Invasive species often start with just a few individuals and multiply to huge numbers across a continent, but they don鈥檛 seem to suffer from a lack of genetic diversity. Or do they? If not, why not?
鈥� The answer is complex, but the bottom line is that an invasive plant species starts with a big advantage: its natural predators, parasites and pathogens are generally absent from its new environment. Therefore, it doesn鈥檛 need the genetic diversity which would otherwise ensure that such threats only affect a proportion of the population.
In addition, a large proportion of invasive species do not reproduce sexually. The extreme example is Canadian pondweed. It blocks watercourses worldwide, yet it is only female plants that have escaped from North America. These can give rise to new plants in an entirely 鈥渧egetative鈥� way 鈥� seeds are never produced.
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In less extreme examples, many invasive species are , producing viable seeds but not sexually, so seeds are not genetically distinct from the parent. Blackberries are a classic example. Mutations do occur in apomictic species, and the lack of sexual reproduction means they could persist in the offspring. This might sound like a recipe for disaster, but these species are typically 鈥減olyploid鈥�, meaning they have more than one copy of each chromosome, so the mutant version is diluted. They do change over time, but these changes don鈥檛 spread through the population. This means that the taxonomy of an apomictic genus is a nightmare.
The other typical characteristic of an invasive species is strong vegetative reproduction through runners and roots. Even copious seed producers, such as tree of heaven and Australian fireweed, spread locally and mostly vegetatively. Sex, again, plays only a minor part.
Guy Cox, St Albans, New South Wales, Australia
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