Monday, November 2, 2009

Oaks: 400+ species or one species with 400 varieties? Part 1

The post heading is a bit of an exaggeration. There might be as many as 2 species of oak. Oaks naturally hybridize to an amazing degree with such a high level of "gene flow" that it's very difficult to determine where one species ends and another begins. From Wikipedia:

Frequent hybridisation among oaks has consequences for oak populations around the world; most notably, hybridization has produced large populations of hybrids with copious amounts of introgression, and the evolution of new species.[4] Frequent hybridisation and high levels of introgression have caused different species in the same populations to share up to 50% of their genetic information.[5] The high rates of hybridisation and introgression, produces genetic data that often does not differentiate between two clearly morphologically distinct species, but instead differentiates populations.[6] Numerous hypotheses have been proposed to explain how oak species are able to remain morphologically and ecologically distinct with such high levels of gene flow, but the problem is still largely a mystery to botanists.
Fagaceae, or oak family, is a very slowly evolving clade compared to other angiosperms,[7][8] and the hybridisation patterns in Quercus pose a great challenge to the concept of a species. A species is often defined as a group of “actually or potentially interbreeding populations which are reproductively isolated from other such groups.”[9] By this definition, many species of Quercus would be lumped together according to their geographic and ecological habitat, despite clear distinctions in morphology and, to a large extent, genetic data. Thus, although it may be difficult to place a definition on a species within a genus like Quercus, it is trivial and uninformative to apply the biological species concept to all forms of life.

The natural tendency of oaks to hybridize (and the ease with which artificial hybridization can be achieved) is probably - no, is without question - the most underexploited area of plant improvement in the world. If oaks had received 1/1,000,000th the attention from plant breeders that corn and wheat have received we'd have oak trees capable of growing 6ft per year and producing acorns in year 3. And I'm not joking.

I came across this cool story in an old article. H. Ness of the Texas Experiment Station (a man who loved his given name so much that he always went by the initial H; I'm guessing Herman) wrote an article for the Journal of Heredity in 1927 entitled "Possibilities of Hybrid Oaks: Further Observations on Hybrid Oaks at College Station, Texas." Ness wrote that the Texas A&M campus in Brazos Cty TX was situated in an area with very little forest species diversity. post oak (Q. minor), black-jack oak (Q. marilandica) and water oak (Q. nigra) in the river bottoms. (Of course in northern Minnesota that level of oak diversity looks positively rainforest-like!)

In 1891 a few live oak (Q. virginiana) trees were planted on campus. More live oaks were subsequently planted on campus, all of them descendents of the first ones. Many of these young trees were planted at great distances from the nearest live oaks, and in places where wind direction and obstructions from buildings etc, "greatly reduce the chances of any pollen reaching them from the trees of the first planting." Live oak trees produce female flowers at about 5 years of age, but do not product male flowers until several years later.

According to my old friend "Herman" Ness: "Yet, the female flowers on these young trees, apparently beyond the reach of pollen from their own species, fail but rarely to produce an ample crop of well developed acorns." So... only female flowers, no live oak pollen. How then are the female flowers fertilized? Well, let's just say that the offspring look suspiciously like post oaks (insert your own "mail man" joke here). More accurately, the offspring display a range - a continuum - of characteristics from (nearly) pure live oak to (nearly) pure post oak, and everything in between.

Hybrid oaks - the offspring of these natural of artifical inter "species" fertilization - often exhibit faster growth and earlier maturity than either of the parent species.

Oaks definitely challenge our conception of species. It's more of a continuum with millons of variations and tremendous genetic elasticity. And within that elasticity lies the potential to feed the world. This will be the focus of future "Oaks: One Species?" posts.


  1. Mr. Ness's name was not "Herman." It's actually worse. It's Helge. He was from Norway and it may have gone over better there than in Texas. Are you a familiar with J. Russell Smith's Tree Crops: A Permanent Agriculture? That is where I learned about Mr. Ness's efforts. Only one other guy is mentioned by Smith as having attempted to breed oaks: Thomas Q. Mitchell. The only thing I could find that Mitchell had written was an article titled "Consider the Acorn" in the Feb. 1948 issue of Harper's magazine. You have to be a subscriber to access it. Anyway, I enjoyed your post and thought you might like knowing about "Helge."
    D. Olsen

  2. Thank you so much more commenting. Now I know the "Rest of the Story!" (Although I'd love to know how you learned this.) Yes, it was in Tree Crops that I read about Ness. I will try to get a copy of that 1948 article by Mitchell. I will post if I have found it, and would happily make it available to you!
    Thanks for reading!