June 23, 2000
What is convergent evolution?
It is the appearance of apparently similar structures in organisms
of different lines of descent. It arises when natural selection favors
similar shapes in different organisms that have similar lifestyles but
are on different lines of descent. The end result is that each ends up
looking very similar to the other even though they are only distantly related.
An excellent example is the leatherback sea turtle, by far the biggest of the
sea turtles. With the exception of the leatherback turtle, the green turtle we
have discussed today is the largest marine turtle. The leatherback is not just
bigger than a green turtle, it is the largest of all living reptiles
(more massive even than the largest salt water crocodile).
However, because they all these "turtles" look so similar they have been
called sea turtles since time immemorial. But they are very distantly
unrelated. Yes, both are reptiles, but that's where it ends.
The Leatherback turtle evolved to have long flippers and a protective
"Shell" similar to true sea turtle shells but it branched off from an
entirely different line of reptiles. It became sea-turtle like simply
because if you are going to live in the sea all your life
(except for the times that you return to land to lay your eggs),
and if you are going to swim slowly because the food you eat cannot
evade you (sea turtles eat grasses, leatherbacks eat jellyfish)
you will need some armor and some long oars to push you about.
So your best shape is the sea-turtle shape with a hard shell and
long flexible flippers. So whenever the random mutations in the
ancestors of leatherback turtles produced structures that mutated
in a direction towards armor and oars, the sea-turtle shape,
those mutations were favored, i.e. selected for, until now modern
leatherbacks and their very distant relatives the true sea turtles
look as though they were very closely related.
I.e. during the course of their evolution the appearance of the
animals in these two very distinct lines of descent has converged.
Listen to Roger Payne's Voice from the Sea piece entitled:
How Finches and Tortoises Contributed to the Theory of Evolution
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