Most people have heard of inbreeding (shorthand for inbreeding depression), the phenomenon where when two individuals who are too genetically similar have offspring, the offspring is less fit or viable. Inbreeding is a problem in humans as well as other organisms.
Outbreeding depression is a similar effect that arises in the opposite scenario: decreased fitness resulting when two individuals that are genetically very distant from each other have offspring.
Relative importance of inbreeding vs. outbreeding in plants and animals
In most animals (including humans), inbreeding depression tends to be more serious of a problem than outbreeding depression. There is some evidence of outbreeding depression in genetically distant individuals, but it doesn’t seem to produce the same severity of problems with birth defects or other genetic disorders that inbreeding can result in.
However, with plants, due to their different genetic characteristics, the problems can be pronounced, and in many species of plants, outbreeding produces more serious problems than inbreeding.
The reasons and mechanisms for plants’ distinct genetic behavior and performance are numerous. Plant genetics can be a bit more chaotic and varied than animal genetics. Plants tend to have more chromosomes than animals, and their number of chromosomes can vary more within a species.
Evolutionary pressure on plants
In order to understand why plants are likely to suffer more from outbreeding depression than animals, consider the evolutionary pressures that have been placed on plants.
Animals are mobile, and as such, can travel distances to find new mates, whereas most plants are anchored to a fixed place, and are thus limited in their reproductive choice to individuals physically near them. In addition, many plants are self-fertile or self-pollinating, allowing an individual to produce genetically-distinct offspring by recombining its own genetic material. In order to produce viable offspring by self-fertilization, plants need to carry more genetic diversity within each individual. Doing so also protects against inbreeding–which is likely in plants due to the physical constraints.
Interestingly, some animals that are less mobile, like mussels and other shellfish that attach to fixed substrates, also can have serious problems with outbreeding depression.
Implications of outbreeding depression on human activity
Plants do not normally encounter outbreeding depression in natural environment, because the geographic constraints and physical constraints on reproduction and seed distribution ensure that genetic material flows only slowly and gradually through different plant populations.
Humans, however, have changed this, by moving plant species around for the purposes of gardening, agriculture, farming, and landscaping.
Up until recently, most discussion of human influences on plant populations has focused on humans introducing new species, which have the potential to become invasive. But there is another, harder to detect influence humans are having: moving individuals of a given plant species around within its range.
Through this, humans can introduce new individuals of a species to a distant region within the range of that species. For example, if humans plant a tree or flower far from where it originated, it can now cross-pollinate with natural populations of its species, and the resulting offspring will be more likely to suffer from outbreeding depression. Humans can thus harm the fitness of wild populations of plants growing adjacent to gardens that they plant with seeds from distant populations.
The structure of the commercial nursery industry
Nowadays, most plants that people grow in their gardens are purchased from nurseries. The financial benefits of economy of scale have driven nurseries to consolidate their operations. Most nurseries that sell plants do not grow plants from seed, but rather, purchase them from huge operations that are often many states away.
So when you buy a tree or flower, it might be from a population in a completely different ecoregion, and, even if you are purchasing a plant species native to your local region, if the individual you purchase is from a distant population of that species, it is likely to contribute to outbreeding depression when it cross-pollinates with local wild populations.
Add cultivars to the mix, destroying genetic diversity
The practice of developing and selecting cultivars of plants, the “named” varieties that you see in most garden centers, has further complicated things. These named varieties are selected for special horticultural features, and they tend to have much less genetic variability than plants taken from wild populations.
The net effect is that these cultivars can now contribute both inbreeding and outbreeding depression to wild populations.
Outbreeding depression, the decreased fitness resulting from offspring of two genetically-distant individuals of the same species, is a major problem in many species of plant, a much bigger problem than inbreeding depression. The best practice for landscaping and gardening is to plant genetically-distinct individuals grown from local populations, so as to minimize the negative impact from outbreeding depression that your gardening choices will have on wild plant populations.