Speciation: The Origin of Species

Chapter 24: The Origin of Species

Introduction to Speciation

• Speciation: The process by which new species arise from existing species.

• Microevolution: Changes in allele frequency in a population over time.

• Macroevolution: Large-scale evolutionary changes that occur above the species level.

The Biological Species Concept

Definition and Importance

• Species: Latin for "kind" or "appearance"; confirmed by physiological, biochemical, and DNA sequence comparisons.

Reproductive Isolation

Reproductive isolation consists of biological barriers that impede members of two species from interbreeding and producing viable, fertile offspring. These barriers limit the formation of hybrids (offspring from interspecific mating) and can be classified as prezygotic or postzygotic.

• Prezygotic barriers: Prevent fertilization from occurring.

• Postzygotic barriers: Prevent hybrid zygotes from developing into viable, fertile adults.

Types of Reproductive Barriers

Barrier Type	Mechanism	Example
Habitat Isolation	Species occupy different habitats	Apple maggot flies vs. blueberry maggot flies
Temporal Isolation	Species breed at different times	Western vs. eastern spotted skunks
Behavioral Isolation	Unique courtship rituals	Blue-footed boobies
Mechanical Isolation	Morphological differences prevent mating		Snails with different shell spirals
Gametic Isolation	Sperm cannot fertilize eggs	Sea urchin species with incompatible gametes
Reduced Hybrid Viability	Hybrids fail to develop or survive	Ensatina salamander hybrids
Reduced Hybrid Fertility	Hybrids are sterile	Mule (horse × donkey)
Hybrid Breakdown	Next-generation hybrids are feeble or sterile	Cultivated rice hybrids

Limitations of the Biological Species Concept

• Cannot be applied to fossils or asexual organisms (e.g., prokaryotes).

• Gene flow can occur between distinct species (e.g., grizzly and polar bears producing "grolar bears").

Other Species Concepts

• Morphological species concept: Distinguishes species by structural features; applies to sexual and asexual species but is subjective.

• Ecological species concept: Defines species by ecological niche; applies to sexual and asexual species and emphasizes disruptive selection.

Modes of Speciation

Allopatric Speciation

Allopatric speciation occurs when populations are geographically isolated, interrupting gene flow. Over time, genetic divergence leads to reproductive isolation.

• Geographic barriers (e.g., lakes, canyons) can separate populations.

• Genetic divergence occurs via mutation, natural selection, and genetic drift.

• Evidence: Laboratory fruit flies on different diets, snapping shrimp separated by the Isthmus of Panama, unique species on islands.

Sympatric Speciation

Sympatric speciation occurs in populations that live in the same geographic area. It is less common and typically results from reduced gene flow due to polyploidy, sexual selection, or habitat differentiation.

• Polyploidy: Extra sets of chromosomes due to cell division errors; common in plants.

• Sexual selection: Mate choice based on traits (e.g., cichlids in Lake Victoria).

• Habitat differentiation: Use of different resources or habitats (e.g., apple maggot flies).

Types of Polyploidy

• Autopolyploidy: Multiple chromosome sets from a single species; can result in tetraploid (4n) individuals.

• Allopolyploidy: Chromosome sets from different species; hybrids may become fertile if chromosome number doubles.

Many crops (oats, cotton, potatoes, tobacco, wheat) are polyploids.

Hybrid Zones and Speciation Outcomes

Hybrid Zones

A hybrid zone is a region where members of different species meet and produce hybrid offspring. These zones reveal factors that cause reproductive isolation and can shift or change with environmental conditions.

• Hybrids often have reduced fitness compared to parent species.

• Hybrid zones may be narrow bands or scattered patches.

• Environmental changes can shift hybrid zones or create new ones.

Outcomes for Hybrids

• Reinforcement: Strengthening of reproductive barriers; hybrids are less fit, so selection favors prezygotic barriers.

• Fusion: Weakening of reproductive barriers; gene flow increases, and species may merge.

• Stability: Continued production of hybrids; gene flow from outside the hybrid zone maintains hybridization.

Rates and Genetics of Speciation

Patterns and Rates of Speciation

• Speciation can be rapid (punctuated equilibrium) or gradual.

• Fossil record and molecular data show intervals between speciation events range from thousands to millions of years.

• Example: Sunflower Helianthus anomalus formed rapidly by hybridization.

Genetics of Speciation

• Speciation may involve changes in one or many genes.

• Example: Shell spiral in Japanese snails controlled by a single gene; flower color in Mimulus influenced by multiple loci.

From Speciation to Macroevolution

Successive speciation events accumulate differences, leading to the formation of new groups of organisms. Macroevolution is the cumulative effect of many speciation and extinction events over time.