Biology Reference
In-Depth Information
10.3.2 Advantages of Sex Must Be Large
At least 20 hypotheses have attempted to explain why sexual reproduction per-
sists (
Crow 1994, Schurko et al. 2008, Lehtonen et al. 2012
). 1) One possibility
is that sex provides an ability to
incorporate and accumulate favorable muta-
tions
. Mutations that arise in an asexual species in different individuals cannot
be combined in one individual easily; successive advantageous mutations would
have to occur in the same asexual lineage, one after the other. 2) Sexual repro-
duction may allow the accumulation of favorable mutations even when deleteri-
ous mutations are present (whereas the maintenance of a favorable mutation in
an asexual population is dependent upon the relative fitness of the individual
in which the mutation occurs). Thus, the value of sex “lies more in the ability to
reassort existing genes as the environment changes and in the elimination of
harmful mutations” (
Crow 1994
).
Sexual reproduction allows harmful mutations to be eliminated. This effect is
based on a concept termed
Muller's ratchet
.
Muller (1964)
noted that in an asex-
ual population, unless it is very large, it is unlikely that any individual is free of
harmful mutations. In such a population, the most-fit individual is one that has
only one mutation. In the next generation, mutations occur again and this time
the most-fit individual may have one new mutation, or two in total, and the
“ratchet” has turned another cog. In the absence of reverse mutation, such a
population would accumulate more and more deleterious mutations. In a sexual
population, a mutant-free type can be created by recombination.
Thus, sexual reproduction can reduce the mutational load. The deleterious
effects of mutations are related to their frequency of occurrence, not to the mag-
nitude of their effects. The smaller the effect of a mutation, the more individuals
it will affect before it is eliminated from the population. Over time, the number
of mutations removed per eliminated individual is much larger in a sexual than in
an asexual population and the mutation load is reduced correspondingly. Sexual
reproduction may provide a means by which some transposable elements (TEs)
can be eliminated from the population, although it could require thousands of
generations and would be affected by the rate of transposition and excision of
TEs, strength of selection, and population size (
Lehtonen et al. 2012
).
Sexual reproduction and diploidy have evolutionary advantages by providing
protection from somatic mutations (
Crow 1994
). Diploidy is common in higher
organisms, which have the most extensive and highly differentiated soma. Each
generation somatic tissues develop and are identical, except for somatic muta-
tions, which are often recessive. Diploidy thus diminishes the deleterious effects
of recessive somatic mutations, which could destroy essential cells or initiate
