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.

    13/76 page 7
    DARWIN'S OWN ANSWERS 7 contemporaries and many recent biologists that environments are stable and largely homogeneous. (iii) Transitional forms will almost certainly be less common than either the ancestral or descendent form, and hence liable to be overlooked or to become extinct. As collecting and the description of variation has progressed, it has been recognized that some species previously collected only from widely separated areas may be in fact be connected by intermediate forms. We now recognize dines of change in particular traits, and also that a species may be polytypic, i. e. contain several geographically distinguishable forms which interbreed to a limited extent. Linnaeus confused matters by giving the name 'variety' indis¬ criminately to geographical races, domesticated races, non-genetic variants, and inherited 'sports'. The idea of a polytypic species radically altered the Linnean concept of a species: the older idea is of a species characterized by a gap separating it from other groups, while a polytypic species is defined by actual or potential genetic continuity between allopatric populations. This does not, however, affect Darwin's point that forms in the process of change are likely to be uncommon. As we shall see, the discovery of active speciation (for example, Drosophila flies in Hawaii and cichlid fishes in Lake Victoria: section 6.2.2); the rarity of transitional forms; and the study of hybrid zones (section 5.2) is throwing a great deal of light on the nature and integrity of species. 1.3.2 The effectiveness of natural selection The most persistent criticism of Darwinism has always been that natural selection is merely a negative instrument removing inefficiency, but incapable of producing novelty or the seemingly perfect adaptation of such features as the eye of a mammal or bird, or the pattern of a butterfly's wing: 1) Cases of special difficulty Darwin recognized three situations: (a) An organ (such as the wing of a bat) may be so specialized for its functions as to bear little resemblance to the prototype (the forelimb of an insectivore) from which it must be presumed to have arisen. The difficulty is envisaging a series of organisms with organs of intermediate grades connecting these widely- separated extremes. (Ò) An organ of extreme perfection (such as the eye in the higher vertebrates) may show such perfect and detailed adaptation that by comparison with the obstacles which the design of such an apparatus would present to human ingenuity, the mind is staggered by the effort of conceiving it as the product of so undirected a process as trial and error. (c) Some organs of seemingly trifling importance (such as the 'fly-whisk' tail of the giraffe) are yet so clearly adapted to the function they perform that they cannot be regarded as accidental. In these cases it may be asked how such a trifling function can ever have been a matter of life and death to the organism, and so have determined its survival in the struggle for existence. The first of these classe&of objection applies to all evolution, whilst the second and third are difficulties more of imagination than of reason. It is impossible to deal with them in detail; R. A. Fisher has commented that 'the cogency and
    digitised image 1
    digitised image 2
    digitised image 3