Suppose, in an average year, that weather is best for survival of young seedlings in early June. If a plant could make only one seed, it should make one targeted to germinate at the beginning of June. (Plants have some control over when their seeds will germinate, based on plant-hormone concentrations, seed-coat thickness, etc.)
Given variability among years, however, a plant that produces many seeds may have more descendants if those seeds germinate at various times, rather than risking everything on one date. On the other hand, too much variation in germination time may be as risky as too little. For example, seeds germinating really late may be killed by frost. How does actual variation in germination timing among seeds of individual wild plants compare with the optimum amount of variation? This week's paper is apparently the first to answer this question.
"Fluctuating natural selection accounts for the evolution of diversification bet-hedging" was published in Proceedings of the Royal Society by Andrew Simons, of Carleton University in Canada.
To see how seedling survival varies as a function of germination date and year, he germinated Lobelia seeds in a greenhouse and transplanted them to the field on several dates per year, over five years. (Seedlings were transplanted in 8 cm fiber pots, so they were probably more protected from competition than if they had germinated in the field. Did competition change enough over the season to affect optimal germination times?) Survival data for these seedlings were used to calculate the optimum variation in germination timing, that is, the amount of variation in a plant's seeds that would maximize its total descendants, given the observed variation in field conditions. He then compared that number to previously measured within-maternal-plant variation in seed germination timing. Actual variation was just slightly less than the calculated optimum: 12 vs. 13 (days?). Simons concluded that the fitness benefits of bet-hedging were sufficient to explain the observed variation.
One would expect some variation in germination even in a uniform environment where the optimum variation was zero (i.e., all seeds should germinate on the one best day, if possible). Simons argued that "such an invariable environment neither exists nor can be experimentally created." It may be true that there is always some variation, but we could certainly minimize variation by growing plants indoors under artificial lights, with the same yearly pattern of light and temperature every year. I wonder how long it would take for plant populations to evolve significantly lower variation in seed germination, under those conditions.