Sympatric speciation
Krishna Kumar Verma
HIG 1/327, Housing Board Colony,
Borsi, Durg, Chhattisgarh 491001, India
Email: kk.sheel@gmail.com
Date
of publication (online): 26 April 2010
Date
of publication (print): 26 April 2010
ISSN
0974-7907 (online) | 0974-7893 (print)
Editor: Pierre Jolivet
Manuscript details:
Ms
# o2367
Received
15 December 2009
Finally
accepted 31 March 2010
Citation: Verma, K.K. (2010). Sympatric speciation. Journal of Threatened Taxa 2(4): 820-823.
Copyright: © Krishna Kumar Verma 2010. Creative Commons Attribution 3.0
Unported License. JoTT allows unrestricted use of this article in any medium
for non-profit purposes, reproduction and distribution by providing adequate
credit to the authors and the source of publication.
Author Details: K.K. Verma is a PhD in Zoology, and has
taught Zoology/Entomology for about 35 years in M.P. Govt. P.G. colleges. What perhaps is specially notable is
that out of 67 research/review papers, published by him in national and international
journals 44 have been published after his retirement in 1991.
Abstract: The prevailing notion is that allopatry is the main driver of
divergence between populations, leading to speciation. But a number of recent studies show that
speciation occurs in sympatry too, that sympatric speciation is quite common,
perhaps more common than hitherto believed. Such studies have been reviewed in this communication.
Keywords:Allopatry, character displacement, divergence, host switching, incipient
speciation, orchids, sexual conflict, sympatry.
Introduction
Notions about allopatric and sympatric speciation include a
spatial dimension. In case related nascent species are in different
geographical areas, allopatry is obvious. However, related species may occupy the same general area, but they may
be adapted to different niches; in that situation too some spatial separation
is involved. That is why
Fitzpatrick et al. (2008) have asked “What, if anything, is sympatric speciation?”. Among
diverging species there is a stage of only partial reproductive isolation,
therefore, there may be a hybridization zone between their preferred
niches. To decide about allopatry
and sympatry in field work in a limited area is generally extremely difficult.
Realizing this difficulty Fitzpatric et al. (2008) have pointed out
that to apply the concept of sympatry in its theoretical extreme (i.e. even
incipient and related species, occupying separate subdivisions of the ancestral
range are not to be taken as sympatric) is nearly impossible. In the following discussion diverging
species, with small/partial spatial separation between them, have been taken as
sympatric.
Divergence among associated organisms
In parasite-host, phytophagous insect-host plant and
plant-pollinator associations there is co-evolution and consequent divergence
in both the sides involved and as pointed out by Kruger et al. (2009),
there is a sort of arms race between them, resulting in higher speciation and
extinction rates in the associates. Kruger et al. (2009) have studied species richness
in parasitic cuckoos. They point
out that there is almost no difference between parental and parasitic cuckoos
in the number of species per genus. However, a cladogenesis test by them has shown that brood parasitism is
associated with significantly higher rates of extinction and speciation. Further they point out that subspecies
diversification rate is twice as much in parasitic cuckoos as in parental
ones. Another important
observation by the authors is that parasitic cuckoos, with more recognized
subspecies, have more hosts. Obvious inferences, from the studies of Kruger et al. (2009),
are that parental cuckoos evolve defenses against new brood parasites, that
there are co-evolutionary changes both in the parental and parasitic species
and that the parasitic ones not only show stepped up speciation but also
increased rate of extinctions, if the new host develops effective defenses
against a new brood parasite. These changes take place in sympatry, if we take
sympatry in a little broader meaning, as suggested above.
Host switching among phytophagous insects
Two species of the chrysomelid beetle genus Chrysochus in North America are C. auratus and C. cobaltinus. Chrysochus species live on plants of Apocynaceae and Asclapiadaceae. Some species of these plant families
contain the toxic compounds cardenolides. C. auratus and C. cobaltinus have their food plants with cardenolides, while all other species
of the genus Chrysochus live and feed on plants without these toxic compounds. Labeyrie & Dobler (2004)
have prepared and compared DNA profiles of the cardenolides and
non-cardenolides feeding species of Chrysochus, and have noted only one small and consistent difference between
the two. While the latter species
have the code for asparagine at the position 122, the cardenolides feeding
species have at this position the code for histidine. Thus it seems that a small mutational change at this
point has helped the two species to invade a new niche, viz. the host plants
with cardenolides, and that this change has occurred in sympatry. Agrawal et al. (2009) have
pointed out that production of latex and cardenolides by plants is a defense
device meant to reduce herbivory in the extant communities, but specialized
herbivores employ several mechanisms to circumvent the negative effect of such
defensive devices of plants. A
newly coming up mutation in a herbivore population may result in a newly acquired
trait, which may counteract the adverse effect of the defensive plant toxins,
thus help the mutant form to shift to a new niche and move along a line of
diversification and this may happen in sympatry. Borghuis et al. (2009) have phylogenetically
analyzed species of the beetle genus Galerucella. On the basis of
their results they have inferred host switching among the species, as they find
several sister taxa using unrelated host plants in sympatry. Gomez-Zurita (2008) has
discussed speciation in the chrysomelid beetle genus Timarcha On the basis of genetic and
phylogenetic studies, he has concluded that, while most species of the genus
have evolved in allopatry, in some cases sympatric speciation has occurred due
to chromosomal changes, host plant switching, or both.
