The great taxonomic diversity of the Orchidaceae is often attributed to adaptive radiation for specific pollinators driven by selection for outcrossing. However, when one looks beyond the product to the process, the evidence for selection is less than overwhelming. We explore this problem by discussing relevant aspects of orchid biology and asking which aspects of reproduction explain the intricate pollination mechanisms and diversification of this family. We reaffirm that orchids are primarily pollination limited, the severity of which is affected by resource constraints. Fruit set is higher in temperate than in tropical species, and in species which offer pollinator rewards than those that do not. Reproductive success is skewed towards few individuals in a population and effective population sizes are often small. Population structure, reproductive success and gene flow among populations suggest that in many situations genetic drift may be as important as selection in fostering genetic and morphological variation in this family. Although there is some evidence for a gradualist model of evolutionary change, we believe that the great diversity in this family is largely a consequence of sequential and rapid interplay between drift and natural selection.

Life history theory assumes that there is a trade—off between current reproduction and future growth or reproduction or both; therefore, natural selection is expected to result in the maximization of reproduction within the limits imposed by the trade—off. Accordingly, it has been predicted that fruit and seed production in perennial plants should be resource limited. Many orchid species seem to be pollinator limited, but this hypothesis has been recently challenged by experimental studies that show a cost of increased fruit set in some orchids. In this study, I combined the results of a 2—yr pollination experiment and a 3—yr demographic assessment of a population of the orchid Tolumnia variegata in a matrix model of population dynamics, and by means of simulations I evaluated the effect of pollination intensity and frequency, fruit production, and cost of fruiting on the asymptotic population growth rate. Mean natural fruit set in this population was <1%, whereas intermediate and high pollination intensity resulted in mean fruit set of 35 and 72%, respectively. In any given year, °98% of all flowering individuals fail to set fruit under natural conditions. Despite the dramatic increase in fruit set after hand—pollination, only plants in the high pollination intensity treatment showed a statistically significant reduction in future growth and flowering. The simulation showed that a small seedling production per fruit would be enough to overcome the cost of fruiting; therefore, plants in this population should experience strong selection for increased pollination. This conclusion does not seem to explain the widespread occurrence of low fruit set and high proportion of fruiting failure among nonautogamous orchids. The results of this study suggest an alternative explanation: pollinator limitation could be evolutionarily stable if the correlation between fruit or seed production and seedling recruitment is sufficiently low. If an increase in fruit or seed production does not translate into an increase in fitness, then selection for increased pollination would not occur or would be too weak. The evaluate this alternative, quantitative studies on the transition from seed to seedling in natural orchid populations are required.

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Altitudinal changes in breeding and pollination systems of tropical montane plants were studied in 13 species of Espeletia growing in the Venezuelan Andes from 2,000 to 4,300 m. Hand pollination tests showed that all species were strongly self‐incompatible. The four species found only above 4,000 m had up to 10% median seed set in self‐pollinated heads, which was significantly higher than the lower elevation species. Nine species were insect‐pollinated, with large bees the major pollinator group. An endemic páramo hummingbird, Oxypogon guerinii, was an important visitor of E. schultzii in three populations examined. Experimental bagging experiments showed that the four high elevation species were wind‐pollinated, further evidenced by the lack of pollinator visits and markedly different capitulum morphologies. Open‐pollinated seed set in two wind‐pollinated species, E. spicata and E. timotensis, was strongly dependent on the population's flowering density, which varied significantly from year to year. The shift from insect to wind pollination in Espeletia can be related to the low pollinator availability at high elevations in the Andes, protection of the capitula from snow and daily frosts, and the extremely long flowering periods of individual heads.

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