GE 70B: Evolution of the Cosmos and Life
ONTOGENY AND PHYLOGENY: Evolution of breeds in the
Humans have changed species through controlled breeding programs, a process termed
Usually it is easy to match the domesticated form with a wild relative.
numerous different forms are selected to diverge from one progenitor.
In today’s lab we will be focusing
on the great diversity of dogs breeds in an attempt to understand how it was possible to select for such a
wide array of dogs from a single ancestral form, and why comparable diversity is not seen among some
other domesticated animals, and us!
The entire life history of an organism from conception to death is referred to as its development or
Any evolutionary change in the timing of events during development, or in the rate of
development, is termed
Development from fertilized egg to adult involves a host of
complex processes and interactions among genes, molecules, and tissues.
Even subtle alterations in the
rate or time of onset of one of those developmental processes can have far-reaching consequences.
result may be a different final product, i.e. a different adult form.
Thus, heterochrony can be an important
evolutionary process, because even slight
changes in developmental timing can sometimes give
rise to marked heritable differences.
Perhaps the most celebrated example is the axolotl, a salamander found in Mexico.
undergo early development as gilled, aquatic animals, but most species lose their gills and live on land as
However, when conditions in ponds are particularly favorable, certain key aspects of
morphological development slow down in the axolotl relative to those in ancestral salamanders, even
though sexual maturation continues.
This change in developmental timing, which has been traced to a
simple change in the amount of a hormone released by the thyroid gland, results in a sexually mature
salamander that, retaining gills and continuing its aquatic existences, looks like juvenile.
We usually speak of heterochrony in terms of the change in form between an ancestral species and
a descendant species.
Heterochrony usually involves an evolutionary change in developmental timing or
in the rate of development so that the shape, size, and/or behavior of the descendant no longer matches the
ancestor at an equivalent age.
That means development can either be reduced or increased in the
descendant adult, relative to the ancestor.
A developmental shift resulting in underdevelopment is called
(literally meaning “child formation”).
A developmental shift resulting in
overdevelopment is called
(meaning “over formation”).
Finally, it is important to realize that all organisms are a mosaic of heterochronies in terms of their