Plant outbreeding depression and human effects on plant populations

Plant outbreeding depression and human effects on plant populations

Most people have heard of inbreeding (short forInbreeding depression), the phenomenon in which two offspring that are too genetically similar have offspring that are less fit or viable. Inbreeding is a problem in both humans and other organisms.

Outbreeding depressionis a similar effect that occurs in the opposite scenario: reduced fitness occurs when two individuals who 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 is more of a serious problem than outbreeding depression. There is some evidence of outbreeding depression in genetically distant individuals, but it does not appear to cause the same severity of problems with birth defects or other genetic disorders that inbreeding can lead to.

However, in plants the problems can be pronounced due to their different genetic characteristics, and in many plant species outbreeding causes more serious problems than inbreeding.

The reasons and mechanisms for the different genetic behavior and performance of plants are numerous. Plant genetics can be a bit messier and more diverse 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

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, while most plants are anchored to a fixed location and are therefore limited in their ability to reproduce to individuals who are physically close to them. Additionally, many plants are self-fertile or self-pollinating, allowing an individual to produce genetically distinct offspring by recombining its own genetic material. To produce viable offspring through self-fertilization, plants must carry more genetic diversity in each individual. This also protects against inbreeding - which is likely in plants due to physical limitations.

Interestingly, some animals that are less mobile, such as mussels and other shellfish that attach to solid substrates, can also have serious problems with outbreeding depression.

Effects of outbreeding depression on human activity

Plants are not normally subject to outbreeding depression in the natural environment because the geographical and physical constraints on reproduction and seed dispersal ensure that genetic material flows only slowly and gradually through different plant populations.

However, humans have changed this by moving plant species for the purposes of gardening, farming, farming and landscaping.

Until recently, most discussions of human impacts on plant populations have focused on humans introducing new species that have the potential to become invasive. But there is another, harder-to-see influence that humans have: moving individuals of a particular plant species within their range.

In this way, humans can introduce new individuals of a species to a distant region within that species' range. For example, when humans plant a tree or flower far from its origin, it can now cross-pollinate with natural populations of its species, and the resulting offspring are more likely to suffer from outbreeding depression. Humans can thus affect the fitness of wild populations of plants growing next to gardens that they plant with seeds from distant populations.

The Structure of the Commercial Nursery Industry

Today, most of the plants that people grow in their gardens are purchased from nurseries. The financial benefits of economies of scale have led kindergartens to consolidate their operations. Most nurseries that sell plants do not grow plants from seed, but rather purchase them from huge operations, often many states away.

So if you buy a tree or a flower, it may come from a population in a completely different ecoregion, and even if you buy a plant species native to your region, the individual you are buying comes from a distant population of that species. It is likely to contribute to outbreeding depression if it cross-pollinates with local wild populations.

Add varieties to the mix and destroy genetic diversity

There are other complexities to the practice of developing and selecting plant varieties, the “named” varieties you see in most garden centers. These varieties mentioned are selected for particular horticultural characteristics and tend to have much lower genetic variability than plants from wild populations.

The net effect is that these varieties can now contributebothInbreeding and outbreeding depression in wild populations.

In summary

Outbreeding depression, the reduced fitness that results from offspring of two genetically distant individuals of the same species, is a major problem in many plant species, a much larger problem than inbreeding depression. The best practice for landscaping and gardening is to plant genetically diverse individuals drawn from local populations to minimize the negative impact of outbreeding depression that your gardening decisions have on wild plant populations.

Inspired by Alex Zorach