Serendipity and the origin of crops

In Puerto Rican dry forest, one of the most striking dichotomies is between forests dominated by native species and forests dominated by exotic Leucaena leucocephala. Most forests that have regrown on abandoned agricultural land are dominated by Leucaena. The colonisation of these areas by native tree species is slow - either the native species are poor colonisers, or Leucaena resists invasion. Its importance on the landscape and apparent stability made me fascinated with the species.

One of the things which struck me early on was the fact that many published sources listed Leucaena as a native species in Puerto Rico. To me, its behaviour shouted “exotic”, but when I spoke to Liogier at the University of Puerto Rico herbarium (author of the Flora of Puerto Rico) he said that it was probably introduced, but there were no records to say anything conclusive, one way or the other. So I was very happy when I came across Colin Hughes’ Monograph of Leucaena. Based on the diversity of hyperparasitoids Hughes concluded that Leucaena leucocephala was native to Central Mexico. He concluded that the abundant populations in the Yucatan (which often formed pure stands in disturbed areas) were, in fact, introduced. And that was the last I had heard on the subject for almost a decade.

When a post entitled “How domestication happens” popped up on my RSS reader, I was curious. The post was minimal, and I almost didn’t click through to the link. When I did so I was rewarded with a post entitled “Biodiversity, trash heaps and the evolutionary origin of crops” and a picture I recognised instantly as Leucaena leucocephala. The post described a paper published in the Proceedings of the National Academy of Sciences entitled: “Serendipitous backyard hybridization and the origin of crops“.

According to Hughes and coauthors, Leucaena leucocephala is one of several domestic species in central Mexico which are believed to have originated through the hybridisation of related species that were brought together in backyard dumps.

Seeds of 13 species of Leucaena are used for food across S-C Mexico…. Food
use is widespread and intensive in Chiapas, Oaxaca, Puebla, Guerrero, and Morelos and more sporadic further north, but unknown further south despite the availability of native species of Leucaena. Present-day food use varies from gathering of pods from local free-living populations for home consumption to intensive harvesting of commercial quantities from cultivated trees and transportation of seeds to regional markets…. This spectrum of increasing human intervention involves transitions from wild to managed to cultivated, from home consumption to local to wider regional marketing, and from very
local to regional to much wider translocation of species.

[This whole thing forced me to reevaluate Leucaena. While I have always thought of it as “useful” for revegetation and perhaps for animal fodder or green manure, the idea of growing it as a food crop for its pods is alien to me. Even the green pods don’t strike me as “food”.]

In Mexico, there is a continuum of use, from Leucaena cuspidata, which is collected from wild populations and used locally, through L. confertiflora and L. collinsii, which are more widely cultivated and tranlocated, to L. eculenta, L. pallida and L. leucocephala which are intensively cultivated. The pods of these latter species are commercial crops which are distributed to markets several hundred kilometres away.

Five of the 13 species are polyploids, and the authors concluded that they are likely to be allopolyploids (polyploids which originated through the bybridisation of two species) whose origins can be traced to backyard gardens and dumps. The underlying idea is that human activity can bring together species which would not normally encounter one-another in the wild. In the case of Leucaena leucocephala and L. pallida the hybrid origin theory is strengthened by the lack of wild populations of the species.

They go on to point out that this hypothesis is not only plausible for Leucaena, but also for the other two major perennial crops in south-central Mexico, Agave and Opuntia, which show similar trends, but are less thoroughly studied than Leucaena species.

1. Hughes, Colin E. (1998). Monograph of Leucaena (Leguminosae-Mimosoideae), Systematic Botany Monographs 55:1-244
2. Hughes, C.E., R. Govindarajulu, A. Robertson, D.L. Filer, S.A. Harris, and C.D. Bailey. 2007. Serendipitous backyard hybridization and the origin of crops. Proceedings of the National Academy of Sciences USA 104:14389-14394.