14  Speciation and Macroevolution

14.1 Learning Objectives

By the end of this chapter, you should be able to:

  1. Define species using different species concepts and explain their strengths and limitations
  2. Describe the mechanisms of reproductive isolation and their role in speciation
  3. Compare and contrast different modes of speciation (allopatric, sympatric, parapatric)
  4. Explain how speciation rates vary and what factors influence them
  5. Describe patterns in the fossil record and their interpretation
  6. Analyze how microevolutionary processes lead to macroevolutionary patterns
  7. Explain major events in the history of life and mechanisms of mass extinction
  8. Apply speciation concepts to understand patterns of biodiversity

14.2 Introduction

Speciation—the process by which new species arise—represents the bridge between microevolution (changes within populations) and macroevolution (large-scale evolutionary patterns). This chapter explores how populations become reproductively isolated and diverge to form new species, and how these processes over geological time have produced the remarkable diversity of life on Earth. Understanding speciation is essential for comprehending biodiversity patterns, evolutionary history, and the current biodiversity crisis.

14.3 What is a Species?

14.3.1 The Species Problem

Challenge: No single definition works for all organisms

Importance: Species are fundamental units in evolution, ecology, and conservation

14.3.2 Species Concepts

Biological Species Concept (BSC) (Ernst Mayr, 1942):

  • Definition: Species are groups of interbreeding natural populations that are reproductively isolated from other such groups
  • Strengths: Clear criteria, emphasizes reproductive isolation as key to divergence
  • Limitations: Doesn’t work for asexual organisms, fossils, or geographically separated populations
  • Problems: Hybridization common in nature (∼10% of animal species, ∼25% of plants)

Morphological Species Concept:

  • Definition: Species defined by differences in morphology
  • Strengths: Practical, works for fossils and asexual organisms
  • Limitations: Subjectivity, cryptic species, phenotypic plasticity

Phylogenetic Species Concept:

  • Definition: Smallest monophyletic group on phylogenetic tree
  • Strengths: Objective, works with molecular data
  • Limitations: Depends on data and analysis methods, may recognize too many species

Ecological Species Concept:

  • Definition: Species defined by ecological niche
  • Strengths: Emphasizes adaptation and natural selection
  • Limitations: Difficult to apply, niches can overlap

Other concepts: Recognition, cohesion, evolutionary

14.3.3 Species in Practice

Operational criteria: Used by taxonomists

  • Genetic distance: Typically >2% sequence divergence for sister species
  • Reproductive isolation: In sympatry
  • Morphological distinction: Statistically significant differences

Cryptic species: Morphologically similar but genetically distinct Ring species: Chain of populations where adjacent populations interbreed but ends do not

14.4 Reproductive Isolation

14.4.1 Premating (Prezygotic) Barriers

Prevent mating or fertilization:

  1. Temporal isolation: Breed at different times
    • Example: Magicicada cicadas (13 vs. 17 year cycles)
  2. Habitat isolation: Occupy different habitats
    • Example: Anolis lizards on different parts of trees
  3. Behavioral isolation: Different courtship behaviors
    • Example: Bird songs, firefly flash patterns
  4. Mechanical isolation: Physical incompatibility
    • Example: Insect genitalia, flower structures
  5. Gametic isolation: Gametes fail to unite
    • Example: Sea urchin sperm recognition proteins

14.4.2 Postmating (Postzygotic) Barriers

Reduce fitness of hybrid offspring:

  1. Reduced hybrid viability: Hybrids don’t develop properly
    • Example: Frog hybrids die as tadpoles
  2. Reduced hybrid fertility: Hybrids are sterile
    • Example: Mules (horse × donkey)
  3. Hybrid breakdown: F2 or backcross hybrids have reduced fitness
    • Example: Cotton species hybrids

14.4.3 Genetics of Reproductive Isolation

Dobzhansky-Muller model:

  • Problem: How do incompatible alleles evolve without reducing fitness?
  • Solution: Incompatibility requires interaction of alleles from different loci
  • Example: Ancestral: A₁A₁B₁B₁ Population 1: A₂A₂B₁B₁ (A₂ fixed) Population 2: A₁A₁B₂B₂ (B₂ fixed) Hybrid: A₂A₁B₂B₁ (incompatible if A₂ and B₂ don’t work together)

Haldane’s rule: When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterogametic sex (XY or ZW)

Speciation genes: Genes involved in reproductive isolation - Often related to gamete recognition, hybrid inviability, chromosomal rearrangements

14.5 Modes of Speciation

14.5.1 Allopatric Speciation

Definition: Speciation due to geographic isolation

Most common mode: Estimated >50% of speciation events

Mechanisms:

