Migration: From Demes to Species...and back again?

The Family Cervidae includes a variety of species we commonly call deer, elk, and moose.

Deer first appear in the fossil record in Mongolia about 10-20 million years ago.
Ancient deer crossed the (now vanished) land bridge between Asia and Alaska, populating North America.

About a million years ago, genus Odocoileus diverged into two sister species we recognize today as

The two species are distinguishable via several characters, but we will focus on only three.

The two species have been separated long enough for these traits to evolve independently in each species.
But have they been separated long enough to be reproductively isolated from each other?

Limited hybridization has been documented in areas where the two species' current ranges overlap.

Hybrids and Darwinian Fitness

In 1992, Susan Lingle of the University of Calgary published her study
Escape Gaits of White-Tailed Deer, Mule Deer and Their Hybrids: Gaits Observed and Patterns of Limb Coordination..

Here is an illustration from that publication.

Lingle hypothesized that the presence of large predators has helped to maintain the reproductive isolation and habitat segregation between Odocoileus hemionus and Odocoileus virginianus.

Cool, right?

  • A hybrid zone is an area of secondary contact between two related species, where limited hybridization is taking place between them.

  • Introgression is the introduction of alleles from one species' gene pool into that of another (closely related) species, due to limited hybridization.

    You may have noticed that I haven't said anything about antlers or foreheads.
    In fact, hybrid males tend to have Mule-deer like antlers, and look similar to Mule Deer, overall.

    Which brings us to...

    Bambi and That Other Guy

    (Or: Why You Should Never Go to the Movies with a Biologist)

    In 1942, Walt Disney Studios produced the classic animated film Bambi.
    It was based on the book of the same name written by Austrian author Felix Salten.

    In Salten's book, Bambi and his family were Roe Deer (Capreolus capreolus), native to Salten's Austria.

    But Disney studios is located in Southern California, where the native deer are Mule Deer.

    Disney was famous for his attention to detail and insistence that animated characters move naturally and realistically.

    To be sure his animators knew how real deer behaved and moved, he brought White-tailed Deer (from Maine) into the studio for studies, and the animators also took field trips to local deer farms to study the native Mule Deer.

    The result? Here are Bambi and his father. Note the antler and forehead field marks.
    (A field mark (a.k.a. "differential" or "diagnostic" character) is a morphological trait used to correctly identify a species.)

    Still not convinced?
    In one scene, Bambi is enthralled by some male relatives.

    Although Disney called this scene "The Gallop of the Stags" the bucks are not galloping. They are stotting.

    (stotting appears at 00:43 in the video)

    Disney has transformed Bambi into a Mule Deer.

    Unfortunately, the animators' study of both White-tailed Deer and Mule Deer created some Hollywood confusion when it came time for Bambi's mating season.

    If you've seen the movie, you probably remember when grown-up Bambi met grown-up Faline (a doe he had known as a fawn), and he fell so in love he became "twitterpated".

    The romantic dream was rudely interrupted when Bambi was challenged by another buck ("Ronno") for Faline's affections.

    But wait. Examine the rival's antlers.

    And you thought learning about speciation wouldn't have any practical applications.

    Hybrid Speciation

    When closely related species hybridize, several outcomes are possible.

    Recall the example of Tragopogon.

    Hybridization in Tephritid Fruit Flies

    Rhagoletis is a genus of common agricultural pests that lay eggs in many important crops, from apples to blueberries.

    Two species of tephritid fruit flies are known to occasionally hybridize..

    And finally, back to Our Friends the Deer.