Evolution__3rd_Edition

296 PART 3 / Adaptation and Natural Selection
Many multicellular life forms do not
have Weismannist development
One cell line may proliferate within
the body
kind of life cycle is called Weismannist, after the German biologist August Weismann,
who first expounded the distinction between germ and somatic cell lines. In a
“Weismannist” organism, most cell lines (the soma) inevitably die when the organism
dies; reproduction is concentrated in a separate germ line of cells.
The separation of the germ line limits the possibilities for selection at the suborganismic
level, between cell lines. One cell may mutate and become able to out-reproduce
other cell lines and (like a cancer) proliferate through the body. But this “adaptation”
will not be passed on to the next generation unless it has arisen in the germ line. Any
somatic cell line comes to an end with the organism’s death. For this reason, cell selection
is not important in species like ourselves.
However, Buss (1987) pointed out that Weismannist development is relatively
exceptional among multicellular organisms (Table 11.1). We tend to think of it as usual
because vertebrates, as well as the more familiar invertebrates like arthropods, develop
in a Weismannist manner. However, more than half the taxa listed in Table 11.1 have
the capacity for somatic embryogenesis a a new generation may be formed from cells
other than those in specialized reproductive organs. The most striking examples are
from plants. Steward, for instance, in a famous experiment in the 1950s, grew new
carrots from single phloem cells taken from the root of an adult plant.
In a species in which new offspring can develop from more than one cell lineage,
selection between cell lines becomes possible. When the organism is conceived it will be
a single cell, and for the first few rounds of cell division the organism will probably
remain genetically uniform. No selection can take place between cell lines if they are all
genetically identical. Eventually a mutation may arise in one of the cells. If the mutation
increases the cell’s rate of reproduction, the cell line will cancerously proliferate at the
expense of other cell lines in the organism. In a Weismannist species, that cell line will
die when the organism dies and any selection between cell lines will be unimportant.
However, if any cell line in the body has some chance of giving rise to the next organismal
generation, the mutant cell line would increase its chance of being in an offspring
and be favored by selection. Explained in this way, selection between cell lines within
the body is detrimental to the organism. However, the process could also be advantageous
for the organism. Whitham & Slobodchikoff (1981) argued that in plants selection
between cell lines enables the individual to adapt to local conditions more rapidly
than would be possible with strictly Weismannist inheritance.
The process is at present more of a theoretical possibility than a confirmed empirical
fact, but it may well be important in non-Weismannist species. It may also have been
important in the non-Weismannist ancestors of such modern Weismannist forms
as arthropods and humans. Buss has developed the idea that cell selection can explain
certain features of embryology in Weismannist species.
11.2.3 Natural selection has produced many adaptations to
benefit organisms
We do not need to consider an example of organismal adaptation here: most of the
adaptations described elsewhere in the book, from the beaks of the woodpecker
..