Analyse d’une zone de contact entre deux formes parapatriques : le cas des pouillots véloces Phylloscopus c. collybita et P. c. Brehmii

Résumé En

The populations of Chiffchaff Phylloscopus collybita brehmii (Homeyer, 1871) which breed in a large part of the Iberian Peninsula, are a parapatric form of Ph. c. collybita. These birds are known to have songs and calls that are very different from the vocalizations of the nominal race living in central and western Europe. The objective of the present study is to find out whether or not reproductive isolating mechanisms exist between the two forms. To better tackle this problem of parapatric speciation, we had first to devise a method to define as acurately and objectively as possible the areas of allopatry and sympatry, and we were led to propose a new mathematical model to interpret our field data. Although this model needs to be further refined, it already provides us with basic equations that allow us to accept or reject intergradation. It also leads us to consider a number of other traits of the biology of the species which might be influential in parapatric speciation. One can already assert that, under our model’s hypothesis, there is no intergradation between brehmii and collybita. The so called «hybrids» (i.e. birds which have songs and calls intermediate between the two forms) are actually scarcer, and the «pure» forms more numerous, than expected by the model. Under the same hypothesis, one can assert than panmictic mating does not take place, either in the sympatrie zone or in the «fifty-fifty percent» zone. A tendency to assortative mating was found in the contact zone. Most pairs were homogamic. Among heterogamic pairs, those made up of an Iberian male and an European female were more often observed than those made up of an European male and an Iberian female. In the «fifty-fifty percent» zone, most of the pairs are homogamic, or made up of an Iberian male and a European female, most of the breeding birds being brehmii. These observations support the view that some isolating mechanisms are already in operation before mating ; they may differ according to sex and form. No postmating isolating mechanisms apparently exist ; most of the heterogamic pairs observed raised their chicks normally. The climatic environment of brehmii is milder in winter, and moister all the year round (higher rainfall, larger number of rainy days, less frosts). P. collybita apparently replaces brehmii when one of these conditions is not fulfilled. Although more observations are obviously needed, we can nevertheless conclude that the differences between collybita and brehmii are larger than those existing between most subspecies.