  1. Vicariance: Physical barrier divides population
    • Example: Isthmus of Panama (∼3 million years ago)
    • Evidence: Sister species on either side (snapping shrimp, sea urchins)
  2. Peripatric speciation: Small population isolated at edge of range
    • Similar to founder effect speciation
    • Example: Hawaiian Drosophila, island species

Evidence:

  • Geographic concordance of sister species
  • Ring species
  • Experimental studies (Drosophila, sticklebacks)

14.5.2 Sympatric Speciation

Definition: Speciation without geographic isolation

Controversial: Requires strong disruptive selection

Mechanisms:

  1. Disruptive selection + assortative mating
    • Example: Apple maggot flies (Rhagoletis pomonella)
      • Original host: Hawthorn
      • New host: Apples (∼150 years ago)
      • Genetic differences: 6% allele frequency differences, different emergence times
  2. Polyploidy: Instant speciation
    • Autopolyploidy: Within species (e.g., potato)
    • Allopolyploidy: Between species (e.g., wheat, cotton)
    • Common in plants (∼50% of angiosperms), rare in animals
  3. Host race formation: Insects on different host plants
  4. Sexual selection: Mate preference drives divergence

14.5.3 Parapatric Speciation

Definition: Speciation with adjacent populations and limited gene flow

Mechanism: Selection gradient across environmental gradient

Example: Anthoxanthum odoratum (grass) on mine tailings (tolerant vs. non-tolerant)

14.5.4 Quantum Speciation vs. Gradualism

Phyletic gradualism: Slow, steady change

Punctuated equilibrium (Eldredge and Gould, 1972):

  • Most change occurs during speciation events
  • Species relatively stable between speciation events
  • Evidence: Fossil record shows stasis and sudden appearances

14.6 Speciation Rates and Patterns

14.6.1 Factors Influencing Speciation Rates

Intrinsic factors:

  • Generation time: Faster in organisms with short generations
  • Dispersal ability: Poor dispersers speciate faster (island biogeography)
  • Sexual selection: Can accelerate speciation
  • Chromosomal rearrangements: Can promote reproductive isolation

Extrinsic factors:

  • Geographic area: Larger areas → more speciation
  • Climate change: Creates/removes barriers
  • Ecological opportunity: Adaptive radiation after extinction or colonization

14.6.2 Adaptive Radiation

Definition: Rapid diversification from common ancestor into varied ecological niches

Requirements:

  1. Common ancestry
  2. Phenotype-environment correlation
  3. Trait utility (adaptation)
  4. Rapid speciation

Classic examples:

  • Darwin’s finches: Galápagos Islands (∼15 species)
  • Hawaiian honeycreepers: ∼50 species from one colonization
  • Cichlid fishes: African rift lakes (500+ species in Lake Victoria)
  • Anolis lizards: Caribbean islands

Triggers:

  • Key innovations: New traits allowing new niches
  • Colonization: New area with available niches
  • Mass extinction: Opens ecological space

14.6.3 Island Biogeography Theory (MacArthur and Wilson, 1967)

Principles:

  • Island size: Larger islands have more species
  • Distance from mainland: Closer islands have more species
  • Equilibrium: Immigration and extinction balance

Application to speciation: Islands as natural laboratories

14.7 Macroevolutionary Patterns

14.7.1 The Fossil Record

Limitations:

  • Incomplete preservation
  • Bias toward hard parts, common species, certain environments
  • Temporal resolution limited

Strengths:

  • Direct evidence of past life
  • Temporal sequence
  • Environmental context

Dating methods:

  • Relative dating: Stratigraphy, index fossils
  • Absolute dating: Radiometric (¹⁴C, K-Ar, U-Pb), paleomagnetism

14.7.2 Patterns in the Fossil Record

Biodiversity through time:

  • Overall trend: Increase in diversity (despite mass extinctions)
  • Rate of increase: Higher in Phanerozoic than Precambrian
  • Current estimate: ∼8.7 million eukaryotic species (only ∼1.5 million described)

Evolutionary trends:

  • Size increase: Cope’s rule (not universal)
  • Complexity increase: Generally, but with reversals
  • Specialization: Often leads to evolutionary “dead ends”

Stasis: Long periods of little change (support for punctuated equilibrium)

14.7.3 Major Transitions in Evolution

  1. Origin of life: ∼3.8 billion years ago
  2. Prokaryotic cells: ∼3.5 billion years ago
  3. Photosynthesis: ∼3.0 billion years ago (oxygen revolution)
  4. Eukaryotic cells: ∼2.0 billion years ago (endosymbiosis)
  5. Multicellularity: Animals ∼600 million years ago (Ediacaran)
  6. Cambrian explosion: ∼541 million years ago (most animal phyla appear)
  7. Colonization of land: Plants ∼470 million years ago, animals followed
  8. Evolution of flight: Insects ∼400 million years ago, vertebrates later
  9. Human evolution: Last 6-7 million years

