S. Gavrilets 2003 "Models of speciation: what have we learned in 40 years?"  
Evolution 57: 2197-2215


Theoretical studies of speciation have been dominated by numerical simulations
aiming to demonstrate that speciation in a certain scenario may occur. What is
needed now is a shift in focus to identifying more general rules and patterns
in the dynamics of speciation. The crucial step in achieving this goal is
the development of simple and general dynamical models that can be studied
not only numerically but analytically as well. I review some of the
existing analytical results on speciation.

- I first show why the classical theories of speciation by peak shifts across 
adaptive valleys driven by random genetic drift run into troubles (and into what kind
of troubles). Then I describe the Bateson-Dobzhansky-Muller (BDM) model of speciation
that does not require overcoming selection. I describe exactly how the probability of
speciation, the average waiting time to speciation, and the average duration of speciation
depend on the mutation and migration rates, population size, and selection for
local adaptation. The BDM model postulates a rather specific genetic
architecture of reproductive isolation. I then show exactly why the genetic
architecture required by the BDM model should be common in general.
Next I consider the multilocus generalizations of the BDM model
again concentrating on the qualitative characteristics of speciation such as
the average waiting time to speciation and the average duration of speciation.
Finally, I consider two models of sympatric speciation where the conditions
for sympatric speciation were found analytically.

- A number of important conclusions have emerged from analytical studies.
Unless the population size is small and the adaptive valley is shallow, the waiting
time to a stochastic transition between the adaptive peaks is extremely long.
However, if transition does happen, it is very quick. Speciation can occur by mutation and
random drift alone with no contribution from selection as different populations
accumulate incompatible genes. The importance of mutations and drift in speciation
is augmented by the general structure of adaptive landscapes. Speciation can be
understood as the divergence along nearly neutral networks and holey adaptive
landscapes (driven by mutation, drift, and selection for adaptation to a local biotic
and/or abiotic environment) accompanied by the accumulation of reproductive isolation
as a by-product. The waiting time to speciation driven by mutation and drift is typically
very long. Selection for local adaptation (either acting directly on the loci underlying
reproductive isolation via their pleiotropic effects or acting indirectly via establishing
a genetic barrier to gene flow) can significantly decrease the waiting time to speciation.
In the parapatric case the  average actual duration
of speciation is much shorter than the average waiting time to speciation.
Speciation is expected to be triggered by changes in the environment.
Once genetic changes underlying speciation start, they go to completion very rapidly.
Sympatric speciation is possible if disruptive selection and/or assortativeness
in mating are strong enough. Sympatric speciation is promoted if costs of being
choosy are small (or absent) and if linkage between the loci experiencing
disruptive selection and those controlling assortative mating is strong.