Chapter 15.  Mechanisms of Speciation

• Evolution accounts for adaptation and for biological diversity. Speciation - the creation of new species - is the mechanisms by which diversity is generated.
• Some interesting problems relating to species and speciation?
• Why are there species?
• Are species real biological entities or are they cultural constructs?
• Why are some taxonomic groups (e.g. Coleoptera) more species-rich than others (e.g. Primates).
• Why are some geographical areas (e.g. Guatemala) more species-rich than others (e.g. New York).
• Species concepts:
• Biological species concept.
• Populations which do not interbreed to produce viable and fertile offspring are considered separate species.  This means that they are evolutionarily independent units.
• This is the classic textbook criterion and is the legal definition of a species under the Endangered Species Act.
• Concept is often difficult to apply (e.g. in fossil species and plants).  Interbreeding is usually detected not experimentally but by the existence of morphological hybrids.
• Morphological species concept ("morphospecies" concept).
• Populations and individuals are classified on basis of shared morphological characteristics.
• This is the criterion used in practice for nearly all taxonomic decisions (especially with fossil species).
• Reproductive isolation tend to lead to morphological discontinuities between populations, so many species separable on the morphological species concept may also be separable on the biological species concept.
• Speciation: the model of allopatric speciation.
• Stage 1: Physical (geographic) isolation.  Recall that natural selection may favor different genotypes in different environments -- this will cause population differentiation.  Migration (gene flow) between populations homogenizes allele frequencies, slowing or preventing differentiation.  If populations are geographically isolated, selection "wins" and populations will diverge.  Isolation can occur through dispersal and vicariance (Fig. 12.5)
• Isolation through dispersal:  Hawaiian Drosophila (Figs. 15.6, 15.7).  Plate tectonic theory etc. allows us to age the islands.  The most closely related species occur on adjacent islands, and phylogenetic branching events correspond to order of appearance of the islands.
• Isolation through vicariance: Snapping shrimp on either side of the Isthmus of Panama (Fig. 15.8).  Shrimp populations became isolated by formation of Isthmus ca. 3 MYA.  Isolation has allowed speciation to occur in this group.
• Stage 2: Divergence
• Genetic drift as a mechanism of divergence.  If dispersal is cause of geographic isolation, founder effect may produce differences between populations straight-away. Against this model: human introductions of species have been common, but divergence of these introduced populations from ancestral populations is uncommon (examples - zebra mussel, purple loosestrife).
• Natural selection as a mechanisms of divergence.  This is the main force causing population differentiation.  Selection can cause speciation, even in the absence of geographic isolation: Table 15.1
• Sympatric speciation: ecological (niche) isolation can substitute for geographical isolation if selection is strong enough.  Text example: apple and hawthorn maggot flies -- incipient speciation in new habitat (apple trees), Fig. 15.9. 15.10.  Table 15.1 displays several examples of sympatric speciation.
• Sexual selection can reinforce effects of natural selection.  If a trait arises in an isolated subpopulation that increases male reproductive success, this subpopulation can diverge rapidly.  Example: Drosophila lekking behavior and mate choice.
• Stage 3:  Secondary contact.  If recently diverged populations come back into contact, will hybridization occur?
• Mechanisms of secondary contact: human introductions, fragmentation/coalescence of species ranges during the Pleistocene, etc.
• There are 5 possibilities:
• Divergence is so compete that no mating and hybridization occurs.  Speciation process is complete.
• Mating occurs, hybrids form.  Speciation process incomplete.
• Hybrids are less fit than parental species.  This can lead to prezygotic isolation and reinforcement (Fig. 15.13).
• Hybrids are more fit than parental species.  This can lead to formation of a stable hybrid zone (e.g. sagebrush, Artemisia tridentata, Figs. 15.14, 15.15, Table 15.3).
• Hybrids are as fit as parental species.  This can lead to formation of broad hybrid zone and to "despeciation" (coalescence of parental populations).