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BIOA02 - Chapter_23.docx

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Department
Biological Sciences
Course
BIOA02H3
Professor
Mary Olaveson
Semester
Fall

Description
Biology – Chapter 23 Notes The term species refers to different “kinds” of organisms, however it is difficult to determine boundaries that separate one kind from another It is possible for someone knowledgeable about a group of organisms to distinguish species based on their looks because organisms of the same species change very little over geographic distances Morphological Species Concept: The classification of organisms based on their appearance The originator of the binomial system of classification, Carlous Linnaeus, used this concept in his classification Because sometimes members of the same species do not look alike, scientists need to look at other factors to classify between them Speciation: The process by which one species splits into two or more daughter species, which thereafter evolve into separate lineages This process is very gradual. Because of the slowness of this process, often two populations at various stages in the process of becoming a new species will exist and it is important to classify them as either being a member of species A or species B Reproductive Isolation: When individuals of a population mate with each other, but not with individuals of other population If reproductive isolation occurs, the population constitute a distinct group within which genes recombine and so they are an independent evolutionary unit. Biological Species Concept: Definition of species proposed by Ernest Mayer; “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups” This definition does not apply to organisms that reproduce asexually Not all evolutionary changes result in a new species; speciation occurs only if those evolutionary changes result in a split of one species into 2 or more daughter species It is also possible that if two populations are isolated from each other, and sufficient differences in their genetic structure accumulate, then when they come together again, they may not be able to exchange genes For speciation to occur, it requires such interruptions in gene flow. This is possible in 2 ways: 1. Allopatric Speciation Allopatric Speciation: Species that occur when a population is divided by a physical barrier. Also known as Geographic Separation It is the dominant cause of speciation in most organisms The barriers can form by continental drift, rising sea levels, climate change, glaciers advancement/retreatment etc. Usually, the populations that are separated are fairly large and they evolve differently because of gene drift, environmental change and other reasons Allopatric Speciation may also occur if a new species crosses a pre-existing barrier and finds a new isolated population The effectiveness of the barrier separating the isolated populations depend on the size, mobility, and dispersing abilities of the species It is also possible for an interruption of gene flow even without the presence of a physical barrier. I.e. if two populations are separated by a patch of land which is unsuitable for a species 2. Sympatric Speciation Sympatric Speciation: A partition of a gene pool without any physical isolation In this for a speciation, a physical barrier is not required, but rather some form of disruptive selection within a population which favours individuals with a certain genotype Polyploidy: Most common means of sympatric speciation; the production within an individual of duplicate sets of chromosomes. Can occur from chromosome duplication in a single species (autopolyploidy) or from the combination of the chromosomes of 2 different species (allopolyploidy) Autopolyploid individuals occur when cells that are normally diploid (2 sets of chromosomes) accidentally duplicate therefore becoming tetraploid (4 sets of chromosomes). If a tetraploid individual mates with a normal diploid individual, their offspring turn out triploid (3 sets of chromosomes) because of meiosis and they end up being sterile. Because their offspring are sterile, they fail to further produce offspring with diploid individuals; however a tetraploid individual can produce offspring with other tetraploids, therefore polyploidy result in complete reproductive isolation in two generations. Allopolyploids may be produced if two individuals of different, but closely related species, mate (hybridize). Unlike autoployploidy, they are fertile Speciation through polyploidy is much more common in plants than in animals because plants can reproduce through self fertilization Once a barrier to gene flow is established, the two separate populations can further their difference in genetic variation through different evolutionary mechanisms, which decrease the probability they can mate together However, geographic isola
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