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Perspektif baru tentang evolusi ular

Ular mungkin tidak memiliki bahu , tetapi tubuh mereka tidak sesederhana seperti yang biasa dibayangkan , menurut sebuah studi baru yang bisa mengubah cara ilmuwan berpikir  tentang ular berevolusi . Dari ular berkembang dari nenek moyang kadal ke bentuk tubuh yang lebih sederhana , para peneliti mengatakan temuan mereka menunjukkan hewan lain yang memperoleh  kolom vertebral lebih kompleks karena mereka berevolusi .....read more

New perspective on snake evolution

Date:

January 5, 2015

Source:

University of Nebraska-Lincoln

Summary:

Snakes may not have shoulders, but their bodies aren't as simple as
commonly thought, according to a new study that could change how scientists
think snakes evolved. Rather than snakes evolving from a lizard ancestor to a
more simplified body form, researchers say their findings suggest other animals
gained more complex vertebral columns as they evolved.

..........................

Snakes may not have shoulders, but their bodies aren't as simple as
commonly thought, according to a new study that could change how scientists
think snakes evolved.

Paleobiologists Jason Head of UNL and P. David Polly of Indiana University
Bloomington found distinctions among snakes' vertebral bones that matched those
found in the backbones of four-legged lizards.

Rather than snakes evolving from a lizard ancestor to a more simplified
body form, the researchers say their findings suggest other animals gained more
complex vertebral columns as they evolved.

The study provides new perspective on Hox genes, which
govern the boundaries of the neck, trunk, lumbar, sacral and tail regions of
limbed animals. The functions of Hox genes previously were
thought to have been disrupted in snakes, resulting in seemingly simplified
body forms.

Snakes differ from mammals, birds and most other reptiles because they lack
forelimbs, shoulder girdles and breastbones. It was thought that when they lost
their limbs, they also lost the regional distinctions that separated their
backbones into neck, trunk, lumbar and other regions.

Yet when Head and Polly examined the shapes of individual vertebral bones
in snakes, lizards, alligators and mice, they found snakes had regional
differentiation like that of lizards.

"If the evolution of the snake body was driven by simplification or
loss of Hox genes, we would expect to see fewer regional
differences in the shapes of vertebrae," Head said. "Instead, what we
found was the exact opposite. Snakes have the same number of regions and in the
same places in the vertebral column as limbed lizards."

Not only did Head and Polly find that snakes were as differentiated as
lizards, but when they compared regions in snakes with Hox gene
expression, they found the two matched.

"This suggests that Hox genes are functioning in the
evolution and development of the vertebral column in snakes, but instead of
patterning distinct, rib-less regions like the neck and lumbar spine of mice,
they control more subtle, graded changes in shape," Head said.

When combined with information from fossils, these findings indicate that
the direction of snake evolution is the opposite of what had been concluded
from developmental genetics alone, Head and Polly say.

"Our findings turn the sequence of evolutionary events on its
head," Polly said. "It isn't that snakes have lost regions and Hox
expression; it is that mammals and birds have independently gained distinct
regions by augmenting the ordinary Hoxexpression shared by early
amniotes."

Amniotes are the group of vertebrates that lay shelled eggs. They include
reptiles, mammals and their predecessors.

"Snakes have a lot more vertebrae compared to lizards and they have
lost the shoulder girdle, but they are just as regionalized," Polly said.

Head and Polly reached their conclusions using a method called geometric
morphometrics and a regression-based analysis of the size and shape of
vertebral structures. To determine where one segment ends and the next begins,
they use a statistical method called maximum likelihood estimation.

"Analysis of gene functions are necessary, but not sufficient in
studying evolutionary transitions," Head concludes. "In order to
fully understand the mechanisms by which new body forms evolve, it is crucial
to study the anatomy of modern and fossil organisms."

The study was published online Jan. 5 by the journal Nature.







Story Source:

The above story is based on materials provided
by University of Nebraska-Lincoln. Note:
Materials may be edited for content and length.







Journal Reference:

1.   
Jason J. Head, P. David Polly. Evolution of the snake body form
reveals homoplasy in amniote Hox gene function. Nature, 2015;
DOI: 10.1038/nature14042

http://www.sciencedaily.com/releases/2015/01/150105125836.htm

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