Imperfect Imitation is (Sometimes) the Best Form of Flattery

21 03 2012

A letter published March 21 in the online edition of the journal Nature indicates that there are evolutionary reasons why some animals that mimic other more aggressive or dangerous species do so rather imperfectly.

The study conducted by a team of Canadian researchers indicated that, at least in hoverfly populations, predators impose less selection if the mimic is smaller.  Thus, small mimic species need to have less fidelity with the species they are mimicking than do larger ones.  The team concludes that the most likely reason for this is that the mimics are less profitable prey species and are not apt to be pursued as strongly as are larger species.  So, less fidelity will do.

The team was also able to show little or no correlation of mimicry to several other theories that had been posited over the years as explanations for the imperfections.  Among these, they were able to show that human ratings of mimetic fidelity are positively correlated with both morphometric measures and avian rankings of the mimicry, indicating that variation in mimetic fidelity is not just an illusion based on human perception.

See here for the complete paper.





Cryptic Sex-Ratio Bias Provides Indirect Genetic Benefits Despite Sexual Conflict

5 04 2010
This image is courtesy of Wikipedia Commons and is in the public domain.

Anolis sagrei, was used in the study to demonstrate that sex-ratio bias can provide indirect genetic benefits despite sexual conflict

Two scientists from Dartmouth College have demonstrated a counterpoint to the long-held belief that sexual dimorphism causes high-fitness parents often to produce low-fitness progeny of the opposite sex.  According to the authors, this alleviates the evolutionary costs incurred, restoring the benefits of mate choice.

The article, by Robert M. Cox and Ryan Calsbeek, was published April 2, online in the journal Science.  Below is the abstract from the article.  To read the entire article (if you have a subscription), please click on the following URL:
http://www.sciencemag.org/cgi/content/abstract/sci;328/5974/92?maxtoshow=&hits=10&RESULTFORMAT=&fulltext=evolution&searchid=1&FIRSTINDEX=0&sortspec=date&resourcetype=HWCIT.

Abstract

When selection favors sexual dimorphism, high-fitness parents often produce low-fitness progeny of the opposite sex. This sexual conflict is thought to overwhelm the genetic benefits of mate choice because preferred males incur a cost through the production of low-fitness daughters. We provide a counterpoint in a lizard (Anolis sagrei) that exhibits sexual conflict over body size. By using mate-choice experiments, we show that female brown anoles produce more sons than daughters via large sires but more daughters than sons via small sires. Measures of progeny fitness in the wild suggest that maximal fitness payoffs can be achieved by shifting offspring production from daughters to sons as sire size increases. These results illustrate how the resolution of sexual conflict can restore the genetic benefits of mate choice.





Floral Symmetry Genes and the Origin and Maintenance of Zygomorphy in a Plant-Pollinator Mutualism

5 04 2010

This article, published online March 30, in the Proceedings of the National Academy of Science (PNAS) examines floral zygomorphy in light of pollinator selection.  Following is the abstract for the article.  To read the entire article (if you have a subscription), please click on the following URL:  http://www.pnas.org/content/early/2010/04/01/0910155107.abstract?abspop=1&related-urls=yes&legid=pnas;0910155107v1.

Abstract

The evolution of floral zygomorphy is an important innovation in flowering plants and is thought to arise principally from specialization on various insect pollinators. Floral morphology of neotropical Malpighiaceae is distinctive and highly conserved, especially with regard to symmetry, and is thought to be caused by selection by its oil-bee pollinators. We sought to characterize the genetic basis of floral zygomorphy in Malpighiaceae by investigating CYCLOIDEA2-like (CYC2-like) genes, which are required for establishing symmetry in diverse core eudicots. We identified two copies of CYC2-like genes in Malpighiaceae, which resulted from a gene duplication in the common ancestor of the family. A likely role for these loci in the development of floral zygomorphy in Malpighiaceae is demonstrated by the conserved pattern of dorsal gene expression in two distantly related neotropical species, Byrsonima crassifolia and Janusia guaranitica. Further evidence for this function is observed in a Malpighiaceae species that has moved to the paleotropics and experienced coincident shifts in pollinators, floral symmetry, and CYC2-like gene expression. The dorsal expression pattern observed in Malpighiaceae contrasts dramatically with their actinomorphic-flowered relatives, Centroplacaceae (Bhesa paniculata) and Elatinaceae (Bergia texana). In particular, B. texana exhibits a previously undescribed pattern of uniform CYC2 expression, suggesting that CYC2 expression among the actinomorphic ancestors of zygomorphic lineages may be much more complex than previously thought. We consider three evolutionary models that may have given rise to this patterning, including the hypothesis that floral zygomorphy in Malpighiaceae arose earlier than standard morphology-based character reconstructions suggest.





Scientist Shows Post-Mating Pre-Fertilization Barrier to Cross Species Breeding

25 03 2010

Daniel R. Matute of the department of ecology and evolution at the University of Chicago has published a study of two species of Drosophila, that show, for as far as I am aware the first time, the presence of postmating prezygotic isolation.  Specifically, that means a barrier or barriers that act after mating but before fertilization.

The paper appeared on March 23, in the online journal Public Library of Science (PLoS) Biology.  It is important from an evolutionary standpoint as a demonstration of the diversity of ways that reinforcement by gametic isolation can bolster isolation between species or proto-species.

Following is the abstract from the paper.  For the complete paper, please see the following URL:  http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1000341.

Abstract

Reinforcement, a process by which natural selection increases reproductive isolation between populations, has been suggested to be an important force in the formation of new species. However, all existing cases of reinforcement involve an increase in mate discrimination between species. Here, I report the first case of reinforcement of postmating prezygotic isolation (i.e., barriers that act after mating but before fertilization) in animals. On the slopes of the African island of São Tomé, Drosophila yakuba and its endemic sister species D. santomea hybridize within a well-demarcated hybrid zone. I find that D. yakuba females from within this zone, but not from outside it, show an increase in gametic isolation from males of D. santomea, an apparent result of natural selection acting to reduce maladaptive hybridization between species. To determine whether such a barrier could evolve under laboratory conditions, I exposed D. yakuba lines derived from allopatric populations to experimental sympatry with D. santomea, and found that both behavioral and gametic isolation become stronger after only four generations. Reinforcement thus appears to be the best explanation for the heightened gametic isolation seen in sympatry. This appears to be the first example in animals in which natural selection has promoted the evolution of stronger interspecific genetic barriers that act after mating but before fertilization. This suggests that many other genetic barriers between species have been increased by natural selection but have been overlooked because they are difficult to study.