Orchids and their pollinators
Orchids (Orchidaceae) are a highly diversified and a species rich
family. It is estimated that there are nearly 30,000 naturally occurring
species of orchids (Mondragon-Palomino & Pfennig 2009). These plants have a number of different
devices to attract pollinating insects. About one third of the species have taken to deceiving the pollinators;
the deceit may be visual, aromatic, tactile, or all three together (Pollan
& Ziegler 2009). Rewarding
species offer nectar to the pollinator. Deceiving species deceive through presenting the appearance in their
floral morphology of a nectar producing species or giving out the scent of a
nectar offering species, but do not produce nectar. Some deceivers have taken to sexual deception through some
petals of the flower presenting the appearance of the rear end of the abdomen
of a female of the pollinator species and also through production of the scent
of the pollinator female. A male
of the pollinator is thus invited for a pseudocopulation with the flower. Species specific relation between an
orchid species and its pollinator is traditionally considered as the main
reproductive isolation mechanism among orchids (Cozzolino et al.
2005a) but, because of deception among orchids, a mixed pollen load is expected
on the pollinator due to visiting deceptive, as well as, rewarding species of
orchids. Cozzolino et al. (2005a)
have found bees and flies, which are orchid pollinators in the Mediterranean
region, carrying hemipollinaria belonging to nine different orchid species. In
this situation hybridization between orchid species is expected. Cozzolino & Widmer (2005b)
have recommended observations on the behaviour of orchid pollinators,
experimental crossings and the use of molecular markers for studying orchid
hybridization. Hybrids of the Sardinian orchids Ophris tricolor and O. incubacia have been studied by Cortis et al. (2009). They find the
hybrids with features which are intermediate between the parental species, in labellum
morphology. The hybrids and the
parentals have been noted as producing a similar mix of scents. But the hybrids have been observed to
produce significantly less fruits and seeds than the parentals. Genetically the hybrids have been found
to be of the first generation resulting from the hybridiztion. Thus it has been inferred that a
post-mating barrier was keeping the two orchid species apart. But such barriers
to successful speciation through hybridization should not be common among
orchids, because, as Pollan & Ziegler (2009) have pointed out,
at present there are about 1,00,000 registered hybrid orchids. As Venditti & Pagel (2009)
have said, speciation through hybridiztion is more common than previously
thought. The species specific
relationship between an orchid species and its pollinator is through the exact
nature of the scent produced by the former. This situation may also help
sympatric origin of a new orchid species. As said by Pollan & Ziegler (2009), a mutation in an
orchid may result in a slight alteration in its scent and the altered scent may
attract the attention of a new pollinator, while the original pollinator may
not respond at all. This will
create a reproductive isolation between the mutant and the original orchid,
thus speciation may be initiated in sympatry.
