Ice Age-Influenced Avian Speciation in North America

The evolution of a new species requires geographical or ecological isolation from its parent species.  In nature this can occur in any number of ways–the uplifting of a mountain chain, a rise in sea level that creates an island, climate-induced ecological change, individual differences in foraging preferences within a population., etc.  Scientists have even created new species of bacteria, fruit flies, and worms in the lab by isolating populations of these organisms from their parent populations.  After many generations of reproductive isolation, the new species will not or can not successfully mate with members of their parent population when reintroduced.  In North America there are many examples of eastern and western bird species that are so similar it’s obvious they evolved from a single common ancestor.  The geographical barrier that isolated eastern and western avian populations is well known.

During the Miocene forested environments covered most of the North American continent.  But early during the Pliocene, about 5 million years ago, Ice Ages began to occur.  The middle of the continent became desert grassland that was unsuitable habitat for forest birds, thus isolating eastern populations from western populations.  In 3 cases, this isolation resulted in subspecific differences.  The common flicker (Colaptes auratus) includes the yellow shafted eastern form and the red shafted western form.  The yellow rumped warbler (Dendroica coranata) is split into the eastern myrtle warbler and the western Audubon’s warbler.  The dark-eyed junco (Junco hyeanalis) includes the eastern slate-colored and the western Oregon.  Ornithologists formerly thought the eastern and western forms of these 3 were separate species, but it was discovered they freely interbreed in regions where their populations overlap, despite differences in physical characteristics.

The common flicker. Unlike most woodpeckers, it forages on the ground, feeding on ants and beetles, rather than pecking wood for insect borers.  There are always a few of these nesting in my neighborhood during spring and summer.  The eastern and western forms of this species were formerly thought to be different species.  Geographical isolation resulted in subspecific differences but not complete speciation.

There are at least 22 examples of eastern species of birds with a similar but distinct western counterpart including eastern and western peewees, eastern and western bluebirds, scarlet and western tanagers, eastern and western screech owls, among many others.  These birds are considered distinct species.

Eastern bluebird.  I also see a few of these in my neighborhood during spring and summer.

Range maps of eastern and western bluebirds.  Both species descend from 1 common ancestral species that formerly occurred across the continent before Ice Ages caused unsuitable desert grassland habitat to replace forested habitat in the middle of the continent.

During the present interglacial, forested habitat is once again becoming more widespread.  Moreover, man often plants trees in region that were once prairie, and he suppresses natural fires necessary for the development of grasslands.  As a result, similar eastern and western species of birds are expanding their ranges and occasionally, they come into contact and hybridize.  Hybridization is another mechanism that plays a role in the evolution of new species.  Similar species that were once reproductively isolated but come into contact again may backcross and evolve into yet another new species.  An estimated 9% of bird species are known to hybridize with other bird species.  (Homo sapiens may be a hybrid species.  DNA evidence suggests some Eurasian Homo sapiens have some Homo neanderthalis ancestry.)

Eastern and Western Screech Owl hybrid.  These 2 species were isolated during Ice Ages by a desert grassland barrier.  Now, they are both expanding their ranges and reuniting.  Hybridization is another mechanism that can result in new species.

Hybridization can endanger a species through genetic swamping.  Barred owls (Strix varia) are expanding their range into the habitat of northern spotted owls (Strix occidentalis).  Environmentalists saved spotted owls from clear-cutting, but within the last 30 years, barred owls have successfully invaded the Pacific northwest.  Barred owls usually just eat the smaller spotted owls, but they also mate with them–47 hybrids have been reported.

Ice Age glaciers also provided a barrier that geographically isolated 7 ancestral populations of birds, resulting in 14 species.  Glacial barriers split northern shrikes from loggerhead shrikes, Bohemian waxwings from cedar waxwings, 3-toed woodpeckers from black- backed 3-toed woodpeckers, boreal owls from saw-whet owls, black billed magpies from yellow-billed magpies, northern goshawks from Cooper’s hawks, and boreal chickadees from black-capped chickadees.  During Ice Ages, northern shrikes, Bohemian waxwings, 3-toed woodpeckers, boreal owls, black-billed magpies, northern goshawks, and boreal chickadees found refuge in Beringia and Eurasia, while loggerhead shrikes, cedar waxwings, black-backed 3-toed woodpeckers, saw-whet owls, Cooper’s hawks, and black-capped chickadees lived in North America south of the glaciers.

Top: Loggerhead shrike (Larius ludovicianus).  Bottom: Northern shrike (Larius excubitor).  They both descend from 1 common species with a circumpolar distribution.  Ice Age glaciers separated this ancestral population, resulting in 2 distinct species.  Northern shrikes usually have gray over their bills, while loggerhead shrikes usually have black over their bills.  The former also have larger bills.

Since the end of the Ice Age, the Eurasian species mentioned above have recolonized much of Canada but maintain separate breeding grounds from their American sister species.

References:

Newton, Ian

Speciation and the Biogeography of Birds

Elselvier Science 2003

Pielou, E.C.

After the Ice Age

The University of Chicago Press 1991

Note: * I discovered plagiarism in the book written by Ian Newton.  He plagiarized a passage from After the Ice Age and didn’t even cite that work in his book.*  Wow! What lazy scholarship.  He didn’t even bother to put the original awkwardly written passage in his own words.

Tags: evolution, geographical isolation, hybridization, North American avian speciation, reproductive isolation