2 May 2007
Staying with the theme of arthropod evolution, this week we'll consider the long-standing controversy over the homology of arthropod "head" segments. The cephalic regions of chelicerates, such as spiders and horseshoe crabs, bear two mouthparts and four walking legs, while mandibulates, such as crustaceans, have a head of two sensory and three gnathal segments. "Aligning" segments in these different bodyplans and drawing homologies has not always been clear. Evidence from neural anatomy, embryology, and developmental genetics has conflicted at times. This issue has important implications for our understanding of arthropod evolution, and for the interpretation of the bodyplans and phylogenetic position of extinct arthropod groups.
A new review by the arthropod anatomists Jacques and Colette Bitsch summarizes several lines of evidence related to this issue. Please join us to discussion your interpretations!
18 April 2007
Arthropods are one of the few animal phyla to have met with success in a diverse range of terrestrial environments. Several arthropod groups have independently adapted to life on land, including insects, myriapods, isopods, and spiders. This week Roberta Engel has asked that we consider some of the features of anatomy and life cycle that have allowed arthropod terrestrialization. In what ways have arthropod groups converged to allow life on land? How has the life cycle been affected? What evolutionary novelties were required in various groups? And how did the evolutionary history that enabled their terrestrialization in the first place influence the diversification of these animals after the fact?
A suggested starting point for this discussion is this classic article by David Zeh and coworkers.
11 April 2007
Evo-Devo attempts to provide an understanding of the evolutionary history of ontogenetic mechanisms. Its research program often employs comparative data and the choice of study organisms can be important for the questions being asked. Bias in the selection of model organisms and those used in survey studies necessarily influences our understanding. This week, let's consider why we work with the organisms we do. Our starting point will be two short recent articles on this subject.
Haag, E. S., and colleagues. (2007.) Caenorhabditis evolution: if they all look alike, you aren't looking hard enough. Trends in Genetics 23: 101-104.   PDF
4 April 2007
Studies of genetic mutants and theoretical modeling have suggested that developmental mechanisms are robust to variations in the timing and intensity of many developmental signals. However, how much variation in developmental mechanisms is present in natural populations? This question has important consequences for the "evolvability" of populations faced with changing environments. This week we'll consider the issue, starting with an article for last month's Evolution & Development. Reed and colleagues have explored variation in the expression patterns of regulatory genes during development of the adult butterfly wing spots.
28 March 2007
Development in fossils?
28 February 2007
The structure and function of appendages varies greatly among vertebrate groups. What are the underlying developmental and regulatory genetic differences that produce these different forms? This week, let's consider the evolution of vertebrate limbs. Two short recent papers have addressed aspects of this issue. Dahn et al. examine limb development in chondrichthyans, in an attempt to understand the origins of paired vertebrate appendages, while Weatherbee et al. explore the derived limb patterning mechanism in bats.
Weatherbee, S. D., Behringer, R. R., Rasweiler, J. J. t. and Niswander, L. A. 2006. Interdigital webbing retention in bat wings illustrates genetic changes underlying amniote limb diversification. PNAS 103: 15103-7.   PDF
21 February 2007
After snow last week, the group will resume this Wednesday with discussion of what happens when duplicate genes go their separate evolutionary ways. What is the role that gene duplication plays in the evolution of phenotypic novelty? How does duplication affect genomic structure? Are there implications with the demise of redundant duplicates? To stimulate the conversation, the suggested reading consists of two short PNAS articles from last year.
Lin, Y. S., Byrnes, J. K., Hwang, J. K. and Li, W. H. 2006. Codon-usage bias versus gene conversion in the evolution of yeast duplicate genes. PNAS 103: 14412-6.   PDF