Chapter 13 -- The Origin of Species p. 392


New species evolve through geographical separation and other mechanisms.

13.1 What Is a Species? p. 394

  • There are many different concepts of species, and no single definition will fit all taxa.
  • But species are important taxonomic units; they display a level of genetic cohesion that does not exist across species.
  1. Species are independently evolving lineages
    1. General lineage species concept: species are metapopulations that exchange alleles frequently enough to comprise the same gene pool
  2. Ways to identify species
    1. Biological species concept: species are groups of interbreeding populations that are reproductively isolated from other such groups
    2. Phylogenetic species concept: smallest possible group descending from a common ancestor and recognizable by unique, derived traits


  1. The Morphospecies Concept:  
  2. Morphological differences
  3. Cryptic species
  • The Biological Species Concept (Mayr 1942) 
  • Species concepts in Bacteria and Archaea [ Fig. 13.22-25]
  • The Phylogenetic Species Concept (evolutionary species concept)
    1. monophyletic groups (fig. 16.1 ) 
  • Applying Species Concepts
    1. Diversification in Marine Copepods (Fig. 16.2, 16.3)
    2. How Many Species of Elephant Live in Africa (Fig. 16.4)
    3. The case of the Red Wolf (Second Edition)
      1. Red wolves (Canis rufus)
      2. Coyotes (C. latrans)
      3. Gray wolves (C. lupus
  • 13.2 Barriers to Gene Flow: Keeping Species Apart p. 396

    • Isolating barriers reduce or impede gene flow.
    • They can be extrinsic properties of the landscape in which an organism lives or intrinsic features of the organisms themselves.
    1. Speciation involves barriers to reproduction  (Fig. 13.2)

    GeographicBarriers p. 396

    • New species can form in allopatry, that is, when two or more populations become geographically isolated from each other.
    • Sympatric speciation occurs within the same geographic region.
    1. Isolating barriers
      1. Geographic: extrinsic properties of landscape that prevent gene flow: Allopatry
      1. Reproductive: features of organisms that prevent interbreeding
        1. Barriers effective even in sympatry
    2. Elk and red deer: two species? (Fig. 13.3)


    The main stages of the speciation process

    1. Isolation (gene flow reduced) 
    2. Divergence (e.g., with mutation, drift, selection) 
    3. Reinforcement of reproductive isolation (selection against hybrids) 

    Physical Isolation as a Barrier to Gene Flow

    1. Allopatric Speciation (Mayr 1942, 1963).
      1. Two major patterns [Fig. 16 .5] 
        1. Vicariance (splitting of range)
        2. Dispersal 

    Reproductive Barriers: Before Mating p. 397

    • Reproductive barriers, such as different timing or location of courtship, can reduce the chances that individuals from two populations mate.
    1. Premating barriers: timing of reproduction  (Fig. 13.4)
    2. Premating barriers: pollinator isolation  (Fig. 13.5)
    3. Premating barriers: mismatched morphology (Fig. 13.6)

    Reproductive Barriers: After Mating, but Before Fertilization p. 399

    • Sperm can fail to reach eggs after mating, creating another barrier to reproduction. .
    1. Postmating-prezygotic barriers:
      1. Gametic incompatibility: sperm or pollen from one species fails to penetrate and fertilize the egg of another species

    Reproductive Barriers: After Fertilization p. 400

    • Genetic incompatibilities can serve as postzygotic barriers to gene flow, especially when hybrid offspring are sterile or have low fitness.
    1. Postzygotic barriers
      1. Hybrids are produced but have low fitness
        1. Hybrid inviability
        2. Hybrid sterility
        3. Ecological inviability
        4. Behavioral sterility
    2. Reproductive barriers to gene flow can take many forms (Fig. 13.7)
    3. Key Concepts
      1. Geographic barriers to gene flow physically separate populations
        1. Important for all species concepts
      2. Reproductive barriers are intrinsic features of organisms that reduce likelihood of interbreeding
      3. Geographic barriers are most common
    4. Key Concepts
      1. Separation of populations in space and time reduces interbreeding
        1. Habitat preference
        2. Timing of reproduction
      2. Divergent floral traits can cause pollinator isolation and reduce interbreeding
      3. Molecular incompatibilities between gametes can prevent interbreeding
      4. Genetic incompatibilities that reduce hybrid fitness can be important barriers to gene flow


