For Immediate Release
Studies have shown that most new species arise from geographically, and
therefore genetically, isolated populations. But some seahorses likely
diversify in a process called sympatric speciation, in which new species
arise from a single population that has no geographic barriers to inhibit
gene flow, according to a paper published this week in the Proceedings
of the National Academy of Sciences
"We think there's a fairly strong case that sympatric speciation may have occurred in seahorses," said Georgia Institute of Technology Assistant Professor of Biology Adam Jones, the lead author on the PNAS paper. "We're not arguing that all speciation in seahorses is sympatric. The majority of speciation is probably due to some geographic barrier to genetic migration. But in some instances it looks like sympatric speciation occurred."
Driving the sympatric speciation process in seahorses is the fish's size-similar
mating practice imposed by male pregnancy, extended male parental care
and monogamy, Jones said. Seahorses choose similar-size mates to have
the best chances for successful reproduction. The female inserts ripe
eggs into the male's brood pouch, where the eggs are fertilized, embed
and incubate for 10 days to six weeks, depending on the species.
"Male reproductive rates, the size of the brood pouch and the number
of eggs that a female produces all increase with the size of the seahorse,"
Jones explained. "So if you're a large seahorse, you want to mate
with another large seahorse so you're not wasting your eggs or your brood
pouch space. So this kind of mating is the real mechanism for sympatric
speciation. A lot of forms of parental care might not cause that size-specific
restraint in mating, but this one does."
In addition to size-specific mating, a process called disruptive selection
is also necessary for sympatric speciation to occur, Jones said. Disruptive
selection occurs when large-sized and small-sized individuals survive
better than mid-sized animals.
To test their hypothesis, Jones and his co-authors developed a computer-based
genetic model to determine if the rate of size-similar mating in their
field study population was sufficient enough to produce disruptive selection
and, in turn, sympatric speciation. The model allows simulated populations
to evolve at the rate of size-similar mating that Jones and his colleagues
observed in a seahorse species off the coast of Perth, Australia. Under
these conditions, the model indicated sympatric speciation does occur
with fairly modest levels of disruptive selection.
"So the remaining question is whether disruptive selection occurs
at a sufficient strength in natural populations of seahorses," Jones
noted. "The model shows it's plausible, but as in most cases of sympatric
speciation, we have no definitive proof."
To determine that size-similar mating was occurring in the field study
population, researchers conducted genetic analyses of parentage, much
like the DNA "fingerprinting" technique used in humans. Researchers
tagged males and females in the field, sampled the DNA of the males' progeny
and then determined the mother of those offspring. Then, researchers compared
the sizes of male and female partners to chart a statistical trend that
indicated size-similar mating.
A third line of evidence for sympatric speciation came from the phylogeny,
or family tree, of seahorses, which are found in coastal and ocean habitats
throughout the world, except in extreme latitudes. Researchers gathered
documentation of species pairs that are close relatives and live in the
"If there had been sympatric speciation and it was based on assortative
mating by size, then when speciation occurs, the result should be a large
species and a small species that live in the same place," Jones explained.
Indeed, researchers noted two examples of species that are close relatives
that are sympatric over part or all of their range.
Further research on sympatric speciation could reveal patterns of genetic
variation in species pairs that researchers suspect might have undergone
Ideally, Jones or other researchers who study the topic further would
focus on seahorse populations in which sympatric speciation may have just
begun. The populations described in the PNAS paper probably underwent
sympatric speciation hundreds of thousands or millions of generations
ago, Jones added.
"The genetic signature of sympatric speciation will erode over time,"
Jones explained. "So the evidence disappears. You can't rule out
allopatric speciation (new species arising from geographically isolated
populations) in these relatively old events. Maybe a geographic barrier
disappeared. The case for sympatric speciation will be stronger if we
can find recent events - something that occurred in the past 50,000 years.
The ideal case would be to find speciation that is occurring right now.
But this is awfully hard, and that's why it is so hard to prove."
Jones' co-authors are Glenn Moore of the University
of Western Australia, Charlotta Kvarnemo of Stockholm
University, and DeEtte Walker and John
Avise of the University of Georgia.
The study was funded by the National Science
Foundation, Pew Foundation, University of Georgia, Bergwell Foundation,
Swedish Natural Science Research Council and the University of Western
More information on seahorses is available from a conservation group called Project Seahorse found online at www.seahorse.mcgill.ca/intro.htm. This group is concerned that seahorse populations are in danger of being wiped out from habitat destruction and overharvesting for use in the aquarium and non-traditional medicinal trades.
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