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

From: Dan'l Danehy-Oakes <danldo at gmail.com>
Date: Tue, 25 Jan 2011 22:12:52 +0000

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

---

From: Lee Berman <severiansola at hotmail.com>
Date: Wed, 26 Jan 2011 00:27:07 +0000

>Dan'l Danehy-Oakes: Let's look at the cetacea for an illustration.

Actually whale vs. fish fins are one of those classic examples of convergent 
evolution. Fish (ancient and modern) mostly have fins built on a fan-shaped
pattern of spiny rays. Whales have built their fins on the mammalian bone pattern
and humerus, radius and ulna structures can be found.
 
Being warm-blooded probably locked whales into being air-breathers. Water has much
less dissolved oxygen than can be found in air.  All gill-breathing creatures are 
cold blooded (though certain shark species eat so much and swim so fast that they
can raise their body temperature for a short time).
 
The interesting question to me is...say Earth had a 100 year long flood that killed 
off all land dwelling animals. After the flood receded, could certain whales colonize
the land, their parts reverting back to a land mammal form?
 
Less interesting is my discussion with Gerry on whether the darker grey humpback whale population
of the arctic ocean could ever end up looking like the lighter grey humpback population of the 
indian ocean.