Annual vs. perennial plant evolution
[2] The complex history of switches between annual and perennial habit involve both natural and artificial causes, and studies of this fluctuation have importance to sustainable agriculture.
This hypothesis finds support in observations of increased prevalence of annuals in regions with hot-dry summers,[3] with elevated adult mortality and high seed persistence.
[6][7] This contradicts the commonly held belief that annuality is a derived trait from an ancestral perennial life form, as is suggested by a regarded plant population biology text.
On a microevolutionary timescale, a single plant species may show different annual or perennial ecotypes (e.g., adapted to dry or tropical range), as in the case of the wild progenitor of rice (Oryza rufipogon).
[8] Many traits involving mating patterns (e.g., outcrossing or selfing) and life history strategies (e.g., annual or perennial) are inherently linked.
[13] However, switches to selfing in annuals may result in an "evolutionary dead end," in the sense that it is probably unlikely to return to an outcrossing (allogamous) state.
[16] Instead offspring are usually generated in discrete packages (as a sort of micro-iteroparous strategy), and the temporal spacing of these reproductive events varies by organism.
The seed bank also helps to ensure the annual's survival and genetic integrity in variable or disturbed habitats (e.g., a desert), where good growing conditions are not guaranteed every year.
[21][22][23] In wheat (Thinopyrum), perenniality is associated with production of a secondary set of tillers (stems arising from the crown's apical meristem) following the reproductive phase.
[6][24] One potential explanation is that both polyploids (larger in size) and asexual reproduction (common in perennials) tend to be selected for in inhospitable extremes of a species' distribution.
High environmental stochasticity, i.e., random fluctuations in climate or disturbance regime, can be buffered by both the annual and perennial habit.
[8][31][32] The annual vs. perennial trait has been empirically associated with differing subsequent rates of molecular evolution within multiple plant lineages.
: ecology (desert climate) Artificial selection seems to have favored the annual habit, at least in the case of herbaceous species, likely due to fast generation time and therefore a quick response to domestication and improvement efforts.
High yield herbaceous perennial grain or seed crops, however, are virtually nonexistent, despite potential agronomic benefits.
For instance, annual domesticates tend to experience more severe genetic bottlenecks than perennial species, which, at least in those clonally propagated, are more prone to continuation of somatic mutations.
[1][45] Cultivated woody perennials are also known for their longer generation time, outcrossing with wild species (introducing new genetic variation), and variety of geographic origin.
[1] Some woody perennials (e.g., grapes or fruit trees) also have a secondary source of genetic variation within their rootstock (base to which the above-ground portion, the scion, is grafted).
[45] Compared to annual monocultures (which occupy c. 2/3 of the world's agricultural land), perennial crops provide protection against soil erosion, better conserve water and nutrients, and undergo a longer growing season.
[46] Perennial species also typically store more atmospheric carbon than annual crops, which can help to mitigate climate change.
