(urth) What Dollo's Law means and what it doesn't

[Dan'l Danehy-Oakes]

Let's look at the cetacea for an illustration.

Long, long ago, their ancestors left the ocean, fish becoming
amphibians and, ultimately, mammals. They lost the shape of fish, and
they lost the ability to "breathe" water (i.e., gills).

Millions of years later, their more recent (but still rather distant)
ancestors _returned_ to the ocean, becoming first kind of crocodilian
things and finally the sleek dolphins, orcas, and whales we know
today.

Note that they have regained the rough shape of fish.

Note also that they have _not_ regained gills.

Dollo's Law applies to the gills, but not the shape. Organisms can
adapt to a new environment, even one that resembles an older
environment. In so doing, they may develop characteristics that
_resemble_ the characteristics their ancestors lost; but they do not
regain those precise characteristics.

And so whales must still return to the surface to breathe.

-- 
Dan'l Danehy-Oakes