Abstract
Hybrid zones can yield estimates of natural selection and gene flow. The width of a cline in gene frequency is approximately proportional to gene flow (σ) divided by the square root of per-locus selection ( &s). Gene flow also causes gametic correlations (linkage disequilibria) between genes that differ across hybrid zones. Correlations are stronger when the hybrid zone is narrow, and rise to a maximum roughly equal to s. Thus cline width and gametic correlations combine to give estimates of gene flow and selection. These indirect measures of σ and s are especially useful because they can be made from collections, and require no field experiments. The method was applied to hybrid zones between color pattern races in a pair of Peruvian Heliconius butterfly species. The species are Mullerian mimics of one another, and both show the same changes in warning color pattern across their respective hybrid zones. The expectations of cline width and gametic correlation were generated using simulations of clines stabilized by strong frequency-dependent selection. In the hybrid zone in Heliconius erato, clines at three major color pattern loci were between 8.5 and 10.2 km wide, and the pairwise gametic correlations peaked at R & 0.35. These measures suggest that s & 0.23 per locus, and that σ & 2.6 km. In erato, the shapes of the clines agreed with that expected on the basis of dominance. Heliconius melpomene has a nearly coincident hybrid zone. In this species, cline widths at four major color pattern loci varied between 11.7 and 13.4 km. Pairwise gametic correlations peaked near R & 1.00 for tightly linked genes, and at R & 0.40 for unlinked genes, giving s & 0.25 per locus and σ & 3.7 km. In melpomene, cline shapes did not perfectly fit theoretical shapes based on dominance; this deviation might be explained by long-distance migration and/or strong epistasis. Compared with erato, sample sizes in melpomene are lower and the genetics of its color patterns are less well understood. In spite of these problems, selection and gene flow are clearly of the same order of magnitude in the two species. The relatively high per locus selection coefficients agree with ``major gene'' theories for the evolution of Mullerian mimicry, but the genetic architecture of the color patterns does not. These results show that the genetics and evolution of mimicry are still only sketchily understood.
Full Text
The Full Text of this article is available as a PDF (3.5 MB).
Selected References
These references are in PubMed. This may not be the complete list of references from this article.
- Barton N. H. The effects of linkage and density-dependent regulation on gene flow. Heredity (Edinb) 1986 Dec;57(Pt 3):415–426. doi: 10.1038/hdy.1986.142. [DOI] [PubMed] [Google Scholar]
- Benson W. W. Natural Selection for Miillerian Mimicry in Heliconius erato in Costa Rica. Science. 1972 May 26;176(4037):936–939. doi: 10.1126/science.176.4037.936. [DOI] [PubMed] [Google Scholar]
- Hedrick P. W. Gametic disequilibrium measures: proceed with caution. Genetics. 1987 Oct;117(2):331–341. doi: 10.1093/genetics/117.2.331. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Hill W. G. Estimation of linkage disequilibrium in randomly mating populations. Heredity (Edinb) 1974 Oct;33(2):229–239. doi: 10.1038/hdy.1974.89. [DOI] [PubMed] [Google Scholar]
- Jones J. S., Bryant S. H., Lewontin R. C., Moore J. A., Prout T. Gene flow and the geographical distribution of a molecular polymorphism in Drosophila pseudoobscura. Genetics. 1981 May;98(1):157–178. doi: 10.1093/genetics/98.1.157. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Mallet J., Barton N. Inference from clines stabilized by frequency-dependent selection. Genetics. 1989 Aug;122(4):967–976. doi: 10.1093/genetics/122.4.967. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Slatkin M. Gene flow and the geographic structure of natural populations. Science. 1987 May 15;236(4803):787–792. doi: 10.1126/science.3576198. [DOI] [PubMed] [Google Scholar]