Genetic diversity

The crops that we know and enjoy come in amazing diversity that most of us have never observed. Sure, we might see ten different kinds of apples when we go to the grocery store. When we buy tomatoes, we might have three or four choices – at a typical grocery store, we can choose whether we want small cherry tomatoes or a big beefsteak variety. If we go to Whole Foods, we get a few more choices. But still, it’s only the tiniest fraction of what is out there.

Maybe this picture gives a better sense of what we tend to miss out on. There are all different colors, sizes, shapes, textures, and flavors out there that most of us never experience.

Tomato diversity
Image by Karsten Seiferlin, Flickr

For another example, let’s think about carrots. The only carrot most people know of is orange, with one particular size and one particular shape. But did you know that the original carrot was not orange? Modern carrots are descended from wild carrots, which are white.

Wild carrot root
Image from World Carrot Museum

Through selective breeding, various colors, sizes and shapes of carrots developed. Today carrots come in the familiar orange, but also red, yellow, purple, and white.


Carrot Diversity
Image from Agricultural Research Services, United States Department of Agriculture

All of this genetic diversity is very important, because different varieties of the same species have different ability to resist pests and diseases, live in different day lengths or light intensity, deal with drought or soil salinity, or grow in less fertile soils. Tragedies have happened when countries have focused exclusively on one crop or crop variety.

The most dramatic example is the Irish potato famine. In the 1840s, the fungus-like organism that causes late blight was introduced to Ireland. In wet weather, this fungus-like organism will cause dark spots on the leaves and stems, and in the case of stem lesions, the whole plant may quickly collapse and die. Meanwhile, the potatoes themselves also become infected and soon rot away. Even when potatoes initially look healthy after harvest, they may develop the infection and rot while in storage.

Potato with Late Blight
Image from Agriculture Research Service, United States Department of Agriculture

In 1845, late blight destroyed one third to one half of Ireland’s potato crop. 1846 was even worse, and three-quarters of the crop was lost. In 1848, one third of the crop was again lost. In this period of history, the Irish were dependent on potato as their main source of calories, and one potato variety called the Irish Lumper in particular. Growing a single crop species with limited genetic diversity made it easy for one pathogen to cause such destruction. There was widespread famine in Ireland at this time, killing about 1 million people and forcing another million to emigrate from Ireland (1).

A more recent example is with Southern Corn Leaf Blight. In the 1960s, corn seed companies began primarily using corn with a trait that would allow for easier corn breeding. By 1970, over 80% of the corn that was grown had the trait. By 1968, a new race of the Southern Corn Leaf Blight pathogen came along that was able to cause disease on all the corn with this trait. Because that was the primary corn variety grown, in 1970 the disease was able to destroy about 15% of the corn belt’s crop for the year (2).

Southern Corn Leaf Blight Image from David B. Langston, University of Georgia,

Southern Corn Leaf Blight
Image from David B. Langston, University of Georgia,

The problem that we are facing today is that locally adapted crop varieties are rapidly being lost, in favor of a few elite cultivars. For example, in the early 1900s there were over 8000 apple varieties in the United States. Today, more than 95% of those varieties have been lost.

The problem goes beyond loss of genetic diversity within species. The variety of species that we eat has also been dramatically reduced. Throughout human history, over 7000 plant species have been used for food. Today, only 150 plant species are under extensive cultivation, and most people get 90% of their energy needs from only 15 plant species (1).

So what does all this have to do with backyard food production? I believe this is one area where those of us with a garden can make a real impact. If we have a small amount of garden space, we can grow one or a few rare plant varieties in our gardens, perhaps saving seed from year to year so that the variety becomes adapted to our local growing conditions. If we have a little bit more garden space, we can even do some crosses and create a new plant variety.


(1) Monoculture and the Irish Potato Famine: cases of missing genetic variation.

(2) Bruce Pollock. The Victory Seed Company.

(3) Paul Gepts. Plant Genetic Resources Conservation and Utilization: The Accomplishments and Future of a Societal Insurance Policy. 2006. Crop Science 46: 2278-2292.

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