Development and Evolution

Morphological changes in evolution - produced by changes in the rate or
timing of development processes

RECAPITULATION (Meckel-Serres, Haeckel): stages of an organism's
development correspond to the species' phylogenetic history -
("ontogeny recapitulates phylogeny")

VON BAER'S LAW: "the general features of a large group of animals
appear earlier in the embryo than the special features" - earlier
developmental stages are more similar than later stages

Genetics and development: REGULATORY GENES

The number of genes underlying phenotypic
(e.g. morphological) changes is an active area of research in evolutionary
biology and molecular developmental genetics

Mutations in regulatory genes can cause cascading effects on development,
resulting in "macromutations"

Experimental manipulation of development - creation of ATAVISMS

Hampe - development of Archaeopteryx-like limbs in chicks

Chick fibula - rudimentary

Competition for cells with the tibia during development - tibia wins

Mica sheets used to inhibit competition

Fibula developes to full length, articulates with tarsal bones (tibiale and
fibulare) as in Archaeopteryx

Kollar and Fisher - development of teeth in birds

tooth development - reciprocal interaction between oral epithelium and
mesenchyme

Kollar and Fisher combined chick oral epithelium with mouse oral
mesenchyme - chick oral epithelim produced enamel - formation of teeth

HOMEOTIC TRANSFORMATIONS - segment or region of the body is
transformed into the likeness of some other normal segment

Example:  BITHORAX - mutation in a regulatory gene in Drosophila -
transformation of the halteres (balancing organs) into wings - recreates
ancestral form of Drosophila (another atavism)

homeotic mutations in Drosophila lead to the discovery of the HOMEOBOX
SEQUENCE - highly conserved series of genes involved in development

These genes produce proteins that bind to DNA (as part of their regulatory
function) - binding regions of proteins are highly conserved - known as
HOMEODOMAINS

Related genes in other organisms were then identified: HOX GENES - ancient
set of genes controlling body plan during development

These genes influence (through specific proteins) the expression of specific
genes in cells at specific locations, resulting in appropriate differentiation.

Hox genes are lined up along the chromosome in the same order as the body
segments for which they are responsible - called COLINEARITY.

Vertebrates have four Hox complexes - each composed of approximately 10
genes - which can be aligned into 13 groups on the basis of DNA sequence
similarity

Homeoboxes appear to be arranged in the same order in Drosophila and
vertebrates

Similar homeobox genes have similar functions in both Drosophila and
vertebrates

Apparently homologous sets of Hox genes are found in all animals - from
worms to humans.

The presence of the same gene complexes controlling development in
mammals, fruit flies, nematodes and jelly-fish suggests that the homeotic
genes appeared very early in the evolution of multicellular animals

"Gene Knockout" experiments have been very useful in figuring out the
effect of the Hox genes on development.

Geneticists have inadvertantly created a number of atavisms by
manipulating Hox genes.

Examples:  Hoxa-2 - influences the development of the middle ear - creation
of mammalian/reptilian intermediates in mice

The signature feature of terrestrial vertebrates is the vertebrate limb, which
allowed our ancestors to crawl onto land.

The mechanisms and genetics of limb development are now being elucidated
for a variety of vertebrates, and they appear to the same in very different
species.  Many of the genes responsible for specifying spatial coordinates
during limb development have been identified.

Some genes in the Hox family are critical to limb development.  Hox genes
are position determining genes in the limb, just as they are for the main body
axis.

Shared developmental and genetic pathways underlie the structural
homology of amphibian, reptile, bird and mammal limbs.

Changes in the timing or level of expression in the pattern-forming genes
could be responsible for large scale adaptive changes in the limb (or between
fins and limbs).

In a comparison of zebrafish fin development and mouse limbs, Sordino
found that there is only a single wave of Hox11-d gene expression in the
developing zebrafish fin buds, but a second wave of expression in the mouse
limb buds, which is responsible for the formation of digits.  This shows that a
major morphological difference is not so major at the level of
developmental genetics.