Another case of large mammal evolution by introgressive hybridization:

Coyote + wolf = new breed of predator
New DNA evidence reveals that coyotes have bred with wolves in the the northeastern United States, turning mice-eating coyotes into much larger animals with a hunger for big prey, such as deer.

The linked article describes a study by Roland Kays and others, who went looking at the mtDNA of nearly 700 museum coyotes and donated specimens, looking for wolf. When they found it, they were able to correlate skeletal measurements with wolf ancestry and trace the progress of the wolf introgression from somewhere "north of the Great Lakes".

They hit on some essential points:

"This is an evolutionary mechanism to generate new variation that can work faster than genetic mutation," added Kays, curator of mammals at the New York State Museum.

If there's a niche for a more solitary canid predator, larger and more aggressive than coyotes, bringing in wolf genes may be the fastest way to get there.

This is a dynamic system, caught in mid-transition. The coywolves have not reached any stable equilibrium distribution with coyotes, and the wolf chromosomes have not broken up within the coywolf descendants to allow the dissociation of adaptive from maladaptive wolf genes in the hybrid population. Over the long term, the wolf mtDNA might be lost entirely, if there are any negative epistases between wolf mitochondrial genes and the mostly nuclear coyote genes that interact with them. Assessment of hybridity by mtDNA is conservative (misses hybrids with paternal instead of maternal origin) and temporary (the purely maternal lineage may disappear over evolutionary time due to slightly negative epistasis).

Other: "Adaptive introgression of coat color in wolves"

UPDATE (2009-09-24): See also in the mailbag.