Friday, January 18, 2013

Adaptations - Color variability

A simple definition of the word adaptation is a physical trait or behavior that helps a living organism survive in its habitat.

One physical trait that many species show is a variability in the coloration of individuals.  This variability may enable individual in a species to blend in better with their specific habitat.  If habitat changes over time, individuals with a certain coloration may have a competitive advantage over individuals of another color.

One example of color variation that occurs in Mid-Michigan is among frogs.

The first two photos are both of Green Frogs (Rana clamitans).  The first one is of a frog that was living in a wooded wetland - a combination of vernal ponds and deciduous swamp.  The floor of this wetland is carpeted with fallen leaves and tends to be a patchwork of light and dark.  This frog's mottled colors blend in very well with its background.

Green Frog #1

The second photograph shows a Green Frog from a sunny, well-lit permanent pond.  Many areas of the pond are covered with a uniform covering of green algae.  This frog is a more uniform color, and like the first frog, blends in well with its habitat (but not well enough to avoid being caught with a net and placed on a dock to photograph).

Green Frog #2

The next three photographs show Wood Frogs (Rana sylvatica).  The first two frogs were found in the same wooded habitat as the first green frog.  The first Wood Frog has a very pale coloration with limited variability.  Even the stripes on its rear legs and the dark "mask" under its eyes are lighter than in most members of this species.

Wood Frog #1
The overall color of the second Wood Frog is darker than the first.  It seems to blend in better with its surroundings.  The dark stripes on its hind legs and the dark "mask" help to break up its outline and help it blend in among the fallen leaves.

Wood Frog #2
The third Wood Frog was photographed nearly 400 miles from Mid-Michigan on Isle Royale National park.  This frog shows an even darker overall color and more variation between the light and dark patches on its skin.  These three Wood Frogs show some of the color variation that can exist within an individual species.  If one color variation offers a competitive advantage, frogs of that color are more likely to survive to pass on their genes to the next generation.  Successive generations may then show less color variability if a dominant color scheme expresses itself.

Wood Frog #3

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