Transcript Development & Evolution ppt

Development & Evolution
Recapitulation – “Biogenetic Law”
• Late 19th Century concept of Ernst Haeckel :
Ontogeny Recapitulates Phylogeny Evolutionary history of a
species is the cause of its embryonic development, therefore, during
embryonic development an organism passes through the stages of
its evolution (phylogeny).
• Influence from 1880s – 1920s and beyond:
1) stimulated a great deal of descriptive embryology (+)
2) stimulated many speculative historical reconstructions (+/-)
3) when applied to human anatomy and behavior served as
support for a wide range of sexist and racist ideas (-)
• Rejected among biologists by 1920s as incompatible with our
understanding of inheritance (Genetics).
• Cultural applications continue well into the 1960s.
• Unfortunately some authors continue to use the term ‘recapitulation’
when they discuss similarities in embryonic development as means
of identifying homologies (see Brusca p 107-8)
Heterochrony
One concept to emerge from the controversy over recapitulation and
genetics is that of ‘rate’ genes – genes that somehow control the
rate of embryonic development and thus can effect the relative
timing of embryonic events.
During the 1930s and 40s some researchers argued that major evolutionary
changes (macroevolution) could occur if the relative timing of events were to
change during development = Heterochrony
Although ignored at the time, by the 1960s and 70s the idea of heterochrony
(mutations in ‘rate’ genes) was revived. Evidence was provided from
comparative embryology - especially of larval forms and experimental
manipulation of metamorphosis (especially amphbians).
Heterochrony still used as an explanation for certain events in evolutionary
history but now considered a small subset of the impact of changes in
developmental regulatory mechanisms.
The Developmental Cascade and the Revival of
the Bauplan
• As progress in genetics unfolded - the Watson-Crick DNA model,
the triplet code, the ‘central dogma’ etc. – application of genetics to
embryonic development led to the concept of the developmental
cascade. The directional nature of development – cleavage,
gastrulation, morphogenesis, organogenesis, cell differentiation –
suggested a sequence of a turning on and off of genes so that cells
became increasingly specialized to function at the right time in the
right place.
• The notion was that mutations of genes that functioned early in the
cascade would greater effect than mutations in those genes that
function later. Selection on such early genes would be especially
strong, tending to stabilize basic body forms and thus tend to
channel evolutionary change into modifications of common ‘body
plans’ (Bauplans)
Raff’s Developmental Hourglass I
Changes in the
early development
of sea urchins from
indirect
development to
direct development
nevertheless
yielded typical
normal adults.
Such evolutionary
changes occurred
several times.
Thus early
development more
flexible than
thought.
Raff’s Developmental Hourglass II
The Phylotypic stage
of development would
correspond to the
establishment of the
taxon’s Bauplan
Raff’s Developmental Hourglass III
The period of
organogenesis is
a time of
maximum global
interaction across
the embryo and
thus most under
selection pressure
– the phylotypic
stage
Resulting modules
(e.g imaginal
discs or
segments) can be
altered
independent of
rest of embryo.
Regulatory Genes as Focus for
Macroevolution
The work of Raff and others suggested that macroevolutionary change
– including changes in bauplan – is the result of mutations in
regulatory genes [genes that code for ‘transcription factors’ which
control the expression of other genes]
Major advances in testing this idea came from using mutant
phenotypes in the fruit fly (Drosophila) and the round worm
(Caenorhabditis) to ‘dissect’ embryos of these organisms. Studies
of homeotic mutations in which one structure is substituted for
another (leg where antenna ought to be) led to the discovery of Hox
genes. A group of regulatory genes each containing a homeobox
{sequence of base pairs for transcription factor}. The hox genes are
found in the same order on the chromosome that the structures they
regulate have in the embryo.
Drosophila Hox Genes
From Freeman & Herron 2007 Evolutionary Analysis
Conservati
on of
Regulatory
genes:
Deep
Homology
Note: regulatory genes
other than the hox
group also show deep
homology. E.g. pax6
gene and eye
development
Conserved Hox genes: macroevolutionary
change may be down stream
The Problem of Homology
• The Issue: Are two structures found in different species similar
because they are sympliesiomorphies (from a common ancestor) or
are they a product of convergence (Natural Selection leading to
similar adaptations in separate lineages – “homoplasy”)?
How to distinguish
1) Level of similarity – detailed description generally shows homoplasies to be
more superficial than homologies (e.g. cepahlopod eye & vertebrate eye).
2) Embryonic origin – homologies are expected to develop from the same
embryonic structure/process, homoplasies are less likely to have
development in common. (e.g. hyrdrozoan tentacles & ectoproct
lophophore)
3) Genetic basis – genes (both regulatory & structural) are expected to be the
same for homologies different for homoplasies. (eye lens proteins in
molluscs)
4) Cladistic Analysis – analysis of several characters may yield a better (more
parsimonious) cladogram if characters are considered to be homoplasies
rather than homologies. (e.g. cephalopod eye & vertebrate eye)