Place et rôle de l’adaptation dans l’évolution des organismes

Abstract

Status and role of adaptation in biological evolution: Biological evolution is dependent of adaptation at three different levels of observation. A large amount of intraspecific variability is related to environmental variation: this is the result of permanent adaptation of the genetic structure of any population to more or less fluctuating external conditions by means of natural selection. However, some patterns of intraspecific variation seem not to be attributable to this process, especially molecular diversity within or between populations, so that it is likely that intraspecific variation as a whole is maintained by complex interactions between selective pressures, mainly adaptative, and more or less random processes (mutations, migrations, genetic drift). Many models of speciation have been developped in which adaptation plays some role. In allopatric speciation, the divergence between the incipient species is believed to be generated by differential adaptation to geographically distinct areas. Though the reproductive barrier itself is not necessarily related to adaptive divergence, it is clear that some components of reproductive isolation may result from`` differential adaptation, for instance ecological isolation from which more efficient isolating mechanisms are expected to emerge by means of sympatric reinforcement, a process which consists of coadaptation of two species to each other as a result of a strict partitionning of the available resources of the habitat. Some modes of speciation involve interactions between random processes and selective ones. For instance, the so-called founder effect actually consists of two steps. The first one, referred to as «1st order effect», is strictly stochastic: the genetic pool of a newly arisen population is not a representative sample of the genetic pool of the population from which its founders were transplanted. During the second step, «2nd order effect», each polymorphic locus reaches a state in which it is adapted to its genetic environment: at the level of the genetic structure as a whole the final equilibrium results from coadaptations between all loci. Among macroevolutionary processes, some are probably strictly neutral. Some others seem to be related to general improvements which are not truly adaptive since they are not specific for any well characterized environment: for instance the progressive increase of size and complexity of the brain which occurs in many mammalian lineages. However, some major features of many phyla arise by means of sequence of adaptive radiations: This process probably has been operating since the earliest stages of evolution. © 1986 Taylor & Francis Group, LLC.