Microevolution and Macroevolution

Speciation

According to the theory of natural selection, speciation is the creation of new species by genetic modifications of previously existing species, so the resulting organisms can no longer successfully mate and produce fertile offspring. Consequently, the most modern definition of species includes a retrieval of the genetic understanding from ancestral parents into a biological species concept, which states that a species is a population that can interbreed in nature and produce fertile offspring. New species have three principle mechanisms describing their formation, each of which involves reproductive isolation:

Allotropic Speciation

Suppose a volcano erupts and the overflow of lava blocks a stream to create a large lake. The resulting geographic separation may isolate organisms on either side of the lake as well as those above and below the dam. A physical separation that prohibits the gene migration between populations creates the opportunity for allotropic speciation for that subpopulation. When this happens, natural selection, mutation, and genetic drift act to genetically diversify the two populations so they are no longer capable of mating and producing fertile offspring. Geographic isolation presents the opportunity for the formation of a new species but cannot create a new species. True speciation only occurs when reproductive barriers prevent productive interbreeding. Two major types of reproductive barriers prevent a species from interbreeding even if they are in the same geographic area: prezygotic and postzygotic reproductive isolation.

Prezygotic Reproductive Isolation

There are five main types of prezygotic reproductive barriers that prevent intraspecies fertilization:

Postzygotic Reproductive Isolation

Three main barriers act on hybrid zygotes after interspecies fertilization:

Bionote

A whinny is the less-common, sterile product of a mating between a male horse and a female donkey.

Sympatric Speciation

Sympatric speciation is the opposite of allopatric speciation because organisms, predominately plants, often create new species without the requisite geographic isolation. Plant-seed dispersal mechanisms often prohibit reproductive isolation, leaving sympatric speciation as the only major evolutionary cause agent for plants. Typically, a mutation occurs that prevents the offspring from successfully mating with a parent, but still allows viable reproduction with other individuals who inherited the same mutation. The most common mechanism is the chromosomal mutation that occurs because of a meiotic failure during gamete formation, when the chromosomes divide mitotically instead. When this happens, the duplicated chromosomes do not segregate and migrate into separate sex cells. Instead, they remain duplicated in the same sex cell, creating an overload of genes in certain gametes, which then become diploid, and deficient in others. It is possible then for the diploid gametes to unite with other diploid gametes to produce a polyploid individual, which contains more than the normal diploid complement of chromosomes. In plants, this occurs most frequently because of self-fertilization. The polyploid offspring can no longer successfully interbreed with the parent or any other similar-species organism that did not inherit the extra set of genes. Sympatric speciation is the reproductive isolation created by genetic abnormalities not as a result of geographic isolation. Although not widespread among animals, sympatric speciation has been significant in plant variation. Hugo de Vries, a Dutch botanist, is credited with identifying polyploidy as an agent of sympatric speciation. Through self-pollination, he created a large flowering polyploid evening primrose with 28 chromosomes instead of the normal diploid number of 14 chromosomes.

Speciation Rate

The speed by which new species are created depends upon the genetic makeup of the species, their ability to adapt to environmental changes, and the speed and severity of the environmental changes. In earlier times, it was thought that speciation occurred slowly over long periods of time. This gradualistic theory has recently given way to the punctuated equilibrium model that defines speciation as occurring in jumps or sudden shifts of speciation interspersed within long periods of inactivity. Emerging evidence from fossils lends support to the punctuated equilibrium model. This discussion is continued in the next section.

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Excerpted from The Complete Idiot's Guide to Biology © 2004 by Glen E. Moulton, Ed.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.

To order this book direct from the publisher, visit the Penguin USA website or call 1-800-253-6476. You can also purchase this book at Amazon.com and Barnes & Noble.

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