Speciation is the evolutionary process where in A group of individuals are potential to mate each other and produce viable and fertile off springs. The speciation lead by different patterns. which are given as follows:
Patterns of Speciation
• What happens after 2 species separate from each other?
• In some cases, the species exists for millions of years, gradually changing in response to external conditions but always maintaining as a single distinct species.
• In other cases, many new species will form from a single species in a very short time: this is "adaptive radiation". This often happens on isolated islands, where a new species is blown in by a storm, and finds many different ecological niches to fill. Darwin's finches are an example of this.
• They are thought to have originated with a small group of finches that blew over about 1 million years ago, to islands with no dangerous predators and very few other land birds.
• Also, sometimes a "key innovation" will arise, and new niches will suddenly be open to the possessors of the innovation. Or, a disaster will allow a sudden expansion of a few surviving species. Mammals grew rapidly in number of species following the extinction of the dinosaurs.
Depicted are relationships among the three lineages of plants: glaucophytes (freshwater algae; blue), rhodophytes (red algae; red), and the green plants (chlorophytes, charophytes, and land plants; green). Estimated dates for some nodes are listed in millions of years before present. The primary endosymbiotic event is estimated to have occurred at least 1.6 billion years ago. A deep split within the green lineage created the chlorophyte clade and the charophyte plus land plant clade. Note that both the charophytes and the bryophytes are grades and are not monophyletic. Major events in the evolution of land plants are demarcated with arrows. Species for which complete nuclear genome sequences are available are listed in color (photographs at right; the three angiosperm species are pictured upper left). Species positioned in large phylogenetic gaps where genome sequences would be informative (black) include the following: the basal lineage of land plants, the liverworts, charophycean algal lineages (Chara, Coleochaete) that are sisters to land plants, and the gymnosperms, which are the sister group to flowering plants (angiosperms). Also included is a multicellular chlorophytic green alga. Secondary endosymbiotic events have occurred within both the red algae (e.g., diatoms, pictured) and green plants. Pie chart shows the relative species richness of the major clades.
Sympatric Speciation by Polyploidy
• About half of all flowering plants are polyploid: more than 2 copies of each gene.
• Polyploids are the result of failure of cell division (mitosis or meiosis) to separate the chromosomes into 2 cells.
• New polyploids are usually sterile, or their offspring are sterile: extra chromosomes with no homologue to pair with in meiosis leads to unbalanced (aneuploid) gametes, which will produce dead offspring.
• Triploids (3 copies of each gene) are sterile--the source of seedless fruit.
• In animals, sexual reproduction is essential every generation, so most polyploids fail to reproduce: need a similarly polyploid mate to produce fertile offspring.
• In plants, vegetative propagation is possible for many generations without sex, and in many species one plant contains both sexes. So, reproduction with a polyploid partner is easier in plants than in animals.
• When two populations of a species are separated by a geographical barrier, they diverge genetically. Sometimes the barrier is removed and the two groups come into contact with one another. The region of contact is a "hybrid zone".
• Several possibilities exist:
• If the two groups have only diverged a bit, fertile offspring will result, and the two groups will merge back into a single species. Geographical differences may exist within the species: different subspecies or varieties, but all can interbreed freely.
• If the two groups have diverged to the point that no fertile offspring result from their matings, sexual selection will occur to deter further matings. New pre-zygotic reproductive isolation mechanisms, especially behavioral differences, arise to reinforce the division between the two species.
• This is called "parapatric speciation": 2 species forming while in contact with each other in a restricted region.
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