of known fitness differences among the lines, whereas population growth rates at high densities were not.

One of the strong points of estimating fitnesses from life history tables is the focus it gives to female fecundity and, in particular, to the age at which offspring are produced. Unfortunately, our life history tables do not include male mating success, and this omission is a weakness of our approach. We can imagine experiments in which a life history table of longevity and male reproductive success would be generated and then utilized with the female life history table to generate fitness estimates. Such experiments would be difficult to carry out. We believe that the analysis we have presented, although not complete, is a step in the right direction of a demographic approach to fitness and to natural selection.

We thank Drs. Richard Nickerson and Marvin Druger for generously providing us their l(x) and m(x) data on AR/AR, AR/CH, and CH/CH karyotypes, and Dr. Margaret Anderson for help with the calculations. We thank Drs. Daniel Promislow, Mohamed Noor, and Laurence Mueller for their comments on the manuscript, and Dr. Brian Charlesworth for comments on an earlier version.

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