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Plant evolution is the subset ofevolutionary phenomena that concernplants. Evolutionary phenomena are characteristics ofpopulations that are described byaverages,medians,distributions, and otherstatistical methods. This distinguishes plant evolution fromplant development, a branch ofdevelopmental biology which concerns the changes that individuals go through in their lives. The study of plant evolution attempts to explain how the presentdiversity of plants arose overgeologic time. It includes the study ofgenetic change and the consequentvariation that often results inspeciation, one of the most important types ofradiation intotaxonomic groups calledclades. A description of radiation is called aphylogeny and is often represented by type of diagram called aphylogenetic tree.
Differences between plant and animal physiology and reproduction cause minor differences in how they evolve.
One major difference is thetotipotent nature of plant cells, allowing them to reproduce asexually much more easily than most animals. They are also capable ofpolyploidy – where more than two chromosome sets are inherited from the parents. This allows relatively fast bursts of evolution to occur, for example by the effect ofgene duplication. The long periods of dormancy that seed plants can employ also makes them less vulnerable to extinction, as they can "sit out" the tough periods and wait until more clement times to leap back to life.[1]
The effect of these differences is most profoundly seen during extinction events. These events, which wiped out between 6 and 62% of terrestrial animal families, had "negligible" effect on plant families.[2] However, the ecosystem structure is significantly rearranged, with the abundances and distributions of different groups of plants changing profoundly.[2] These effects are perhaps due to the higher diversity within families, as extinction – whichwascommon at the species level – was very selective. For example, wind-pollinated species survived better than insect-pollinated taxa, and specialised species generally lost out.[2] In general, the surviving taxa were rare before the extinction, suggesting that they were generalists who were poor competitors when times were easy, but prospered when specialised groups became extinct and left ecological niches vacant.[2]
Duringembryogenesis, plants and animals pass through aphylotypic stage that evolved independently[3] and that causes a developmental constraint limiting morphological diversification.[4][5][6][7]
Polyploidy is pervasive in plants and some estimates suggest that 30–80% of living plant species are polyploid, and many lineages show evidence of ancient polyploidy (paleopolyploidy) in their genomes.[8][9][10] Huge explosions inangiosperm species diversity appear to have coincided with ancient genome duplications shared by many species.[11] 15% of angiosperm and 31% of fern speciation events are accompanied by ploidy increase.[12] Most polyploids displayheterosis relative to their parental species, and may display novel variation or morphologies that may contribute to the processes ofspeciation and eco-niche exploitation.[9][13] The mechanisms leading to novel variation in newly formed allopolyploids may include gene dosage effects (resulting from more numerous copies of genome content), the reunion of divergent gene regulatory hierarchies, chromosomal rearrangements, andepigenetic remodeling, all of which affect gene content and/or expression levels.[14][15][16] Many of these rapid changes may contribute to reproductive isolation and speciation.
Alleukaryotes probably have experienced a polyploidy event at some point in their evolutionary history. Seepaleopolyploidy. In many cases, these events can be inferred only through comparingsequenced genomes.Angiosperms have paleopolyploidy in their ancestry. Unexpected ancient genome duplications have recently been confirmed in mustard weed/thale cress (Arabidopsis thaliana) andrice (Oryza sativa).
Cyanobacteria remained principalprimary producers throughout theProterozoic Eon (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable ofnitrogen fixation.[citation needed]Green algae joined blue-greens as major primary producers oncontinental shelves near the end of theProterozoic, but only with theMesozoic (251–65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms didprimary production in marine shelf waters take modern form. Cyanobacteria remain critical tomarine ecosystems as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as theplastids of marine algae.[17]
Chloroplasts have many similarities with cyanobacteria, including a circularchromosome, prokaryotic-typeribosomes, and similar proteins in the photosynthetic reaction center.[18][19] Theendosymbiotic theory suggests that photosynthetic bacteria were acquired (byendocytosis) by earlyeukaryotic cells to form the firstplant cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Likemitochondria, chloroplasts still possess their own DNA, separate from thenuclear DNA of their plant host cells and the genes in this chloroplast DNA resemble those incyanobacteria.[20] DNA in chloroplasts codes forredox proteins such as photosynthetic reaction centers. TheCoRR hypothesis proposes that thisCo-location is required forRedoxRegulation.
Transcription factors and transcriptional regulatory networks play key roles in plant development and stress responses, as well as their evolution. During plant landing, many novel transcription factor families emerged and are preferentially wired into the networks of multicellular development, reproduction, and organ development, contributing to more complex morphogenesis of land plants.[21]
Charles Darwin in his 1878 book The Effects of Cross and Self-Fertilization in the Vegetable Kingdom[22] at the beginning of chapter XII noted “The first and most important of the conclusions which may be drawn from the observations given in this volume, is that generally cross-fertilisation is beneficial and self-fertilisation often injurious, at least with the plants on which I experimented.”Flowers emerged in plant evolution as an adaptation for the promotion of cross-fertilisation (outcrossing), a process that allows the masking of deleteriousmutations in thegenome of progeny. The genetic masking effect of cross-fertilisation sexual reproduction is known asgenetic complementation.[23] This beneficial effect of cross-fertilisation on progeny is also recognized ashybrid vigor orheterosis. Once flowers became established as an evolutionary adaptation to promote cross-fertilization, subsequent switching to inbreeding ordinarily becomes disadvantageous, largely because it allows expression of the previously masked deleterious recessive mutations, i.e.inbreeding depression.
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