Further evidence for coadaptation in crosses between geographic populations ofDrosophila pseudoobscura

Abstract

Body size inDrosophila pseudoobscurais a continuously varying character with a high heritability; it is almost certainly related to fitness. Natural populations ofD. pseudoobscurafrom Canada to Mexico have been sampled and found to vary geographically in body size. The geographic variation for the genes determining size is to some extent correlated with the physiographic division of the West. The populations from the Pacific coast have genetically smaller flies than do those from the interior provinces. Experimental populations derived from the samples of seven widely separated natural populations were crossed to yieldF₁andF₂hybrid generations. Body size in theF₁'s varied irregularly, while theF₂'s showed a consistent ‘breakdown’, theF₂'s being significantly smaller than theirF₁parents. TheF₁'s were significantly less variable than their parents, while theF₂'s were significantly more variable than their parents of theF₁generation. The natural populations possess coadapted genetic systems, with genes mutually adjusted by selection for favorable interactions. Recombination disrupted the balanced genic complexes to give theF₂breakdown and the increasedF₂variability.D. pseudoobscuradiffers fromD. subobscurain showing the effects expected in crosses between coadapted systems. This species difference lends additional support to the hypothesis that the gene pools of these two successful species respond in different ways to environmental variation. The gene pool ofD. pseudoobscurais flexible and changes readily, while that ofD. subobscurais relatively rigid.