Crops like tomatoes, even heirloom tomatoes, aren’t found in the wild. Domestication of crops usually involves only a relative handful of individual plants. Narrowing the species down to a few hundred (or possibly even a few dozen plants) means only a limited number of copies of each gene will be carried through and many of the variant copies of the genes present in the wild population won’t be included in that number. Keeping the population small for multiple generation reduces variability even more as by chance some rare version of genes in one generation won’t be passed to any of the offspring in the next.
Genetic bottlenecks happen in the animal world as well. Skin grafts between unrelated Cheetahs aren’t rejected because the animals are so genetically similar their immune system can’t distinguish the grafted skin as being different from its own skin. Even less fortunate are the tasmanian devils who have so little genetic diversity that they are being decimated by a transmissible cancer. After fighting with an infected devil, which has tumors on its face and neck, tiny bits of the cancer will get into an uninfected devil’s wounds, and since the immune system can’t distinguish the foreign cancer cells from the devil’s own cells, the cancer cells reproduce unchecked, the trait that makes normal cancers, produced by mutated versions of our own cells, so deadly. And the solution mentioned in the article, to save the species by protecting 200 individuals, while better than letting them all die, will sacrifice even more genetic variability by subjecting the already inbred devils to a new population (and genetic) bottleneck.
I’ve gone off-topic with cool biology.* most crops capture only a small slice of the genetic diversity present in their wild progenitors. Crops can even go through extra bottlenecks, as when a few potatoes, carried back from the new world, gave rise to most of the potatoes grown throughout Europe. Even less diversity, which would have given any plant breeders of the say even less to work with if they tried to develop a blight resistant potato.
To overcome those limitations, today breeders of many crops will hunt down the wild ancestors of that crop and do crosses to bring more of the wild species’ genetic diversity and look for valuable genetic variants that were lost during the original domestication hundreds or thousands of years ago, or new genetic variants that have emerged in the wild since.
It is these intentional introgressions of wild genetic material that make modern tomato breeds more genetically diverse than older tomato breeds such as heirlooms that drew from the more limited range of genetic variants available in tomatoes at the time of their initial breeding. h/t to TheScientistGardener for bringing that awesome fact to light.
*Come on, a disease that spreads when broken off pieces of external tumors get into open wounds is pretty cool, especially when you consider fighting in a crucial part of tasmania devils mating rituals. I do hope the devils can be saved though, Australia has lost enough cool species without the Tasmanian Devils going the way of the Tasmanian Tiger.