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Speciation, Phylogenetics, and Macroevolution

Gregor Welcomes You!

Gregor welcomes you Welcome, investigators! In Chapter 16 you learned how allele frequencies change within populations. But when does a population stop being one species and become two? How do we reconstruct the branching history of life? This chapter explores the fascinating process of speciation — the birth of new species — and the tools biologists use to map evolutionary relationships across billions of years. Let's investigate!

Summary

When populations become reproductively isolated, they diverge into separate species — a process this chapter examines through the biological species concept, pre- and postzygotic isolating mechanisms, and the contrasting geographic pathways of allopatric versus sympatric speciation. Adaptive radiation and convergent evolution illustrate how selection can rapidly diversify or independently produce similar forms. The chapter then introduces phylogenetics and cladistics: how shared derived characters and molecular clocks allow biologists to reconstruct evolutionary history and build cladograms. Coevolution shows how interacting species can drive each other's evolution, rounding out the major patterns and processes that operate above the population level.

Concepts Covered

This chapter covers the following 15 concepts from the learning graph:

Speciation

  1. Biological Species Concept
  2. Speciation Overview
  3. Reproductive Isolation
  4. Prezygotic Barriers
  5. Postzygotic Barriers
  6. Allopatric Speciation
  7. Sympatric Speciation

Evolutionary Patterns

  1. Adaptive Radiation
  2. Convergent Evolution
  3. Coevolution

Phylogenetics

  1. Phylogenetics
  2. Cladistics
  3. Cladograms
  4. Shared Derived Characters
  5. Molecular Clocks

Prerequisites

This chapter builds on concepts from:

What Is a Species?

Biological Species Concept

The biological species concept (Ernst Mayr, 1942) defines a species as a group of organisms that can interbreed in nature and produce viable, fertile offspring — and are reproductively isolated from other such groups.

Strengths:

  • Clear, testable criterion (can they interbreed?)
  • Widely applicable to sexually reproducing organisms

Limitations:

  • Cannot be applied to asexual organisms (bacteria, many protists)
  • Cannot be applied to fossils (we can't observe their mating behavior)
  • Some closely related species can hybridize (e.g., wolves and coyotes)

Despite these limitations, the biological species concept remains the most widely used definition in AP Biology.

Speciation: The Birth of New Species

Speciation Overview

Speciation is the process by which one species splits into two or more reproductively isolated species. The key requirement is the evolution of reproductive isolation — barriers that prevent gene flow between populations.

Reproductive Isolation

Reproductive isolating mechanisms are biological features that prevent members of different species from interbreeding. They are divided into two categories based on timing:

Prezygotic Barriers

Prezygotic barriers prevent the formation of a zygote (act before fertilization):

Barrier Mechanism Example
Habitat isolation Species live in different habitats in the same area Ground squirrels on opposite sides of Grand Canyon
Temporal isolation Species breed at different times (seasons, times of day) Two frog species: one breeds in spring, another in summer
Behavioral isolation Species have different courtship rituals or signals Firefly species with different flash patterns
Mechanical isolation Anatomical incompatibility prevents mating Flower shapes that fit only specific pollinators
Gametic isolation Sperm and egg are chemically incompatible Sea urchin species with species-specific egg surface proteins

Postzygotic Barriers

Postzygotic barriers act after fertilization, reducing the viability or fertility of hybrids:

Barrier Mechanism Example
Reduced hybrid viability Hybrid embryo fails to develop properly Sheep-goat hybrids rarely survive
Reduced hybrid fertility Hybrid is viable but infertile Mule (horse × donkey) is sterile
Hybrid breakdown First-generation hybrid is fertile, but subsequent generations have reduced fitness Some rice hybrids show declining vigor

Allopatric Speciation

Allopatric speciation ("other homeland") occurs when a geographic barrier physically separates a population into isolated groups. With gene flow cut off, each group evolves independently through natural selection, genetic drift, and mutation. Over time, reproductive isolation develops.

Examples:

  • The Grand Canyon separating squirrel populations (Kaibab vs. Abert's squirrels)
  • Island colonization (Darwin's finches on the Galápagos)
  • Continental drift separating populations on different landmasses

Allopatric speciation is the most common mode of speciation.

Sympatric Speciation

Sympatric speciation ("same homeland") occurs without geographic separation. Reproductive isolation evolves while populations are still in contact.

Mechanisms:

  • Polyploidy (most common in plants) — errors during cell division produce organisms with extra chromosome sets (e.g., 4n). Polyploid individuals can breed with other polyploids but not with the original diploid population → instant reproductive isolation.
  • Habitat differentiation — some individuals exploit a different niche within the same area
  • Sexual selection — mate preference diverges within a population

Diagram: Speciation Pathways Comparison

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Speciation Pathways Comparison — Specification

Type: MicroSim (p5.js)
sim-id: speciation-pathways
Library: p5.js
Status: Specified

Learning objective: Students will be able to compare (Bloom's L4: Analyze) allopatric and sympatric speciation, and explain (Bloom's L2: Understand) how geographic barriers, polyploidy, and habitat differentiation each lead to reproductive isolation.

Instructional Rationale: A side-by-side animated comparison of two speciation pathways — one showing a population split by a geographic barrier, the other showing polyploidy within a single population — makes the critical distinction between the two modes visible and memorable.

Canvas: 800 × 480 px, responsive.

