Zak Voigt
Cortney Kopischke
Evolution and Development
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Two main questions or themes are:
o What role has developmental evolution played in the history of life on Earth?
o Do the developmental trajectories that produce phenotypes bias the productions of
variation or constrain trajectories of evolutionary change?
What is evolutionary developmental biology (EDB)?
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seeks to understand the mechanisms by which development has evolved, both in terms of
developmental processes and in terms of evolutionary processes
Major contributors:
o Geoffroy Saint- Hilaire (1772-1844)
o Karl Ernst von Baer
o Darwin
o E.B Lewis
o Thomas Kaufman
Most important discovery was the Hox cluster in the 1970s and 1980s
o Gave birth to modern EDB
o Homeobox genes work like stem cell in differentiating body structures
o Ultrabithorax (Ubx) gene in drosophila – E.B. Lewis (See page 475)
o Hox gene complexes in Drosophila
 Antennapedia (Antp) –Thomas Kaufman 1970 and 1980s
 Bithorax- E.B. Lewis 1940’s-1970’s
o See picture on page 475
o Mammals – 4 Hox gene complexes, 13 different Hox genes
o Drosophila- 2 complexes, 8 Hox genes
Hox genes were extremely important discovery
o Brought together vertebrates and invertebrates at a genetic level
If Hox genes are present in 2 different species, why do they develop differently?
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Hox gene expression is not the case, but other genes.
Hox genes encode proteins to regulate transcription
Proteins (transcription factors) bind to DNA control regions of target genes
Transcription factor - a protein that interacts with a regulatory DNA sequence and
affects the transcription of the associated gene (Futuyma, 2005)
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Target gene – in developmental genetics, a gene regulated by a transcription factor of
interest. This regulation may be direct or indirect (Futuyma, 2005)
EDB in Plants
o See figure on page 479
o A, B, and C = Transcription factors
o Sepals develop from A gene, Carpels by C, Petals by mix of B and A, Stamens by mix of B
and C – Wild-type
o AP3 mutant- Petals converted to sepals, stamens to pistils.
o All from mutations in the B gene APETALA3
Homology
o Similar features are inherited with some or no modification from ancestors
o Example- M2 molar in carnivores
o See page 480
o Believed to be lost in Felidae, present in 10% of Lynx
o If molar was lost in Felidae, how does it appear in later lineage?
 Possibility that certain factors (environment, diet) make the M2 molar necessary
This conflicts with homologous traits
 Can’t go back in time and pick up what was lost
 Reverse order
 i.e. – Differentiating digits
o Tetrapods- differentiate sequentially from back to front except
for salamanders
Developmental Pathways
o Also called developmental circuits
o Basically, genes that code proteins which regulate transcription factors (remember info
from before?)
o Determine level of transcription at target genes and structural genes
o These genes(devo-pathways) are crucial to certain physical structures
o See page 484, Figure 20.8
o Distalless family genes present in various organisms, function in forming different
structures
o Another example: eyeless
 Mutation in this gene and its mammalian counterpart, Pax6, lead to eye
reduction/loss
 See page 483, figures 20.9 and 20.10
o Injection of eyeless in Drosophila can induce ectopic expression of eyes
o Amazingly, Pax6/eyeless gene is interchangeable between vertebrates and invertebrates
 When injected in Drosophila, mouse and squid Pax6 genes create ectopic eyes
o Ectopic expression – expression of a certain body part where it is not normally found
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Darwin spent much time on how human eyes evolved
Some organisms have very simple eyes, i.e.- Cnidarians, “eyespots” w/ few
photoreceptive pigments
o Some organisms have more complex eyes, i.e. flies, compound eyes allow depth
perception, detailed vision, motion sensitivity
o Ironically, basic and complex eyes both contain the eyeless/Pax6 gene
Gene regulation
o Genes usually have many different enhancers
o Regulatory modularity is thought to control the evolution of specific tissues and body
structures
 Ex: Drosophila melanogaster and Ubx expression (see page 484)
 Ex: Wild corn (Teosinte) and Maize (see page 485)
 Could be a result of a trans-regulatory difference
 Difference in transcription factor
 Modularity- The degree to which the development of various body structures is
independent (Futuyma, 2005)
o Co-option – use of pre-existing genes to perform new functions, develop new structures
 Very cool evolutionary trait
 Existing genes evolve into new ones
 Ex: Crystallin- gene family in which many genes have dropped some
amino acids to perform different functions (see page 487)
 Genes can be expressed after a critical period to lengthen critical period and
increase chances of novelties (page 488)
Developmental Constraints and Morphological Evolution
o No animal lives in a perfect world
o Sometimes evolution is denied by certain constraints:
 1) Physical constraints- An organism’s body structure doesn’t allow for more
complexity. Any examples?
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2) Selective (or functional) constraints- some lineages don’t express certain
features because they’re always bad or they interfere with a current trait
 3) Genetic constraints- even genes have tolerance levels; if these are exceeded
for a variation, it won’t happen
 4) Developmental constraints- “a bias on the production of various phenotypes
caused by the structure, character, composition, or dynamics of the
developmental system.” (Maynard Smith et al., 1985)
 Basically, some organisms don’t have the proper tools for the job. i.e.lack of cells, proteins, or developmental genes)
Developmental or genetic constraints lead to evolutionary patterns
 1) Absence of features in certain lineages
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Ex: Viviparity in lizards and snakes, but not turtles. Why do you think
this is?
2) Directional trends
 Novelties that came about in ancestral lineages give rise to evolutionary
“branches” that wouldn’t have happened otherwise (see page 492)
3) Parallel evolution of traits in independent lineages
 Traits of differing species of the same lineage may have only one genetic
pathway to evolve in certain aspects. Ex: Drosophila (page 494) + Bony
horns (page 492)
4) “Standardization”- a high evolutionary variation occurs in that no other
variation is needed
 Canalization – used to explain standardization pattern. Evolution of
modifications favor best phenotype (physicality)
 Evolutionary “bottle neck” w/ winners poured out
5) Morphological stasis over long periods of evolutionary time
 Very basic, evolutionary stand still
6) Similarities in embryological stages among higher taxa
 Although certain taxa become more complex, they retain the same basis
for early development from their ancestors
 Ex: the notochord in vertebrates
EDB of Humans
o Very exciting to see what pathways and mechanisms lead to all of humans massively
complex traits
 Brain size, skull shaping, limbs, efficient hands, behavior, and culture
o Looking at connection with humans and apes
 Nucleotide sequence differs by only 1.2% between human and chimpanzee
genomes
o Although it will be a monumental undertaking, comparing genomes of humans and
chimps will reveal great things
o Allows humans to see where we’re going from here by discovering where we’ve been,
and where we started
References:
Carroll, Sean B., Scott D. Weatherbee, and Jennifer K. Grenier. From DNA to Diversity: Molecular
Genetics and the Evolution of Animal Design. 2nd ed. New York: Blackwell, 2006
Fox, Charles W., and Jason B. Wolf, eds. Evolutionary Genetics: Concepts and Case Studies. New
York: Oxford UP, 2006.
The Nature of Science and the Study of Biological Evolution. Colorado Springs: BSCS, 2005.
Futuyma, D. (2005). Evolution. Sinauer Associates Inc.