Selection on plant phenotypic traits through male and female function
Final Report Abstract
Habitat fragmentation is considered to be one of the major threats to biological diversity worldwide. To date, however, its consequences have mainly been studied in an ecological context, while little is known about its effects on evolutionary processes. Habitat fragmentation could, for example, influence trait evolution by affecting abiotic or biotic environmental factors that are known to act as selective agents. It is, for example, well known that fragmentation may disturb interactions between plants and their pollinators, which in turn are of fundamental importance to trait evolution in plants. The first major aim of our project was to investigate whether habitat fragmentation alters pollinator-mediated selection on plant phenotypic traits via effects on pollinator availability. We hypothesized that pollinator-mediated selection on floral display size and flowering phenology is stronger in smaller populations, because plants in small populations are more likely to be pollen limited than plants in large populations and therefore benefit from trait values that increase attractiveness to pollinators. We also explored how local plant density and plant phenotypic traits interact to determine patterns of pollinator-mediated selection. Here, we similarly hypothesized that plants growing singly are subject to stronger selection than plants growing together with many conspecific neighbors. The second major aim of our project was to examine the potential of plants in fragmented populations to respond to such selection by estimating the amount of heritable variation in relevant traits. We hypothesized that the amount of heritable trait variation is lower in small compared to large populations. We used the self-incompatible, perennial forest herb Phyteuma spicatum as study species, which occurs in fragmented forests in NW-Germany (populations range in size from about 10 to 2500 flowering individuals), and we combined descriptive and experimental approaches. Our results exclusively focus on selection via female reproductive success; our third major aim to also examine selection through male function failed. We had planned to estimate paternal fitness by using molecular genetic markers. However, fundamental problems with genotyping individuals using the developed microsatellite markers resulted in that we were not able to conduct reliable paternity analyses and to determine male fitness. Overall and relating to the first major aim of our project, the strength and direction of selection on floral display size and flowering phenology appeared to be spatially and temporally variable, both across populations of varying size and within populations that varied in local plant density. Selection gradients for floral display size were always positive, i.e. seed production increased with increasing inflorescence size. The strength of selection, however, differed among populations and years and in one case it was also affected by local plant density within a population. Most importantly, selection for increased inflorescence size decreased with increasing population size in one year, showing that habitat fragmentation may indeed affect selection on plant phenotypic traits. We also detected significant selection on flowering phenology in some populations and years. Generally, fitness appeared to be higher in earlier flowering plants and in plants flowering synchronously with others; however, in some populations and years we also obtained evidence for stabilizing selection on flowering time. Contrary to our expectations, however, differences in selection on inflorescence size or flowering time were not linked to differences in pollination intensity, but were rather likely linked to differences in some other component of the abiotic or biotic environment. At last, and relating to the second major aim of our project, we found that narrow-sense heritabilities for two traits, namely flowering duration and mean seed mass, decreased with decreasing population size, suggesting that plants in small populations may have a reduced capacity to respond and adapt to changes in the environment which alter selective pressures on these traits. In summary, our results show that habitat fragmentation may affect selection on plant phenotypic traits, highlighting potential evolutionary consequences of human-induced environmental change. Evolutionary change, however, may be limited, given that the capacity of plants in small populations to respond to altered selective pressures is likely reduced. In general, patterns of selection appear to be spatially and temporally highly variable and not linked to differences in pollination as initially hypothesized. Evolutionary trajectories, as well as the underlying mechanisms, are thus complex and further studies are certainly needed to gain more conclusive insights into the potential evolutionary consequences of habitat fragmentation in this and other species.
Publications
- (2011) Evolutionary consequences of habitat fragmentation: population size and density affect selection on inflorescence size in a perennial herb. Evolutionary Ecology 25:417-428
Weber A, Kolb A
- (2013) Local plant density, pollination and trait-fitness relationships in a perennial herb. Plant Biology 15:335-343
Weber A, Kolb A
(See online at https://doi.org/10.1111/j.1438-8677.2012.00645.x) - (2013) Population size, pollination and phenotypic trait selection in Phyteuma spicatum. Acta Oecologica 47:46-51
Weber A, Kolb A
(See online at https://doi.org/10.1016/j.actao.2012.12.004) - (2014) Differences in heritable trait variation among populations of varying size in the perennial herb Phyteuma spicatum. Conservation Genetics 15:1329-1337
Weber A, Kolb A
(See online at https://doi.org/10.1007/s10592-014-0620-1)
