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
White-tailed Deer (Odocoileus virginianus) and
Black-tailed or Mule Deer (Odocoileus hemionus)
The two species are distinguishable via several characters, but we will focus on only three.
White-tailed bucks have curving, "candelabra" shaped antlers (left)
Mule deer bucksmales have branching, bifurcating antlers (right)
(Note: A male deer is called a buck or stag. A female is called a doe.)
White-tailed Deer have foreheads the same brown as the rest of the face.
Mule Deer have foreheads distinctively darker than the rest of the face, even at birth.
White-tailed Deer, which (putatively) evolved in flat, lowland habitats, GALLOP to escape a predator.
In galloping, at least one foot is always touching the ground.
On flat terrain, a healthy deer can easily outrun a chasing predator.
Mule Deer, which (putatively) evolved in mountainous habitats, STOT to escape a predator.
In stotting, all four feet strike and then leave the ground together.
On hilly terrain, predators cannot outrun a healthy mule deer able to stot across a small ravine or gully in a single bound.
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?
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.
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.
When closely related species hybridize, several outcomes are possible.
species reinforcement - hybrids have lower fitness than either parent species. Reproductive isolation is maintained due to lack of hybrid survival/reproduction.
species fusion - two related species in a hybrid zone may have weak reproductive isolating barriers. Over time, the two species gene pools essesntially become one, and they may no longer be recognizable as two biological species.
species stability/hybrid equilibrium - Hybrids are continually produced by the two parent populations in a hybrid zone. A narrow hybrid zone can foster constant hybridization with reduced hybrid survival (e.g.,Bombina) hybrid zone in Eastern Europe)
hybrid speciation - Hybrids may actually be reproductively superior to parent populations. If hybrid x hybrid matings are more successful (in terms of Darwinian fitness), hybrid speciation could eventually occur.
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.