CHAPTER 16 -- MECHANISMS of SPECIATION

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16.1 -- SPECIES CONCEPTS 

1. INTRODUCTION 
2. The Biological Species Concept (Mayr 1942) 
3. The Phylogenetic Species Concept (evolutionary species concept)
   1. monophyletic groups (fig. 12.2) 
4. The Morphospecies Concept: 
   1. Morphological differences (Fig. 12.3) 
   2. Cryptic species
5. Species concepts in Bacteria [Fig. 12.1] 
6. Applying Species Concepts: The case of the Red Wolf 
   1. How many species 
      1. Red wolves (Canis rufus) [Fig. 12.4]
      2. Coyotes (C. latrans)
      3. Gray wolves (C. lupus) 
   2. morphospecies evidence 
   3. DNA evidence 
   4. Conclusions 

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) 

12.2 -- MECHANISMS OF ISOLATION 

Physical Isolation as a Barrier to Gene Flow

1. Allopatric Speciation (Mayr 1942, 1963).
   1. Two major patterns [Fig. 12.5] 
      1. Vicariance (splitting of range)
      2. Dispersal 

Geographic Isolation through Dispersal and Colonization 

1. Hawaiian Drosophila [Fig. 12.6]
2. Founder hypothesis 
3. Geological history of the Hawaiian Islands [Fig. 12.7].
   1. Evidence for speciation and dispersal events 

Geographic Isolation through Vicariance

1. Vicariance
2. Panamanian isthmus and snapping shrimp (Knowlton et al.) [Fig. 12.8].
   1. Shrimp on Caribbean versus Pacific side of 4 Ma Isthmus of Panama. 

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 

12.3 -- MECHANISMS OF DIVERGENCE 

Genetic Drift

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

Natural Selection

1. Rhagoletis: apple maggot flies [Fig. 12.9] 
   1. Races found on hawthorn (ancestral) and apple (new) trees 
   2. Isolation by choice of food host species.
   3. Assortative mating by fruit type. 
   4. Strong disruptive selection on each food: 
      1. Advantages of feeding on hawthorn 
      2. Advantages of feeding on apple 
   5. Test of hypothesis that selection acts on genes associated with timing of development: (fig 12.11) 
2. Speciation in action [Table 12.1] 

Sympatric Speciation [Box 12.2] 

Parapatric Speciation ("adjacent") [Box 12.2] 

Sexual Selection 

1. Sexual selection can be especially strong force leading to divergence of male traits.
2. Hawaiian Drosophila [Fig. 12.12] 
   1. Evidence for sexual selection on head width [Fig. 12.13] 

12.4 -- SECONDARY CONTACT 

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

Reinforcement 

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 12.14] 

Hybridization 

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) [Fig 12.15] 

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 12.2] 

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. 12.16] 
   2. Hybrid sagebrush fitness [Fig. 12.17] 

12.5 -- THE GENETICS of DIFFERENTIATION and ISOLATION 

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.

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