is most striking when comparing the floras of equal climatic zones on different continents, like those of North America and Europe. Introduction of foreign species into an area has shown, that this type of reproductive barrier is often rather feeble and collapses easily. Hybridizations with endemic species are therefore common unless prevented by other barriers. Spartina townsendii is a good example.
The same phenomenon is met when analyzing the vegetation of islands. Endemic species (plants and animals), i.e. species occurring exclusively in a certain, often clearly demarcated area, are very common on islands. Both C. DARWIN and A. WALLACE based their selection theories on this observation. The DARWIN-finches, each species of which occurs only on certain islands of the Galapagos Archipelago, became famous. The Galapagos Archipelago – of volcanic origin and geologically rather young - does also house plant groups, certain species of which live only on single islands. The occurrence and dispersal of Scalesia, a genus of the Compositae, is of special interest. The roughly 20 species are easiest distinguished by the shape of their leaves. Moreover, they are woody, some are even large trees. This is atypical for Compositae, a plant families harbouring one of the largest number of species, since most Compositae are annual herbs or perennial non-wooden plants. Composite trees do not only live on the Galapagos Archipelago, but also on other Pacific islands, like the San Fernandez-Islands close to the coastline of Chile and in a few continental habitats.
The following model experiment shows, that the collapse of a reproductive barrier may even lead to the extinction of a species. In California, the two evening primrose species Clarkia biloba and Clarkia lingulata do never occur in the same habitat. They do not form stable hybrids. H. LEWIS (University of California, Los Angeles, 1962), who studied the evolution of these species extensively, showed, that the pollinators of these two species are unable to distinguish them, when they occur in mixed culture. Beside intraspecific progeny, numerous sterile hybrids were thus found in the next generation. The fitness of each species is primarily determined by its number of flowers. In an experimental population containing two thirds of Clarkia biloba and one third of Clarkia lingulata at the start of the experiment, the latter disappeared in the course of four generations, although growth conditions were the same for both species and both developed the species-specific optimal number of flowers.
An extinction of species or varieties occurs not only in plants pollinated by insects: remember the experiment with Hordeum vulgare.
Just like geographic isolation, biotope isolation is no absolutely safe method for preventing the hybridization of related (sympatric) species. A. KERNER von MARILAUN showed already in the 19th century, that hybrid populations do often occur along the border of the habitats of (two) neighbouring vicarious species. These hybrids are rarely stable, and the development of new species is even rarer. The primula hybrids described by MARILAUN are a good example. Primula auricula flowers yellow and occurs in the Alps on limestone, Primula hirsuta and several similar species have two-coloured flowers (yellow and red-lilac) and are common on primeval rock (silicate, granite). Mostly unstable hybrids can be found, wherever these two types of rock occur simultaneously. These hybrids are the origin of Primula pubescens, the garden auricle.
Seasons do hardly occur in the tropics. The sympatric Miconia-species (family: Melastomataceae) from the primary forest of the Amazon area at Manaus provide an example of a timely isolation. These species are all visited by the same pollinators, among them bees of the genus Melipona and Halictides (S. RENNER, 1984). The temporally staggered flowering periods of the Miconia-species do largely prevent interspecific pollination. At the same time, a competition for pollinators is diminished.
Oenothera breviceps and Oenothera clavaeformis provide an example for different flowering times during the course of the day. The two species live side by side in the deserts of western North America. They flower at the same time of the year and are pollinated by the same insects, solitary bees, mainly Andrena. The flowers of Oenothera breviceps open before sunrise and are pollinated by early insects. Oenothera clavaeformis flowers during late afternoon and is thus pollinated by bees, that are active at this time of the day. An exchange of pollen between the two species is largely prevented. Hybrids occur rarely (P. RAVEN, 1962). Wind-pollinated plants of the gramineous genus Agrostis , too, have been shown to give their pollen away dependent on species and time of the day (W. R. PHILIPSON, 1937).
These two mechanisms take often place simultaneously, because, on one hand, the influence of the pollinator on the development and selection of species has to be considered, on the other hand, the flower structure allowing pollen nectar, and stigma access to only certain pollinators is of importance. Aquilegia formosa, a North American colombine species, has simple, nutant, yellow and red flowers with a short, 1 – 2 cm long spur. The flowers contain nectar and are pollinated by humming birds with beaks slightly longer than the flower’s spur. Aquilegia chrysantha, Aquilegia longissima, and Aquilegia pubescens have all pale yellow, erect flowers with long spurs. These species are usually pollinated by butterflies of the Sphingidae-group. Respective measurements show, that the lengths of the spurs and the lengths of the butterflies’ proboscides tally extremely well. Humming birds have no chance to reach the nectar of Aquilegia chrysantha and Aquilegia longissima, and they do not even try. They are , nevertheless, sometimes successful in the case of Aquilegia pubescens. The last three species can without difficulties be crossed experimentally. The hybrids produce fertile progeny. In nature, the demarcation is kept by different geographical distributions and different pollinators.
In contrast, Aquilegia pubescens and Aquilegia formosa have overlapping distributions. Since humming birds may act as pollinators of both species, hybridization and thus also gene exchange occurs (V. GRANT, Rancho Santa Ana Botanical Garden, Clairemont/ Cal., 1952). Hybridization cancels an otherwise effective mechanism of isolation and leads thus to a reduction of the species’ fitness.
Another example: the flowers of Gilia modocensis and Gilia malior (Polmoniaceae family, Asteridae) are adapted to autogamy. They are usually not visited by insects. Shape and sizes of style and anthers match so well, that the anthers touch and pollinate the style automatically as soon as the flower opens. The hybrids of the F2 to F4 generation produce the most different types of recombinants that lack the just mentioned matching. The anthers’ touch of the style does not take place. The failure of autogamy causes a sterility of the hybrids, even though completely functional pollen and egg cells are produced (G. L. STEBBINS, University of California, Davis, 1950).
The importance of reproductive isolation and how fatal a collapse of a reproductive barrier can be is illustrated by the fact, that several independent barriers do often exist in parallel. A pair of species (a and b) from the genus Gilia provide a good example:
(a) Gilia capitata chamissoni
(b) Gilia millefoliata
Both sympatric species are isolated ecologically. (a) occurs on sand dunes, (b) on meadows. The two species are seasonally isolated due to different flowering periods, (b) flowers earlier than (a). Finally, an ethological and mechanical isolation exists: (a) has large flowers and is pollinated by bees, while (b) has small flowers and is autogamous (V. GRANT, 1952, 1963).
The mechanisms of isolation mentioned so far seem mostly to be maintained by extern influences. This is only partly true, since genetic factors decide whether a species flourishes on limestone or primeval rock or whether the flowers open in the morning or in the evening. The genome of the plant determines, too, how the flower looks like and whether it is attractive and viable for the pollinator. The behaviour and occurrence of the pollinator, on the other hand, is what we would call an extern influence.
Sterility barriers are almost exclusively founded on endogenous, i.e. genetically determined factors. Pollen, for example, may happen to reach a wrong style. It does consequently develop no pollen tube, or the developed pollen tube degenerates or cannot reach the egg cell. This barrier is called pollen incompatibility or pollen sterility. Pollen incompatibility is, beside other mechanisms, one of the causes for the existence of obligatory allogamous species.
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