Evolution__3rd_Edition

208 PART 2 / Evolutionary Genetics
It is an empirical question, how
often real fitness interactions are
multiplicative or epistatic
Fitness epistasis is not the same as
genetic interaction
In the case we discussed, w 12 is the fitness of a butterfly with a tail and the color pattern
of a tailless model, Therefore, w 12 < w 11 and w 22 . w 21 is the fitness of a butterfly without
a tail, but the color pattern of a tailed model. Therefore, w 21 < w 11 and w 22 . Selection
now favors the T + /– genotypes when they are with C 1 /– but not when with C 2 /C 2 , and
T – /T – when it is with C 2 /C 2 but not when with C 1 /– (the dash implies it does not matter
which gene is present, because of dominance). The fitness relations are epistatic. There
can now be a doubly polymorphic equilibrium. All four alleles will be present, and the
haplotypes T + C 1 and T – C 2 will have disproportionately high frequencies. T + C 2 and T – C 1
haplotypes are selected against, because they often find themselves in poorly mimetic
butterflies. Linkage disequilibrium (D > 0 in this case) exists at the equilibrium.
In general, selection can only produce linkage disequilibrium at equilibrium when
the fitnesses of the genotypes at different loci interact epistatically. Not all epistatic
fitness interactions generate doubly polymorphic equilibria with linkage disequilibrium.
But all (or nearly all) such equilibria do have epistatic fitnesses.
We have been discussing the different sorts of fitness interactions a multiplicative or
epistatic (and there are others too) a as properties of formal models. Real genes in real
organisms will have fitness interactions too, and the more important question is what
sort of interactions these are. There are cases like Papilio in which epistasis is present
and powerful; but these may be isolated examples rather than representing a general
condition. Evolutionary biologists are interested in whether fitness interactions
between loci are generally epistatic and generate strong linkage disequilibrium, or
whether they are generally independent and generate linkage equilibrium. These two
extremes roughly correspond to a more “holistic” and a more “atomistic” (or “reductionist”)
school of thought, though that is not to say that they correspond to two clearly
demarcated camps of biologists.
No general answer is yet available, but it is possible to make some observations.
Different loci will tend to interact multiplicatively when they have independent effects
on an individual’s survival and reproduction. Some biologists suggest that loci which
influence events at different times in an organism’s life are more likely to show
multiplicative fitness relations (though it is also possible for such events to interact).
Epistatic interactions may be more likely for loci controlling closely interdependent
parts of an organism. The extent to which we expect loci to interact epistatically or not
then loosely depend on how atomistic or holistic a view we have of the organism (see
also Section 8.12, below).
Notice that epistatic fitness interaction is not the same as mere physiological or
embryological interaction. Fitness epistasis requires heterozygosity at two loci. Imagine
a case in which the A locus controls, say, muscle strength and the B locus controls
metabolic rate. Muscles and metabolism interact in a physiological sense: when
muscles are put to work, the metabolic rate goes up. However, if the population is fixed
for homozygotes at both loci (all individuals are A 1 B 1 /A 1 B 1 ) then there cannot be any
fitness epistasis. Epistatic fitness requires heterozygosity at both loci, and the kind of
fitness relations we saw in the Papilio memnon example. This is a special condition.
Though it is often called fitness “interaction,” the term interaction is being used in a
technical, not a colloquial, sense.
We can also test empirically how common epistatic fitness interactions are in nature.
Linkage disequilibrium is produced by epistatic selection, and the degree of linkage
..