The secondary metabolites of plants are one manifestation of genetic diversity. Although the metabolic pathways leading to the commoner plant secondary compounds are known, very little is known about the control of the expression of these pathways. The absence of the expression of any pathway cannot be taken to indicate that the necessary genes are absent, and it is not possible to know what genetic potential is being lost when any species is driven to extinction.
Bradshaw (1991) recalls the confidence of Charles Darwin in his theory of natural selection 'acting during long ages and rigidly scrutinising the whole constitution, structure and habits of each creature, favouring the good and rejecting the bad. I can see no limit to this power . . .' and asks whether, in the enthusiasm to demonstrate the successes of natural selection in action, enough attention has been given to the failures of evolution and to the limits of its power. He asserts that there is no reason for believing that the processes supplying genetic variation are omnipotent and capable of supplying whatever is needed, and that restriction of supply seems more likely. This is supported by a wide variety of evidence from both natural and artificial populations. In evolutionary time, maybe 100 times more species have become extinct than exist at present. Moreover, most of the time little or no evolution is occurring: the stability of most species and populations in both the short and long term is a dominant characteristic of the living world. Both stability and failure must be fitted into a Darwinian view of the world, assuming Darwin was essentially correct.
Endler and McLellan (1988) suggest that the process of evolution is two-step: (1) the exploitation of existing variation, usually immediate, fast and predictable in response to new ecological pressures and opportunities, and (2) the dependence on new variation, occurring through random mutations over long periods of time. Selection processes acting on both of these could produce the pattern of punctuated equilibrium which is observed over geological time: bursts of evolutionary change followed by long periods of stasis.
The understanding and maintenance of existing variation is important at the present time for several reasons. One is the re-introduction into crop species of defensive traits, many of which were deliberately bred out by our ancestors. There are many examples of where genes from wild relatives have been successfully incorporated into a crop species to improve its natural pest resistance and where some species have adapted to polluted areas but others have not (Bradshaw, 1991). More important is the retention of the potential to adapt in the face of new climatic upheavals which some scientists predict will occur through global warming. If the gene pool continues to be eroded, large-scale extinctions of species may occur.
Secondary chemicals have been important since the beginning of life on earth. The elimination of large sections of the gene pool by which they are generated may have serious consequences for life as we know it.
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