinanthus) major Rchb.
growing in virgin biotopes and developing seeds at the end of July to produce in
the conditions of yearly hay-making two forms (probably species), one of which
completed the whole cycle in June before hay-making ( A . verna), and the other
216
A. S. KONDRASHOV AND M. V. MINA
which began growing after haymaking and developed seeds in August
(A. polyclados). The parameters of life-history were closely correlated with some
morphological characters such as the number of knots on the stem. The new
forms-A. verna and A . polyclados-were found to develop sympatrically from
populations of A . major under disruptive selection many times at different places,
as Zinger observed various stages of speciation in different populations. A
correlation between the blooming of A . polyclados and the mowing-time in
different and sometimes neighbouring meadows suggested a high intensity of
selection. It has also been shown that artificial disruptive selection for flowering
time in Brassica campestris yields a rapid divergence and substantial temporal
isolation between opposite marginals (Murty, Arunachalam, Do101 & Ram,
1972).
Experiments on sympatric specialion
Results of experiments aiming to produce reproductive isolation under
disruptive selection in cage populations are controversial (for references on early
works on selection for the bristle number in Drosophila see Thoday & Gibson,
197 1 ) . Some later works concerned with selection for geotaxis in Musca domestica
(Soans et al., 1974; Hurd & Eisenberg, 1975), and some experiments (especially
those on geotaxis) have yielded successful results. We would like to clarify two
points here. First, negative results in such experiments may not mean more than
the natural populations, that were used, had no polymorphism sufficient for
speciation (Thoday & Gibson, 1971). Second, disruptive selection in such
experiments is directed at one character, whereas in natural conditions a
difference of resources is likely to require differences in many characters.
Consequently, selection should be created because of introduction of different
resources, as it is disruptive selection combined with selection for coadaptedness
that has the strongest effect on speciation (Kondrashov, 198313). Besides, some
characters subject to selection are more likely to be involved in the formation of
reproductive isolation. The latter seems to be the case for behavioural characters
(Coyne & Grant, 1972), which may well explain the success of the experiments
in which such characters were involved (Halliburton & Gall, 1981).
SYMPATRIC SPECIATION AND GENOME COADAPTEDNESS
There is a tendency to view sympatric speciation as based either on the notion
of ‘macromutations’ (polyploidization included) as a cause of speciation or on
‘bean-bag genetics’. For example, Mayr (1978) wrote: “The models of the
proponents of sympatric speciation generally work with the fitness (and
polymorphisms) of single genes and are little concerned with problems of the
relative frequency of genes and with gene interactions. The opponents of
sympatric speciation, by contrast, are impressed by the role of heterozygosity
and more generally by the difficulty of setting from one peak of a well
integrated, well coadapted gene complex through a valley of disintegration to
another peak of coadaptedness”.
Such a view misleads one into thinking that one of the difficulties of sympatric
speciation would be the problem of the availability of mating partners for
genetically isolated individuals ...This problem does exist with some snails in the
SYMPATRIC SPECIATION
217
formation of the left form out of the right form (or vice versa). Such a processa real speciation-is possible only in sedentary non-mobile species where
individuals can mate to their sib of the same phenotype (Alexandrov &
Sergievsky, 1978). However, if only homozygotes A A and aa do not mate and all
other matings are possible, there is no problem. This is the more so when new
species differ in several loci. It is important to note that sympatric speciation, as
well as allopatric speciation, goes on at the populational level and not at that of
individuals.
The second part of Mayr’s statement implies that interspecies and intraspecies
differences are of a different nature (see also Mayr, 1970). This is supposed to
prevent sympatric speciation which requires that assortativeness in a parent
population should gradually transform into reproductive isolation between the
species.
It is a truth universally acknowledged that all individuals of a particular
species have one and the same ‘coadapted gene complex’ and a ‘canalysed
ontogenesis’. It follows that in a genotype space there are some isolated adaptive
peaks corresponding to actual or potential species, while all other genotypes are
‘disintegrated’ and maladapted under any environment. In such circumstances
the first phase of speciation, i.e. a settling from one peak to another, is only
possible in a small geographically isolated population as a result of a random
drift (Mayr, 1970; Templeton, 1980).
The situation looks like a paradox: a crucial point of evolution-speciationis viewed by Darwinists as beginning from anti-adaptive changes. Numerous
records of physiological and ontogenetical inferiority of hybrids are generally
considered as the main evidence for this. This would be the case if hybrid
inferiority were due to heterozygosity at one locus, but this looks quite
exceptional (Portin, 1974). However, if two species differ in many genes, the
inferiority of their hybrids does not mean that the speciation involves a decrease
of fitness because of multidimensionality of genotype space (Maynard Smith,
1983: 21).
Suppose that coadapted genotypes form a complex system of ridges in a
genotype space, rather than a set of isolated peaks. Hence, maladaptedness of a
hybrid d between species a and b does not necessarily imply that species passed
through a ‘valley of disintegration’ in the course of their evolution from a
common ancestor c. The species ( e a n d f) may be situated at the opposite ends
o f a straight narrow ridge. Then their F , hybrids will be coadapted but most of
F, progeny (being more viable as a result of segregation) will miss the ridge.
Hybrid breakdown in F, and subsequent generations is often observed (Ohta,
1,980; Grant, 1981).
The isolated peaks of coadaptedness imply a typological concept, with the
only exception that the ‘type’ of a species is determined not at the
morphological or biochemical levels (which was proved not to be true by Mayr
(1970), Lewontin (1974) and a number of others), but at the genetical and
ontogenetical levels.
