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Chapter 23.doc

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STAT 2230
Dan Meegan

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Chapter 23: Species and Their Formation 23.1 What Are Species? We can recognize and identify many species by their appearance • Linnaeus described hundreds of species on the basis of their appearancemorphological species concept. Species form over time • Each species starts at a speciation event and ends at either extinction or another speciation event, at which it produces two daughter species. This process is often gradual. • Speciation is the process by which one species splits into two or more daughter species, which thereafter evolve as distinct lineages. • The gradual nature of most speciation guarantees that in many cases, two populations at various stages in the process of becoming new species will exist. • An important component to speciation is reproductive isolation. If individuals of a population mate with one another, but not with individuals of other populations, they constitute a distinct group within which genes recombine. 23.2 How Do New Species Arise? Allopatric speciation requires almost complete genetic isolation • Speciation that results when a populations is divided by a physical barrier. • Is thought to be the dominant mode of speciation among most groups of organisms. • The populations separated by such barriers are often, but not always, initially large. • They evolved difference for reasons including gene drift, but especially because the environments in which they live are, or become, different. • Allopatric speciation may also result when some members of a population cross an existing barrier and found a new, isolated population. • A physical barrier’s effectiveness at preventing gene flow depends on the size and mobility of the species in question. Sympatric speciation occurs without physical barriers • A partition of a gene pool without physical isolation. • What is required is some form of disruptive selection in which certain genotypes have high fitness on one or the other of two resources. • Sympatric speciation via ecological isolation may be widespread among insects, many of which feed on a single plant species. • BUT most common means of sympatric speciation is polyploidy—the production within an individual of duplicate sets of chromosomes. • Polyploidy can arise from chromosomes duplication in a single species (autopolyploidy) or from the combing of the chromosomes of two different species (allopolyploidy). • Allopolyploids may also be produced when individuals of two different species interbreed or hybridized. 23.3 What Happens when Newly Formed Species Come Together? • Reproductive isolation can evolve as an incidental by-product of genetic changes in allopatric populations. • Geographic isolation does not necessarily lead to reproductive isolation, however, b/c genetic divergence does not cause reproductive isolation to appear as a by-product. Prezygotic barriers operate before fertilization • Mechanisms that operate before fertilization—prezygotic reproductive barriers—may prevent individuals of different species or populations from interbreeding: o Habitat Isolation o Temporal Isolation o Mechanical Isolation o Gametic Isolation o Behavioural Isolation Postzygotic barriers operate after fertilization • If individuals of two different populations lack complete prezygotic reproductive barriers, postzygotic reproductive barriers may still prevent gene exchange. o Low hybrid zygote viability o Low hybrid adult viability o Hybrid infertility • Individuals that mate with individuals of the related species should evolve
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