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Division of Biological Sciences, Section of Molecular and Cellular Biology, University of California, Davis, California 95616
Received for publication October 15, 1998. Accepted for publication April 20, 1999.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONLITERATURE CITED |
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Key Words: plant evolution
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONLITERATURE CITED |
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, Arnold (1997)
, and Weiner (1995)
. I have been really surprised at the similarity between the ideas set forth by these authors and the ideas that have been running through my own mind during the last 20 years. My own version, influenced greatly by these authors, is as follows.
Evolution is a two-step process that requires initial genetic variation within populations as well as the molding of this variation via gene recombination and natural selection. Neither the genetic structure of populations nor the molding by natural selection is dominant. The two situations are interdependent and of equal importance. Natural selection was strongly emphasized in the mid-century synthesis but only on the basis of theory and logic rather than the hard facts that have emerged more recently. The contribution of natural selection to the present theory has been superbly reviewed by Weiner (1995)
with great and essential emphasis on the investigations by Peter and Rosemary Grant on Darwin's finches. These investigators have developed a technique of careful documentation of details that other authors had regarded as superficial. The detailed data demonstrated the activity of natural selection in the wild. In particular, the data showed that a variable climate such as the one experienced by the Galapagos during the past 20 years and the alternation between divergence and secondary contact (hybridization) during this time have been the keys to the evolutionary changes that they recorded. They have even reviewed for us a foil, such as those that dramatists introduce to enhance the significance of their main characters. This foil is the genetic structure and ecological situation of Darwin's finches, known collectively as Geospiza, on Cocos Island to the north of the Galapagos and halfway to Central America. On Cocos these finches have been blessed by an abundance of many kinds of food but restricted by the absence of other islands between which migrations might have occurred. The result is a population that is not only extremely variable but also variable in terms of adaptive radiation and different ways of food acquisition pursued by different lines of individuals within populations. We therefore now know that evolutionary change requires not only genetic differentiation but also the distribution of that variation by natural selection in ways that facilitate reproductive isolation and therefore the origin of geographic varieties, subspecies, and species.
All of this information is based on data derived from animals, whereas my own thinking and data are from plants. I must therefore present a plant evolutionist's point of view, which is based on profound differences in both genetic structure and adaptive variation between the two kingdoms. These differences are as follows. First, plants do not undergo segregation of the germplasm during embryogenesis. On the contrary, in a species like Sequoiadendron giganteum each individual has differentiated thousands of germ plasms during its thousands of years of existence and the same is true for long-lived herbaceous plants such as stoloniferous or rhizomatous sod grasses. This means that mutations occurring during the lifetime of an individual can be transmitted by a small proportion of its gametes. Second, most plants are hermaphroditic so that unless genetic inhibition to it arises, uniparental production, the maximum form of inbreeding, can occur and does so in a large percentage of weedy annuals. Third, the diversity of cell differentiation during development is much greater in animals than in plants. An adult animal contains 150–200 kinds of cells, while an adult plant does not contain more than 40 or 50 different kinds of cells, and often many fewer. These differences affect greatly the relationship between total genetic diversity and the origin and persistence of adaptive-varieties or species. The occurrence of hybrids between well-established species of either animals or plants is a rare event. However, the ability of self-pollination plus the long life and less diverse cellular structure in plants as compared to animals greatly increases the amount and diversity of differences among progeny of hybrids in the two groups. The most prevalent of diverse aftereffects in plants is polyploidy, including both allopolyploidy arising from wide interspecific crosses and autopolyploidy arising from crosses between closely related species or between different varieties or ecotypes within the same species. Of course, Anderson (1949)
demonstrated long ago that backcrossing or introgression, if it takes place in new or changing environments, is an important way of obtaining diversity in both animals and plants. However, it is more prominent in higher animals, particularly birds and mammals, because their developmental complexity renders polyploidy much more difficult to obtain. Therefore, the plant evolutionist must now adopt the technique of Peter and Rosemary Grant on Galapagos finches by conducting similar experiments with similar attention to what may seem unimportant morphological and other differences between species and genera of plants. This I believe should now be developed as the principal objective of research for plant evolutionists. Such research should combine morphological, physiological, and histological aspects with careful observation of natural populations as well as artificial populations established in gardens, greenhouses, and growth chambers. I recognize that the amount of time and money needed for such investigations may often be excessive; however, the need for carrying out such research may be much greater than we previously thought because of recent information about the effects of agricultural practices on cultivated varieties. For instance, Dr. Weiner has pointed out in his book that the wholesale application of DDT and other insecticides has caused evolution toward resistance to them. We do not know whether the same has been true of the application of herbicides to seedlings of crop plants. Is it a general truism that artificial application of insecticides and herbicides to populations of crop plants is not inducing the elimination of pests and weeds but rather their evolution toward resistance to human interference with their life cycles?
That this may be true not only for the effects of pesticides on crop plants but also the application of antibiotics to diseased humans is a major question raised by Dr. Weiner in his book. This possibility is real enough so that evolutionary knowledge should be applied to the questions raised. A major task of evolutionists concerned either with animals or with plants is to guide the human species toward preservation; major concerns include not only deteriorating sources of food but also the possible deterioration of our own bodies.
I wish you all success with your future research, whatever it may be, and shall follow it as closely as I can for as long as I can do so.
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FOOTNOTES |
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONLITERATURE CITED |
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Arnold, M. L. 1997 Natural hybridization and evolution. Oxford University Press, New York, NY.
Avise, J. C. 1994 Molecular markers, natural history and evolution. Chapman and Hall, New York, NY.
Weiner, J. 1995 The beak of the finch: a story of evolution in our lifetime. Knopf, New York, NY.
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