Annual vs. perennial plant evolution

Time scale

Above the species level, plant lineages clearly vary in their tendency for annuality or perenniality (e.g., wheat vs. oaks). 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). Indeed, ability to perennate (live more than one year) may vary within a single population of a species.

Underlying mechanisms: Trade-offs

Annuality and perenniality are complex traits involving many underlying, often quantitative, genotypic and phenotypic factors. They are often determined by a trade-off between allocation to sexual (flower) structures and asexual (vegetative) structures. Switches between the annual and perennial habit are known to be common among herbaceous angiosperms.

Increased allocation to reproduction early in life generally leads to a decrease in survival later in life (senescence); this occurs in both annual and perennial semelparous plants. Exceptions to this pattern include long-lived clonal (see ramets section below) and long-lived non-clonal perennial species (e.g., bristlecone pine).

Associated traits

Many traits involving mating patterns (e.g., outcrossing or selfing) and life history strategies (e.g., annual or perennial) are inherently linked.

Self-fertilization

Self-fertilization (selfing, or autogamy) is more common in annual compared to perennial herbs. Since annuals typically have only one opportunity for reproduction, selfing provides a reliable source of fertilization. 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. Selfing and inbreeding can also result in the accumulation of deleterious alleles, resulting in inbreeding depression.

Semelparity

All annual plants are considered semelparous (a.k.a., monocarpy or big-bang reproduction), i.e., they reproduce once before death. It should be noted (see "Anomalies" section) that even semelparity exerts some plasticity in terms of seed-production timing over the year. That is, it is uncommon for all offspring to be generated at exactly the same time, which would be considered the extreme end of semelparity. 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. This is attributed to phenotypic plasticity.

Biennial plants (living two years and reproducing in the second) are also considered semelparous.

Seed bank

Although annuals have no vegetative regrowth from year to year, many retain a dormant population back-up underground in the form of a seed bank. The seed bank serves as an annual's source of age structure in the sense that often not all seeds will germinate each year. Thus, each year's population will consist of individuals of different ages in terms of seed dormancy times. 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. Not all annuals, however, retain a seed bank. As far as population density, annuals with seed banks are predicted to be more temporally variable yet more spatially constant over time, while plants with no seed bank would be expected to be patchy (spatially variable).

Cross-fertilization

Certain non-selfing reproductive adaptations, such as dioecy (obligate outcrossing via separate male and female individuals), may have arisen in long lived herbaceous and woody species due to negative side effects of selfing in these species, notably genetic load and inbreeding depression. Among angiosperms, dioecy is known to be substantially more common than pure self-incompatibility. Dioecy is also more typical of trees and shrubs compared to annual species.

Iteroparity

Most perennials are iteroparous (or polycarpic), which means they reproduce multiple times during their lifespan.

Persistence of ramets

Ramets are vegetative, clonal extensions of a central genet. Common examples are rhizomes (modified stem), tillers, and stolons. A plant is perennial if the birth rate of ramets exceeds their death rate. Some groups of ramets have been seen to live to hundreds of years of age, and a steady rate of branching may aid in avoiding senescence. Some perennials even display negative senescence, in which they physiologically improve with age.

Examples of plants with rhizomatous growth include perennial Sorghum and rice, which likely share similar underlying genes controlling rhizome growth. 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. This is called post-sexual cycle regrowth (PSCR). Such long-lived genets in a population may provide a buffer against random environmental fluctuations.

Polyploidy

There is a possible connection between polyploidy (having more than two copies of one's chromosomes) and perenniality. 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. One example could be the intricate polyploidy of native Australian perennial Glycine species.

Niche Conservatism

Woody species have been found to occupy fewer climatic niches than herbaceous species, which was suggested to be a result of their slower generation time; such differences in adaptation may result in niche conservatism among perennial species, in the sense that their climatic niche has not changed much over evolutionary time.

Semelparity and iteroparity

Semelparity in perennials is rare but occurs in several types of plants, likely due to adaptive changes for greater seed allocation in response to seed predation (although other drivers, such as biased pollination, have been proposed).

List of semelparous perennials:

Mating system

The Polemoniaceae (phlox) family shows considerable flexibility in both life history and mating system, showing combinations of annual / selfing, annual / outcrossing, perennial / selfing, and perennial / outcrossing lineages. These switches indicate a more ecologically determined, rather than a phylogenetically fixed, change in habit.

Environmental drivers

High environmental stochasticity, i.e., random fluctuations in climate or disturbance regime, can be buffered by both the annual and perennial habit. However, the annual habit is more closely associated with a stochastic environment, whether that is naturally or artificially induced. This is due to higher seedling compared to adult survival in such stochastic environments; common examples are arid environments such as deserts as well as frequently disturbed habitats (e.g., cropland). Iteroparous perennial species are more likely to persist in habitats where adult survival is favored over seedling survival (e.g., canopied, moist). This adult/juvenile trade-off can be described succinctly in the following equations:

λ_a = cm_a

λ_p = cm_p + p

m_a > (or <) m_p + (p/c)

(Silvertown & Charlesworth, 2001, p. 296)

Where: λ_a = rate of growth of annual population. λ_p = rate of growth of perennial population. c = survival to reproductive age (flowering). m_a = seeds produced for each annual individual (average). m_p = seeds produced for each perennial individual. p = adult survival.

If m_a > m_p + (p/c), the annual habit has greater fitness. If m_a < m_p + (p/c), the perennial habit has greater fitness. Thus a great deal of the fitness balance depends on the reproductive allocation to seeds, which is why annuals are known for greater reproductive effort than perennials.

Different climate and disturbance patterns may also cause demographic changes in populations.

Evolution rate

The annual vs. perennial trait has been empirically associated with differing subsequent rates of molecular evolution within multiple plant lineages. The perennial trait is generally associated with a slower rate of evolution than annual species when looking at both non-coding and coding DNA. Generation time is often implicated as one of the major factors contributing to this disparity, with perennials having longer generation times and likewise an overall slower mutation and adaptation rate. This may result in higher genetic diversity in annual lineages.

Plant taxon groups that have evolved both annual and perennial life forms.

Artificial selection

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. However, woody perennials also exemplify a major group of crops, especially fruit trees and nuts. High yield herbaceous perennial grain or seed crops, however, are virtually nonexistent, despite potential agronomic benefits. Several common herbaceous perennial fruit, herbs, and vegetables exist, however; see perennial plants for a list.

Annual and perennial species are known to respond to selection in different ways. 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. Cultivated woody perennials are also known for their longer generation time, outcrossing with wild species (introducing new genetic variation), and variety of geographic origin. 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).

Current agricultural applications

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. Wild perennial species are often more resistant to pests than annual cultivars, and many perennial crop wild relatives have already been hybridized with annual crops to confer this resistance. Perennial species also typically store more atmospheric carbon than annual crops, which can help to mitigate climate change. Unfavorable characteristics of such herbaceous perennials include energetically unfavorable trade-offs and long periods of juvenile non-productivity. Some institutions, such as The Land Institute, have begun to develop perennial grains, such as Kernza (perennial wheat), as potential crops. Some traits underlying perenniality may involve relatively simple networks of traits, which can be conferred through hybrid crosses, as in the case of perennial wheat crossed with annual wheat.