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Week 10 Course Notes: Chapter 27

by: Sydney Diekmann

Week 10 Course Notes: Chapter 27 Biol 2002

Marketplace > University of Minnesota > Biology > Biol 2002 > Week 10 Course Notes Chapter 27
Sydney Diekmann
U of M
GPA 3.9

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These are the notes for week 10 of the course, separated by chapter. Concepts covered include speciation.
Foundations of Biology
Dr. Susan Wick, Dr. David Matthes
Class Notes
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This 3 page Class Notes was uploaded by Sydney Diekmann on Sunday November 8, 2015. The Class Notes belongs to Biol 2002 at University of Minnesota taught by Dr. Susan Wick, Dr. David Matthes in Fall 2015. Since its upload, it has received 19 views. For similar materials see Foundations of Biology in Biology at University of Minnesota.


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Date Created: 11/08/15
Chapter 27: Speciation  gene flow makes allele frequencies more similar among populations o if gene flow ends, allele frequencies in isolated populations begin to evolve independently of one another  if mutation, selection, and genetic drift cause isolated populations to diverge sufficiently, speciation occurs  speciation: splitting event that creates two or more distinct species from a single ancestral species o results from genetic isolation and genetic divergence 27.1 How are Species Defined and Identified?  species: evolutionarily independent population or group of populations o identified by three common criteria… 1. The Biological Species Concept  main criteria for identifying species is reproductive isolation  no gene flow occurs between populations that are reproductively isolated from each other  prezygotic isolation: prevents individuals of different species from mating  postzygotic isolation: offspring between matings between members of different species do not survive/reproduce  cannot be evaluated in fossils or species that reproduce asexually 2. The Morphospecies Concept  evolutionarily independent lineages differ in size, shape, or other morphological features  differences are more likely to arise if populations are independent and isolated from gene flow  widely applicable, equally applicable to sexual, asexual, or fossil species  disadvantaged because…  it can lead to the naming of two+ species when there is only one polymorphic species with differing phenotypes  cannot identify cryptic species which differ in traits other than morphology  morphological features used to distinguish traits are subjective 3. The Phylogenetic Species Concept  identifies species based on the evolutionary history of populations  all species are related by common ancestry  monophyletic group/clade/lineage: consists of an ancestral population, all of its descendants, and only those descendants  species are defined as the smallest monophyletic groups on the tree of life  can be applied to any population and is logical because different species have different synapomorphies only if they are isolated from gene flow and have evolved independently  synapomorphy: a trait that is found in certain groups of organisms and their common ancestor, but is missing in more distant ancestors 27.2 Isolation and Divergence in Allopatry  speciation begins when gene flow between populations is reduced/eliminated, causing genetic isolation o occurs routinely when populations become geographically separated => alloparty => allopatric speciation  geographic isolation occurs in one of two ways… 1. dispersal: a population can disperse to a new habitat, colonize it, and found a new population  followed by genetic drift and natural selection to new habitat 2. vicariance: physical splitting of a habitat (geographic barrier) into subgroups that are physically isolated  interruption of gene flow => sister species that close relatives 27.3 Isolation and Divergence in Sympatry  sympatry: when populations/species live in the same geographic area or close enough to make interbreeding possible o sympatric speciation occurs even though populations live within the same area… 1. external events = disruptive selection for extreme phenotypes based on different ecological niches  prezygotic reproductive isolation is occurring as a result of natural selection for adaptions to two different niches 2. internal events = chromosomal mutations  polyploidy occurs when an error in meiosis or mitosis results in the doubling of chromosome number  autopolyploid individuals result when the chromosomes all come from the same species  allopolyploid individuals result when the parental chromosomes come from two different species  polyploid individuals are reproductive isolated from the original diploid population and thus evolutionarily independent because breeding results in sterile offspring  speciation by polyploidy is more common in plants because it is fast and sympatric 27.4 What Happens when Isolated Populations come into Contact?  if divergence has taken place and affects when, where, or how individuals in the population mate, then interbreeding is unlikely to take place o prezygotic isolation exists  mating between populations is rare, gene flow is minimal, and divergence continues  if prezygotic isolation does not exist and the population beings interbreeding, then the populations fuse over time o gene flow erases any distinction  other possibilities exist… A. Reinforcement  if two popoulations have diverged extensively and are genetically distinct, then their hybrid offspring will generally have lower fitness than their parents  natural selection for traits that isolate populations in a way to avoid wasted parental effort is called reinforcement which reinforces differences that evolved while the populations were isolated  if populations are sympatric, individuals from two species are seldom willing to mate with each other  if populations are allopatric, individuals from two species are often willing to mate with each other B. Hybrid Zones  not all hybrid offspring are dysfunctional – some are capable of mating and producing viable offspring that have intermediate features between two parental populations  => development of hybrid zones, geographical area where interbreeding occurs and hybrid offspring are common  hybrid zones can be narrow or wide, long or short lived depending on the fitness of the hybrid offspring and the extent of breeding between parental species C. Hybridization  new species can result if hybrid offspring have higher fitness due to unique combination of traits that happen to be adaptive to their particular environment BIG IDEA: Secondary contact of two populations can produce several possible outcomes; fusion of the populations, reinforcement of divergence, founding of stable hybrid zones, extinction of one population, or the creation of new species.


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