So far in this course we have dealt entirely with the evolution of characters that are controlled by simple Mendelian inheritance at a single locus. There are notes on the course website about gametic disequilibrium and how allele frequencies change at two loci simultaneously, but we didn’t discuss them. In every example we’ve considered we’ve imagined that we could understand something about evolution by examining the evolution of a single gene. That’s the domain of classical population genetics. For the next few weeks we’re going to be exploring a field that’s actually older than classical population genetics, although the approach we’ll be taking to it involves the use of population genetic machinery. If you know a little about the history of evolutionary biology, you may know that after the rediscovery of Mendel’s work in 1900 there was a heated debate between the “biometricians” (e.g., Galton and Pearson) and the “Mendelians” (e.g., de Vries, Correns, Bateson, and Morgan). Biometricians asserted that the really important variation in evolution didn’t follow Mendelian rules. Height, weight, skin color, and similar traits seemed to

Human biochemical genetics

Genetics and analysis of quantitative traits

1. Introductory remarks 2. Convolvulacaea 2. Scrophulariaceae, Gesneriaceae, Labiatae, etc. 4. Cruciferae, Papaveraceae, Resedaceae, etc. 5. Geraniaceae, Leguminosae, Onagraceae, etc. 6. Solanaceae, Primulaceae, Polygoneae, etc. 7. Summary of the heights and weights of the crossed and self-fertilised plants 8. Difference between crossed and self-fertilised plants in constitutional vigour and in other respects 9. The effects of cross-fertilisation and self-fertilisation on the production of seeds 10. Means of fertilisation 11. The habits of insects in relation to the fertilisation of flowers 12. General results Index.

The Effects of Cross and Self Fertilisation in the Vegetable Kingdom: CONVOLVULACEÆ

▪ Abstract Mixed mating, in which hermaphrodite plant species reproduce by both self- and cross-fertilization, presents a challenging problem for evolutionary biologists. Theory suggests that inbreeding depression, the main selective factor opposing the evolution of selfing, can be purged with self-fertilization, a process that is expected to yield pure strategies of either outcrossing or selfing. Here we present updated evidence suggesting that mixed mating systems are frequent in seed plants. We outline the floral and pollination mechanisms that can lead to intermediate outcrossing, review the theoretical models that address the stability of intermediate outcrossing, and examine relevant empirical evidence. A comparative analysis of estimated inbreeding coefficients and outcrossing rates suggests that mixed mating often evolves despite strong inbreeding depression. The adaptive significance of mixed mating has yet to be fully explained for any species. Recent theoretical and empirical work suggests that...

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The Evolutionary Enigma of Mixed Mating Systems in Plants: Occurrence, Theoretical Explanations, and Empirical Evidence

We review some recently published results on sex chromosomes in a diversity of species. We focus on several fish and some plants whose sex chromosomes appear to be 'young', as only parts of the chromosome are nonrecombining, while the rest is pseudoautosomal. However, the age of these systems is not yet very clear. Even without knowing what proportions of their genes are genetically degenerate, these cases are of great interest, as they may offer opportunities to study in detail how sex chromosomes evolve. In particular, we review evidence that recombination suppression occurs progressively in evolutionarily independent cases, suggesting that selection drives loss of recombination over increasingly large regions. We discuss how selection during the period when a chromosome is adapting to its role as a Y chromosome might drive such a process.

/pdf/steps-in-the-evolution-of-heteromorphic-sex-chromosomes-29whtyn0tu.pdf

Steps in the evolution of heteromorphic sex chromosomes

A comparison of the phytosociology and dynamics of three floodplain (Várzea) forests of known ages, Rio Juruá, western Brazilian Amazon

A polymorphism for anthocyanin production was used as a genetic marker to document the relationship between anther-stigma separation and outcrossing rate in the predominantly self-fertilizing weed Datura stramonium. White-flowered plants that differed in anther-stigma separation were placed into populations consisting exclusively of purple-flowered plants. Self vs. outcross origin of progeny was evident in the hypocotyl color of the seedlings. Outcrossing rates measured for single flowers were significantly positively correlated with anther-stigma separation, albeit with some scatter around the regression line, especially for flowers with exserted stigmas. We also performed an 8 × 8 diallel cross to determine whether anther-stigma separation is genetically determined. Heritability in two field plots was ∼0.3 and in the greenhouse was ∼0.2. Maternal effects, epistasis, and dominance appeared to be relatively unimportant. Genotypes performed consistently across the three environments, although total plant size varied more than fivefold. It appears that the mixed-mating system of D. stramonium has a heritable basis and would be capable of responding to selection.

https://bsapubs.onlinelibrary.wiley.com/doi/pdf/10.2307/2656629

Heritability of stigma position and the effect of stigma-anther separation on outcrossing in a predominantly self-fertilizing weed, Datura stramonium (Solanaceae).

Assessment of pollen viability in hand-pollination experiments: a review

Competitive effects and responses to neighbours were examined by growing plants of three species (Achillea millefolium, Dactylis glomerata and Vicia cracca) singly and in intra- and interspecific pairs at low and at moderate substrate fertility. The effects of neighbours were distinguished from the effects of a reduction in available space by growing plants across a range of pot sizes. Focal plants were affected by a simple reduction in available space differently than by the presence of competing neighbours (...)

/pdf/competition-among-old-field-perennials-at-different-levels-1tk0vkvlcr.pdf

Competition among old-field perennials at different levels of soil fertility and available space

The breakdown of self-incompatibility has occurred repeatedly throughout the evolution of flowering plants and has profound impacts on the genetic structure of populations. Recent advances in understanding of the molecular basis of self-incompatibility have provided insights into the mechanisms of its loss in natural populations, especially in the tomato family, the Solanaceae. In the Solanaceae, the gene that controls self-incompatibility in the style codes for a ribonuclease that causes the degradation of RNA in pollen tubes bearing an allele at the S-locus that matches either of the two alleles held by the maternal plant. The pollen component of the S-locus has yet to be identified. Loss of self-incompatibility can be attributed to three types of causes: duplication of the S-locus, mutations that cause loss of S-RNase activity, and mutations that do not cause loss of S-RNase activity. Duplication of the S-locus has been well studied in radiation-induced mutants but may be a relatively rare cause of the breakdown of self-incompatibility in nature. Point mutations within the S-locus that disrupt the production of S-RNase have been documented in natural populations. There are also a number of mutants in which S-RNase production is unimpaired, yet self-incompatibility is disrupted. The identity and function of these mutations is not well understood. Careful work on a handful of model organisms will enable population biologists to better understand the breakdown of self-incompatibility in nature.

Judy L. Stone

Papers

A comparison of the phytosociology and dynamics of three floodplain (Várzea) forests of known ages, Rio Juruá, western Brazilian Amazon

Heritability of stigma position and the effect of stigma-anther separation on outcrossing in a predominantly self-fertilizing weed, Datura stramonium (Solanaceae).

Assessment of pollen viability in hand-pollination experiments: a review

Competition among old-field perennials at different levels of soil fertility and available space

Molecular mechanisms underlying the breakdown of gametophytic self-incompatibility.