Evolution__3rd_Edition

316 PART 3 / Adaptation and Natural Selection
Box 12.1
The Ethics of Human Cloning
The question of why sex exists has until
recently been scientifically rather than
practically or ethically important. That
has been changed by developments in
cloning technology. Cloning still has to
overcome some technical problems,
and the theory of evolution is of little
relevance there. But the evolution of
sex is highly relevant for the ethics of
reproductive cloning. Sexual reproduction
probably only exists because it is
advantageous. Given the 50% cost of
sex, it probably at least doubles the
fitness of an average sexual offspring
relative to a cloned offspring. As we
shall see, the two most plausible theories
at present suggest that sex helps
organisms limit the effects of genetic or
Mutations to remove sex can
probably occur easily
infectious disease. If so, then doing
away with sex would increase the
chance that offspring would succumb
to disease. A decision to produce
cloned offspring might be ethically
analogous to producing sexual offspring
and taking them to a plagueridden
city where the chance of dying
of infectious disease was twice the
normal rate a or to damaging enough
of their genes to double the chance of
dying from genetic disease. This argument
could be wrong. The parasitic and
mutational theories of sex may both be
incorrect. But then sex probably has
some other advantage, which could be
lost by cloning. Cloning would be
Section 10.7.2, p. 274, this explains sex by genetic constraint.) This hypothesis is
unlikely, for two reasons.
A mutation to produce asexual reproduction in a sexual form is not a biologically
difficult mutation. All that the mutation has to do is eliminate the meiotic cell division
at the end of the cell line that produces the gametes. The reproductive cells would then
be produced by mitosis rather than meiosis. This is a “loss” mutation, in which a piece
of biological information (that is, all the cellular processes of meiosis) is lost. Nothing
new is being created. The reproductive cell division will become mitotic; but mitosis
already exists a all the other cell divisions in the body are by mitosis.
Secondly, asexual reproduction exists in many forms of life. Asexual reproduction
has evolved many times within sexual branches of the tree of life, showing that the
necessary mutations can occur. Mutations to produce asexual reproduction are therefore
plausible in theory and occur in fact. Their absence is probably not the reason for
persistence of sex.
12.1.3 Sex can accelerate the rate of evolution
unproblematic if sex exists because we
are stuck with it, or because its evolutionary
advantage no longer matters in
modern human society. However, we
need research results before we could
draw any such conclusion. The general
point a that we need to understand the
evolution and function of a character
before medically altering it a is the
principle of Darwinian medicine (Nesse
& Williams 1995). The argument here
particularly applies to reproductive
cloning. However, cloning may also be
used to produce new cells for a single
individual, to replace faulty cells or
organs, and in this case theories about
sex may or may not be relevant.
A population of sexually reproducing organisms can, under some conditions, evolve
faster than a similar number of asexual organisms. Sexual reproduction can greatly
increase the rate at which beneficial mutations, at separate loci, can be combined in a
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