https://avianhybrids.wordpress.com/2020/01/23/a-genetic-model-for-puntuated-equilibria/ has a brief article which includes an easy to understand very brief summary of the Developmental Gene Hypothesis.
http://web01.cabeard.k12.in.us/science/APBiology/bc_campbell_biology_7/0,7052,4350321-,00.html has a much older article called "Unit Four: Steven M. Stanley, Evolution Interview: Steven M. Stanley" which is informative about the theory of punctuated equilibrium theory of macroevolution. it says in part the following.
"It seems that the debate between a gradual and a punctuated view of
evolution is not just about tempo. It's also a debate about mechanism.
It is to a degree, but there has been much misunderstanding
about this. Some geneticists have assumed that paleontologists adopting a
punctuational position argue for a totally new genetic mechanism for
evolution. That's not really true. The punctuational view is quite
compatible with the view that natural selection operating on mutations
over a period of generations is the prevailing mode of transitions. It's
simply a matter of our compressing this into a shorter time and
considering it as happening in small populations.
One of the basic notions of the modern synthesis has been that
large, well-established species with subdivided populations offer the
most effective conditions for evolution to occur. The punctuational
scheme would argue that small populations evolve not just as parts of a
whole complex that's evolving, but as individual units that are evolving
and diverging rapidly. In fact, Ernst Mayr, a major proponent of the
modern synthesis, laid the foundation for this whole viewpoint as early
as 1954. He published a paper that suggested that the lack of continuity
in the fossil record could well be a result of change taking place in
small populations rapidly on the geological scale of time; yet the idea
was never assimilated into the modern synthesis.
So transitional forms are so rarely observed in the fossil
record because most speciation events involve very small splinter
populations separated from a larger established population?
I think that's often the case. Evolution happens rapidly in
small, localized populations, so we're not likely to see it in the
fossil record. If you think about successful speciation events in terms
of local diversity, there is a very revealing and simple notion. If you
could sit and watch a particular group of animals or plants, say, in a
family that includes 50 species, through 5 million years of time, each
of those species would give rise to only one other species, on the
average. Speciation is a very rare event. Because biologists often focus
on the immediacy of things, they sometimes overlook how improbable and
rare a speciation event actually is."
