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Systematics and evolutionary history

The Picidae are just one of nine living families in the order Piciformes. Other members of this group, such as the jacamars, puffbirds, barbets, toucans, and honeyguides, have traditionally been thought to be closely related to the woodpecker family (true woodpeckers, piculets, wrynecks, and sapsuckers). The clade Pici (woodpeckers, barbets, toucans, and honeyguides) is well supported and shares a zygodactyl foot with the Galbuli (puffbirds and jacamars). More recently, several DNA sequence analyses have confirmed that Pici and Galbuli are sister groups.

The phylogenetic relationship between the woodpeckers and the eight other families in the order Piciformes is shown in the cladogram below. The number of species in each family is taken from the list maintained by Frank Gill, Pamela C. Rasmussen and David Donsker on behalf of the International Ornithological Committee (IOC).

The name Picidae for the family was introduced by English zoologist William Elford Leach in a guide to the contents of the British Museum published in 1819. The phylogeny has been updated according to new knowledge about convergence patterns and evolutionary history. Most notably, the relationship of the Picinae genera has been largely clarified, and the Antillean piculet was found to be a surviving offshoot of protowoodpeckers. Genetic analysis supports the monophyly of the Picidae, which seem to have originated in the Old World, but the geographic origins of the Picinae is unclear. The Picumninae are returned as paraphyletic. Morphological and behavioural characters, in addition to DNA evidence, highlights genus Hemicircus as the sister group of all remaining true woodpeckers, besides a sister-group relationship between the true woodpecker tribes Dendropicini and Malarpicini.

The evolutionary history of this group is not well documented, but the known fossils allow some preliminary conclusions; the earliest known modern picids were piculet-like forms of the Late Oligocene, about 25 million years ago (Mya). By that time, however, the group was already present in the Americas and Europe, and they actually may have evolved much earlier, maybe as early as the Early Eocene (50 Mya). The modern subfamilies appear to be rather young by comparison; until the mid- Miocene (10–15 Mya), all picids seem to have been small or mid-sized birds similar to a mixture between a piculet and a wryneck. A feather enclosed in fossil amber from the Dominican Republic, dated to about 25 Mya, however, seems to indicate that the Nesoctitinae were already a distinct lineage by then.

Stepwise adaptations for drilling, tapping, and climbing head first on vertical surfaces have been suggested. The last common ancestor of woodpeckers (Picidae) was incapable of climbing up tree trunks or excavating nest cavities by drilling with its beak. The first adaptations for drilling (including reinforced rhamphotheca, frontal overhang, and processus dorsalis pterygoidei) evolved in the ancestral lineage of piculets and true woodpeckers. Additional adaptations for drilling and tapping (enlarged condylus lateralis of the quadrate and fused lower mandible) have evolved in the ancestral lineage of true woodpeckers (Hemicircus excepting). The inner rectrix pairs became stiffened, and the pygostyle lamina was enlarged in the ancestral lineage of true woodpeckers (Hemicircus included), which facilitated climbing head first up tree limbs. Genus Hemicircus excepting, the tail feathers were further transformed for specialized support, the pygostyle disc became greatly enlarged, and the ectropodactyl toe arrangement evolved. These latter characters may have facilitated enormous increases in body size in some lineages.

Prehistoric representatives of the extant Picidae genera are treated in the genus articles. An enigmatic form based on a coracoid, found in Pliocene deposits of New Providence in the Bahamas, has been described as Bathoceleus hyphalus and probably also is a woodpecker.

The following cladogram is based on the comprehensive molecular phylogenetic study of the woodpeckers published in 2017 together with the list of bird species maintained by Frank Gill, Pamela Rasmussen and David Donsker on behalf of the International Ornithological Committee (IOC). The Cuban green woodpecker in the monotypic genus Xiphidiopicus was not included in the study. The relative positions of Picumninae, Sasiinae and Picinae in the cladogram are uncertain. In the 2017 study the results depended upon which of two different statistical procedures were used to analyse the DNA sequence data. One method found that Sasiinae was sister to Picinae (as shown below), the other method found that Sasiinae was sister to a clade containing both Picumninae and Picinae.