While disintegrated genotypes are maladapted everywhere because of their
intrinsic properties, the fitnesses of coadapted genotypes may be high or low,
depending on their environments. In any particular environment some
coadapted genotypes correspond to selective peaks, i.e. a certain selective
topography (see Wright, 1982) exists. It might be supposed that speciation goes
L
218
A. S. KONDRASHOV AND M. V. M l N A
through peak-shifting of a population because of random drift. However, recent
quantitative estimations (see Barton & Charlesworth, 1984) suggest that this
process is implausible even under allopatric conditions, no matter what factors
(intrinsic or environmental) may have created the adaptive valley. We therefore
assume that sympatric speciation is an attempt of a population to follow the
changes of selective topography, namely a splitting up of a peak into two
(Rosenzweig, 1978, see above). The more flexible selective topography is, the
more evolutionary possibilities open up for a population. If all the changes are
adaptive, the population will not be able to cross from adaptive zone A to zone B,
until the coadapted genotype g can realize its potential advantages in a suitable
environment (Fig. 2). As selective topography varies both in time and space,
evolution may be facilitated in a system of connected subpopulations. We
assume that if geographic structure facilitates evolution, this is due to fuller
utilization of environmental and selectional potentials, and not to drift. Wright
(1948) proposed a similar view, though population subdivision is more
traditionally considered as a tool to increase the number of random peakshifting attempts (Wright, 1982).
In the course of sympatric speciation the formation of assortativeness and,
consequently, of isolated gene pools precedes the development of a profound
distinction between the species that are responsible for the inferiority of their
hybrids. In other words, reproductive isolation appearing in the course of
sympatric speciation is due to incompatibility, i.e. minor differences preventing
matings between genetically well-matched partners. Independent evolution of
Figurc 2. Prak (A) and ridge (B) patterns of coadaptedness. Coadapted genotypes are shadowed
(SCC ICXI).
SYMPATRIC SPECIATION
219
the formed species may lead to their incongruence, i.e. to pronounced
distinctions which make mating between them impossible or unsuccessful
(Hogenboom, 1975). Only at this stage does divergence seem to be irreversible.
The reality of incompatible but congruent species is proved by the fact that
artificial hybrids between natural sympatric species are often quite viable and
fertile (e.g. Tauber & Tauber, 1977a, b; Johnson, 1978). It is also confirmed by
the data on species that have no differences in genes of a pronounced effect
(lethals), in isozymes and in karyotypes (Carson, 1976; Nevo & Cleve, 1978).
We presume the difference between a pair of congruent isolated gene pools
which is a primary product of sympatric speciation to be of the same nature as
intraspecific variations. This is also in line with the facts of introgressive
hybridization and reuniting of species that show the reversibility of divergence
at this stage, such as ‘blending’ of two clupeid species (Alosa alosa and A . f a l l a x )
in the Rhine River (Redeke, 1938), mass hybridization of toads Bufo fowleri and
B. americana (Blair, 1941) and numerous examples of this in plant species
(Anderson, 1949; Grant, 1981: ch. 17).
CONCLUSIONS
The main theme of this work is that it is the nature of selection that
determines the fate of a population. If selection favours speciation, the
population can split into species under quite realistic conditions. Therefore, the
occurrence of sympatric speciation in a group depends on how often disruptive
selection acts on its populations. The number of such cases is likely to vary a
great deal for different taxa. For instance, there are apparently no sympatric
groups belonging to the same species of birds or mammals which are as
distinctly different in their ecology and morphology as are host races in insects,
seasonal races or trophotypes in fishes. It is therefore not surprising that the
most active antagonists of sympatric speciation are to be found among
ornithologists (such as Mayr) while the study of insects and fish supplies the
data to support this theory. Populations of animals which do not move around
may seldom be involved in sympatric speciation because the neighbourhood
range in such cases is too small and the environmental conditions of a local
population are too homogeneous for disruptive selection to appear.
Sympatric speciation seems to always follow the one principal scheme.
Therefore, it is unreasonable to distinguish speciation which is due to
assortativeness and that due to disruptive selection (Mayr, 1970) since both
these factors are indispensable, as well as to separate allochronic (Mayr, 1970)
and competitive (Rosenzweig, 1978) types of speciation whose names refer only
to the mechanisms of formation of separate factors of the process.
We believe that a further study of sympatric speciation must involve an overall investigation of the dumb-bell structure. A tendency to regard the allopatric
speciation hypothesis as a null hypothesis which needs no proof and is valid
unless disproven is unreasonable. Neither sympatric nor allopatric hypotheses
are easily subjected to falsifying tests in the analysis of natural situations. After
all, the investigator has to be contented with a conclusion that one of the
hypotheses demands fewer assumptions and is more likely to be true than the
other. The allopatric hypothesis, though valid for a wider range of ecological
and genetical conditions, is by no means more realistic a priori, than the
A. S. KONDRASHOV AND M. V. MINA
220
sympatric hypothesis, as the latter needs much fewer assumptions about
geographic history background.
Important progress could be made if we could reveal the peculiarities of some
actual situations that may only arise under either sympatric or allopatric
speciation. This has not yet been achieved. We believe that there are many
situations when such an approach is impossible without obtaining additional
data.
ACKNOWLEDGEMENTS
The authors are grateful to B. M. Mednikov and A. G. Kreslavsky for
helpful discussions, to Natasha Kondrashova for the translation and to the
referee for many valuable suggestions.
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