Character displacement and sympatric divergence
Pfennig & Pfennig (2009) have discussed at length
ecological and reproductive character displacement. They point out that, if two
species or two incipient species live in the same habitat, selection operates
between them to minimize competitive use of resources or to minimize
reproductive interference by character displacement or displacement of traits
associated with resource use and reproduction. Such displacement may take place both in allopatry and in
sympatry. In sympatry it may occur
between two closely related species or incipient species and in both such cases
it leads to further divergence and phenotypic differences between them. The role of selection in character
displacement has been discussed by the authors. An illustrative example of character displacement is in
Verma & Shrivastava (1985). They have studied niche separation between two species of
the tortoise beetle Aspidomorpha (Aspidomorpha), A. miliaris and A. sanctae-crucis, which live and feed on leaves of the same host plant, Ipomoea fistulosa, and have similar geographic
distribution in India (Maulik 1919). While A. sanctae-crucis adults feed on the marginal parts of a leaf, A. miliaris, as adults, feed on the deeper
parts of a leaf lamina, making roundish/ovoid holes (Figs. 1 a & b). Larvae
of A. miliaris are gregarious, living in groups up to the time of pupation. When feeding, they form a tight row
along a leaf margin and when at rest, they make an oval cycloalexic group
(Verma 1992) on a leaf (Figs. 2 a & b). In contrast the larvae of A. sanctae-crucis live isolated and scattered on
leaves. They feed on deeper parts
of the leaf lamina, making roundish holes, like the adults of A. miliaris. A. miliaris is found on the Ipomoea weed in more humid areas, whereas A. sanctae-crucis prefers weed growth in drier
areas. Besides these behavoural
and ecological differences, there are also morphological differences between
the two species, e.g. in their mandibular structure and in the shape of the
basal part of their elytra. It is
also notable that, in spite of these differences, there is a some overlap
between the niches of the two species. Pfennig & Pfennig (2009) point out that character displacement
furthers divergence between incipient or partly diverged species. Hence sympatric speciation mechanism
may include character displacement.
Incipient speciation
Speciation is a continuum, during which there is a gradual
acquisition of reproductive isolation and a species is an arbitrarily chosen
stage in this continuum (Verma 2006). In the continuum the stage chosen, to be designated as a
species, is generally at a pronounced level of reproductive isolation and
divergence. Cases of incomplete
speciation have been reviewed by Nocil et al. (2008). They point out that ecological
divergence often leads to weak reproductive isolation and small genetic
differentiation between related populations. In such cases divergence occurs in a limited spatial
dimension. Arias et al. (2008) have studied, through mimetic colour
pattern and molecular genetic data, incipient ecological speciation in Heliconius butterflies. They have found a hybrid zone between
two subspecies of Heliconius erato,
namely H. erato venus and H. erato chestertonii, suggesting an incomplete reproductive isolation between the
two. Further the authors have
found that H. erato venus is monophyletic in origin, and well differentiated from H. erato chestertonii, suggesting no introgression
between the two in the past. H. erato chestertonii occurs at higher altitudes than
other races of H. erato. This subspecies has
well maintained its integrity in spite of a high level of hybridization and is
thus well on the way to further divergence and speciation. Such instances of incipient speciation
have been often taking place with limited spatial separation; hence they may be
taken as taking place in sympatry. Incipient speciation may lead to further divergence, not only through
growing reproductive isolation, but also through hybrid speciation. Hybrid speciation generally does not
happen in animals, due to some pre- or post-mating barriers. But in a small number of cases , due to
crosses between parapetric populations of only partly diverged or incipient
species or members of the same species complex, viable hybrids are produced,
which start new lineages. This
situation has been recorded by Gomez-Zurita (2008) in the chrysomelid beetle
genus Timarcha. From his genetic and phylogenetic study on species of this genus
he has inferred that within the T. goettingensis species complex hybrid speciation has occurred.
Sexual conflict and divergence
The female chichlid fishes in the African Lake Malawi, living
against a rocky bottom, have a mutant body colour, orange-blotch (OB), which is
advantageous to them, as it gives them crypsis against a mottled rocky
substratum (Robert et al. 2009). But this body colour has a disadvantage; it has disrupted
the species specific male colour pattern for mate choice. This sexual conflict has resulted in a
selection in favour of brightly coloured males. Elsewhere the original body colour, brown barred (BB)
continues as the favoured colour, as it makes the fishes inconspicuous to
predators. The genetic divergence
between chichlids, living against rocky lake bottom and those in other areas is
developing. Roberts et al. (2009)
believe that similar genetic conflicts have played an important role in
diversification and speciation in chichlids in African lakes. Here it would be relevant to recall the
views of Rodriguez (2009), according to whom a trait may evolve in a
nonsexual context, but may lead to a sensory bias affecting mate choice. Thus it may help fuel rapid evolution
through sexual selection.
Conclusion
It has been traditionally believed that speciation has been mostly
in allopatry. But now it is being
realized that sympatric speciation also plays an active role in producing
species divergence and is more common in speciation than previously believed (Venditti
& Pagel 2009).
References
Agrawal, A.A., M. Fishbein, R. Halitschke, A.P. Hastings, D.L.
