Table 1.

Hypotheses for the primary phylogenetic origin of ecological diversity in Western Palaearctic birds

Hypothesis	Potential processes	Potential supporting evidence
Climate occupation divergence	Global climate cycling and geological processes separate breeding populations geographically (vicariance); speciation occurs with adaptation to regional climate	Climate niche has the greatest disparity, smallest phylogenetic signal and greatest divergence from a Brownian evolutionary model
Habitat divergence	Interspecific competition occurs as species occupy habitat used for feeding and reproduction, causing habitat use to diverge as in ecological character displacement	Habitat niche has the greatest disparity, smallest phylogenetic signal and greatest divergence from a Brownian model
Trophic divergence	Interspecific competition occurs for food resources, resulting in character and food preference displacement between closely related species	Trophic niche has the greatest disparity, smallest phylogenetic signal and greatest divergence from a Brownian model
Niche conservatism	Lineages gradually diverge due to responses of ecological traits to selection that is random in direction, and uncorrelated temporally and among species	All three niches show similar levels of disparity and phylogenetic signal, both of which are consistent with a Brownian model of evolutionary divergence; trait divergence is consistent with evolutionary gradualism