14.7.4 Convergent Evolution

Definition: Independent evolution of similar traits in unrelated lineages

Examples:

  • Flight: Birds, bats, insects
  • Camera eyes: Vertebrates, cephalopods
  • Streamlined body: Dolphins, ichthyosaurs
  • C4 photosynthesis: Evolved independently >60 times in plants

Significance: Demonstrates adaptation to similar environmental challenges

14.8 Mass Extinctions

14.8.1 The “Big Five” Mass Extinctions

  1. End-Ordovician (∼444 million years ago):
    • Loss: ∼85% of marine species
    • Cause: Glaciation and sea level drop
  2. Late Devonian (∼375 million years ago):
    • Loss: ∼75% of species
    • Cause: Possibly anoxic oceans, volcanic activity
  3. End-Permian (∼252 million years ago):
    • “Great Dying”: ∼96% of marine species, 70% of terrestrial vertebrates
    • Cause: Siberian Traps volcanism, climate change, anoxia
  4. End-Triassic (∼201 million years ago):
    • Loss: ∼80% of species
    • Cause: Central Atlantic Magmatic Province volcanism
  5. End-Cretaceous (K-Pg) (66 million years ago):
    • Loss: ∼75% of species, including non-avian dinosaurs
    • Cause: Chicxulub asteroid impact + Deccan Traps volcanism

14.8.2 Patterns and Consequences

Selectivity: Some groups more affected than others

Recovery: Takes 5-10 million years for diversity to recover

Evolutionary consequences:

  • Opens ecological space for survivors
  • Changes evolutionary trajectories
  • Example: Mammal diversification after dinosaur extinction

14.8.3 The Sixth Mass Extinction?

Current rates: 100-1000× background extinction rate

Causes: Habitat loss, climate change, pollution, overexploitation, invasive species

Differences from past: Human-caused, much faster

Projections: 20-50% of species could be extinct by 2100

14.10 Applications

14.10.1 Conservation Biology

Evolutionary Significant Units (ESUs): Populations with unique evolutionary heritage

Genetic rescue: Introducing new genetic material to inbred populations

Captive breeding: Maintaining genetic diversity

14.10.2 Agriculture and Pest Management

Crop domestication: Artificial selection parallels natural selection

Pest evolution: Resistance to pesticides, need for evolutionary thinking in management

14.10.3 Medicine

Pathogen evolution: Antibiotic resistance, vaccine escape

Cancer evolution: Tumors as evolving populations

Evolutionary medicine: Why we get sick from evolutionary perspective

14.11 Chapter Summary

14.11.1 Key Concepts

  1. Species concepts: Multiple definitions with different strengths/limitations
  2. Reproductive isolation: Premating and postmating barriers prevent gene flow
  3. Speciation modes: Allopatric (most common), sympatric, parapatric
  4. Speciation rates: Vary with intrinsic and extrinsic factors
  5. Macroevolution: Large-scale patterns from fossil record and comparative biology
  6. Mass extinctions: Five major events have reshaped life’s history
  7. Evolutionary trends: Not progressive or goal-directed
  8. Applications: Conservation, agriculture, medicine

14.11.2 Speciation Mode Comparison

Mode Geographic Context Primary Mechanism Examples
Allopatric Physically separated Genetic drift + differential selection Galápagos finches, snapping shrimp
Sympatric Same area Disruptive selection + assortative mating Apple maggot flies, polyploid plants
Parapatric Adjacent areas Selection gradient across environment Grass on mine tailings

14.11.3 Reproductive Isolation Mechanisms

Type Timing Mechanism Example
Temporal Premating Different breeding times Cicada species
Habitat Premating Different habitats Tree lizard ecomorphs
Behavioral Premating Different courtship Bird songs
Gametic Premating Gamete incompatibility Sea urchins
Hybrid inviability Postmating Developmental failure Frog hybrids
Hybrid sterility Postmating Meiotic problems Mules
Hybrid breakdown Postmating F2 reduced fitness Cotton species

14.11.4 Mass Extinction Comparison

Event Time (mya) % Marine Species Lost Probable Cause
End-Ordovician 444 85% Glaciation
Late Devonian 375 75% Anoxic oceans
End-Permian 252 96% Volcanism, climate
End-Triassic 201 80% Volcanism
End-Cretaceous 66 75% Asteroid impact

14.11.5 Evolutionary Timescales

Microevolution: Generations to thousands of years

Speciation: Thousands to millions of years

Macroevolution: Millions to billions of years

Mass extinction recovery: 5-10 million years

14.12 Review Questions

14.12.1 Level 1: Recall and Understanding

  1. Compare and contrast the biological and phylogenetic species concepts.
  2. List three premating and three postmating reproductive isolating mechanisms.
  3. What are the three main modes of speciation, and how do they differ?
  4. What evidence supports the theory of punctuated equilibrium?
  5. Name the “Big Five” mass extinctions and their approximate dates.

