1. The evolutionary fate of natural hybridization is determined by the fitness of hybrids. Specifically, the survival of newly formed hybrids is affected by intrinsic genetic incompatibilities and/or genotype–environment interactions. The influence of these factors on the evolutionary potential of two natural hybrids of Narcissus has been assessed. Fitness components were estimated in early experimental F1 hybrids as well as in natural hybrids of unknown descent.
2. To test the efficiency of reproductive isolation, fruit set, seed number and seed mass from interspecific crosses were measured, as were germination rate, radicle vigour and leaf appearance time in F1 experimental hybrids in the greenhouse.
3. Germination rate, survival and the number of leaves produced were also measured in F1 plants transplanted to eight natural sites and compared with plants growing under greenhouse conditions. In addition, to overcome the difficulty of obtaining experimental late-generation hybrids, fruit set, leaf production, bulb production and bulb size were measured in natural hybrids of unknown descent.
4. Experimental crosses indicated weak post-mating barriers. In the case of N. cavanillesii, the barriers were virtually absent and interspecific crosses produced the same level of fruit set, seed set and seed mass as conspecific crosses. Fitness of F1 experimental hybrids was similar to or greater than conspecific progeny, although some reciprocal hybrids differed on some of the traits measured. No differences were found in hybrid performance between the various natural sites and the greenhouse, indicating a lack of exogenous selection against hybrids. Mature natural hybrids of unknown descent exhibited higher levels of growth and bulb propagation than parental species but very low fertility.
5.Synthesis. Hybrid vigour in early life stages and the absence of exogenous selection against hybrids provide an initial advantage for the establishment of hybrids in a wide variety of habitats. These factors, together with higher levels of growth and bulb propagation recorded in hybrids as compared to parental species, may compensate for low fertility, thus conferring evolutionary potential to Narcissus natural hybrids.