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) Explain how natural selction may bring about changes in a population and what conditions may be necessary to bri about speciation

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a)} How may Variation Arise in Natural populations b) Explain how natural selction may bring about changes in a population and what conditions may be necessary to bri about speciation A species is a group of organisms that look alike and can reproduce successfully to produce fertile offspring. Within a species there is variation which can be caused my meiosis, random fertilisation, mutations and the natural environment. Genetic variation increases the chance for survival of a population if the environment changes. For example the introduction of a new virus in to the new environment may kill some organisms but others may be resistant to it and survive. If all the organisms had been genetically identical they may have all been killed. The species that survive are likely to live longer and pass on their useful genes to future generations. The change in a species over time is called evolution. There are two types of variation; continuous and discontinuous variation. Discontinuous variation is when there is a complete range of measurements from one extreme to the other for a normal characteristic, such as human height. Discontinuous variation is where characteristics are fit into separate categories with no intermediates such as human blood groups. ...read more.


Far more offspring are generally produced than survive to maturity - they suffer from predation, disease and competition. Populations are usually fairly constant is size. Darwin concluded that individuals that were better adapted to their environment compete better than the others, survive longer and reproduce more, so passing on more of their successful characteristics to the next generation. Darwin used the memorable phrases survival of the fittest, struggle for existence and natural selection. There are three kinds of Natural Selection. Directional Selection, This occurs whenever the environment changes in a particular way. There is therefore selective pressure for species to change in response to the environmental change e.g. The peppered moth.These light coloured moths are well camouflaged from bird predators against the pale bark of birch trees, while rare mutant dark moths are easily picked off. During the industrial revolution in the 19th century, birch woods near industrial centres became black with pollution. In this changed environment the black moths had a selective advantage and became the most common colour, while the pale moths were easily predated and became rare. Populations do not have to decide to adapt, or mutate, after an environmental change. ...read more.


For example horses and donkeys can apparently interbreed, but the offspring (mule) doesn't develop properly and is infertile. This definition does not apply to asexually reproducing species, and in some cases it is difficult distinguish between a strain and a species. New species usually develop due to: geographical isolation (allopatric speciation) andreproductive isolation (sympatric speciation). Allopatric Speciation: It is meaningless to say that one species is absolutely better than another species, only that it is better adapted to that particular environment. A species may be well-adapted to its environment, but if the environment changes, then the species must adapt or die. In either case the original species will become extinct. Since all environments change eventually, it is the fate of all species to become extinct (including our own). Reproductive Isolation (Sympatric Speciation) Reproductive Isolation is a type of genetic isolation. Here the formation of a new species can take place in the same geographical area, e.g. mutations may result in reproductive incompatibility. A new gene producing, say, a hormone, may lead an animal to be rejected from the mainstream group, but breeding may be possible within its own groups of variants. When this mechanism results in the production of a new species it is known as sympatric speciation. ?? ?? ?? ?? Ben Dreyfuss 6mms ...read more.

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