Accordingly a new adaptive peak
is achieved and the system enters large scale, low gain.
1994; Orr 1995; Orr and Orr 1996; Gavrilets and Hastings 1996) assume that viable genotypes form "clusters" in genotype space so that the population can move from one adaptive peak
to another one separated by an adaptive valley following a "ridge" of well-fit genotypes without crossing any deep adaptive valleys.
According to this idea, genetic factors are extremely interactive, such that some combinations generate high fitness and become "peaks" on the adaptive landscape, while other combinations have low fitness and represent "valleys." Wright, in his shifting balance theory (SBT) of evolution, posited that a species becomes stuck on the local equilibrium of an adaptive peak
, and can only move to the domain of attraction of a higher peak by the actions of genetic drift followed by subsequent selection (Wright 1931a, 1932; Simpson 1953; Barton and Rouhani 1987, 1993).
(This requires the approximations that fitnesses are independent of genotype frequencies and that selection is weak enough relative to recombination that linkage disequilibrium is negligible.) If the landscape stays constant, a local maximum of mean fitness (an adaptive peak
) will eventually be reached (at least approximately - mutation and recombination usually decrease fitness).
Second, a population in the neighborhood of a single adaptive peak
will eventually climb the peak regardless of the pattern of genetic (co)variances (Lande 1979; Via and Lande 1985; Zeng 1988), in which case the role of quantitative genetics is only temporary.
This is in contrast to Barton (1992) who argued that even the less adaptive peak
can easily take over.
Each of these allows a species to evolve through a valley to a new adaptive peak
by a combination of stochastic and deterministic processes.
We therefore calculated the percentage of all 30 populations (i.e., trials) at each migration rate that shifted entirely to the higher adaptive peak
Based on the principle of universal pleiotropy, Wright (1932) envisaged an adaptive landscape where each local population was defined by a coadapted combination of loci representing an adaptive peak
. Such a multiplicity of peaks has been termed "multiple-peak epistasis." Universal pleiotropy and multiple-peak epistasis are two of the four premises that underlie the shifting-balance theory of evolution (Wright 1970), which also assumes the existence of polymorphisms at most loci and a subdivided population structure to allow different peaks to be reached in different populations.
Since we expect populations to be centered near an adaptive peak
, the common question has been how shifts from one peak to another may occur.
We can all articulate the central error of such a perspective: since organisms help to create their own environments, adaptive peaks
are built by interaction and undergo complex shifts as populations move in morphospace; organisms cannot climb stable mountains of an engineer's fancy.
It is thus unclear how commonly natural populations are characterized by multiple adaptive peaks
; and hence, in general what the potential is for multiple peaks to influence evolutionary processes in natural populations.