Evolution__3rd_Edition

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Asexual reproduction has a spindly
phylogenetic distribution
The argument for group selection is
not watertight, ...
. . . and there are general ...
. . . and specific arguments against
group selection
CHAPTER 12 / Adaptations in Sexual Reproduction 319
apparently asexual form does not use sex in some rare or cryptic circumstances. The
bdelloid rotifers are the best documented exception. The Bdelloidea is an entire suborder
of rotifers, containing 300 or so species. Mark Welch & Meselsohn (2000) used a
new method, in which they reconstructed gene trees (Section 11.5, p. 457) to show that
bdelloid rotifers indeed are exclusively asexual.
Despite an exception or two, asexual reproduction does mainly have a spindly taxonomic
distribution in multicellular life. The spindly taxonomic distribution of asexual
reproduction suggests that asexual lineages have a higher extinction rate than sexual
lineages a that asexual lineages usually do not last long enough to diversify into a genus
or larger taxonomic group. The higher extinction rate could be because asexual populations
do not evolve fast enough to keep up with environmental change, as discussed
in the previous section. Alternatively, it could be because asexual forms accumulate
more deleterious mutations than sexual populations, as we shall discuss in Section 12.2
below. Either way, according to the group selection theory, sexual reproduction prevails
despite its cost for the individual because sexually reproducing groups have a
lower extinction rate.
The argument is not completely convincing. To say that sexual populations have a
lower extinction rate than asexual populations is to say one thing: to say that sex exists
because of its lower extinction rate is to say something much stronger. Sex might exist
in sexual species because sex is advantageous to the individuals of those species, and
asexual reproduction exists in asexual species because it is advantageous to the individuals
in those species. The different extinction rates would then be species-level consequences
of different individual adaptations in the two types of species. By analogy,
carnivores could have higher extinction rates than herbivores, but that would not mean
that herbivory was disadvantageous to individual herbivores and only maintained
because of its advantage to the group. The taxonomic distribution of asexuality, therefore,
although it is consistent with the group selectionist theory of sex, does not confirm
it. The same pattern could have arisen if sex had an individual advantage.
There are also arguments against group selection. As we saw in Section 11.2.5
(p. 301), biologists are generally suspicious of group selectionist theories. When individual
and group advantages conflict, individual selection is usually more powerful.
Adaptations that are disadvantageous for the individual are not expected to evolve even
if they do benefit the group. Although sexual populations last longer than asexual ones,
sexual individuals reproduce more slowly than asexual individuals. Asexuality, once it
has arisen, will tend to take over sexual groups. Asexuality can arise in a group by either
mutation or immigration, and neither of these processes is likely to be, on an evolutionary
timescale, all that rare. Asexual reproduction probably arises at a fairly high
rate. The reason to be suspicious of group selection is that it requires the rate at which
asexual females arise in sexual groups to be very low.
Williams (1975) also put forward a specific objection against group selection in the
case of sex. His objection has come to be called the balance argument. Some species,
such as many plants, aphids, sponges, rotifers, and water fleas (Cladocera), can reproduce
both sexually or asexually according to the conditions. These species are called
heterogonic. Many heterogonic species time their sexual reproduction for periods
of environmental uncertainty, and reproduce asexually when conditions are more
stable; but that is not the important point here. What matters is that an individual can