Reinforcement of reproductive isolation between adjacent populations in the Park Grass Experiment

Abstract

It has been debated, ever since Charles Darwin and Alfred Russell Wallace disagreed about the matter, whether natural selection plays a role in reinforcing reproductive isolation during the earliest stages of speciation. Recent theory suggests that it can do so, but until now the empirical evidence has conspicuously lacked a case in which reinforcement has actually been observed to split a population. We show that this has occurred at least once in populations of the grass Anthoxanthum odoratum growing in the Park Grass Experiment where flowering time has shifted at the boundaries between plots. As a consequence, gene flow via pollen has been severely limited and adjacent populations that had a common origin at the start of the experiment in 1856 have now diverged at neutral marker loci. Heredity (2005) 95, 198-205.

Evolution of reproductive isolation in plants

“…The literature about reinforcement is characterized by more debate than firm evidence. A recent study, however, provided compelling evidence for reinforcement of a flowering time shift in the grass Anthoxanthum odoratum (Silvertown et al, 2005), and a macroevolutionary analysis of the Cape Flora of South Africa suggested that pollinator shifts can occur upon secondary contact to prevent gene flow between incipient species that initially diverged as a consequence of adaptation to other ecological conditions, such as soil composition. In this context, the enhancement of reproductive isolation by While reinforcement is thought to complete the speciation process between incipient species upon secondary contact by strengthening prezygotic barriers to halt gene flow, reproductive character displacement (sensu Butlin, 1987) occurs as a consequence of interactions in sympatry among reproductively isolated species, and serves to reduce gamete loss in unsuccessful heterospecific matings.…”

Speciation: Flowering time and the Wallace Effect

“…These plots were originally set up in 1856 to test the reactions of meadow vegetation to different fertilizer applications, procedures which continue to this day and which have generated enormous amounts of data on plant physiological responses, population dynamics and community ecology. Building on previous work (eg Snaydon and Davies, 1976), Silvertown et al (2005) have shown that there has been a shift in flowering time of A. odoratum at the border between adjacent experimental plots. Crucially, this 'inverse cline' of flowering is a signature of the first steps along a particular road to speciation, that has been predicted by modelling studies exploring how natural selection against hybrids could contribute to reproductive isolation between populations in proximity.…”

“…How this reproductive isolation is achieved has been the focus of considerable debate, and now a new study on page 198 of this issue, by Jonathan Silvertown and colleagues at the Open University and Rothamsted Research in the UK, provides strong evidence for a controversial proposal. Their works suggests that natural selection can reinforce reproductive isolation within what is essentially one population (Silvertown et al, 2005).…”

“…The mechanism of reproductive isolation caused by natural selection against hybrids within adjacent populations is generally called 'reinforcement', sometimes termed the 'Wallace Effect' (Silvertown et al, 2005) due to its championing by Alfred Russell Wallace at the end of the 19th century (Wallace, 1889). History has been unkind to Wallace, who is often perceived as a lesser figure in Darwin's shadow, rather than as an important scientist and natural historian in his own right.…”

Spore Germination Determines Yeast Inbreeding according to Fitness in the Local Environment

“…Similarly, hybridization between plant species can be prevented if they have different flowering times (e.g. , Young 1996;Pascarella 2007), and locally adapted races can be maintained within a population by differences in their flowering times, most notably in natural Anthoxanthum odoratum (McNeilly and Antonovics 1968) as well as in the Park Grass Experiment (Snaydon and Davies 1976;Silvertown et al 2005). If the four spores within a Saccharomyces tetrad germinate at the same time, they can form two mating pairs and mate entirely within the tetrad (as modeled in Tazzyman et al 2012).…”