Xeluan

Over at Arn there is thread on <a href="http://www.arn.org/ubb/ultimatebb.php?ubb=get_topic;f=13;t=000382" target="_blank">Whale Evolution</a> which I've left the cheap seats for to get involved (as I have just read Carl Zimmer's "At The Water's Edge" and mistakenly assumed I know something).

Regarding the increase in length that marked Basilosaurus, if I am correct this increase in length occured as the early whales moved from the shallower water into the deeper oceans.

Is it too naive to think that the increase in length was related to the move to open and deeper water allowing larger body size and, if so, have other species recorded a similar development in their evolution?

Xeluan

Camaban

He wishes.

As for extending when they get into open water.

Well... look at it this way. at first, they weren't particularly well adapted to diving, etc to get away from predators, so the only way to avoid being eaten was to be bigger than anything that would try to eat them.

Being longer would probably also give a speed advantage (once the muscles were built up, anyway)

I could just be talking out of my arse there, but it doesn't sound that unreasonable.

pz

I've never found this argument particularly persuasive. Growing to be 31 feet long isn't going to dissuade any of the predators that found you toothsome at 30 feet.

It's more likely that finding a niche with a rich food supply simply permits larger individuals to thrive, and as the individuals within a population grow larger, other benefits, such as better thermoregulation and the ability to survive for longer periods between meals, begin to synergize.

Xeluan

Harnessing the power of the web, I emailed Dr Hans Thewissen with some questions on Basilosaurid evolution. Dr Thewissen was very kind enough to reply and I got way more information than I bargained for which was great.

Basically, what pz states seems to be right. That Basilsaurus found a unique niche that allowed them to thrive (it seems that they liked to eat sharks). One thing I was wrong about was where Basilosuarus live. Dr Thewissen thinks that they lived in shallow, coastal waters.

Also, there have been discoveries of a Pappocetus fossils in Nigeria which are intermediate in size between Abulocetids and Basilosaurids.

Regarding the speed of evolution (ie the 12 million years between Abulocetus and Basilosaurids) Dr Thewissen feels that this is not any faster than documented rates of evolution found in the lab.

One thought I have is that those who focus on the speed of whale evolution, and claim that it was way to fast for 15m Basilosaurids to evolve from 3m Abulocetus, miss the point. Sure the increase in size seems dramatic but that is simply a human bias thinking that big is better. Early whales has 12 million years. The evolution of the human brain with intelligence and sentinence took far less time and (if my bias may intrude) far more 'amazing' than the progress from Abulocetids to Basilosaurids.

Xeluan

[ October 14, 2002: Message edited by: Xeluan ]</p>

Doubting Didymus

I agree with you that this is probably not the reason in this specific case, but I do not think it is as impossible as you suggest.

Remember that the benifit of largeness is not only to make oneself a difficult meal. Largeness is often directly related to strength. A 31 foot whale may find it easier to fight off a shark. Alternatively, there is also a broad continuum of predator size. Being a foot longer might not help you against Jaws, but it may easily give you the edge over a grey nurse (forgive me if my technical knowledge of fish is limited).

Remaining for the moment on the scale continuum point, as well as there being a smooth scale in predator size, there is also a scale in predator numbers. Sharks often hunt both alone and in groups of various sizes. This may provide a constant selection pressure towards enhanced size, because a 31 foot whale may be able to defend itself or intimidate a single whit pointer, but still be vulnerable to multiple sharks where an even larger individual whale would have fewer problems.

Remember also that whales are pack animals (or family animals at least). While I am not specifically aware of sharks deliberately attacking the weaker individuals, as wolves or lions do, it certainly seems that that kind of factor may be at play in the oceans. Thus, smaller whales are more at risk than their larger siblings / cousins.

Liquidrage

Please forgive me if this view is naive..

I would think that a species growing in size over generations would be the result of competition from within the species itself. This growth would be dependant on a source of food large enough in quantity and nutrional value to support the increased food requirements that come with increased size.

For example, when two males compete for a female, the larger/stronger one typically wins. This deciding factor over many generations can lead to a greatly increased size in the specie.
However, the larger animal also requires a food source to support the large body.
If this food source is not there, despite the size the larger animal will be weaker then his smaller opponent that requires slightly less food. The smaller animal would win and there would be no increase in size.

If this is the case we can surmise that Abulocetids filled a niche that was vastly underexploited and highly nutritous. They would also be a pack creature that competed for the right to mate.

Doubting Didymus

That does not explain why species that share the same habitat and the same set of conditions do not progress to a similar size. One such species would be dolphins.

My best guess is that, like most long term evolutionary trends, the selection pressures are a composite of a large variety of factors which may have resulted in any number of different forms. Thus, it is difficult to speculate.

In other words, there are a huge number of evoloutionary pathways that a species could take, and discovering the reason why it took one path over another is a complex affair.

Liquidrage

I disagree.

The dolphins (or what would become dolphins) might not have found a niche that had the required food source.

If I may change your last statement I would make it,
"In other words, there are a huge number of niches that a species could fill, and discovering the reason why it took one niche over another is a complex affair"

I'm not claiming that my above guess is correct, but I could still keep it in regards to your objection.

Doubting Didymus

I can agree with that. I suppose it depends onwhat you consider to be a niche.

I would suggest that a given niche does not always imply a single evolutionary pathway.

Liquidrage

I would agree with that as well.