Are species real?
sdmanning at ASUB.EDU
Mon Apr 10 13:13:07 CDT 2006
Apologies for past (and any present) glitches in trying to post the
following, which is sent with the permission of Troy Wood, one of the
co-authors. I thought this was worth the trouble however. Do you see any
problems with the gist of the analysis?
I also speculate that the fern low hybridization rate is related to at
least some of them having so many chromosomes(?).
BLOOMINGTON, Ind. -- Notoriously "promiscuous" plants like oaks and
dandelions have led some biologists to conclude plants cannot be divided
into species the same way animals are.
That perception is wrong, say Indiana University Bloomington scientists in
this week's Nature. Their analysis of 882 plant and animal species and
1,347 inter-species crossings -- the first large-scale comparison of
species barriers in plants and animals -- showed that plant species are
just as easily categorized as animal species.
The study also yielded a surprise. The hybrid offspring of different animal
species are more likely to be fertile than the hybrid offspring of plant
"We found that not only are plants just as easily subdivided into species
as animals when analyzed statistically, but plants are more likely to be
reproductively isolated due to hybrid sterility," said evolutionary
biologist Loren Rieseberg, who led the study. "Most plant species are
indeed 'real.' The problem has been that botanists have been way
over-attracted to the plant species that readily hybridize and where the
hybrids perpetuate themselves asexually. While it's true that dandelions
and blackberries pose problems, these horror stories only make up 1 percent
of the whole."
The scientists did find categorization problems with nearly half of the
plant and animal species they surveyed.
Rieseberg and his co-authors, doctoral student Troy Wood and postdoctoral
research associate Eric Baack, examined hundreds of peer-reviewed papers
reporting the measurement of various plant and animal characteristics, or
reporting on the success or failure of hybridization of plant and animal
species with similar species. The scientists culled the papers for
information, grouped and combined data for each given species, and then
looked at how often characteristics clustered in accordance with named species.
The scientists found that while real, quantifiable clusters did exist in
most groups of plants and animals, the one-to-one correspondence of species
names and character clusters was quite low -- about 54 percent. One
explanation for this, Rieseberg said, is that too many taxonomists are
"splitters" -- they give too many species names to a single group of
After analyzing the hybridization data, the scientists found that only 30
percent of the approximately 500 plant species they surveyed are able to
produce fertile hybrids when mated with other species. By stark contrast,
61 percent of animal species surveyed are able to reproduce successfully
with other species.
The hybridization of animal species is often portrayed as rare and strange,
or else the result of human-forced matings, as is the case with ligers
(lion-tiger hybrids) and mules (horse-ass hybrids). It is not common
knowledge that many bird and fish species successfully hybridize in the
wild. The scientists found that birds were most likely to produce fertile
hybrids when crossed with other bird species. Ferns, of all things, were
least likely to generate fertile hybrids.
Many of the hybridization papers that Rieseberg, Wood and Baack looked at
reported crossings under laboratory conditions, and therefore the crossings
may not accurately represent what happens in nature. For example, two
species that can hybridize may not actually do so, perhaps because they
exist on different continents or because they prefer to mate with members
of their own species. For that reason, the percentages of plant and animal
species that hybridize in the wild are likely to be lower than those
reported by the scientists.
The Nature study is meant to address gaps in scientists' knowledge in two
areas: the fundamental nature of species and the divisibility of plant and
animal species using commonly accepted definitions of species. Debates in
both areas began with the 1859 edition of Darwin's Origin of Species, and
they have not yet been settled.
"These discussions should have been settled earlier, but no one bothered to
summarize the relevant literature, perhaps because it is so vast,"
Over the past 50 years, numerous scientific papers have been published in
which species are categorized by statistical analysis of observable traits
(i.e., numerical taxonomy) and/or by the ease with which species can be
hybridized (i.e., breeding studies). "After going through all this
literature, we realized someone just needed to compile and analyze it all,"
The scientists decided to use the mass of data to see whether taxonomists
were doing a good job, and whether cross-species mating in the plant
kingdom is especially likely to be successful.
"The species concept debate has devolved from an empirical discussion into
a philosophical one," Rieseberg said. "But this is fundamentally an
empirical question. These data support the notion that species can be both
units of evolution and products of evolution."
Rieseberg holds the Class of '54 Chair and is a distinguished professor of
biology at IU Bloomington. Troy Wood and Eric Baack also contributed to the
report. It was funded primarily by a Guggenheim Fellowship grant.
Supplementary support came from the MacArthur Foundation, the National
Institutes of Health and the National Science Foundation.
Dr. Steve Manning
Arkansas State University--Beebe
Mathematics and Science
Professor of Biology
P.O. Box 1000
Beebe, AR 72012
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