Hybrids and Phylogenetic Systematics Ii. The Impact of Hybrids on Cladistic Analysis

Abstract

I examined three aspects of the cladistic treatment of a set of 17 F hybrids of known parental origin: (1) impact of hybrids on consistency index (CI) and number of most parsimonious trees (Trees), (2) placement of hybrids in cladograms, and (3) impact of hybrids on hypotheses of relationship among species. The hybrids were added singly and in randomly selected sets of two to five to a data set composed of Central American species of Aphelandra (including the parents of all hybrids). Compared to analyses with the same number of OTUs all of which were species, the analyses with hybrids yielded results with significantly higher CI. There was no difference in Trees between analyses with hybrids versus species. There was thus no evidence that hybrids would appear to be more problematic for cladistic methods than species. Accordingly, hybrids will not be readily identifiable as taxa that cause marked change in these indices. About % of the hybrids were placed as the cladistically basal members of the lineage that included the most apomorphic parent. Relatively apomorphic hybrids were placed proximate to the most derived parent (ca. 13% of hybrids). Other placements occurred more rarely. The most frequent placements of hybrids thus did not distinguish them from normal intermediate or apomorphic taxa. When analyses with hybrids yielded multiple most parsimonious trees, these were no more different from each other than were the equally parsimonious trees that resulted from analyses with species. Most analyses with one or two hybrids resulted in minor or no change in topology. When hybrids caused topological change, they frequently caused rearrangements of weakly supported portions of the cladogram that did not include their parents. When they disrupted the cladistic placement of their parents, they often caused their parents to change positions, with at least one topology bringing the parental lineages into closer proximity with the hybrid placed between them. Hybrids between parents from the two main lineages of the group caused total cladistic restructuring. In fact, the degree of relationship between a hybrid's parents (measured by both cladistic and patristic distance) was strongly correlated with CI (negatively) and with the degree of disturbance to cladistic relationships (positively). Thus, hybrids between distantly related parents resulted in cladograms with low CI and major topological changes. This study suggests that hybrids are unlikely to cause breakdown of cladistic structure unless they are between distantly related parents. However, these results also indicate that cladistics may not be specially useful in distinguishing hybrids from normal taxa. The applicability of these results to other kinds of hybrids is examined and the likely cladistic treatment of hybrids using other sources of data is discussed.

Promiscuous Speciation with Gene Flow in Silverside Fish Genus Odontesthes (Atheriniformes, Atherinopsidae) from South Western Atlantic Ocean Basins

“…This could be interpreted as a hard polytomy and it would be explained by several hypothetic differentiation scenarios such as, the existence of shared ancestral polymorphisms, incomplete lineage sorting in radiating speciation process and/or reticulation events involving these taxa. Our results may be congruent with some recent events of hybridization since hybrid individuals tend to collapse in a basal polytomy when included in a cladistic analysis [59].…”

Molecular evidence for bicontinental hybridogenous genomic constitution in Lepidium sensu stricto (Brassicaceae) species from Australia and New Zealand

“…The inclusion of hybrids in cladistic analyses may effect tree topology (McDade, 1992), e.g., as a consequence of recombination between divergent parental ITS types (Baldwin and Sanderson, 1998). All Australian species of presumed hybrid origin were included in the final data set because the exclusion of these species in initial analyses had no effect on topology of the remaining species.…”

Can Extensive Reticulation and Concerted Evolution Result in a Cladistically Structured Molecular Data Set?

“…As expected, these results same survey also correspond to ribosomal DNA. contrast with McDade's (1992) study of the impact of Therefore, it is apparent that low homoplasy levels are F 1 hybrids in cladistic analyses, the primary reason the rule when analyzing nrDNA sequences and that being that morphological characters, on which her homogenization and fixation of repeat types within analysis is based, lack homogenization mechanisms reproductive groups may contribute to them (Hillis and are subject to different modes of inheritance.…”