MECHANISMS OF EVOLUTION
Plants become isolated geographically or ecologically and can diverge. In any one place physiological barriers, barriers to pollination or chromosomal changes, especially polyploidy, can lead to isolation. The different forms may be known as varieties, subspecies or species depending on the degree of difference and the distribution.
Breakdown of isolating mechanisms
Environmental change or a second colonization can lead to reconnection and a breakdown of any distinction. Many plants can hybridize with related species, the hybrids sometimes being sterile or partially sterile and often less well adapted than their parent species. Natural selection normally favors a physiological breeding barrier between species.
If the chromosomes divide but the cell does not it becomes tetraploid. If this happens in the reproductive cells the plant will be tetraploid and may become reproductively isolated from its parent diploid species. This has happened many times after hybridization and it can restore fertility. A majority of flowering plants are polyploid and it occurs in all other land plant groups.
Patterns of speciation
Plants are more diverse in the tropics than the temperate, partly owing to the period of isolation. Trees and wind-pollinated plants have fewer species than herbaceous or insect-pollinated plants. The most specialized insect-pollinated plants, orchids, have a large number of species, many of them rare.
Most plant species live in populations in which the individuals are capable of interbreeding . If one population becomes isolated from the others and does not interbreed with them, new varieties or species may form. Plants may become isolated through being physically separated. If seeds are transported to an isolated island by freak weather or via a bird, they may become isolated geographically by an area of unfavorable habitat. This may be an oceanic island surrounded by sea, or an island of suitable habitat such as a mountain peak surrounded by lowland. A population can become at least partially isolated ecologically within the distribution of the parent species if it colonizes a habitat with different soil conditions or dominant vegetation type requiring markedly different adaptations.
A reproductive barrier to other members of population may develop in one location. This can be a physiological barrier between two plants, perhapsassociated with a self-incompatibility system . Flowers may open at different times of the year and not overlap in their flowering seasons, or they may become adapted to different pollinating insects, especially if the flowers are highly specialized to one or a small group of insect species. Plants may be isolated by chromosome number through polyploidy since this is flexible and variable in plants and those with different numbers may be incapable of reproducing together. If a species is capable of self-fertilization or asexual reproduction , a single isolated individual can found a new population. Once a group of individuals becomes reproductively isolated it can diverge from theparent population either in its morphology or in its physiology, or both. A small isolated population may be subject to genetic drift or to different selection pressures, particularly if it is in a new area or habitat.
Where recognizably different populations are geographically isolated, they are regarded as species if the differences are marked, but races orsubspecies if they are minor; there is no absolute distinction between a species and a race. For populations in different ecological conditions, the termecotype is used . Subspecies, or more generally variety, is also used for different chromosome types or morphological types that coexist, if distinct consistent forms can be recognized. A physiological reproductive barrier, used as a definition of a species in the ‘biological species concept’, is useful in some plant groups, but many plants can hybridize and, with a range of different breeding systems , the definition cannot be used consistently. No one definition of species works for all plants.
If environmental conditions change, what was previously a geographical barrier may cease to be one, e.g. with the expansion and retreat of glaciers in the northern hemisphere during the last million years, or where two previously isolated plant species have met following their introduction by humans. Geographically isolated populations can reconnect with the parental population through a second colonization or a return to the place of origin and the two populations may interbreed and become one species.
Many plant species are capable of hybridizing with related species. These hybrids may be sterile (like the horse/donkey hybrid, the mule), and the reproductive barrier between the species is maintained, but frequently the sterility is partial, and some fertile pollen (rarely ovules) is produced. In some plants the hybrids are fully fertile with their parents. Some hybrids are capable of interbreeding with one parental species but not the other, and in these circumstances, the interbreeding parent may become highly variable in those sites where there is hybridization. In some plants, hybrid swarms between species are fairly common and the species barrier has locally broken down completely.
Hybrids are frequently rare and confined to disturbed or intermediate habitats between those of its parent species. They are likely to be less well adapted than either parent and hybridization may break up any collections of genes that allow the plant to be well adapted to its habitat. Under these circumstances, natural selection will favor or reinforce a physiological or other barrier to interbreeding, often coupled with a divergence in flowering time, pollinator type or pollen placement.
Plants have many cells that contain more than one set of chromosomes . These appear to derive from a division of the chromosomes not being followed by a cell division. If this happens at some point in the reproductive cycle, around or during meiosis or just after fertilization, the basic number for the plant can be doubled. This has occurred frequently and can happen spontaneously within a species, when it is known as autopolyploidy. Some individuals of the species will then be tetraploid and have four (or more) sets of chromosomes alongside the diploids with two. Morphologically, the diploid and tetraploid plants are often identical or nearly so, but they may be completely isolated from each other reproductively and have different distributions. Some authors regard these as separate species but most, for practical reasons, regard them as chromosomal varieties. Some are not completely isolated from each other and can reproduce successfully across the ploidy levels, though there may be abnormalities at meiosis leading to partial sterility.
Following hybridization polyploidy can restore fertility. If a hybrid is partially or completely sterile, this is normally due to the inability of the chromosomes of the two parents to pair correctly at meiosis because they are too different. If the chromosomes double without cell division, then at meiosis there will be an exact copy of each chromosome, so pairing at meiosis will be possible and fertility restored. This has happened numerous times in plants, many tetraploids being the product of a hybridization followed by a doubling of chromosomes. This is known as allopolyploidy. The new polyploidy is likely to be reproductively isolated from both parents immediately, so, in effect, forms a new species in a single generation. Polyploids are frequently selfcompatible even if the parents have a self-incompatibility mechanism so one plant is often viable as the founder of a new species.
A small majority of flowering plant species are probably derived by polyploidy at some stage in their ancestry since they have a high number of chromosomes (50 or more). It occurs in numerous angiosperm families and is also common among all other land plants, the highest chromosome number known (over 1000) being found in an adderstongue fern .
The number of species is highly variable between families and between genera and these can be related to climate, mode of growth and reproduction. There are more species in the tropics and in certain places like the Cape region than in the northern temperate region. Although there are many reasons for this, one of them must be the period of isolation. Where the climate has forced large-scale plant migrations, many species are widespread but where the migrations have been less owing to less severe climatic fluctuation they have been more isolated and more species have formed as a result. This will promote gdiversity , and, in the most species-rich regions, each species has a more restricted distribution than in the less diverse regions.
In general, trees are less diverse than herbaceous plants and wind-pollinatedplants are less diverse than insect-pollinated plants. Trees and wind-pollinated plants disperse their pollen greater distances and more generally than herbaceousplants (most insect-pollinated trees can be pollinated by a range ofinsects), thereby connecting populations over a large area. Among insectpollinated families the most specialized to particular insects are the most species-rich. The extreme example is the orchids, in which each species has become specialized to one or a few insect species as pollinators. This has allowed numerous species to form , most of which are rare members of the plant communities in which they grow. Many are capable of hybridization with related species producing fertile offspring, but they do not normally hybridize because the insect species remain constant.