Over generations, two populations of rabbits in a desert evolved into separate species due to adaptation to their environments through natural selection. Hawks preyed on rabbits, eliminating those with coats that did not camouflage them against the color of the local soil. Over time, the rabbit populations on opposite sides of the desert developed distinctly light and dark coats, with genes for the other coat color disappearing. When a flood separated the populations, their genetic differences could no longer mix through breeding, allowing them to continue evolving as distinct species.

1. Rabbits!
An Example of
How Evolution Works
Emily Gaydos, 4-10-12

2. Once upon a time, there were rabbits in a desert. They
were very happy and bred like… well, rabbits.

3. There was variation in the population—some rabbits were darker, some were
lighter. It didn’t matter, though—they were all happy.

4. But then, hawks discovered the rabbits and the hunt was on!

5. On one side of the desert, the sand was lighter in color, and that made it
easier for the hawks to see and catch dark or very light rabbits.

6. On the other side, the sand was darker, and that made it easier for the hawks
to see and catch light or very dark rabbits.

7. Because of the selection pressure of hawk predation, the frequency of traits in
the population of rabbits began to change.

8. On the side with lighter soil, the dark and very light rabbits disappeared—
eaten by the hawks. On the other side, the light and very dark rabbits were
eaten first. Over time, only the best camouflaged rabbits remained.

9. Since rabbits tend to breed with nearby rabbits, most of the light-colored
rabbits bred with other light-colored rabbits and had light-colored offspring.
The bunnies born without good camouflage were quickly eaten up.

10. On the other side, the dark-colored rabbits bred with other dark-colored
rabbits and had mostly dark-colored offspring. The bunnies born without
good camouflage there were also quickly eaten up.

11. Thus, over time, the rabbits began to look like two different populations—one
with a higher frequency of the trait/genes for lighter coats and the other with
a higher frequency of the trait/gene for darker coats.

12. Since the selection pressure was maintained by Hawk Predation, genes for
Dark Coats were pretty much wiped out in rabbit population on light
soil, while genes for Light Coats dwindled in the population living on dark soil.

13. And then one year, a mighty rain cut a new flood channel in the
desert, separating the rabbits from each other!

14. Since they could no longer mingle and exchange DNA, the separated
populations of rabbits became more and more unlike each other.

15. ??
??
Over time, they became so different genetically that they couldn’t interbreed
when they did meet again. The frequency of traits/genes had changed so
dramatically that the rabbits had become two distinct species!

16. Evolution is the change of trait/gene frequencies in a population over time.

17. Natural selection happens when something about the environment—like soil
color or predation—makes an animal’s traits either more fit or less fit
(advantageous) in that given environment.

18. If an animal’s traits are a better fit to the environment and therefore result in
more offspring, the number of critters having those traits will increase.

19. If an animal’s traits are a worse fit to the environment and therefore result in
less offspring, the number of critters having those traits will decrease.

20. Selection pressures can be complex, leading to complex adaptations. Let’s say
coyotes also move in. Now, faster, skinnier rabbits might have a better chance
at surviving to have more offspring with their faster, skinnier genes…

21. …while on the other side, the sandier soil makes it easy to dig burrows, and
smaller, fatter rabbits with a tendency to burrow have a better chance at
surviving to breed more small, fat burrowing rabbits…

22. The difference in fitness between competing animals doesn’t have to be big to
matter. If a trait gives a critter even a slight advantage that results in a few more
offspring, there will be slightly more of those advantageous genes in the population.

23. Over time, even these slight adaptations can add up as the trait/gene frequencies
are pushed in different directions in response to selection pressure.

24. The relationship between animals and the selection pressures of their environments
can be outrageously complex. All creatures on the planet today are the result of
millions and millions of years of these interactions—generation after generation of
natural selection for traits/genes just a little bit more fit for each population’s unique
and changing environment. Yet, for each species and each adaptation, the
underlying mechanism is the same:
In response to selection pressure, the frequency of traits and the genes controlling
those traits changes in a population over time. That’s evolution.
Emily Gaydos, 4-10-12