    1. The third step in speciation is selection against hybrids if there is contact between recently diverged populations. 


    1. Occurs if hybrids have reduced fitness. 
    2. Natural selection will favor assortative mating (Dobzhansky). 
    3. Two main categories of reproductive isolating mechanisms 
      1. prezygotic (prevents fertilization and the formation of hybrid zygotes)
        1. ecological isolation
        2. temporal isolation
        3. pollinator isolation
        4. ethological isolation
        5. mechanical isolation
        6. Gametic mortality or isolation
      2. postzygotic (after the formation of hybrid zygotes) 
    4. prezygotic isolation evolves much faster in sympatric species pairs than in allopatric species pairs [Fig 16.12] 

    13.3 The Origin of Isolating Barriers: How New Species Form p. 402

    Geographic Isolation p. 403

    • The best-documented process by which speciation occurs is geographic isolation, which separates populations and allows them to diverge genetically.
    1. Allopatric speciation (Fig. 13.8)
    2. Phylogenetic signature of allopatric speciation (Fig. 13.9)
      1. Panamanian isthmus and snapping shrimp (Knowlton et al.)
      2. Image below Courtesy of The Smithsonian Institution/Carl Hansen and Nancy Knowlton
    3. Islands provide opportunity for allopatric speciation (Fig. 13.10)
      1. Geological history of the Hawaiian Islands
    4. Colonization leads to speciation (Fig. 13.11)
      1. Crickets on the Hawaiian Islands


    Physical Isolation as a Barrier to Gene Flow

    1. Allopatric Speciation (Mayr 1942, 1963).
      1. Two major patterns [Fig. 16 .5] 
        1. Vicariance (splitting of range)
        2. Dispersal 

    Geographic Isolation through Dispersal and Colonization 

    1. Hawaiian Drosophila [Fig. 16.6]
    2. Founder hypothesis 
    3. Geological history of the Hawaiian Islands [Fig. 13.10].
      1. Evidence for speciation and dispersal events 

    Geographic Isolation through Vicariance

    1. Vicariance
    2. Panamanian isthmus and snapping shrimp (Knowlton et al.) [Fig. 13.9].
      1. Shrimp on Caribbean versus Pacific side of 4 Ma Isthmus of Panama. 


    Ring Species [Fig. 13.13]
    1. Evidence that one species can split into two.  A map showing the ranges of the Greenish warbler's geographic variants. The birds interbreed everywhere they meet around the Tibetan Plateau, except where the northwestern form meets the northeastern form in the region indicated by hash marks. There, the birds behave as different species. 
    2. Figures to right from Wikipedia.  Yellow: P. t. trochiloides; Orange: P. t. obscuratus; Red: P. t. plumbeitarsus; Green: P. t. "ludlowi"; Blue: P. t. viridanu
    3. Ensatina salamanders (Evolution in Action on YouTube)


    Speciation in Sympatry p. 409

    • Sympatric speciation occurs when populations diverge without geographic isolation.
    1. Sympatric speciation in Rhagoletis (Fig. 13.14)

    Natural Selection

    1. Rhagoletis: apple maggot flies [Fig. 13.14]
      1. Sympatric speciation
      2. Races found on hawthorn (ancestral) and apple (new) trees 
      3. Isolation by choice of food host species.
      4. Assortative mating by fruit type. 
      5. Strong disruptive selection on each food: 
        1. Advantages of feeding on hawthorn 
        2. Advantages of feeding on apple 
      6. Test of hypothesis that selection acts on genes associated with timing of development 
    2. Speciation by natural selection [Table 16.1];

    Barriers as By-products p. 412

    • Barriers to reproduction may develop incidentally as populations diverge in their use of ecological space.
    1. Ecological speciation in stickleback (Fig. 13.15)
    2. Ecological speciation in Darwin’s finches (Fig. 13.16)
    3. Key Concepts
      1. Populations become allopatric when new geographic barriers arise
      2. New species can form when geographic barriers arise
      3. Isolation by distance occurs because individuals tend to mate with individuals from the same or nearby populations
      4. Ring species demonstrate how species can form even with some gene flow
    4. Key Concepts
      1. Reproductive isolation can arise even in sympatry because individuals in different subpopulations rarely interbreed
      2. Ecological speciation can lead to speciation in sympatry
      3. Reproductive isolation can arise as a by-product of adaptation to divergent environments


    1. Galapagos Finches
      1. Charles Darwin and the Voyage of the Beagle, 1831-1836
      2. 13 species of Galapagos and one Cocos finches [Figs. 3.6, 3.7]
      3. Isla Daphne Major [Fig. 3.8]
      4. Geospiza fortis on Daphne Major is a seed eater.
      5. beak size and shape correlate with seed size
      6. size and shape of the beak has an important consequence for fitness.


