Population size is an important factor that contributes greatly to the maintenance of your crops’ genetic diversity. When adequately sized, it allows a plant variety to better adapt to environmental changes thanks to its genetic variability and vitality. To preserve this diversity, seeds should therefore be saved from the greatest possible number of different individuals that meet the selection criteria (i.e., resistance to a specific disease). The selected seeds grown the next season will carry the genetic component that contains a certain level of resistance to the said disease, ensuring the plants to survive and reproduce. This trait can be further “enhanced” through subsequent seed selections over the next generations. Genetic heterogeneity is therefore a critical element in the selection, adaptation, and evolution of varieties.
Saving seeds from a great pool of different plants will also help to prevent inbreeding depression – a reduced biological fitness that detrimentally alter the plant’s ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. That translates in plants germinating and yielding poorly, experiencing a loss of vigor and fertility, which then lead to a reduction of the population size encouraged by environmental events. With some exceptions (such as cucurbits), outbreeding plants are more susceptible to inbreeding depression (corn, carrots and onions are notorious examples) and therefore rely on cross-pollination in order to preserve and increase their genetic heterogeneity. Encouraging cross-pollination between individuals through adequate spacing and planting pattern (blocks instead of rows) is therefore important to maximize the mixing of genes. Due to their nature and fertilization method, inbreeders naturally contain less genetic diversity and are less affected by inbreeding depression. They can often be maintained with populations of very few plants (i.e., common beans). If signs of inbreeding depression emerge, the introduction of seeds from the same variety from a reliable outside source can be made in order to reinvigorate and reacquire the genetic diversity of the variety.
Plant populations that are too small are also at risk of being exposed to genetic drift, a change in the frequency of an existing gene variant (allele) in the population due to random sampling of organisms. This may result in the chance disappearance of particular genes for certain traits and thereby reduce genetic variation as individuals die or do not reproduce. Genetic drift can also cause initially rare alleles to become much more frequent and even fixed.
To ensure and maintain a diverse representation of the gene pool within a population, a minimum number of plants need to be grown using a healthy seed sample with a good germination rate. While this number varies from one crop to another and differs depending on experts, the general recommended rule is that seeds should be saved from at least 20 inbreeding or 100 outbreeding individuals per variety. Home gardeners have often limited space at their disposal and should therefore aim – depending on their goal – to grow as many plants as their garden allows it in order to yield an adequate range of genetic diversity.
Specific population size recommendations for applicable crops are discussed in Module VII.