consider 6 species of woodpeckers that nest around Chapel Hill

• Melanerpes erythrocephalus   Red-headed Woodpecker
• Melanerpes carolinus   Red-bellied Woodpecker
• Picoides pubescens   Downy Woodpecker
• Picoides villosus   Hairy Woodpecker
• Colaptes auratus   Northern Flicker
• Dryocopus pileatus   Pileated Woodpecker

they also do not interbreed (how do we know?)

first two (genus Melanerpes) differ in patterns of coloration -- second two (genus Picoides) differ mostly in size (especially of the beak), feeding behavior, and habitat

differences between sympatric species in morphology and behavior are often related to differences in ecology and communication

for 200 years there has been no question that the woodpeckers occurring in Orange County are six separate species -- in contrast, the number of genera is not so easily determined -- the genus Melanerpes has often been split into two -- with erythrocephalus and carolinus in two separate genera -- although everyone agrees these two species are more closely related than either one is to any of the other species

so we face an important question -- why is there less agreement about genera than about species?

to see why, consider the problem of classifying species into genera and other higher categories (families, orders, and so forth)

this biological classification is

• hierarchical -- species inside genera, genera inside families, and so forth (ever since Linnaeus in 1758)
• phylogenetic -- with categories that match the branching pattern in evolutionary lineages -- so each taxon (cateogry) includes species that are more closely related to each other than to any other species (ever since Darwin 1859)

consequently, to classify our woodpeckers into genera, we must first identify the probable branching pattern during their evolution (their phylogenetic relationships)

evidence for phylogenetic relationships sometimes comes from the fossil record but usually from similarities of morphology, behavior, proteins, or DNA

not all similarities indicate close phylogenetic relationships -- only those that are restricted to a group of species because the characters first appeared in their most recent common ancestor -- such characters are called shared derived characters

ornithologists now agree on the probable phylogeny (evolutionary branching) of our six woodpeckers (and their relatives in other areas) -- also all agree that a genus is a group of species that are on the same evolutionary branch (are each other's closest relatives) -- but they do not always agree on how large a branch to include in one genus

in other words, how much similarity should there be between two species in the same genus? -- "splitters" and "lumpers" disagree -- "splitters" feel that congeneric species (in the same genus) should not differ much -- "lumpers" accept bigger differences -- it does not matter whether you study morphology, behavior, or genes -- there is no objective way to decide whether a group of closely related species is one genus or two

the same problem applies to all higher categories (genus, family ...)

in contrast, there is little disagreement about sympatric species, because . . .

sympatric species do not usually interbreed

notice that sympatric species have phylogenetic relationships because they do not normally interbreed -- if interbreeding often occurred between sympatric species, the result would be many intermediates or even a single variable population