Indiscriminant early life stages prevent local adaptation along an environmental gradient

Background/Question/Methods

Given the constant forces of dispersal, selection, and mutation, why do so many species have stable niches? Lack of evolutionary range expansion given sufficient dispersal signifies a failure of adaptation when novel environmental conditions are commonly encountered. For annual plants, range boundaries may be conserved where there is a trade-off in performance such that traits, such as sensitive seed germination cuing, that lead to high performance in core habitat limit performance in novel sites.

We used a field planting study to assess the ecological and evolutionary causes of local range conservation for the annual plant Gilia tricolor. In three years, family structured plantings were installed in plots in the population core, in the marginal habitat zone at the population edge, and in novel habitat exterior to the population. Seed emergence and post-emergence fitness were tracked for all seeds for four years, including those dormant in previous seasons. We questioned if seeds have adaptive emergence cuing by assessing the correlation between micro-site specific emergence rates and post-emergence fitness. We assessed how performance varied for seeds and paternal families that originated in different habitat zones performed across the population boundary, among observational years, and with seed age. 

Results/Conclusions

Emergence and post-emergence fitness varied significantly among study years, but consistently declined as expected across the habitat gradient from core to unoccupied habitat. Performance declined with seed age. There was a dramatic lack of correlation between emergence rates and post-emergence fitness in all habitat types, showing that seed emergence was indiscriminate of habitat quality.   Furthermore, there were no differences in seed dormancy or mortality between years, or in the rates of dormancy for seeds from different families and habitat origins. Mean values of post-emergence fitness for paternal families, estimated as the random effects coefficients in generalized linear mixed models, were uncorrelated between the three habitat zones.  This suggests that there were no spatially structured additive genetic variation for post-emergence success among families despite spatial differences their origin relative to the local population boundary and environmental gradient. Our findings demonstrate how adaptive expansion at a range margin can be inhibited where early life stages such as seed cuing and emergence are unable to distinguish between suitable and unsuitable microsite conditions.