15 Gems Of Evolution (Part 4, 5, and 6)

To try and hold you over for the weekend, here are parts 4, 5, and 6 of the 15 Gems of Evolution.

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4) The evolutionary history of teeth

One motivation in the study of development is the discovery of mechanisms that guide evolutionary change. Kathryn Kavanagh at the University of Helsinki and her colleagues investigated just this by looking at the mechanisms behind the relative size and number of molar teeth in mice. The research, published in 2007, uncovered the pattern of gene expression that governs the development of teeth — molars emerge from the front backwards, with each tooth smaller than the last. The beauty of the study lies in its application. Their model predicts the dentition patterns found in mouselike rodent species with various diets, providing an example of ecologically driven evolution along a developmentally favoured trajectory. In general, the work shows how the pattern of gene expression can be modified during evolution to produce adaptive changes in natural systems.

Reference
Kavanagh, K. D., Evans, A. R. & Jernvall, J. Nature 449, 427–432 (2007).
Additional resources
Polly, P. D. Nature 449, 413–415 (2007).
Evans, A. R., Wilson, G. P., Fortelius, M. & Jernvall, J. Nature 445, 78–81 (2006).
Kangas, A. T., Evans, A. R., Thesleff, I. & Jernvall, J. Nature 432, 211–214 (2004).
Jernvall, J. & Fortelius, M. Nature 417, 538–540 (2002).
Theodor, J. M. Nature 417, 498–499 (2002).
Author website
Jukka Jernvall: http://www.biocenter.helsinki.fi/bi/evodevo

5) The origin of the vertebrate skeleton

We owe much of what makes us human to remarkable tissue, found only in embryos, called the neural crest. Neural-crest cells emerge in the developing spinal cord and migrate all over the body, effecting a remarkable series of transformations. Without the neural crest, we would not have most of the bones in our face and neck, or many of the features of our skin and sensory organs. The neural crest seems to be unique to vertebrates, and helps to explain why vertebrates have distinctive ‘heads’ and ‘faces’. Untangling the evolutionary history of the neural crest is especially hard in fossil forms, as embryonic data are obviously absent. One key mystery, for example, is how much of the vertebrate skull is contributed by neuralcrest cells and how much comes from deeper layers of tissue. New techniques have allowed researchers to label and follow individual cells as embryos develop. They have revealed the boundaries of the bone derived from the neural crest, down to the single-cell level, in the neck and
shoulders. Tissue derived from the neural crest anchors the head onto the front lining of the shoulder girdle, whereas the skeleton forming the back of the neck and shoulder grows from a deeper layer of tissue called the mesoderm.

Such detailed mapping, in living animals, casts light on the evolution of structures in the heads and necks of animals long extinct, even without fosilized soft tissue such as skin and muscle. Skeletal similarities that result from a shared evolutionary history can be identified from muscle attachments. This allows the tracing of, for example, the location of the major shoulder bone of extinct land vertebrate ancestors, the cleithrum. This bone seems to survive as part of the shoulder blade (scapula) in living mammals. This kind of evolutionary scan may have immediate clinical relevance. The parts of the skeleton identified by Toshiyuki Matsuoka from the Wolfson Institute for Biomedical Research in London and his colleagues as being derived from the neural crest are specifically affected in several developmental disorders in humans, providing insights into their origins.Mitsuoka’s study shows how a detailed analysis of the morphology of living animals, informed by evolutionary thinking, helps researchers to interpret fossilized and now-extinct forms.

Reference
Matsuoka, T. et al. Nature 436, 347–355 (2005).
Author website
Georgy Koentges: http://www2.warwick.ac.uk/fac/sci/systemsbiology

6) Natural selection in speciation

Evolutionary theory predicts that divergent natural selection will often have a key role in speciation. Working with sticklebacks (Gasterosteus aculeatus), Jeffrey McKinnon at the University of Wisconsin in Whitewater and his colleagues reported in 2004 that reproductive isolation can evolve as a by-product of selection on body size. This work provides a link between the build-up of reproductive isolation and the divergence of an ecologically important trait.
The study was done on an extraordinary geographical scale, involving mating trials between fish taken in Alaska, British Columbia, Iceland, the United Kingdom, Norway and Japan. It was underpinned by molecular genetic analyses that provided firm evidence that fish that have adapted to living in streams had evolved repeatedly from marine ancestors, or from fish that live in the ocean but return to fresh water to spawn. Such migratory populations in the study had larger bodies on average than did those living in streams. Individuals tended to mate with fish of a similar size, which accounts well for the reproductive isolation between different
stream ecotypes and their close, seafaring neighbours.

Taking into account the evolutionary relationships, a comparison of the various types of stickleback, whether stream or marine, strongly supports the view that adaptation to different environments brings about reproductive isolation. The researchers’ experiments also confirmed the connection between size divergence and the build-up of reproductive isolation — although traits other than size also contribute to reproductive isolation to some extent.

Reference
McKinnon, J. S. et al. Nature 429, 294–298 (2004).
Additional resources
Gillespie, R. G. & Emerson, B. C. Nature 446, 386–387 (2007).
Kocher, T. D. Nature 435, 29–30 (2005).
Emerson, B. C. & Kolm, N. Nature 434, 1015–1017 (2005).
Author websites
Jeffrey McKinnon: http://facstaff.uww.edu/mckinnoj/mckinnon.html
David Kingsley: http://kingsley.stanford.edu
Dolph Schluter: http://www.zoology.ubc.ca/~schluter