So, we humans tend to think of the living being as the unit of life. I.e. each cat, tree, human, cockroach, microbe is exactly one life.
In fact, the unit of life is the gene, and we’re merely a manifestation of a colony of genes. Nothing new so far.
It occurred to me today that we’ve got a pretty good understanding of the family tree of living beings, how related dogs are to petunias for example, and how past beings were grandparents to disparate species. Will we ever have something similar for genes? Or is past DNA so fragile that there’s no hope of investigating ancient genes and mapping them into a real family tree of life?
Lots of genes are routinely compared between species. In fact, one of the means for determining that a given region of a gene is crucial to the function of the gene is to look at how that region has remained relatively unchanged as it has marched through different species. This is called genetic conservation of that region.
With the human genome project nearing completion and sequencing of the genomes of other species, pan-genomic comparisons are now becoming possible. These comparisons will likely yield a treasure-trove of new hypotheses regarding the molecular (genetic) basis of speciation.
As far as the fragility of DNA, DNA happens to be a very stable molecule (Jurassic Park was close to the mark on this point–you really can recover some genetic information from samples that are millions of years old)but very often it’s not an issue. Organisms with ancient genes are still with us today. Often, we can estimate when species diverged without any genetic criteria at all. Once the lineage trees are established, one may investigate differences in gene sequences in the modern forms of these diverged organisms.
I think that you’re a little confused here. You seem to be saying that a gene is “exactly one life.” Genes are most definitely not alive. DNA/RNA (which consist of more than just genes) are generally termed the “blueprints” of life, as they encode the information necessary for an organism to produce what it needs to exist as an organism. But a gene by itself doesn’t do squat. I can’t tell if you were not aware of this or just expressed yourself in an awkward manner; check out a basic bio text at the library if you wish to learn more.
Now to get to your question:
In the present day (as opposed to Linnaeus), the structure of the family tree of living beings is determined to a great extent by genetic relationships - for instance, chimps and humans share ~98.4% of their nuclear DNA (and thus a similar % of genes). However, there can’t really be a family tree of genes, because genes don’t ‘evolve’ in the same way that organisms do. Natural selection doesn’t act upon a particular gene, it acts upon organisms as a whole (that is, the entire genome of a particular organism is selected for or against at once).
I think that your confusion in the first quotation might have stemmed from such information as the 98.4% figure above. Figures like this do mean that genes can be ‘tracked,’ for lack of a better term, to some extent. Almost identical genes often share analagous functions (and identical genes share homologous functions) in different organisms.
Let me give an example. The genes Pax-6 (mouse) and Eyeless (drosophila fly) control eye formation in their respective organisms. They are phenomonally close genetically, as demonstrated by transgenic experiments in which Pax-6 genes were caused to be expressed in different body segments of flies, causing eyes to grow in those segments. But the gene is not the unit of life, as demonstrated by the fact that the eyes which grew on the fly were fly eyes, even though it was a mouse gene which was causing them to grow.
As for the ancient DNA angle, yes, it is fragile. DNA degrades over time, to the extent that after ‘x’ years, it is so chopped up as to be non-sequenceable. I’m not exactly sure what ‘x’ is, as research is being done into this as we speak. I can vouch first-hand that DNA can be successfully extracted and sequenced from bones and teeth dating from the early holocene (~10,000 years old; not that old geologically). However, Jurassic Park wasn’t that far off in the notion that animals trapped in amber could contain DNA - it’s been done. It’s just that we can’t subsequently hatch a baby dinosaur.
I stand by my statement that the gene is the unit of life. It is true that a gene can’t exist on it’s own, and evolution does occur over a group of genes, but that doesn’t change the fact that genes are the individual elements that compete and cooperate (and evolve) as life. “The Selfish Gene” is, of course, a basic description of the concept.
ellis555 wrote
Thanks. Could you give me some more detail on the “It’s been done” part? Are we today in possession of what we believe is intact DNA of (any) animal say a million years old? How far are we able to go decoding that info, like the human genome project? Are you saying that pieces of extremely old genetic material have been decoded, and it’s just a matter of resources to discover the entire genetic makeup of ancient animals?
You might be interested in the chapter “The one true tree of life” in Richard Dawkins’ “The Blind Watchmaker”. Or the whole book for that matter. Or Dawkins’ other books.
That chapter is the one in which he discusses taxonomy from an evolutionary standpoint, and makes a big pitch for molecular taxonomy.
Basically, yes, we have sequenced incredibly old DNA extracted from amber. Try checking out a few of these articles, culled from a search at http://biosis.lanl.gov
[ul]
[li]Cooper, Alan. Studies of avian ancient DNA: From Jurassic Park to modern island extinctions. Avian molecular evolution and systematics. 1250 Sixth Ave., San Diego, California 92101, USA : Academic Press, Inc., 1997. (book)[/li]
[li]Gutierrez, Gabriel; Marin, Antonio. The most ancient DNA recovered from an amber-preserved specimen may not be as ancient as it seems. Molecular Biology and Evolution; July 1998; v.15, no.7, p.926-929.[/li]
[li]Wang, Hai-Lin; Yan, Zi-Ying; Jin, Dong-Yan. Reanalysis of published DNA sequence amplified from Cretaceous dinosaur egg fossil. Molecular Biology and Evolution; 1997; v.14, no.5, p.589-591.[/li]
[li]Walden, Kimberly K. O.; Robertson, Hugh M. Ancient DNA from amber fossil bees? Molecular Biology and Evolution; 1997; v.14, no.10, p.1075-1077.[/li][/ul]
And I stand by my statement that individual genes aren’t doing squat. It is the combination of genes within an organism that is being selected for. Otherwise, how to you explain heterozygous populations, such as human hemoglobin? If it were just the gene that were being selected for, heterozygous populations should only appear while and where natural selection is actively promoting a progression between ‘species’ of genes. But if this is the case, then the progression from A to S hemoglobin (or C, or whatever the hell you think it is doing) is taking a damn long time.
I didn’t take the time to check the cites above, so I apologize if this was addressed in one of them. I do remember reading that actual dinosaur DNA had been extracted from the stomach of a fly in amber, a la JP. However, they only got a few hundred base pairs, which is a very small amount. The human genome, to compare, is around 10^9 base pairs, IIRC.
To address the OP, tracing changes in DNA through species has become a very standard method for determining evolutionary trees. It does not involve ancient DNA, though. Scientists compare DNA from living things to put together what happened in the past.
Smeghead’s reply made me realize that I should have thrown that caveat in there. I don’t believe that anyone has any plans to attempt to sequence the entire genome of any extinct critter, as you’d need a hell of a lot more samples than are currently available to do so. Any degredation of DNA means fragmentation. And it’s a little hard to sequence the entire genome if you’ve only got some of the fragments.
There are only a couple of organisms that are sequenced as is (drosophila and a flatworm, off the top of my head. I imagine that they’re pretty far along on mice, if not complete. and of course, the various human genome efforts are humming along nicely)