Rabosky & S. Rasmann (2009). Evidence for adaptive radiation from phylogenetic study of plant
defenses. Proceedings of the National Academy of Sciences (Early Edition). DOI:
10.1073/pnas.0904862 106
Arias, C.F., A.G. Munoz, C.D. Jiggins, J. Mavarez, E. Bermingham
& M. Linares (2008). A hybrid zone provides evidence for incipient ecological
speciation in Heliconius butterflies. Molecular Ecology DOI: 10.1111/j.1365-294X.2008.03934.x
Borghuis, A., J.V. Groenendael, O. Madsen & J. Ouborg (2009). Phylogenetic anlyses of the leaf
beetle genus Galerucella: Evidence for host switching at speciation? Molecular Phylogenetics and
Evolution 53:
361-367.
Cortis, P., N.J. Vereecken, F.P. Schiestl, M.R. Barone Lumaga, A. Scrugli
& S. Cozzolino (2009). Pollinator convergence and the nature of species’ boundaries in
sympatric Sardinian Ophrys(Orchidaceae). Annals of Botany 104: 497-506.
Cozzolino, S., F.P. Schiestl, A. Muller, O. De Castro, A.M. Nardella & A.
Widmer (2005a).
Evidence for pollinator sharing in Mediterranean nectar-mimic orchids: absence
of premating barriers? Proceedings of the Royal Society B 272: 1271-1278.
Cozzolino, S. & A. Widmer (2005b). Orchid diversity: an evolutionary
consequence of deception? Trends in Ecology and Evolution 20(9): 487-494.
Fitzpatric, B.M., J.A. Fordyce & S. Gavrilets (2008). What, if anything, is sympatric
speciation? Journal of Evolutionay Biology. DOI: 10.1111/j.1420-9101.2008.01611.x
Gomez-Zurita, J. (2008). Species and speciation in Timarcha, pp.17-39. In: Jolivet, P., J. Santiago-Blay & M. Schmitt
(eds.). Research on Chrysomelidae (vol. 1). Brill, Leiden, The Netherlands.
Kruger, O., M.D. Sorenson & N.B. Davies (2009). Does coevolution promote richness
in parasitic Cuckoos? Proceedings of the Royal Society B. DOI: 10.1098/rspb.2009.1142
Labeyrie, E. & S. Dobler (2004). Molecular adaptation of Chrysochus leaf beetles to toxic compounds in
their food plants. Molecular Biology and Evolution 21(2): 218-221.
Maulik, S. (1919). Fauna of British India, volume on Coleoptera, Chrysomelidae,
Cassidinae and Hispinae. Taylor and Francis Ltd., London.
Mondragon-Palomino, M. & G. Theiben (2009). Why are orchid flowers so diverse?
Reduction of evolutionary constraints by paralogues of class B floral homeotic
genes. Annals of Botany 104: 583-594.
Nosil. P., L.J. Harmon & O. Seehausen (2008). Ecological explanations for
(incomplete) speciation. Trends in Ecology and Evolution DOI: 10.1016/j.tree.2008.10.011
Pfennig, K.S. & D.W. Pfennig (2009). Character displacement: Ecological
and Reproductive responses to a common evolutionary problem. The Quarterly Review of
Biology84(3): 253-276.
Pollan, M. & C. Ziegler (2009). Love and lies. How do you spread
your genes around when you’re
stuck in one place? By tricking animals, including us, into falling in love. National Geographic
Magazine,
September 2009. (http://ngm.nationalgeographic.com/2009/09/orchids-text) Downloaded on 29thAug. 2009.
Roberts, R.B., J.R. Ser & T.D. Kocher (2009). Sexual conflict resolved by
invasion of a novel sex determiner in Lake Malawi chichlid fishes. Science Express, 1st October 2009 (www.sciencexpress.org) Downloaded on 2ndOctober 2009.
Rodriguez, R.L. (2009). Trait duplication by means of sensory bias. Behavioral Ecology (Advance access publication on 4thOctober 2009) DOI: 10.1093/beheco/arp130
Venditti, C. & M. Pagel (2009). Speciation as an active force in
promoting genetic evolution. Trends in Ecology and Evolution DOI: 10.1016/j.tree.2009.06.010
Verma, K.K. (1992). Cycloalexy in the tortoise beetle Aspidomorpha miliaris (Coleoptera, Chrysomelidae). Chrysomela 26: 6.
Verma, K.K. (2006). Have modern species concepts failed? Bionotes 8(4): 88-91.
Verma, K.K. & R.K. Shrivastava (1985). Separate niches for two species of Aspidomorpha living on Ipomoea fistulosa M. and de Bary (Coleoptera,
Chrysomelidae). Entomography 3: 437-446.