Using fluctuating bilateral asymmetry as a measure of developmental stability, we tested the hypothesis that genomic coadaptation mediates developmental stability in natural populations. Hybrid populations were more asymmetrical than populations of the parental species, and ranks of overall developmental instability were positively correlated with ranks of mean heterozygosity in these populations. The failure to find increased asymmetry in previous studies of natural hybrid populations (Jackson, 1973a, 1973b; Felley, 1980) suggests that such populations may have re-evolved coadapted genomes. Increased asymmetry in hybrid Enneacanthus populations may reflect the youthfulness of these populations.

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Using fluctuating bilateral asymmetry as a measure of developmental stability, we tested the hypothesis that genomic coadaptation mediates developmental stability in natural populations. Hybrid populations were more asymmetrical than populations of the parental species, and ranks of overall developmental instability were positively correlated with ranks of mean heterozygosity in these populations. The failure to find increased asymmetry in previous studies of natural hybrid populations (Jackson, 1973a, 1973b; Felley, 1980) suggests that such populations may have re‐evolved coadapted genomes. Increased asymmetry in hybrid Enneacanthus populations may reflect the youthfulness of these populations.

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The effects of heated effluents on morphology of fishes in natural populations were investigated. Largemouth bass (Micropterus salmoides), bluegills (Lepomis macrochirus), and redbreast sunfish (Lepomis auritus) were sampled in bodies of water that had never experienced thermal effluents, in those that were currently receiving heated effluents, and in those that had received heated effluents in the past, but received them no longer. Differences in means between heated and ambient temperature locations were seen only in two meristic characters in the redbreast sunfish. Differences in amounts of bilateral asymmetry (hypothesized to be a measure of developmental homeostasis) were not demonstrated between populations in heated and ambient‐temperature locations. However, fish from a pond heavily contaminated with mercury had higher levels of asymmetry than fish from the other populations sampled.

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