Wednesday, August 15, 2012

What's an individual?

We expect then that within our bodies, each cell has the same genetic code, the same sequence of DNA, since all of our cells originated from the same fertilized egg. We understand that children are novel genetic combination of their parents, that twins share the same genetic code, and that individuals differ genetically one from the other. Overtime, these genetic differences provide the basis for evolution.

How strange then is the recent report that different parts of the same tree have different DNA sequences!

Ed Yong, reporting from the Ecological Society of America Annual Meeting , tells of the results from the laboratory of Brett Olds, where they determined the DNA sequence from different parts of the same black cottonwood. They found differences in thousands of genes between the topmost bud, the lowermost branch, and the roots.

As Olds told Yong, “This could change the classic paradigm that evolution only happens in a population rather than at an individual level.” 

The differences in the DNA sequences between the branches could conceivably lead to advantageous characteristics. Perhaps different branches of the same tree compete with one another for light, nutrients and pollinators, and this competition leads to Darwinian selection, whereby the most fit branches out-compete their neighboring branches.  The differences in DNA sequence would then be more likely carried on in the next generation by the branches that produced  more or heartier seeds.

Of course the caveat is that this is a blog reporting on a report of a report. i can't wait to see the research article, and for this paradigm to be tested by additional labs using other tree species. If it holds up, we'll have to rewrite some of our textbooks!


  1. Tree people are all over this! I like to think of my 'individual' trees as evolutionary trees in and of themselves!

    For a cool example, see O’Connell, Lisa M. 2004. “Somatic Mutations at Microsatellite Loci in Western Redcedar (Thuja Plicata: Cupressaceae).” Journal of Heredity 95 (2) (March): 172–176. doi:10.1093/jhered/esh024

    1. Thanks for the ref. Somatic mutations also increase after stress, and this propensity can be passed on at least one generation. But as I usually only read Arabidopsis work, I hadn't considered what would happen in a perrenial! So in retrospect, its not such a far-fetched finding (the changes in DNA seq). But that's how it usually is in a paradigm shift.