Species cohesion in sexually reproducing organisms
Most described species reproduce sexually. If the organisms in a certain population reproduce sexually exclusively or almost exclusively with one another, then it is probable that, sooner or later, they will differ in their phenotype and genotype from the members of groups with which they do not reproduce or with which they reproduce (cross) only exceptionally. Simultaneously, it is not important whether the members of given groups do not reproduce together because “they don’t want to” (for example, they reproduce preferentially with individuals that look or smell similar to themselves) or because they are not capable of doing so (for example, because natural barriers occur between their areas of occurrence, e.g. a river or mountain range). The gene pool of sexually reproducing species necessarily develops as a single unit (Mayr 1963). If a new mutation is formed in a gene of a certain species, then its evolutionary fate does not depend only on how it affects the fitness of the individual in whose genome it is formed, but primarily on how it will affect the fitness of most of its future offspring, i.e. how well it works in combination with the alleles of all the genes that occur in the particular gene pool with the greatest frequency. Thus, in sexually reproducing organisms, constant testing occurs, not only of how new mutations affect the fitness of the individual in whose genome they are momentarily located, but also in the long term primarily of how these new mutations are capable of cooperating with the other alleles occurring in the particular gene pool. This ensures that the individual organisms within the species will not be able to diverge too much through chance or through the variability of the selection pressures acting on them. Thus, a species might not be able to evolve in the most effective way; for example, it cannot simultaneously adapt to several types of environment encountered by its members, but rather it evolves as a whole.
Genetic and thus phenotype divergence of individual populations can occur in species with a structured population and low intensity of gene flow across the area of occurrence (Ehrlich & Raven 1969). A relatively large percentage of species of animals and plants belong in this category. Consequently, some authors are of the opinion that species cohesion is determined by only a certain subset of genes, i.e. selectionally advantageous genes, whose spreading across the area is ensured by quite small gene flows. Thus, the individual populations can mutually diverge in most other (selectionally neutral) genes without disturbing the overall cohesion of the species (Rieseberg & Burke 2002).
- more or less2202
- not at all2513