Concerning cervical vertebrae
The July/August issue of Seed has an interesting column by PZ Myers (who also writes the excellent science blog Pharyngula) concerning the number of cervical vertebrae in mammals.
It seems that almost without exception, all mammals have 7 cervical vertebrae. That's not true of other orders, just mammals. So it doesn't matter if it's a human, a cow or even a giraffe. Neck length clearly doesn't affect the number of cervical vertebrae--which is rather strange. Other species have a clear relationship between neck length and number of cervical vertebrae. Why don't species in order Mammalia?
One of the reasons for this that PZ Myers mentions is that mammals with one less cervical vertebrae tend to die young, often before they're even born. In fact 80% of embryos with this "defect" die before their first birthday. I think it's obvious that having one less vertebrae isn't lethal in itself, so the survival disadvantage must be due to other reasons than simply the lack of one vertebrae.
The point to Dr Myers' assay is that sometimes groups of genes are linked, often for reasons that aren't obvious, and that anything that decouples the linkage can be very dangerous to the developing critter. An example given is that when spontaneously aborted fetuses were examined, 55% of them had only 6 cervical vertebrae. This is quite amazing since only about 1% of humans have this mutation.
Too often people view evolution as a simplistic competition where the fastest or smartest survives. PZ Myers wants to remind us that survival is anything but simple. Linked genes are just one of the many wrinkles that can affect survival.
It seems that almost without exception, all mammals have 7 cervical vertebrae. That's not true of other orders, just mammals. So it doesn't matter if it's a human, a cow or even a giraffe. Neck length clearly doesn't affect the number of cervical vertebrae--which is rather strange. Other species have a clear relationship between neck length and number of cervical vertebrae. Why don't species in order Mammalia?
One of the reasons for this that PZ Myers mentions is that mammals with one less cervical vertebrae tend to die young, often before they're even born. In fact 80% of embryos with this "defect" die before their first birthday. I think it's obvious that having one less vertebrae isn't lethal in itself, so the survival disadvantage must be due to other reasons than simply the lack of one vertebrae.
The point to Dr Myers' assay is that sometimes groups of genes are linked, often for reasons that aren't obvious, and that anything that decouples the linkage can be very dangerous to the developing critter. An example given is that when spontaneously aborted fetuses were examined, 55% of them had only 6 cervical vertebrae. This is quite amazing since only about 1% of humans have this mutation.
Too often people view evolution as a simplistic competition where the fastest or smartest survives. PZ Myers wants to remind us that survival is anything but simple. Linked genes are just one of the many wrinkles that can affect survival.
Comments
What is that big 13 to left <---- ?
Nosy minds want to know!