Evolution__3rd_Edition

372 PART 4 / Evolution and Diversity
Asexual forms ...
...may...
. . . or may not ...
. . . exist as distinct species
Microbes show clearer species in
some cases than in others
divide the snails into three main regions. From the left, the first main region has a
relatively high frequency of allele 130; the second has a high frequency of allele 100;
and the right-hand region has a higher frequency of allele 80. These regions transcend
the mountian barrier to gene flow, which is shown as a gray region from left to right
in Figure 13.8a. The similarity of the populations within each of the three regions,
on both sides of a barrier to gene flow, is difficult to explain by the biological species
concept. The ecological species concept might be able to explain the pattern, but we
need further research on how the different alleles are adapted to different regions across
the map.
A further test case comes from asexual species. The ecological species concept predicts
equally clear-cut species in both sexual and asexual forms. There is no reason why
only sexual, and not asexual, forms should inhabit niches, and selection should therefore
maintain asexual species in integrated clusters much like sexual species. But the
biological species concept predicts a difference. Asexual forms do not interbreed and
no gene flow occurs. If gene flow holds species together, then asexual forms should
have blurred edges; nothing will stop asexual species from blurring into a continuum.
Sexual forms should be more clearly discrete than their asexual relatives.
Unfortunately, the evidence published so far is indecisive. On the one hand, many
authors a especially critics of the biological species concept, Simpson (1961b) being an
example a have asserted that asexual species form integrated phenetic clusters just like
sexual species. This is supported by the study of Holman (1987) on rotifers. Bdelloid
rotifers are a large asexual taxon (Section 12.1.4, p. 319). The monogont rotifers are the
sister taxon of the bdelloids, but monogonts are at least sometimes sexual. Holman
showed that species in bdelloids have been recognized at least as consistently as in
monogonts.
On the other hand, examples also exist of asexual forms that do not form distinct
species. Maynard Smith (1986) has pointed to the example of hawkweed (Hieracium).
It reproduces asexually and is highly variable, such that taxonomists have recognized
many hundreds of “species,” and no two taxonomists agree on how many forms there
are. In all, asexual species are a potentially interesting test case, but the evidence that has
so far been assembled does not point to a definite conclusion.
Bacteria, and other microbes, illustrate much the same point. Bacteria mainly reproduce
asexually, and yet distinct species of bacteria are named just like in multicellular,
sexual life forms. This could mean that the biological species concept is inadequate
because it is unable to account for bacterial species. However, genetic exchange does
take place between bacterial cells. The units recognized as species in bacteria may then
be maintained by gene flow. Alternatively, bacteria may not really form distinct species,
and the habit of naming bacterial “species” may be misleading. The evidence about
genetic variation in bacteria is too limited to allow a broad conclusion about bacteria
as a whole. Much is known about the population genetics of a few bacteria, such as
Escherichia coli, but the population genetics of most microbes remains obscure. One
popular interim conclusion is that some bacteria have extensive genetic exchange
between cells and form good species, but other bacteria have little genetic exchange
and the application of species concepts in them may be problematic. Cohen (2001) and
Lan & Reeves (2001) discuss microbial species. Maynard Smith et al. (1993) look at the
kind of data that are needed. Meanwhile bacteria, like asexual forms generally, pose a
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