Abstract
Syngameons are sets of species linked by interspecific hybridization. Common observations regarding the structure of syngameons are that hybridization propensity is not uniform across species and that patterns of hybridization are dominated by a few species. I use computer simulations to test these claims in naturally occurring syngameons selected from the literature and from personal observation. Natural syngameons, especially those involving plants, typically exhibit nonrandom structure: The first three order statistics for the number of hybrid partners and the variance in the number of hybrid partners are larger than chance alone would predict. The structure of two insect syngameons examined is not significantly different from random. To test a hypothesis that variation in hybridization propensity across species in natural syngameons is simply an artifact of hybridization opportunity, I examine the structure of four artificial syngameons (fertility relationships) produced by full diallel crosses. Three of four artificial syngameons exhibit nonrandom structure, as the observed variation in number of successful crosses is larger than chance alone would predict. In general, there are no significant results involving the order statistics. Finally, I discuss biogeographic, ecological, and phylogenetic hypotheses for variation in hybridization propensity across species in natural syngameons.
Highlights
Interspecific hybridization is a common feature in eukaryotic evolution and it has important consequences to cladogenesis, species interactions, invasion dynamics, and conservation biology
Observed values of the order statistics and observed values of the standard deviations in number of hybrid partners were significantly larger than chance alone would predict (Table 1). (The first two order statistics for the Potamogeton syngameon were weakly significant, while the standard deviation in number of hybrid partners was highly significant.) The two insect syngameons did not exhibit nonrandom structure; none of the order statistics or standard deviations in the number of hybrid partners were significantly different from that expected by chance alone
I have demonstrated that patterns of hybridization within these syngameons are typically dominated by a few species
Summary
Interspecific hybridization is a common feature in eukaryotic evolution and it has important consequences to cladogenesis, species interactions, invasion dynamics, and conservation biology. Syngameons exhibit emergent properties; a common observation is that hybridization events are not distributed among species and that a few species dominate the pattern of hybridization. Consider the pattern of hybridization between southwestern white oaks as depicted by a network graph (Figure 1) – the set of hybridizations is dominated by three species: Quercus gambelli, Q. arizonica, and Q. grisea; and the number of hybrid partners ranges from 8 (Q. grisea) to 1 (Q. striatula). The appearance of a structured pattern of hybridization within a given syngameon may be an artifact of the number of species involved and the number of hybrid combinations. To reject chance in favor of more biologically interesting mechanisms producing patterns of hybridization within syngameons, it is necessary to enumerate all possible network graphs constrained by the observed number of species and hybrid combinations. The objective was to place observations regarding the structure of syngameons on a sound probabilistic foundation
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