CASE STUDY IN MOLECULAR
EVOLUTION NO. 6
Page 1: And the bands played on
When Darwin shook the western world with his theory of evolution (1), Mendel was still doing experiments. Darwin's concept of natural selection depended upon a renewable source of heritable variation and differential survival associated with that variation. The concepts of mutation and Mendelian genetics, well known to us, were unknown to Darwin even though he did have a rudimentary qualitative appreciation of heritable traits known as "sports." During the early decades of the 20th century genetics as a discipline grew, differentiated, and matured. Natural selection was redefined in mathematical terms associated with the relative fitness of different alleles and gene combinations. The resulting neo-Darwinism theory of evolution merged classical Darwinian ideas with those of genetics. It gained considerable acceptance through a number of influential books (2, 3, 4, 5) published before 1950. To the chagrin of paleontologists and taxonomists, evolution had become reduced to changes in gene frequency in the gene pool of a species. The assumptions of the Hardy-Weinberg equilibrium defined the sources of change (6), but always mutation and natural selection were considered important. At the dawn of the era of molecular genetics and biochemistry, the underlying mechanisms of evolution seemed well understood. In science, however, progress occurs when observations cannot be explained by theory.
In theory, if each allele has a distinctive fitness associated with it, then it is unlikely that two alleles will have exactly the same fitness. Consequently, selection will constantly favor alleles of higher fitness, making polymorphism (the coexistence of alleles at a locus) rare and probably transitory. This prevailing view (7) could not explain the extensive, yet arguably underestimated, polymorphism soon revealed by gel electrophoresis (8, 9, 10).
For initial group discussion: The figure below displays the electrophoretic patterns for Esterase E (the darkest set of bands across the middle of the figure) from 24 male orange sulfur butterflies, Colias eurytheme, collected in a single field on a single day in Austin, Texas (10a). The origin is at the bottom and migration in this non-denaturing gel is toward the anode at the top.
Complexity of Esterase E allelic variation in the Orange Sulfur Butterfly (10a)
What can you conclude from these data? What questions do you have?
1. Darwin, C. (1859) The Origin of Species.
2. Dobzhansky, T. (1937) Genetics and the Origin of Species 1st ed., Columbia University Press, New York. Also (1970) Genetics of the Evolutionary Process, Columbia University Press. (see 60th anniversary symposium by the National Academy of Science.)
3. Huxley, J. S. (1942) Evolution: The Modern Synthesis, Harper, New York.
4. Mayr, E. (1942) Systematics and the Origin of Species, Columbia University Press, New York. Also (1970) Populations, Species, and Evolution, Harvard University Press, Cambridge.
5. Simpson, G. G. (1949) The Meaning of Evolution, Yale University Press, New Haven.
6. Dobzhansky, T., Ayala, F. J., Stebbins, G. L. and Valentine, J. W. (1977) Evolution, W. H. Freeman and Co., San Francisco (see Chapter 4).
7. Simpson, G. G. (1964) Organisms and molecules in evolution, Science 146, 1535-1538.
8. Hubby, J. H. and Lewontin, R. C. (1966) A molecular approach to the study of genic heterozygosity in natural populations. I. The number of alleles at different loci in Drosophila pseudoobscura, Genetics 54, 577-594.
9. Lewontin, R. C. and Hubby, J. H. (1966) A molecular approach to the study of genic heterozygosity in natural populations. II. Amount of variation and degree of heterozygosity in natural populations of Drosophila pseudoobscura, Genetics 54, 595-609.
10. Harris, H. (1966) Enzyme polymorphisms in man, Proc. Roy. Soc. Ser. B. 164, 298-310.
10a. Burns, J. M. and Johnson, F. M. (1967) Esterase polymorphism in natural populations of a sulfur butterfly, Colias eurytheme. Science 156, 93 - 97.