produced by conspecifics to see how close they come to being optimally
exciting given these preferences.For example,measure the beats per
minute in a large sample of commercially produced song and see
whether the speeds match the optimal responsiveness curves of human
receivers.Many such experiments are pretty obvious,but they become
more interesting if they are extended across closely related species to see
whether the preference is phylogenetically ancient,or whether it evolved
to an extreme form through runaway selection in one species but not in
others.For example,if humans respond best to dance music played at
120 beats per minute,but chimpanzees and gorillas do not respond dif-
ferently to different rhythmic speeds,we would have some evidence for
runaway selection affecting rhythmic preferences in the human lineage.
Computer simulations of evolution under sexual selection may also
prove useful in showing how aesthetic displays evolve (e.g.,Enquist and
Arak 1993).My colleagues Peter Todd and Greg Werner extended our
previous sexual selection simulations (Miller and Todd 1995;Todd and
Miller 1993,1997;Todd,this volume) to model the evolution of musical
complexity and variety under mate choice (Werner and Todd 1997).In
these simulations,a population of males produces acoustic sequences
that are received by females.Both males and females are represented
as recurrent neural networks with network architectures,connections,
weights,and biases determined by heritable genes.Each simulation run
is started with randomly generated male and female genotypes,and all
evolution is simply the outcome of the female networks imposing mate
choice on male networks based on the sequences they produce.The
runaway effect is possible because male and female networks can
become genetically correlated through assortative mating.We found
that,under such conditions,pure sexual selection can favor ever more
complex acoustic sequences and can maintain considerable diversity in
such sequences between individuals and across generations (Werner and
Todd 1997).Order and Chaos:The Interplay between Ritualization and Creativity in
Human Music
Human music shows an unusual combination of order and chaos,
with some elements highly ritualized and stereotyped,such as tonality,
rhythm,pitch transitions,song structure,and musical styles,and others
highly variable and innovative,such as specific melodies,improvisation,
and lyrical content.Hartshorne (1973:56) commented,“Songs illustrate
the aesthetic mean between chaotic irregularity and monotonous regu-
larity.”How could sexual selection favor both in a single display344 Geoffrey Miller