splitting up the original population into subpopulations of individuals
with distinct traits and preferences (see Todd and Miller 1991a,for a
simulation model of this speciation process).Coevolution’s ability to
generate synchronic diversity through speciation is a source of much of
the variety and beauty of our natural world (Miller and Todd 1995;
Skutch 1992).
Second,coevolution can generate diversity across time,diachronic
diversityin which traits in a population continuously change,generation
after generation.This pattern of constant change can be seen in arms
races between different species,for instance,predators and prey,where
adaptations in one species—ability to chase faster,say—are countered
by new adaptations in the other species—ability to change direction
quickly when fleeing (Futuyama and Slatkin 1983).In musical evolution
systems,diachronic diversity is equivalent to generating a succession of
new artificial composers and perceivers.As mentioned,this succession is
something that human listeners can accomplish by changing their criti-
cal criteria over time;coevolving artificial critics allow us to take humans
out of the evolutionary loop.
Thus,to generate musical diversity both across time and at any given
instant,both diachronically and synchronically,we must build a system
that can create a multitude of distinctly defined “species”within one pop-
ulation,and that can further induce those species to move around in
musical space from one generation to the next.Sexual selection through
mate choice allows the former,leading a population to cluster into sub-
populations with specific (musical) traits and preferences.But we need
some further force to push a population out of its attained stable pattern
of speciation.Competing species,for instance,predators or parasites,can
play this role (see Hillis’ 1992 simulation of parasites driving a popula-
tion out of suboptimal behaviors).Within the realm of sexual selection,
this motive force can be achieved through directional mate preferences
(Kirkpatrick 1987;Miller and Todd 1993,1995) that,for example,cause
females always to look for brighter or more colorful or more behav-
iorally complex males.These changing preferences can induce a popula-
tion to continue evolving.For the evolution of musical creators,as we
will see,this constant striving force can be effected through neophilia:
females always looking for males who create musical patterns that are
new and unexpected.Our coevolutionary model thus ends up looking
like and being inspired by the evolution of birdsong through sexual selec-
tion of critical females choosing which singing males to mate with.But
its application may be much wider:Miller (this volume) proposes that
human musical behavior itself is the result of runaway coevolution
between preferences and abilities.375 Simulating the Evolution of Musical Behavior