Patterns of geographic variation and evolution in polytypic butterflies

Patterns of geographic variation and evolution are examined in polytypic butterflies. It is concluded that genetic and evolutionary cohesion at the full species level is limited to monotypic organisms that are strongly vagile or migratory. Polytypic species with a fragmented population structure lack such cohesion, and each isolated population tends to function as an independent evolutionary unit. Taxonomic subspecies are usually the products of geographic isolation and differentiation, and represent separate phylogenetic lineages. Secondary intergradation between differentiated populations takes place in hybrid suture zones that often correlate with past geographic isolating barriers. Vicariance is a common phenomenon at the subspecific level in polytypic butterflies. Older populations survive as relicts in disjunct refugia, and are frequently surrounded by newer populations with more recently expanded distributions. There is no clear-cut distinction between a subspecies and a full species. A complete continuum is observed among differentiated populations with regard to reproductive isolating mechanisms and genetic or ecological compatibility. Speciation is strictly a secondary process that may or may not result from the primary process of macroevolution, the geographic differentiation of populations adapting to diverse environmental conditions. It is also suggested that macroevolution is usually characterized by peripatric cladogenesis and punctuated equilibria. As a consequence, extant insect populations have the dimension of time and ancestor-descendant relationships.