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PRESENTATION ON SPECIATION. GROUP NO. Definition of speciation. This is the process by which one or more species arise from the previously existing species. .
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PRESENTATION ON SPECIATION GROUP NO
Definition of speciation • This is the process by which one or more species arise from the previously existing species.
If single species may give rise to a new species (interspecific speciation) common in many flowers or two different species may give rise to a new species (interspecific speciation). • if the intraspecific speciation occurs at the same time as the population are separated it is known as allotropic speciation. If the process occurs at the same time as the population are occupying the same geographical area it is called sympatric specitation
TYPES OF SPECIATION • ALLOPATRIC SPECIATION Allopatric speciation(Greek allos, "other" + Greek patrā, "fatherland") Speciation by geographic isolation. In this mode of speciation, something extrinsic to the organisms prevents two or more groups from mating with each other regularly, eventually causing that lineage to speciate. Isolation might occur because of great distance or a physical barrier, such as a desert or river, as shown below.
2. Parapatric Speciation In Parapatric speciation there is no specific extrinsic barrier to gene flow. The population is continuous, but nonetheless, the population does not mate randomly. Individuals are more likely to mate with their geographic neighbors than with individuals in a different part of the population’s range. In this mode, divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range.
3. Peripatric speciation • new species are formed in isolated, smaller minor populations that are prevented from exchanging genes with the main population.
Sympatric speciation • Sympatric speciation does not involve the geographical separation of population at the time at which genetic isolation is taking place, it requires the development of some form of reproductive isolation mechanism which has risen by selection within a geographically confined area. This may be structural , physiological, behavioral or genetic
Evidence for Speciation • In the summer of 1995, at least 15 iguanas survived Hurricane Marilyn on a raft of uprooted trees. They rode the high seas for a month before colonizing the Caribbean island, Anguilla. These few individuals were perhaps the first of their species, Iguana iguana, to reach the island. If there were other intrepid Iguana iguana colonizers of Anguilla, they died out before humans could record their presence.
Speciation will only occur as the results of the formation of the barriers which lead to reproductive isolation between members of the population. Reproductive isolation is brought about by some form of what the geneticist Theodosius dobzhansky called isolation mechanism.
Isolation is the segregation or separation of populations by certain barriers, which prevent interbreeding
ISOLATION MECHANISM CAN BE IN • SEASONAL ISOLATION Occurs where two species mate or flowers at different times of the year • ECOLOGICAL ISOLATION Occurs where two species inhabit similarly regions but have different habitat preferences; for example violaarvensisgrows on the calcareous soil whereas viola tricolor prefers acidic soils .
BEHAVIOURAL ISOLATION . Occurs where animals exhibits courtship patterns, mating only results in the courtship display by only • MECHANICAL ISOLATION Occurs in animals where the differences in genitalia prevents successful copulation and in plants where related species of flowers are pollinated by different animals.
Post zygotic isolation • Cytological Isolation • It is the situation where after mating, fertilization fails to occur due to differences in the chromosomal number. • Zygote Mortality Isolation • In some cases, even if successful fertilization occurs following an interspecific mating, the zygote may not survive. It may die at any stage of development. • Hybrid Inviability Isolation • Here the hybrid organism resulting from an interspecific breeding fails to survive. • Hybrid Sterility Isolation • Sometimes viable hybrids may be formed but may become sterile, failing to produce young ones. e.g., Mule.
CAUSES OF SPECIATION 1. Geographic IsolationIn the fruit fly example, some fruit fly larvae were washed up on an island, and speciation started because populations were prevented from interbreeding by geographic isolation. Scientists think that geographic isolation is a common way for the process of speciation to begin: rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations.
Reduction of Gene Flowspeciation might also happen in a population with no specific extrinsic barrier to gene flow. Imagine a situation in which a population extends over a broad geographic range, and mating throughout the population is not random. Individuals in the far west would have zero chance of mating with individuals in the far eastern end of the range. So we have reduced gene flow, but not total isolation. This may or may not be sufficient to cause speciation. Speciation would probably also require different selective pressures at opposite ends of the range, which would alter gene frequencies in groups at different ends of the range so much that they would not be able to mate if they were reunited.
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