by Carl Strang
Today I will attempt to tie together four different studies published last year that focused on a wide range of different organisms but had in common the theme of rapid genetic change, casting a new light on evolutionary processes. We’ll begin with fox squirrels.
Fox squirrels are revealed to have changed rapidly in postglacial times.
This first study (Moncrief, Nancy D., Justin B. Lack, and Ronald A. Van Den Bussche. 2010. Eastern fox squirrel (Sciurus niger) lacks phylogeographic structure: recent range expansion and phenotypic differentiation. J. Mammal. 91:1112-1123) looked at the divergent populations of fox squirrels across eastern North America. Body size and coat color variation across 10 subspecies groups had been associated with a model of expansion from separate glacial refugia in Florida and Texas. In other words, once the glacier melted away, woodlands expanded northward and the squirrels went with them. Populations north of Texas were expected to have connected to those that survived the thousands of years of glaciation in Texas, with a similar connection between more eastern populations and a Florida refuge. Study of the genetics of these populations showed no such connections, however. The data suggest a rapid postglacial expansion, with the observed differences in size and coat color evolving in place, indicating “rapid morphological divergence within the past 14,000 years.”
Millenium Park, Chicago. How does a forest ant make it in the big city?
Now we get out the magnifying glass and look at a smaller creature (Grzegorz Buczkowski. Extreme life history plasticity and the evolution of invasive characteristics in a native ant. Biological Invasions, 2010; DOI: 10.1007/s10530-010-9727-6. As described in a ScienceDaily article). The odorous house ant is a native forest species, in its original configuration with a colony of a single queen and around 50 workers, living in an acorn shell or other small space. In urban areas they expand to supercolonies with as many as 6 million workers and 50,000 queens. They are one of the most common house ants, named for a cocoanut or rum odor when crushed. Buczkowski, a researcher out of Purdue University, has found an intermediate state in urban parks, with 500 workers and a single queen. Other species have not made this kind of transition, which is ecologically extreme, from a K-selected life history strategy (focused on persisting in a narrowly defined competitive situation, with the production of few, well-equipped offspring) to an r-selected life history strategy (focused on high reproductive output, wide dispersal, and high mortality of less well provisioned offspring).
The next study focused on butterflies in the western mountains.
Sticking with insects, we look next at the process by which one species evolves into two (McBride CS, Singer MC (2010) Field Studies Reveal Strong Postmating Isolation between Ecologically Divergent Butterfly Populations. PLoS Biol 8(10): e1000529. doi:10.1371/journal.pbio.1000529). They looked at populations of the Edith’s checkerspot butterfly Euphydryas editha on western mountain slopes. Caterpillars of different populations develop on different plants, some on Collinsia torreyi (blue-eyed Mary) and others on Pedicularis semibarbata (pine lousewort). The plants grow together, but at any given spot the caterpillars are found on only one of them. When brought together in the lab the butterflies hybridize, but experiments demonstrated that the hybrid offspring are at a strong disadvantage. Collinsia-feeding populations lay small clutches of eggs near the tops of the plants, Pedicularis feeders lay larger clutches near the bottoms. Hybrids are intermediate, and so have poor survival. They miss the fresh leaves of each species, and they either overwhelm the Collinsia food source or are inadequate to match egg mortality on Pedicularis. This study is significant in providing a clear example of incipient speciation in which the process of extrinsic postzygotic isolation is important. “Extrinsic” means that the hybrid offspring are themselves fully healthy and functional, but external environmental factors work against them. “Postzygotic” is a reference to the fact that the different varieties of the butterfly are willing and able to mate with one another, but selection acts on the offspring.
All of the above studies are given some context by the final one I include today (Stephan Ossowski, Korbinian Schneeberger, José Ingnacio Lucas-Lledó, et al. 2010. The rate and molecular spectrum of spontaneous mutations in Arabidopsis thaliana. Science 327:92-97). They directly measured the number of mutations appearing in genomes of this plant species (mouse-ear cress) over 30 generations. The results, if translated to human genetics, would represent 60 DNA base pairs in an average individual that are mutated to be different from those of the parents. This is a larger quantity than anyone would have expected, and it has the highly significant meaning that in a large population, genetic material could change rapidly, allowing for significant genetic flexibility on which natural selection can work. “’Everything that is genetically possible is being tested in a very short period,’ adds Lynch [a co-author, interviewed in a ScienceDaily article about the study], emphasizing a very different view than perhaps the one we are all most familiar with: that evolution reveals itself only after thousands, if not millions of years.” Squirrels, ants and butterflies appear to show that this result is not limited to plants.
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