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Bio142 Chapter 27

by: Saida Muktar

Bio142 Chapter 27 Bii

Saida Muktar

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Chapter 27 Notes
Intro to biological sciences
Dr. Omland
Class Notes
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This 6 page Class Notes was uploaded by Saida Muktar on Tuesday February 16, 2016. The Class Notes belongs to Bii at University of Maryland - Baltimore County taught by Dr. Omland in Winter 2016. Since its upload, it has received 32 views.


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Date Created: 02/16/16
Speciation results from genetic isolation and genetic divergence. Genetic isolation results from lack of gene flow, and genetic divergence occurs because selection, genetic drift, and mutation proceed independently in the isolated populations. SPECITATION is a splitting event that creates two or more distinct species from a single ancestral species.  When speciation is complete, a new branch has been added to the tree of life. 27.1 How are Species Defined and Identified? SPECIES is defined as an evolutionary independent population or group of populations. The three most common criteria used to identify species are: 1 The Biological Species Concept o According to the biological species concept, the main criterion for identifying species is reproductive isolation. No gene flow occurs between populations that are reproductively isolated from each other. o If two different populations do not interbreed in nature, or if they fail to produce viable and fertile offspring when matings take place, then they are considered distinct species. o Reproductive isolation can occur from 1. PREZYGOTIC ISOLATION, which prevents individuals of different species from mating 2. POSTZYGOTIC ISOLATION, in which the offspring of matings between members of different species do not survive or reproduce. o The disadvantage of the BSC is that the criterion of reproductive isolation cannot be evaluated in fossils or in species that reproduce asexually. Plus it is difficult to apply when closely related populations do not happen to overlap with each other geographically. 1 The Morphospecies Concept o Under the morphospecies concept, researchers identify evolutionary independent lineages by differences in size, shape or other morphological features. o The logic behind the morphospecies concept is that distinguishing features are most likely to arise if populations are independent and isolated from gene flow. o It is useful when biologist have no data on the extent of gene flow, and it is equally applicable to sexual, asexual, or fossil species. o Its disadvantages are:  It may lead to the naming of two or more species when there is only one polymorphic species with differing phenotypes  It cannot identify cryptic species, which differ in traits other than morphology  The morphological features used to distinguish species are subjective 2 The phylogenetic Species Concept oThe phylogenetic species concept identifies species based on the evolutionary history of populations oAll species form a monophyletic group -- tree of life oA MONOPHYLETIC GROUP, also called a CLADE or LINEAGE, consists of an ancestral population, all of its descendants, and only those descendants. oMonophyletic groups are identified by traits called synapomorphies.  SYNAPOMOPHY is a trait that is found in certain groups of organisms and their common ancestor, but is missing in more distant ancestors  Synapomorphies are homologous traits that can be identified at the genetic, developmental, or structural level. oUnder the phylogenetic species concept, species are defined as the smallest monophyletic groups on the tree of life. oThe phylogenetic species concept has two distinct advantages:  It can be applied to any population (fossil, asexual or sexual  It is logical because different species have different synapomorphies only if they are isolated from gene flow and have evolved independently o The disadvantage is that carefully estimated phylogenies are available only for a tiny subset of populations on the tree of life.  Researchers use a combination of species concepts to identify evolutionary independent populations in nature.  SUBSPECIES are populations that live in discrete geographic areas and have distinguishing features but are not considered distinct enough to be called separate species. 27.2 Isolation and Divergence in Allopatry  Speciation begins when gene flow between populations is reduced or eliminated, causing genetic isolation.  Genetic isolation happens routinely when populations become geographically separated.  Populations that are geographically separated are said to be in ALLOPATRY. Thus, speciation that begins with geographic isolation is known as ALLOPATRIC SPECIATION.  Geographic isolation occurs in two ways: 1. Dispersal o 1 VICARIANCE, which is the physical splitting of a habitat. o  Geographic isolation of populations via dispersal or vicariance produces genetic isolation due to the interruption of gene glow -- the first requirement of speciation. 27.4 What Happens When Isolated Populations Come into Contact?  When two populations that have been isolated come into contact again after divergence has taken place and if divergence has affected when, were or how individuals in the populations mate, then interbreeding is unlikely to take place.  In such cases prezygotic isolation exists.  If prezygotic isolation doesn’t exist the populations fuse overtime, as gene flow erases any distinctions between them. However, reinforcement, hybrid zones, and speciation by hybridization may occur.  Reinforcement o If two populations have diverged extensively and are distinct genetically, it is reasonable to expect that their hybrid offspring will have lower fitness than their parents.  If two populations are well adapted to different habitats, then hybrid offspring will not be well adapted to either habitat.  If the two populations have diverged enough genetically, hybrid offspring also may fail to develop normally or may be infertile. o When postzygotic isolation occurs, there should be strong selection against interbreeding because hybrid offspring represent a wasted effort on the part of parents.  Individuals that do not interbreed, because of different courtship ritual or pollination syster or other form of prezygotic isolation, should be favored becaused they produce more viable offspring o Natural selection for traits that isolate populations in this way is called REINFORCEMENT.  The selected traits reinforce differences that evolved while the populations were isolated from one another. o Jerry Coyne and Allen Orr found an intersting pattern:  If closely related species are sympatric - meaning that they live in the same area - individuals from the two species are seldom willing to mate with one another  If the species are allopatric - meaning that they live in different areas - then individuals from the two species are often willing to mate with one another. o Sympatric species exhibit prezygotic isolation but allopatric species do not.  Hybrid zones o Hybrid offspring are not always dysfunctionl. In some cases they are capable of mating and producing viable offspring that have features that are intermediate between those of the two parental populations. In this case hybrid zones can form o A HYBRID ZONE is a geographic area where interbreeding occurs and hybrid offspring are common. o Depending on the fitness of hybrid offspring and the extent of breeding between parental species, hybrid zones can be narrow or wide, and long or short lived 2/13/2016 12:30 PM - Screen Clipping


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