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Evolution
Evolution in Action
Origin of Species
The Origin of New Species In the modern synthesis of original Darwinism mixed with molecular biology and population genetics, the origin of a new species begins with some form of isolation.

Probably a physical barrier separates a small group of individuals away from the original population, and they begin to live in "separate lands". Split apart, the two populations go their separate ways and diverge from one another as mutations, altered gene combinations, natural selection, and accumulation of different adaptations change their phenotypes.

Sooner or later, some of these changes affect reproduction. Alterations in reproductive organs, mating behavior, or season in which they mate produce an intrinsic mating barrier between the two groups. Should an individual from one group meet up with an individual from the second group, exchange of genes will now be impossible. The two groups can be considered separate species.

Because the originating event was a physical barrier and a physical separation of the two groups into "separate lands", this is termed allopatric speciation.

Occasionally, a new resource becomes available within an ecosystem. For example, seeds of a new type of plant blow across a wide ocean, survive drowning or being eaten by birds, and land eventually on a tiny island far off shore.

There they germinate and grow into plants never seen on this island before.

Local insects cannot, at first, eat the leaves of this new plant because they have none of the "recognizer" genes that enable them to identify it as a source of food. But sooner or later, a mutation in the insect makes such a gene available, and a subgroup of the original insect population moves over to this new plant and begins to eat it.

Having no competitors, this mutant insect enjoys the new food resource and keeps it all to itself. The small subgroup increases in numbers and might initially cross mate with the original insect population, passing on the beneficial gene, but only until a second mutation arises that alters its reproductive behavior.

By one mechanism or another, an intrinsic reproductive barrier comes between the two groups of insects, preventing the newer group from mating with members of the original population. By most definitions, this newer group of plant-eating insects is now a separate species even though it is still living in the "same land" as the original population.

Populations that split into two species, but still live within the same area, are said to be showing sympatric speciation.

Rapid sympatric speciation takes place in some plant species by a mistake at the time of mitosis. Instead of separating the newly doubled chromosomes during the division stage, some cells in these plants retain both sets of chromosomes, giving them four sets of homologous chromosomes instead of two.

Such cells are said to be tetraploid (tetra- meaning four, instead of the usual di- meaning two, as in diploid), and the condition of increased chromosome number is called polyploidy.

From these tetraploid cells, whole new plants can arise that have four sets of chromosomes in all tissues, including their reproductive cells. Although they may look exactly like their diploid cousins, and even exchange pollen, they cannot form fertile hybrids. Thus, they are reproductively isolated - instant sympatric speciation by chromosome doubling.

What is a species and how to new species arise?

A species is a population whose members share a common gene pool but cannot exchange genes with members of other populations.

New species arise either by intrinsic or extrinsic isolation of a few members of a population. Because each population is unable to share genes, the gene pool of the original and new populations diverge as different mutations, and different gene combinations accumulate.

Natural selection continues its inexorable quest for the fittest phenotypes within both populations, but the outcomes many be very different. Sooner or later, reproductive barriers arise that make cross-fertilization between the two groups impossible.

They have become two new species.


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© 2001, Professor John Blamire