Neo-Darwinism / R.J. Berry.

  • Berry, R. J. (Robert James)
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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    52 ALLOMETRY 6.23 Speciation in animals; speciation in plants Animal speciation differs from plant speciation: 1) animals can easily develop reproductive isolation based on behaviour patterns, which will be reinforced by natural selection. In contrast, the possibility for establishing pre-zygotic barriers in plants (particularly wind- pollinated ones) is obviously much less. 2) Because of their great capacity for vegetative growth, sterility is much less of a hindrance to plants than animals. For example, the Canadian pond-weed {Elodea canadensis) has spread throughout European water-ways from an introduction in 1840 of a few female plants only. Many plant species (e.g. dandelion. Taraxacum officinale) reproduce almost entirely asexually. 3) As already noted, sterile hybrids of plants may acquire fertility by polyploidy more easily than can animals. For this reason, 'reticulate evolution' (i.e. repeated splitting and fusion of lines) is more common in plants than animals. 6.2.4 Genetic differences between species Older biologists used to speculate about the number of genes differing between species. When electrophoresis gave the possibility of scoring the alleles present at a large number of loci, some workers attempted to devise a quantitative measure of genetic separation to define sub-species, semi-species, species, genera, etc. This approach has failed utterly, because two species may be different at only a few loci directly producing reproductive isolation, or very many which involve sterility incidentally. In the same way, chromosome number and total arm length may vary little in a group, or enormously between two closely-related species. For example, all the living hominoids (chimpanzee, gorilla, orang, and man) have very similar karyotypes, the main difference being that a centric fusion has occurred to produce a metacentric chromosome in man {In = 46) which is absent in the other three {In = 48); in contrast a race of Madagascan lemur {Lemur macaco collaris) has at least four different karyotypes in different parts of its range (2л = 60, two different forms of 52, and 48); the Indian muntjac {Muntiacus muntjac) has 2л = 7 in the 2л = 6 in the Ç, while its close relative, the Chinese muntjac (M reevesi) has 2л = 46. Gene and chromosome studies lead to the same conclusion as investigations of molecular change (section 3.2), that evolution is not a product of linear and constant adaptation, but proceeds by fits and starts; just as the environment may change in both the short and long term and just as selection is an inconstant force, so evolution itself varies in direction, rate, and amount at different times. 6.3 Allometry Haeckel's theory of recapitulation argued that evolution takes place by adding new stages at the end of normal development, so that an individual repeats the adult stages of its ancestors before reaching maturity (summarized by the aphorism that 'every species climbs its own phylogenetic tree'). This