Richard D. Alexander (2005)
"Ronald A. Fisher (1930) used the adjective runaway to apply to a hypothetical
selective process that he assigned to sexual selection (see also Trivers 1972; Miller
2000). I suggest that sexual selection and reciprocity selection are special forms of
the broader concept of social selection. Social selection is a consequence of competition
among individuals for rewards arising out of various kinds of social or mutualistic
beneficence.
Fisher’s runaway sexual selection has two special features. First is the tendency
of the choosing parties to begin favoring individuals with traits that are extreme
on some axis of desirability, rather than favoring some particular condition (other
than extremeness) of the individuals to be chosen. This form of choosing can
only occur in an organism that is able to compare an array of individuals and identify
desirable extremes (e.g., Alexander, Marshall, and Cooley 1997). It can evolve
to become the standard method of choice only if selection continues for a long time
in one direction and the best choices continue to be beyond the previous expressions
of the trait. Thus, if females that choose extreme males outreproduce those
that do not, the choosing of extremes will spread. So will extremeness in males;
otherwise the females choosing them would not gain. Spreading or fixing the choosing
of extremes will inject a certain degree of inertia into the process. For this reason,
once an “extreme-choosing” tendency is in place, extremes in the traits of the
chosen individuals can pass beyond the form in which they are adaptive in any other
sense and indeed can become disadvantageous in other contexts. Some such conflict
actually exists among all the compromises of selection on different traits of the organism
because the effect of selection can only be enhancement of the reproductive
integrity of the organism as a whole rather than the state of any of its individual
traits. When selection is social, however—when it is a matter of individuals choosing
other individuals in a mutualistic or reciprocal social interaction rather than, say,
competition to detect or capture food, or to escape enemies more effectively—overshoots in adaptiveness in other respects, because of choosing extremeness,
will be more prominent.
The second special feature of Fisher’s runaway sexual selection is the feedback
resulting from the genetic partnership between males and females in jointly
produced offspring. Trivers (1972, 166) described it as follows:
>[I]f there is a tendency for females to sample the male distribution and to prefer
one extreme (for example, the more brightly colored males), then selection
will move the male distribution toward the favoured extreme. After a one
generation lag, the distribution of female preferences will also move toward a
greater percentage of females with extreme desires, because the granddaughters
of females preferring the favoured extreme will be more numerous than
the granddaughters of females favoring other male attributes. Until countervailing
selection intervenes, this female preference will, as first pointed out
by Fisher (1958), move both male attributes and female preferences with increasing
rapidity in the same direction.<
We can note, first, that some aspects of human evolution that intrigue us, such
as brain functions that result in cleverness in social interactions, including scenario
building and testing the social future by weighing alternatives internally, could
have evolved partly through sexual selection. In view of the tendency of the human
brain to become larger across history, and of human behavior to become more
complex—and seemingly ever more rapidly after these features had exceeded their
counterparts in other species—we might be concerned to examine the likelihood
that runaway sexual selection is involved. A related question is whether some or all
features of runaway selection can also occur in nonsexual social interactions such
as the high-risk forms of social reciprocity that are prominent only in the human
species.
At first one may imagine that there are no parallels in social selection allowing
it to become runaway. But the way an individual gains by selecting its social
partners parallels the ways an individual can gain from cooperative interactions
with a mate and from a mate’s parental care. In both cases there is likely to be genetic
change in both the ability to choose good partners and the background of
the favored phenotypic attributes because a mutually beneficial interaction can be
maintained only if, on average, both interactants gain.
Sexual selection is a distinctive kind of runaway selection because joint production
of offspring by the interacting pair causes the process to accelerate (see
above Trivers quote). The defining feature of runaway selection is not acceleration,
however, but the tendency of the process to go significantly beyond adaptiveness
in all contexts except within the particular selective race—much further beyond
adaptiveness in other contexts than is ordinarily the case in the myriad compromises
among the conflicting adaptive traits that create and maintain the unified organism.
This aspect of runaway selection may hold for reciprocity selection, in which,
unlike in sexual selection, both parties can carry tendencies not only to choose extremes but also to display extremes. In social selection, again unlike in sexual selection
(except in simultaneous hermaphrodites), an individual can play both roles,
of chooser and chosen, with respect to the same traits, and alternations of roles
can occur during extended interactions between particular partners. To the extent
that social success in ecologically dominant humans (see earlier) becomes the
central determinant of reproductive success, runaway reciprocity selection may
be a more viable possibility than Fisherian runaway sexual selection.
