How does classification reflects evolutionary history?

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How does classification reflects evolutionary history?

Post by harrypotter101 » Fri Oct 19, 2007 4:32 pm

I've looked all over and I can't find an answer to my question so I need a bit of help. :oops: :oops: :oops: :oops: :oops:
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Post by alextemplet » Fri Oct 19, 2007 4:41 pm

In theory, a given level of classification generally includes all of the descendants from a single common ancestor. For example, all of the species of a particular genus are believed to have evolved from a single common ancestor, as are all of the species in a family, order, class, etc. Obviously, the bigger the grouping, the farther back in history the common ancestor had to be!
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Post by AstusAleator » Sat Oct 20, 2007 8:47 am

Harry, I'd suggest looking up cladistics.
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Post by MrMistery » Sat Oct 20, 2007 1:18 pm

long story short: it doesn't.
An ideal clasification would do that. But our current taxonomy is nowhere near perfect. Why? Well, until about 2 decades ago, everyone thought we had reached a place where our classifcation was perfect, or anyways near perfection.
And then came the blow: molecular systematics.
What molecular systematics does is this: take a small piece of DNA(usually ribosomal DNA or chaperone DNA is used) and look for changes in it in different organisms. This is something much closer to reality and has revealed many things - the most dramatic being that not all prokaryotes are closely related, and that the Archea are more closely related to EK than PK. In 2002, the 3 domain system, based on molecular systematics was oficially accepted.
However, there are problems. different rates of evolution in different habitats and convergent evolution are 2 of the things that molecular systematics cannot take into account. and when you are using small pieces of DNA and not the whole genome, problems arrise. To point out one, a molecular taxonomy of the eudicots revealed that Curcubitaceae and Fagaceae are the closest related family. Clearly, that is not true.
Furthermore, creating a clasification based on the evolution of rRNA and the evolution of HSP70(a family of chaperone proteins) does not yield the same result.
That is why, the current taxonomy uses molecular methods in combination with classical ones.
THe holy grale in taxonomy is a science called phylogenomics. What those guys do is take the whole genomes of organisms and compare them. With such large number of bases being compared, this is a virtually fool-proof method. however, we will not be able to sequence the DNA of all living organisms for maaaaaaany years to come.
Right now, we work with what we have, and we don't have much.
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

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