[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: How The Giraffe Got (or didn't get) Its Long Neck



In that pace we will go back to "lamarckism".<<<

Huh? You can derive elongate necks that aid in feeding with a Darwinian selective regime. Just because people use the case of giraffe necks to contrast Darwinian selection with Lamarckian change (or rather a caricature thereof) doesn't imply that both can't be used to explain it (note, I am not endorsing Lamarck's ideas, merely agreeing with S.J. Gould that the textbook description of his idea is a caricature of the position Lamarck actually held).


And what about fossils? Are there more or less compleat fossil
skeletons from ancient Giraffidae?<<<

Yes, there are several species of fossil giraffid, some with long necks and they didn't all live in Africa. That's something that people seem to overlook when trying to explain giraffe necks, as nearly every discussion I've seen presumes the modern African setting is the one that lead to the evolution of their long necks, but of course this may not be true. It could be that in other environments with different feeding regimes, or even in the current one but when competing against now-extinct herbivores that the long neck conferred a greater advantage than it does now. It's also worth noting that an adaptation can be more or less neutral most of the time but play a key role during times of stress (eating above other herbivores might be _very_ advantageous during times of drought, for example) and if=2
0those periods of selective pressure are common enough then they will still drive phenotypic change, even if during the majority of the animals life that selective pressure isn't acting. And while I think the current study is correct that sexual selection wasn't the driving factor, it could well be a maintaining selective pressure, as mate selection often favors (and therefore serves to reinforce) characters that have traditionally played an adaptive role.


I don't know enough about giraffid biogeography off hand, but personally my first inclination would be to look for the likely origin of long-necked species and see what THAT environment was like before attempting to speculate on what factor(s) drove the evolution of neck elongation. Most current discussions (even in technical papers) ignore the phylogenetic and temporal context that giraffes evolved in when trying to solve this mystery.

Scott



Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333

www.skeletaldrawing.com

-----Original Message-----
From: Roberto Takata <rmtakata@gmail.com>
To: rtravsky@uwyo.edu
Cc: dinosaur@usc.edu
Sent: Tue, 19 May 2009 11:06 am
Subject: Re: How The Giraffe Got (or didn't get) Its Long Neck








What about okapi? No useful information from comparative anatomy and embryology?



[]s,

Roberto Takata=0
D

On Tue, May 19, 2009 at 1:49 AM, Richard W. Travsky <rtravsky@uwyo.edu> wrote:

with maybe some relation to long necked dinosaurs...

http://news.bbc.co.uk/earth/hi/earth_news/newsid_8050000/8050298.stm

For centuries experts have argued over how the giraffe got its long
neck.

Some said it helped the giraffe feed on leaves other animals cannot,
while
some suggested it evolved as a consequence of giraffes evolving long
legs.

But evidence for such ideas remains flimsy. Now one of the more recent hypotheses has also bitten the dust.

Giraffes did not grow elaborately long necks as a sexual signal,
scientists
have shown, leaving its origins a mystery.

In the journal Zoology, Professor Graham Mitchell of the University of
Wyoming, in Laramie, US, and colleagues Professor John Skinner and Dr
S J
van Sittert of the University of Pretoria in South Africa report that
there
is still no consensus on the origin of the giraffe's neck.

The theory which most support, they say, is that the neck confers a
feeding
advantage, allowing the giraffe to reach leaves beyond more numerous
smaller
browsers such as gazelles or antelope.
[...]
More recently, another popular idea emerged: that sexual selection,
rather
than natural selection, drove the evolution of the giraffe's neck.

The idea is that down the generations, males evolved ever longer
necks to


dominate rivals for the affections of female giraffes. The fact the
male
giraffes uniquely wrestle for dominance by "necking" and "head
clubbing" one
another, with males with the longest necks and heaviest heads tending
to
win, has been forwarded as evidence to support the hypothesis.

So Professor Mitchell and his colleagues decided to put the sexual
selection
hypothesis to the test by examining 17 male and 21 female giraffes.

If long necks were a sexually selected trait, they expected to find a
number
of things:

 Â* Long necks should be more exaggerated in males than females
 Â* They should evolve to be bigger in size more than other parts of
a
giraffe's body
 Â* They should confer no immediate benefit to survival, and may
come at a
cost

Their results didn't support any of these propositions.

They could find no significant differences in the relative size of
male or
female necks. [...]
But they can say that any advantages that were gained don't appear to
have
been sexual in nature. "Better explanations for neck elongation must
be
sought elsewhere," they write.