Fisherian runaway sexual selection presumably begins with a likelihood of
heritability (i.e., genetic variability) in both variations in choices and variations in
chosen traits. It is easy to see that in this circumstance such competitions, or
races, will lead to genetic changes, at first changing both the ability and tendency
to choose extremes and the nature of the extremes available for choice. Continuing
mutations and genetic recombination (outbreeding among temporarily isolated
groups) will tend to offer the choosing parties increasingly extreme possible
choices, though to a reduced degree as selection continues to remove heritable
variations in trait expressions and the trait becomes sufficiently maladaptive in contexts
other than sexual selection. From earlier arguments it is not easy to understand
the extent to which relevant heritability is likely to disappear, or even become
trivial.
Diminution (or disappearance) of heritability in variations will not necessarily
change the tendency to choose extremes: in effect, if an ability and tendency to
choose extremes in any of a variety of social races (at least in humans) could become
genetically fixed in the population (including the ability and tendency to learn
from others, or from observation, the values of such choosing), it could still offer
advantages to the chooser of extremes (without evolutionary change in the trait
per se) because of the usefulness of even nonheritable trait variations chosen in a
social partner. Of course, there may be further evolutionary improvements in the
ability to identify and use extreme traits even after all choosers are already choosing
extremes.
On the other hand, heritable variations in what is chosen will result in a continued
march toward greater extremes because this relative quality will not be fixed
in the way the ability to identify and favor extremes can be fixed; extremes can be
identified only by comparing whatever is available. In Fisher’s version of runaway selection,
extremes win reproductively at first because they are ecologically superior,
meaning functionally superior outside the choice situation itself. Later they continue
to win because they are sexually (or, here, socially) superior, even though, as
a result of the progress of selection involving choice of extremes, they may have be come so extreme as to be otherwise functionally (ecologically) inferior. Here, “sexually
or socially superior” means that the choosing parties will have acquired a genetic
composition that will cause them to choose the extreme, the resulting choice
of the extreme individuals itself causing the chosen individuals to outreproduce.
Extremes, however, will be chosen whether or not, as extremes, they represent
heritable variants. If extreme social performances yield special social opportunities
to those displaying them (e.g., via their reputations as achievers or winners),
then regardless of the basis for the superior performances (i.e., genetic variant or
not), winning performances can yield benefits to the kin or other social associates
of the individual with the extreme traits. Thus merely joining social competitions
or races can pay, though only heritable variations, including the (variant) ability to
choose the best from among multiple available races that might be entered, will
yield evolutionary change. Heritable variations in the ability to choose appropriate
races—including not only those generally likely to be profitable but those in which
the choosing individual, because of his or her special traits, has a special likelihood
of competing successfully—may be all that is needed to drive runaway reciprocity
selection. The more different ways that success can be achieved through reciprocity
competition, the more robust will be this type of social selection. This is a kind
of selection that will yield at least part of the human type of social intelligence, perceptiveness,
and perseverance. As we all know, status (or “reputation”) can exist
independent of the adoption of a particular extreme in behavior, so the importance
of any behavioral extreme in changing some aspect of culture can also depend on
whether a prestigious person adopts, favors, or approves of it.
Since Fisherian runaway sexual selection in nonhuman organisms has remained
controversial (or theoretical), but social parallels to it may be robust in human society,
one may wonder if Fisher actually derived his idea from observing human
social situations rather than from thinking about sexual selection in the birds he
ultimately used as his examples. If so, it is likely not the only instance in which an
evolutionist was inspired by human traits and tendencies to develop a general evolutionary
explanation (e.g., Darwin’s observations on human selection of variations
in domestic animals and the fact that Hamilton’s rule applies in its fullest extent
as extensive differential nepotism across multiple levels of relatedness only in humans).
This suggestion is ironic in another way, in view of the success with which
academic biology departments have managed to exclude the human species from
their consideration, leaving its analysis to the almost exclusively nonevolutionary
approaches of social science and medical departments, and surely delaying acceptable
explanations of the human species in evolutionary terms.
When extremes in particular directions are being chosen in social selection,
new extremes never before experienced may be chosen above all other expressions
of a trait. This behavior was originally referred to in European ethology as a
“superoptimal stimulus” effect and is not specifically related to runaway selection.
Whenever superoptimal stimulus effects can be identified, however, they suggest
a history of choosing extremes, therefore the possibility of some form of runaway
selection. If, for example, we perceive that during the past several centuries art—
or even human sociality in general—has flourished and become an enormous and
diverse enterprise compared to any “ancestral” condition it might have exhibited,
we might suspect that this is evidence of a history of choosing extremes continually
in short supply and consequently of some kind of runaway process in however
artistry affects the securing of the resources of reproduction. Rather than evidence
that evolution cannot be used to explain, say, the arts, explanations of recent flowerings
of diversity and complexity in human enterprises can be sought by expanding
our understanding of the various subprocesses of evolutionary selection. We
should not be discouraged because the pathways to explanation become progressively
more difficult and call for expansions of our understanding of the workings
of the evolutionary process."
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