Farmers spread with farming

I was going to attempt to read and comment on a recent paper in PLOS Biology myself, but fortunately smarter people than me, who understand the subject better, got there first. So all I need to do is point you to Razib Khan’s explanation of how recent DNA analyses confirm “tentatively” the idea that farming didn’t spread into Europe as a result of people imitating their neighbours. Instead, the DNA suggests that spread was:

[A] classic demic diffusion process. This is basically a very simple model whereby farmers with larger population growth rates expand into the “space” of hunter-gatherers.

Now to do the same with their crops and livestock?

Nibbles: Coffee rust, PECS, Agrofuels, Israel, Mayan farming, Cannabis breeding, Drought resistance

Mashing up banana wild relatives

Over at the Vaviblog is a detailed discussion (though not nearly as detailed as the paper) of a new paper outlining a new theory for the origin of the cultivated banana. ((De Langhe, E., Hribova, E., Carpentier, S., Dolezel, J., & Swennen, R. (2010). Did backcrossing contribute to the origin of hybrid edible bananas? Annals of Botany DOI: 10.1093/aob/mcq187))

Edible bananas have very few seeds. Wild bananas are packed with seeds; there’s almost nothing there to eat. So how did edible bananas come to be cultivated? The standard story is that some smart proto-farmer saw a spontaneous mutation and then propagated it vegetatively. Once the plant was growing, additional mutants would also be seen and conserved. In fact this “single-step domestication” is considered the standard story for many vegetatively-propagated plants, such as potato, cassava, sweet potato, taro and yam. And while it may be true for those other crops, evidence is accumulating that it may not be the whole story for bananas.

Leaving the details aside, De Langhe and his colleagues propose that instead of a single step, at least two were involved, with a proto-cultivated banana back-crossing with one of its wild relatives and then being seen by the proto-farmer as an improvement to be added to her proto-portfolio of agricultural biodiversity. Something very like that is going on today among cassava farmers, for example; they allow volunteer seedlings, the product of sexual reproduction between already favoured clones and wild relatives, to flourish in their fields and then select among them. ((Pujol, B., Mühlen, G., Garwood, N., Horoszowski, Y., Douzery, E., & McKey, D. (2005). Evolution under domestication: contrasting functional morphology of seedlings in domesticated cassava and its closest wild relatives New Phytologist, 166 (1), 305-318 DOI: 10.1111/j.1469-8137.2004.01295.x)) Banana farmers could easily have done the same.

To quote again from The Vaviblog:

The big question, of course, is “what does any of this matter?”. And the surprise is that it really does. Banana breeding is difficult at the best of times; no seeds, no pollen, you can imagine. But if the backcross hypothesis is true, then the current approach to banana breeding, which De Langhe et al. describe as “substituting an A genome allele by an alternative derived from a AA diploid source of resistance or tolerance to biotic and abiotic stress”, might be misguided. If the chromosomes are not “pure” A or B, and if backcrosses were involved in the origin of banana varieties, maybe breeders should look again at some of the diploid offspring from their crosses and see whether they could be further backcrossed to come up with types that are more use to farmers.

Now, what I really need is for one of the handful of people who really understand this stuff to tell me where I’ve misunderstood it.