Summary: Evolutionary Developmental Biology

1. Evolution is caused by the inheritance of changes in development. Modifications of embryonic
or larval development can create new phenotypes that can then be selected.

2. Darwin's concept of "descent with modification" explained both homologies and adaptations.
The similarities of structure were due to common ancestry (homology), while the modifications
were due to natural selection (adaptation to the environmental circumstances).

3. The Urbilaterian ancestor can be extrapolated by looking at the developmental genes common
to invertebrates and vertebrates and which perform similar functions. These include the Hox
genes that specify body segments, the tinman gene that regulates heart development, the Pax6
gene that specifies those regions able to form eyes, and the genes that instruct head and tail
formation.

4. Changes in the targets of Hox genes can alter what the Hox genes specify. The Ubx protein, for instance, specifies halteres in flies and hindwings in butterflies.

5. Changes of Hox gene expression within a region can alter the structures formed by that region. For instance, changes in the expression of Ubx and abdA in insects regulate the production of prolegs in the abdominal segments of the larvae.

6. Changes in Hox gene expression between body regions can alter the structures formed by that
region. In crustaceans, different Hox expression patterns enable the body to have or to lack maxillipeds on its thoracic segments.

7. Changes in Hox gene expression are correlated with the limbless phenotypes in snakes.

8. Changes in Hox gene number may allow Hox genes to take on new functions. Large changes
the numbers of Hox genes correlate with major transitions in evolution.

9. Duplications of genes may also enable these genes to become expressed in new places. The
formation of new cell types may result from duplicated genes whose regulation has diverged.

10. In addition to structures being homologous, developmental pathways can be homologous.
Here, one has homologous proteins organized in homologous ways. These pathways can be used
for different developmental phenomena in different organisms and within the same organism.

11. Deep homology results when the homologous pathway is utilized for the same function in
greatly diverged organisms. The instructions for forming the central nervous system and for
forming limbs are possible examples of deep homology.

12. Modularity allows for parts of the embryo to change without affecting other parts.

13. The dissociation of one module from another is shown by heterochrony (changing in the
timing of the development of one region with respect to another) and by allometry (when
different parts of the organism grow at different rates).

14. Allometry can create new structures (such as the pocket gopher cheek pouch) by crossing a
threshold.

15. Duplication and divergence are important mechanisms of evolution. On the gene level, the
Hox genes, the Distal-less genes, the MyoD genes, and many other gene families started as single genes. The diverged members can assume different functions.

16. Co-option (recruitment) of existing genes and pathways for new functions is a fundamental
mechanism for creating new phenotypes. One such recruitment is the limb development pathway being used to form eyespots in butterfly wings.

17. Developmental modules can include several tissue types such that correlated progression
occurs. here, a change in one portion of the module causes changes in the other portions. When
skeletal bones change, the nerves and muscles serving them also change.

18. Tissue interactions have to be conserved, and if one component changes, the other must. If a
ligand changes, its receptor must change. Reproductive isolation may result from changes in
sperm or egg proteins.

19. Developmental constraints prevent certain phenotypes from occurring. Such restraints may be physical (no rotating limbs), morphogenetic (no middle finger smaller than its neighbors), or
phyletic (no neural tube without a notochord).

20. The Hsp90 protein enables cells to accumulate genes that would otherwise give abnormal