Soybeans are an extremely important crop throughout the world.  So it’s worth knowing where they come from.  This story goes back to prehistoric times.

When humans transitioned from hunting and gathering to growing cereal grains and raising livestock, they had to select the type of grains they would grow from the wild growing grains they had been eating for generations.  Human ingenuity and probably a little trial-and-error helped them pick soybeans.  This would have likely occurred in Asia, where soybeans are believed to have originated and are a native plant.  

Just as in every population, there is genetic variation in the native soybeans that results in physical differences.  Some of these ancestral soybeans would have been taller or shorter.  Some would have had stronger or weaker stems.  Some would have had bigger seed pods, or tasted better to early humans.  Some would have been better at resisting parasites, such as the soybean cyst nematode or SCN.  

Our early human ancestors gathered up and saved the seed from the best plants they could find.  Planting these seeds, instead of eating them, would have given rise to even more plants with the same valuable traits.   The domestication of soybean was an act of artificial selection.  Humans decided which plants would survive and reproduce in their fields.  Mating these plants with one another over many generations would eventually mean that all the plants in our fields were genetically similar to one another.  But at the same time, nature was at work on the wild populations, and the wide variety of selection pressures in the wild meant that there was a wide variety of soybeans in the ancestral populations. 

Humans have the ability to travel large distances in short amounts of time, taking their seed and farming practices with them as they move.  SCN move only a small distance in the course of their life cycle.  The first soybean fields were likely free of these parasites and the soybean was spread across the world while the SCN lagged behind. This meant the parasite and its host were separated for a while.  Humans were constantly selecting for the best soybeans from their fields and were purposefully selecting plants to breed that would make better seeds for the next harvest.  The farmers, scientists, and plant breeders were not concerned with the SCN parasite, because it was not a problem in the field at that time. 

But with the advent of modern machinery and global travel, it didn’t take the SCN long to catch up.  The eggs of the SCN can survive a decade in soil.   It is possible that soil containing the eggs of the SCN was carried from one location to another, allowing the parasite to spread into new soybean fields.  It’s also possible that humans unknowingly planted their soybeans in soil that already contained the SCN eggs.  

Regardless of how the SCN parasite got into the soybean fields, the result is the same.  The plants in the fields are now reunited with their old nemesis, the SCN.  The soybeans that humans had selected for so long ago, did not contain the genes necessary to slow or stop the nematode’s growth.  The nematodes find themselves surrounded by what seems like an endless supply of food.  This means lower yields, sick plants, and even more spreading of this parasite. 

Let’s not forget that there are still some populations of ancestral soybeans.  Some of them do contain the gene or genes that help them resist the SCN parasite.   By breeding these wild plants with the domesticated populations, it is possible to add this trait to the populations growing in our fields. 

Another option is to search our domestic populations for mutants that can resist the nematode.  Here’s a short explanation of how that might work. 

Today farmers harvest all of their seeds and sell them or feed them to their animals.  Most farmers will buy seeds that are nearly genetically identical to each other, but, mutations do happen.  Imagine you were looking for a mutation that would allow the plants to survive an SCN attack.  You might go out and look into fields that are known to have SCN parasites in the soil.  If you found such a field and there in the middle of the yellow and withered plants you found one green plant, happily growing as if the parasite had not even touched it, you might have found a mutant.  Harvesting the seeds of such plants and testing the offspring to see if they too could withstand the parasite might result in future generations, and a new variety of soybean seeds, that can resist the parasite. 

Because we now understand how biological evolution works, this whole process of creating new varieties of plants that are able to resist their pests makes much more sense.  We now know that natural processes create variations in every population, plant or animal.  We also know that if only some of those organisms survive to reproduce, that the population will change, some genes will be lost and others will become more common.  We also know that if two populations are separated for long enough, they each have this ability to change.

The pressures put on any population will ultimately determine which organisms survive and which genes are winners, becoming more popular in the next generation, and which genes are losers, becoming less popular in the next generation. 

Now consider what might happen if a farmer chooses to plant a strain of soybeans that are resistant to the SCN parasites. What might happen to the parasite population?

Most of the SCN in this field have not been exposed to this type of soybean before and they have been growing more and more dense as the years progress.  Suddenly, this new soybean is their only food source.  Because they are a population of animals, they will already have some variety, represented here by the different colors.  Many of the SCN that were able to survive and reproduce last year, will not be as successful this year.  Instead, only those worms who are able to avoid detection, or, in some other way are able to enter the soybean root, will successfully grow and reproduce. 

Let’s pretend the blue worms survive and the red worms do not.  Notice the after one year the number of worms drops dramatically!

During year two and going forward, what do you think might happen to the number of worms? 

That’s right, the numbers start climbing again as the blue worms have a seemingly endless food supply and there are very few surviving red worms.

The theory of evolution predicts these types of changes because 1, variation exists, 2, certain varieties will survive better than others, and 3, the selection pressure remains constant.