In our previous post “Flowers: Sex, Pollinators and Pollinizers“, we focused our topic on asexual reproduction in plants — primarily those that need a pollinator or pollinizer to boost the reproduction cycle. Asexual reproduction, as we discussed, works best in environments that do not change. But as you may know, our surrounding environment rarely ever stays exactly the same.
So in order to colonize offspring in different locations, plants sexually evolved as a way to ensure diversity among their seedlings.
Weeping Willow — Dioecious
Dioecy in a plant species forces outbreeding, increasing the genetic diversity, and thus the amount of variation among the seedlings.
This can be a good thing in a patchy, heterogeneous environment, an environment that changes, or when colonizing a new environment that has different characteristics.
Brown Turkey Fig — Monoecious
Monoecious plants (unless they are self-incompatible) are more inbred and their seedlings less diverse.
This can be a good thing in a homogenous environment, or one that doesn’t change.
As hinted above, even monoecious plants can be self-incompatible. There are several different ways various plant species can be self-incompatible (dioecy is one). The bottom line among self-incompatible plant species is that outbreeding is the rule and the seedlings they produce are more diverse than they would have been if the parent plant had pollinated itself with its own pollen. The downside of self-incompatibility is that such species require more than one plant to start a new colony. For self-compatible species, all it takes is a single seed to start a new colony somewhere. Self-compatible species are more likely to be found across larger geographic areas, on islands and multiple continents. Self-compatible species are less likely to go extinct when the climate changes or a local disaster occurs because such species have more colonies elsewhere. A self-compatible species may lose local populations, but it is more likely to continue surviving somewhere.
In the real world, about one-half of all flowering plant species are self-incompatible.
The rest can do it alone, although most of them benefit from occasional outcrossing and the increase in genetic diversity that brings. Well known crops that are primarily self-compatible (i.e., self-pollinating) include peas, beans, cotton, peppers, tomatoes and wheat. Crops that are primarily self-incompatible (and must be cross-pollinated) include carrots, potatoes, onions, cabbage, watermelons, cucumbers and apples. If we want to harvest fruits or seeds from self-incompatible plants, we must have at least two plants that are not vegetative clones taken from a single parent plant, but are instead actual seedlings.
Pollination and the subsequent production of seeds is, of course, sexual reproduction. Many plant species are also capable of asexual reproduction (aka vegetative reproduction). Plants that multiply by rhizomes, bulbs, or tubers, etc. produce offspring that are genetically identical to each other and to the parent plant. (Artificial propagation techniques such as layering, root division and the rooting of stem cuttings are also asexual reproduction.) Asexual reproduction does nothing to increase genetic diversity. It works fine for rapidly colonizing a completely new location, and for surviving in place as long as the environment doesn’t change.