Neo-Darwinism / R.J. Berry.

  • R. J. Berry
Date:
1982
Catalogue details

Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)

Credit: Neo-Darwinism / R.J. Berry. Source: Wellcome Collection.

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    6 Species and Macroevolution The evidence for infra-specific genetic adjustment {micro-evolution) is over¬ whelming. Populations have to be thought of as being in a state of dynamic genetic flux with their inconstant environments; the old idea of a genetic type manifested in a fixed taxonomic or museum type is clearly invalid. However, there is much less certainty about whether the well-understood processes that produce differences between populations within a species also lead to the formation of new species, and whether 'higher' taxa (genera, families, classes, etc.) are the result of the same processes again. Part of the problem is the practical difficulty of recognizing the limits of a species. Although it is acceptably defined as 'a group of actually or potentially inter-breeding natural populations which is reproductively isolated from other such groups', or 'the largest and most exclusive reproductive community which shares a common gene pool', in practice: 1) Species are more often thàn not described on the basis of a few dead specimens, and hence depend on morphological criteria. 2) Species exist and vary in both time and space. Confusions arise because of lack of knowledge about the extent of variation within different species, and because of gradations between forms which on some criteria would be regarded as a distinct species. Both these problems apply to interpretations of the fossil record, since such species have to be described almost entirely on morphological grounds. (The study of fossil assemblages or palaeoecology is beginning to contribute further information, but is still in its infancy.) 6.1 Species are evolutionary units Species are keys in the evolutionary process for two reasons: 1) They are natural units because all members of a species share a common gene-pool, thus separating them from other species (even though a very limited amount of gene-flow may take place through the hybrid zone type of situation described in section 5.2. Groups which share gene-pools in this restricted way can be described as semi-species-, they are an interesting but fairly unimportant exception to Darwin's emphasis on the discreteness of species described in section 1.3.1). The existence of natural units with varying degrees of relationship make possible a natural (or phylogenetic) classification. 2) Being independent units means that their survival, change, and extinction are constrained by their ecological and not their genetical associations. Evolution above the species level is, of course, still dependent on inherited variation, but the emphasis moves to survival of the whole group in its particular environment(s).
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