Biomedical subjects

P Bierzychudek

Publications and source records attributed to P Bierzychudek.

4 recordsLinked to original sources

PubMed2001

Perspective: Evolution of flower color in the desert annual Linanthus parryae: Wright revisited.

Linanthus parryae, a diminutive desert annual with white or blue flowers, has been the focus of a long-standing debate among evolutionary biologists. At issue is whether the flower color polymorphism in this species is the product of random genetic drift, as Sewall Wright argued, or of natural selection, as proposed by Carl Epling and his colleagues. Our long-term studies of three polymorphic populations in the Mojave Desert demonstrate that flower color is subject to selection that varies in both time and space in its direction and magnitude. For all sites taken together, blue-flowered plants produced more seeds than white-flowered plants in years of relatively low seed production, whereas white-flowered plants had higher fitness in years of high seed production. Evidence of selection on flower color was found in two of the three study sites. Differences in fitness between the color morphs were sometimes large, with selection coefficients as high as 0.60 in some years. Our longest period of observations was at Pearblossom site 1, where plants reached appreciable densities in seven of the 11 years of study. Here we found significant differences in the seed production of the color morphs in six years, with three years of blue advantage and three years of white advantage. For all sites taken together, total spring precipitation (March and April) was positively correlated with both absolute and relative seed production of the color morphs. At Pearblossom site 1, blue-flowered plants typically had a fitness advantage in years of low spring precipitation, whereas white-flowered plants had a fitness advantage in years of high spring precipitation. This temporal variation in selection may contribute to the maintenance of the flower-color polymorphism at Pearblossom site 1, whereas gene flow from neighboring populations is proposed as the principal factor maintaining the polymorphism at the other study sites. We found no significant differences between the color morphs in pollinator visitation rate or in their carbon isotope ratio, a measure of water-use efficiency. Although the mechanism of selection remains elusive, our results refute Wright's conclusion that the flower color polymorphism in L. parryae is an example of isolation by distance, a key component of his shifting balance theory of evolution.

Adaptation, Physiological↗

PubMed2001

Stable two-allele polymorphisms maintained by fluctuating fitnesses and seed banks: protecting the blues in Linanthus parryae.

Motivated by data demonstrating fluctuating relative and absolute fitnesses for white- versus blue-flowered morphs of the desert annual Linanthus parryae, we present conditions under which temporally fluctuating selection and fluctuating contributions to a persistent seed bank will maintain a stable single-locus polymorphism. In L. parryae, blue flower color is determined by a single dominant allele. To disentangle the underlying diversity-maintaining mechanism from the mathematical complications associated with departures from Hardy-Weinberg genotype frequencies and dominance, we successively analyze a haploid model, a diploid model with three distinguishable genotypes, and a diploid model with complete dominance. For each model, we present conditions for the maintenance of a stable polymorphism, then use a diffusion approximation to describe the long-term fluctuations associated with these polymorphisms. Our protected polymorphism analyses show that a genotype whose arithmetic and geometric mean relative fitnesses are both less than one can persist if its relative fitness exceeds one in years that produce the most offspring. This condition is met by data from a population of L. parryae whose white morph has higher fitness (seed set) only in years of relatively heavy rain fall. The data suggest that the observed polymorphism may be explained by fluctuating selection. However, the yearly variation in flower color frequencies cannot be fully explained by our simple models, which ignore age structure and possible selection in the seed bank. We address two additional questions--one mathematical, the other biological--concerning the applicability of diffusion approximations to intense selection and the applicability of long-term predictions to datasets spanning decades for populations with long-lived seed banks.

Adaptation, Physiological↗

PubMed1987

Resolving the paradox of sexual reproduction: a review of experimental tests.

Though theoretical biologists have responded enthusiastically to the challenge of resolving the paradox of sex, there has not been an equivalent amount of experimental testing of the hypotheses they have put forward. This paper briefly reviews the essential elements of the models proposed to explain the predominance of sexual reproduction, and describes the existing experimental evidence. Some of the experimental approaches have involved tests of the models' assumptions; work done to date, for example, suggests that the theoretically twofold 'cost of sex' may be an over-estimate for animals, but an under-estimate for plants [see also the chapter by Lewis-Ed.]. The assumption that truncation selection occurs in nature has found no experimental support. Other experiments have tested the models' predictions. There is little evidence that sibling competition is mitigated by genetic diversity among progeny. Several studies have provided evidence that minority genotypes have higher fitness, but generally fail to reveal a mechanism for that advantage. Overall, no one model emerges as a likely single 'cause' for the maintenance of sexual reproduction. Future progress is likely to come, not from further tests of the models' predictions (which are necessary logical outcomes of particular sets of assumptions), but rather from examining the frequency with which their assumptions are met in nature.

Animals↗

PubMed1987

Patterns in plant parthenogenesis.

Plant taxa that reproduce asexually display some distinct geographical and ecological patterns. A literature review reveals that such taxa 1) tend to have larger ranges, 2) tend to range into higher latitudes, and 3) tend to range to higher elevations than do their sexual relatives. Asexual taxa have a greater tendency than sexual taxa do to colonize once-glaciated areas. These trends have previously been identified as characteristic of parthenogenetic animals as well. While many authors have interpreted these trends as providing support for the 'biotic uncertainty' hypothesis for the maintenance of sex, these trends are consistent with several other interpretations as well. Furthermore, all of these interpretations have ignored the positive correlation that exists between ploidy level and breeding system: asexual plant and animal taxa are generally polyploid, while their sexual relatives are generally diploid. Evidence is presented for plants, and by extension for animals as well, that high ploidy levels alone-independent of breeding system-could endow individuals with the ability to tolerate these 'extreme' environments. For this reason, it appears premature to interpret observed distribution patterns as evidence to support hypotheses about what forces maintain sexual reproduction. Only experimental tests, using sexuals and asexuals of comparable ploidy levels, can permit us to discriminate among the alternatives.

Ecology↗