Types of Speciation

<h1>Text Preview</h1> <p>slide 1</p> <p>Both geographic and reproductive isolation keep two populations from mating with each other, so gene flow between populations stops. Let's take a closer look at how allopatric and sympatric speciation occur. Allopatric speciation is caused by geographic isolation. Once the Grand Canyon was formed, squirrels could mate only with squirrels on the same side of the canyon. Each population adapted to its environment.</p> <p>slide 2</p> <p>Speciation doesn't occur every time a population is split into two groups by a geographic barrier. Allopatric speciation is most likely to occur when a small population, or splinter population, is separated from the parent population. Since the population of the splinter group is small, its gene pool may not be representative of the parental population. This is called a founder effect.</p> <p>Also, genetic drift will continue to change the gene pool at random until the splinter population becomes larger. Genetic drift is chance events in reproduction that change allele frequencies in a population. Since the splinter population is generally found at the edge of the parental population's range, the conditions it encounters are often more severe. The traits that are selected in the splinter population are likely to be different from those selected in the parental population.</p> <p>slide 3</p> <p>Allopatric speciation is common on island chains. Let's think about the finches Darwin studied on the Galapagos Islands. One theory of how the 13 species of finches evolved is through adaptive radiation, the evolution of many different species from a common ancestor. The South American ancestors most likely colonized one of the islands. The finches evolved on the island and became a new species, distinct from the common ancestor. This new species may have colonized a nearby island. </p> <p>On the new island, the finches evolved to become another distinct species. The new species could have migrated back to the first island. We can use this cycle of migration and evolution to explain the 13 closely related species of finches on the Galapagos Islands.</p> <p>slide 4</p> <p>Speciation can also occur if individuals in a population become reproductively isolated even if they aren't geographically separated. This is called sympatric speciation. Sympatric speciation can arise from reproductive isolation caused by a genetic change that makes two populations unable to mate with each other.</p> <p>Many plant species have originated as a result of sympatric speciation. An accident during cell division can result in an organism with more than two sets of chromosomes. Such an accident is called polyploidy, and the resulting organisms are called polyploids. The extra chromosomes make it impossible for the plant to breed with plants from the original population. Many of the plants we eat are polyploids, including some wheats, oats, potatoes, and tomatoes.</p> <p>slide 5</p> <p>Sympatric speciation also occurs in animal evolution, but generally it doesn't involve chromosome doubling. In animals, mutations can cause two populations to become reproductively isolated. Mutations in an ancestor of the fruit fly Drosophila heteroneura caused some males to have a wider head. The wider head is preferred by certain females in mate selection. Females with the same set of mutations mate only with the wide-headed males. It can take anywhere from a handful to a few thousand mutations to reproductively isolate a subset of a population.</p> <p>slide 6</p> <p>Both allopatric and sympatric speciation lead to a new species. Let's think about how different two organisms have to be for each to be considered a specific kind of organism, and therefore its own species. Many scientists believe that members of two populations must be unable to breed with each other for them to be considered different species. But this definition doesn't work in all situations. </p> <p>For example, how do we classify organisms that only reproduce asexually? Scientists have come up with four different ways to separate organisms into different species. One centers on the ability to reproduce, another is based on anatomic differences, a third emphasizes mate selection, and the last includes both physical and behavioral components. Which definition we use depends on what organism we're studying.</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>