Papers: Speciation & Tracks

[Michael Corriss <mcorriss@gate.net>]

A couple of interesting items from:
Editors' Choice: Highlights of the recent literature
SCIENCE, Volume 303, Issue 5661,  
dated February 20 2004, is now available at:

         http://www.sciencemag.org/content/vol303/issue5661/twil.shtml

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ECOLOGY/EVOLUTION: Symmetry and Speciation 
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Andrew M. Sugden

Evolutionary theory predicts that there will be higher potential for
diversification in groups of organisms that have traits that promote
reproductive isolation between populations. For plants, one such feature
is floral symmetry. Bilaterally symmetrical (zygomorphic) flowers such
as orchids allow for more precise collection and placement of pollen by
visiting animal pollinators than do radially symmetrical (actinomorphic) 
flowers such as buttercups. Mutations that cause changes in the shape
and dimensions of zygomorphic flowers have the potential to generate
reproductive isolation between populations by decreasing
pollinator-mediated gene flow.

Sargent compared species richness between zygomorphic and actinomorphic
lineages in related animal-pollinated groups of plants. The zygomorphic
lineages, in 15 out of 19 cases, were more species-rich than the 
actinomorphic lineages. Assuming that extinction rates are not higher in
the actinomorphic lineages, this suggests that animal pollination indeed
promotes the likelihood of reproductive isolation and speciation in
plants with bilaterally symmetrical flowers. -- AMS

Proc. R. Soc. London Ser. B 10.1098/rspb.2003.2644 (2004).

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PALEONTOLOGY: The World at Their Feet 
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Linda Rowan

Although mammals and dinosaurs evolved at about the same time about 200
million years ago, the fossil record of mammals is scant. Because
mammals tended to be small in numbers and stature, their delicate bones
are rarely preserved and the record consists mostly of teeth.

One way to sidestep this gap in the fossil record is to look for tracks
made by mammals. Lockley and Foster have discovered several sets of
mammal tracks from the latest Cretaceous (about 75 to 65 million years
ago) in Colorado. The three- to five-toed footprints resemble those of
modern rodents; however, rodents did not evolve until later. The
footprints probably belong to a multituberculate or a marsupial. From
the tracks it is clear that these mammals were small and agile and lived
among birds, dinosaurs, and other reptiles on a fertile coastal plain.
More work will beneeded to connect the partial record from head to toe
to determine more about how mammals evolved in the Mesozoic. -- LR

Ichnos 10, 269 (2003).

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-mpc