1- Chapter 21: Species and Speciation

Reproductive isolation is the key to the biological species concept (BSC).
Reproductive isolation arises from barriers to reproduction before or after egg fertilization.
Speciation underlies the diversity of life on Earth.
Speciation can occur with or without natural selection.

Definition (Mayr): "Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups."
Origin: concept refined after Wallace; central to Modern Evolutionary Synthesis.
Example: closely related orangutan populations illustrate interbreeding groups separated by isolation.

Practical tool for identifying species in the field.
Based on the general rule that most individuals of the same species look alike (morphology).
Not perfect; variation and cryptic species can mislead; imperfect in some groups.

Two categories:
- Pre-zygotic (before fertilization): Behavioral, Physical, Temporal (time), Ecological (space).
- Post-zygotic (after fertilization): Genetic incompatibility, often leading to failure of zygote development or sterile offspring.

Most speciation is thought to be allopatric.
Allopatry: allo = other/different; patry may be interpreted as united in distribution.
Two main routes to allopatry:
- Dispersal: a subset colonizes a new area.
- Vicariance: a barrier splits a population.

Peripatric speciation: a new population forms at the edge of the range; founder population is small.
Often followed by Adaptive Radiation: rapid speciation into many ecologically distinct forms as organisms adapt to local conditions.
Key idea: dispersal to a new, isolated area drives isolation and diversification.
Example: New Guinea kingfishers → 8 subspecies due to isolation and local adaptation.

Diversification driven by ecological opportunity or novel niches.
Rapid generation of multiple, ecologically distinct species from a common ancestor.

One lineage speciates in response to changes in another lineage; the two groups co-evolve.
Classic example: lice changing in response to host (gophers) changes; reflects ecological interaction.

Typically arises from disruptive natural selection within the same geographic area.
Requires very strong selection to maintain divergence despite gene flow.

Polyploidy: multiple sets of chromosomes; degrees of ploidy exceed the parent.
Can produce offspring that cannot reproduce with the parent generation, leading to instant speciation.
Common mechanism in plants; creates immediate reproductive barriers.

Hybridization between species can generate new lineages that become reproductively isolated.
Occurs in animals as a pathway to new species; more common in plants but not exclusive to them.

Speciation can occur with or without selection.
Natural selection does not always lead to speciation.
In allopatry, genetic drift can drive non-adaptive changes that contribute to divergence.
Natural selection can reinforce reproductive isolation, especially pre-zygotic isolation:
- Sympatric speciation often relies on disruptive selection to maintain divergence.
- Allopatric speciation is often facilitated by natural selection, but drift can also drive divergence in the absence of strong selection.

Sympatric speciation via disruptive selection requires a wide range of phenotypes to maintain divergence.
Allopatric speciation can be facilitated by mild or strong natural selection along with geographic isolation.
Reinforcement: selection favors traits that prevent hybridization, strengthening pre-zygotic isolation.

Biological Species Concept (BSC): reproductively isolated groups.
Pre-zygotic vs Post-zygotic isolation.
Allopatry, vicariance, dispersal, peripatric.
Adaptive radiation, co-speciation, sympatric speciation, polyploidy, hybridization.
Ring species and Wallace Effect as illustrating complexity of speciation.

Peripatric speciation and adaptive radiation: dispersal to new areas (e.g., New Guinea kingfishers).
Polyploidy: instantaneous speciation in plants.
Hybridization: animal examples of speciation via hybrid lineages.
Reinforcement: natural selection strengthens mating barriers.

The essential theme: speciation is the process that creates diversity; it can occur with or without natural selection, and reproductive isolation is the core criterion distinguishing species.