Although varieties abound, wheat can be more simply considered as either hard or soft, hardness being a measure of the kernel’s resistance to crushing.
All wheat originally was soft-kerneled. And there is, so far as we know, no evolutionary advantage to either the hard or the soft trait.
But clearly, somewhere along the line, that section of genetic material that determines the hardness of the kernel underwent a random mutation. Specifically, the Puroindoline a or Puroindoline b genes, which have long been a focus of Craig Morris’s research.
In order to understand the hard/soft divide, Morris, a plant physiologist, suggests that we consider the historical relationship of wheat and humans.
Or rather, wheat, humans, and mice.
“Hexaploid wheat never existed until about 8,000 years ago,” says Morris, referring to the genetic structure of modern wheat. “Ploidy” refers to the number of sets of chromosomes that make up an organism’s genetic material.
“Almost certainly what happened,” says Morris, “Neolithic farmers were growing a tetraploid ancestor. Goatgrass, a diploid, was a weed in the field.”
Goatgrass can cross with wheat, but the union rarely forms a stable cross. Although they can form a hybrid, it generally is sterile.
In fact, a fertile cross between goatgrass and ancestral wheat, resulting in offspring that can reproduce, has probably been captured by farmers only twice through history, says Morris.
But cross they did.
“Some Neolithic farmer had it figured out,” says Morris. “And hence, the world grows wheat.”
But wheat’s origins shed no light on why some is soft and some hard.
Given its appetite for grain, the house mouse has likely been an unwelcome companion to humans since the day humans started storing grain. In earlier work, Morris and his colleagues had observed that the house mouse “showed a marked (up to fivefold) preference for soft wheat kernels over hard.
”Although the hardness mutation in wheat is rare, if mice preferred the soft wheat, the number of hard kernels in a given batch would increase. When the farmer planted his wheat in the spring, he or she would plant an inordinate number of hard wheat kernels, which would in turn increase the likelihood of more hard kernels in the next harvest—which the mice would ignore in favor of the soft kernels.” Thus, Morris formed his hypothesis: “The house mouse, due to feeding preferences, exerted phenotypic selection for hard kernel texture in wheat, thereby increasing the frequency of the hard mutant Puroindoline allele at the Hardness locus.”
To test that hypothesis, Morris and his colleagues conducted a series of trials wherein mice were fed a mixture of hard and soft kerneled wheat. The proportion of the hard kernels initially was 0.9 percent to 10 percent.
After “allele selection” by the mice, the proportion of hard kernels averaged 31 percent overall. In other words, the mice shifted allele frequency by as much as 10-fold.
“One can envisage,” write Morris and his colleagues in a recent report in the journal Ecology and Evolution, “that within a limited number of planting/harvesting cycles, mouse predation would indeed shift the population of a theoretical landrace to the hard allele. Once fixed, there would be little opportunity for the Puroindoline gene to mutate back to the soft phenotype.”
Although determining how long it took the hard wheat to evolve is difficult, the researchers estimate, conservatively, that the allele frequency could have reached 99 percent in a mere few centuries.