14.12.2 Level 2: Application and Analysis

  1. Why is allopatric speciation considered the most common mode of speciation?
  2. How could you determine whether two morphologically similar populations represent different species?
  3. Explain how polyploidy can cause instantaneous speciation in plants.
  4. What factors might explain why some lineages have higher speciation rates than others?
  5. How does the concept of adaptive radiation help explain patterns of biodiversity on islands?

14.12.3 Level 3: Synthesis and Evaluation

  1. Evaluate the statement: “The current biodiversity crisis represents a sixth mass extinction.”
  2. How does the fossil record both support and challenge our understanding of evolutionary processes?
  3. Why is the concept of “progress” problematic in evolutionary biology?
  4. Design a research project to study speciation in a natural population.

14.13 Key Terms

  • Species: Basic unit of biological classification
  • Biological Species Concept: Defines species as interbreeding groups reproductively isolated from others
  • Reproductive isolation: Biological barriers that impede members of different species from interbreeding
  • Allopatric speciation: Speciation that occurs when populations are geographically isolated
  • Sympatric speciation: Speciation that occurs without geographic isolation
  • Adaptive radiation: Rapid evolutionary diversification from a common ancestor into various ecological niches
  • Punctuated equilibrium: Pattern of evolution characterized by long periods of stasis interrupted by brief periods of rapid change
  • Mass extinction: Event in which a large percentage of all living species become extinct in a relatively short period
  • Convergent evolution: Independent evolution of similar features in species of different lineages
  • Macroevolution: Large-scale evolutionary changes over long time periods
  • Microevolution: Changes in allele frequencies in populations over generations

14.14 Further Reading

14.14.1 Books

  1. Coyne, J. A., & Orr, H. A. (2004). Speciation. Sinauer Associates.
  2. Mayr, E. (1942). Systematics and the Origin of Species. Columbia University Press.
  3. Eldredge, N., & Gould, S. J. (1972). Punctuated equilibria: An alternative to phyletic gradualism. In Models in Paleobiology (pp. 82-115). Freeman, Cooper.

14.14.2 Scientific Articles

  1. Mayr, E. (1942). Systematics and the origin of species. Columbia University Press.
  2. Dobzhansky, T. (1937). Genetics and the Origin of Species. Columbia University Press.
  3. Grant, P. R., & Grant, B. R. (2008). How and Why Species Multiply: The Radiation of Darwin’s Finches. Princeton University Press.

14.14.3 Online Resources

  1. Understanding Evolution: https://evolution.berkeley.edu
  2. Tree of Life Web Project: http://tolweb.org
  3. Paleobiology Database: https://paleobiodb.org

14.15 Quantitative Problems

  1. Island Biogeography: An island has an immigration rate I = 10 - 0.1S (where S = number of species) and an extinction rate E = 0.05S.
    1. What is the equilibrium number of species?
    2. If the island doubles in size, extinction rate becomes E = 0.025S. What is the new equilibrium?
    3. If distance increases, immigration becomes I = 5 - 0.1S. What is the new equilibrium?
  2. Speciation Rate Calculations: A clade has 1000 extant species. Fossil evidence suggests the clade originated 50 million years ago.
    1. Assuming constant speciation rate and no extinction, what is the speciation rate (species per million years)?
    2. If extinction rate is 0.5 species/million years, what is the net diversification rate?
    3. How many species would be expected after 100 million years at this net rate?
  3. Genetic Divergence: Two populations were separated 1 million years ago. Mutation rate is 10⁻⁸ per base per generation, generation time is 5 years.
    1. How many mutations are expected per base in each lineage?
    2. What is the expected sequence divergence between populations?
    3. If reproductive isolation typically occurs at 2% divergence, is speciation likely?

14.16 Case Study: Cichlid Fish Radiation in African Lakes

Background: Lake Victoria contains >500 species of cichlid fishes that evolved from a common ancestor in <15,000 years.

Questions:

  1. What evidence suggests these cichlids represent an adaptive radiation?
  2. What mechanisms might have driven such rapid speciation?
  3. How does this example challenge traditional views of speciation timescales?
  4. What threats do these fish face, and why are they particularly vulnerable?

Data for analysis:

  • Lake age: <1 million years (dried up 15,000 years ago)
  • Species diversity: >500 species, many endemic
  • Trophic specializations: Algae scrapers, insect eaters, scale eaters, egg eaters
  • Mate selection: Based on color patterns (sexual selection)
  • Threats: Nile perch introduction, eutrophication, hybridization

Next Chapter: Phylogenetics and the Tree of Life