Genetic diversity is the variation within a crop variety, and a seed bank protects it by saving seed from many plants, not only the best few, because a stock saved from too few plants loses vigour, yield and disease resistance over the generations. The cure for a narrowed stock is to save from more plants and to bring in seed of the same crop from another grower.
Part of Community Seed Bank Management
Genetic diversity in a seed bank is the variation among individual plants of one variety that lets it cope with disease, weather and change.
A Uniform Stand Can Be A Fragile One
A field that looks uniform and healthy may have little variation in its genes. Variation is what lets some plants survive a disease or a dry year when others fail. A crop with none depends on its luck. The Irish potato famine of the 1840s is the usual lesson: one clonal variety was planted across a country, and the blight attacked all of it alike (Genetic Diversity).
What Wears Diversity Down
- Saving from too few plants. The seed bank then keeps only the genes those plants carried, and inbreeding rises each generation. The effect is strongest in crops that normally cross-pollinate (Minimum Population Sizes).
- Selecting too hard. Saving only the single best plants narrows the stock. Save from average plants too (Selecting for Adaptation vs Uniformity).
- Crossing with another variety by accident. This adds variation of the wrong kind; see Isolation & Cross-Pollination.
- Losing varieties altogether. A variety grown by one household and lost to one bad year is gone.
What Rebuilds It
- Save from many plants of each variety, in the numbers set for the crop.
- Spread the growing. Several households or plots should grow each important variety, so one disaster does not take it.
- Exchange seed with neighbouring communities. New seed of the same variety brings in new genes.
- Keep older seed lots in the bank, so a failed generation can be replaced.
- Record how many plants each lot came from (Seed Inventory and Catalog System).
Self-Pollinators And Cross-Pollinators Differ
A self-pollinating variety is largely uniform already, so inbreeding depression is not the concern; the risk is losing the small variation it has. A cross-pollinating variety carries much more variation and suffers most from small populations. The numbers for each are in the linked article.
Common Mistakes
- Believing that a uniform variety is healthy. Uniformity is not diversity.
- Replanting only the seed of the best year. One lot is a single point of failure.
- Letting one household hold the only copy. Duplicate every important lot.
What’s Next
- Learn next: Minimum Population Sizes to Avoid Inbreeding — the arithmetic of how many plants
- Combine with: Landrace Development — where diversity becomes local adaptation