    1. Threeespine Sticklebacks (Figs. 16.16-16.18)


    13.4 The Speed of Speciation p. 416

    • Animal populations may take hundreds of thousands of years to diverge into clearly delineated species.
    • Plants can take far less time.
    1. The speed of speciation (Fig. 13.17)
    2. Allopolyploidy can lead to extremely rapid speciation (Fig. 13.19)
      1. hybridization
    3. Key Concepts
      1. The speed of speciation depends on genetic architecture of organisms
      2. Genome duplication common in plants

    Changes in Chromosomes as a Barrier to Gene Flow

    1. Populations can become genetically isolated because of differences in chromosome numbers 
    2. Polyploids: can lead to instantaneous sympatric speciation 

    Creation of New Species Through Hybridization 

    1. Hybridization is a major source of evolutionary novelty in plants. 
    2. Reiseberg's work on sunflowers 
      1. Helianthus anomalus, H. annuus, and H. petiolaris

    Hybrid Zones [Table 16.3] 

    1. Four possibilities if there is secondary contact.
      1. Already completely diverged; no interbreeding 
      2. Interbreeding--hybrids less fit than parents: reinforcement of divergence through selection against hybrids.
      3. Interbreeding--hybrids as fit as parents: hybridization leading to coalescence of populations 
      4. Interbreeding--hybrids more fit than parents: hybridization leading to stable hybrid zone or to formation of new species. 
    2. Example 
      1. Hybrid sagebrush morphology [Fig. 16.13]
      2. Hybrid sagebrush fitness [Table 16.4, Fig. 16.14] 

    13.5 Uncovering Hidden Species p. 418

    • Very similar-looking organisms sometimes turn out to be previously unknown species
    1. Genetic data reveals cryptic diversity in giraffes (Fig. 13.20)
    2. Cryptic diversity in skipper butterflies (Fig. 13.21)



     13.6 The Puzzle of Microbial “Species” p. 422

    • Bacteria and other microbes do not fit the biological species concept.
    • Their species may be defined by ecological separation
    1. Bacterial species often defined by specific adaptations (Fig. 13.22)
    2. Horizontal gene transfer makes classification difficult (Fig. 13.24)
    3. Horizontal gene transfer in E. coli (Fig. 13.23)
    4. Horizontal gene transfer is common (Fig. 13.25)
    5. Key Concepts
      1. Ultimately, species concepts are human artifacts
      2. Methods for recognizing species continue to improve



  • 1


    Genetic Drift

    1.  Founding populations of Galapagos finches 
    2. Bottlenecking 
    3. Drift is no longer viewed as the most important force in speciation. 

    Sexual Selection 

    1. Sexual selection can be especially strong force leading to divergence of male traits.
    2. Hawaiian Drosophila [Fig. 16.10 ] 
      1. Evidence for sexual selection on head width [Fig. 16.11] 


    1. What are the consequences if hybrid has equal or greater fitness than parent?
    2. Economic example 
      1. herbicide resistance in sorghum (Sorghum bicolor) can be transferred to the weed johnsongrass (S. halepense) [ third edition ] 

    1 6 .5 -- THE GENETICS of SPECIATION  

    1.  What degree of genetic differentiation is required to isolate populations and produce new species?
      1.  Traditional (Mayr): radical reorganization of genome--genetic revolution 
      2. Current research
        1. Large scale changes are unnecessary. 
        2. Number, location, and nature of genes that distinguish closely related species.
    2. Pea Aphids (Fig. 16.15)
    3. Threeespine Sticklebacks (Fig. 13.15)


    Patterns of Evolution
    1. Micro and macroevolution from Dennis O'Neil.
    2. Don Prothero on punctuated equilibrium at 20, a paleontological perspective

    Evidence from the Fossil Record

    Environmental Change


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