Layout: Two parallel panels:

  • Left: "Allopatric Speciation" — shows a population being divided by a geographic barrier (mountain range rises), then diverging over generations (color/shape changes)
  • Right: "Sympatric Speciation" — shows a polyploidy event within a population (chromosome doubling), then reproductive isolation developing

Interaction: - "Play" button advances through stages (5 stages each) - Next/Previous for manual stepping - At each stage, a text card describes what is happening genetically - "Quiz" toggle: at each stage, student predicts whether gene flow is still occurring (yes/no)

Colors: Population A: blue individuals. Population B (diverged): orange individuals. Geographic barrier: brown. Polyploid individuals: larger circles with extra chromosome lines visible.

Responsive design: Panels stack vertically on narrow screens.

Evolutionary Patterns

Adaptive Radiation

Adaptive radiation occurs when a single ancestral species rapidly diversifies into many new species, each adapted to different ecological niches. This typically follows:

  • Colonization of a new, relatively empty environment (e.g., islands)
  • Mass extinction that opens niches for survivors

Examples:

  • Darwin's finches — 14+ species from one ancestral finch on the Galápagos, each with a beak adapted to a different food source
  • Hawaiian honeycreepers — spectacular beak diversity from a single colonizing species
  • Mammalian radiation after the K-Pg extinction

Convergent Evolution

Convergent evolution produces analogous structures — similar traits in unrelated species exposed to similar environmental pressures. The underlying genetics and developmental pathways are different; only the final phenotype is similar.

Examples:

  • Streamlined body shape in dolphins (mammals), sharks (fish), and ichthyosaurs (reptiles)
  • Camera-type eyes in vertebrates and cephalopods (evolved independently)
  • Cactus-like succulents in American deserts (Cactaceae) and African deserts (Euphorbiaceae)

Coevolution

Coevolution occurs when two or more species exert selective pressures on each other, driving reciprocal evolutionary change.

Examples:

  • Predator-prey arms races — cheetah speed and gazelle speed escalate together
  • Pollinator-flower relationships — flower shape, color, and scent evolve to match pollinator preferences; pollinator mouthparts evolve to access nectar
  • Parasite-host coevolution — host immune defenses and parasite evasion strategies escalate reciprocally

Key Insight

Gregor thinking Adaptive radiation (one species → many) and convergent evolution (many species → similar traits) are opposite patterns. Radiation demonstrates how diverse niches can pull one lineage into many forms. Convergence demonstrates how similar niches can push unrelated lineages toward the same solution. Both illustrate the power of natural selection.

Phylogenetics and Cladistics

Phylogenetics

Phylogenetics is the study of evolutionary relationships among organisms. These relationships are represented as branching tree diagrams that show patterns of common ancestry.

Cladistics

Cladistics is the method of classification based on common ancestry. Organisms are grouped into clades — groups that include an ancestor and all of its descendants.

Shared Derived Characters

Cladistic analysis relies on identifying shared derived characters (synapomorphies) — traits that are:

  • Shared by two or more taxa
  • Derived (evolved more recently, not present in the distant ancestor)

Shared derived characters indicate that taxa share a more recent common ancestor. In contrast, shared ancestral characters (symplesiomorphies) — primitive traits shared by all members of a larger group — do NOT help distinguish close relationships.

Example: Among vertebrates:

  • Four limbs = shared derived character uniting tetrapods (amphibians, reptiles, mammals)
  • Vertebral column = shared ancestral character (all vertebrates have it, so it doesn't distinguish subgroups)

Cladograms

A cladogram is a branching diagram that shows the hypothesized evolutionary relationships among a group of organisms based on shared derived characters.

How to read a cladogram:

  • Each node (branch point) represents a common ancestor
  • Each branch represents a lineage evolving over time
  • Sister taxa are groups that share the most recent common ancestor
  • The outgroup is the most distantly related taxon, used to root the tree
  • Traits are mapped onto branches where they first evolved

Diagram: Interactive Cladogram Builder

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Interactive Cladogram Builder — Specification

Type: MicroSim (p5.js)
sim-id: cladogram-builder
Library: p5.js
Status: Specified

Learning objective: Students will be able to construct (Bloom's L6: Create) a cladogram from a character matrix and interpret (Bloom's L2: Understand) evolutionary relationships from an existing cladogram.

Instructional Rationale: Building a cladogram from raw character data (rather than just reading a pre-made one) develops the reasoning skills tested on the AP exam, where students must interpret or construct phylogenetic trees from data tables.

Canvas: 800 × 480 px, responsive.

Layout:

  • Left panel: Character matrix (table of species × traits, with checkmarks for presence)
  • 6 species (rows)
  • 6 characters (columns)
  • Outgroup identified
  • Right panel: Cladogram building area
  • Drag species labels to branch tips
  • Drag trait markers to branches where they evolved

Pre-loaded example: Vertebrate classification

Character Lamprey (outgroup) Trout Frog Lizard Pigeon Mouse
Vertebrae ✓ ✓ ✓ ✓ ✓ ✓
Jaws ✓ ✓ ✓ ✓ ✓
Four limbs ✓ ✓ ✓ ✓
Amniotic egg ✓ ✓ ✓
Feathers ✓
Hair/Mammary glands ✓

Interaction: - Drag species to correct positions on the cladogram - Drag trait markers to the correct branches (where the trait first evolved) - "Check" button verifies the student's cladogram against the correct solution - Incorrect placements highlighted with red; hints provided - 3 pre-loaded datasets of increasing difficulty - "Molecular Data" toggle: replaces morphological traits with DNA sequence similarity percentages

Colors: Each trait: distinct color marker. Correct placement: green flash. Incorrect: red shake. Outgroup: gray.

Responsive design: Character matrix and cladogram stack vertically on narrow screens.

Molecular Clocks

Molecular clocks use the rate of DNA or protein sequence change to estimate the time of divergence between two lineages. The method assumes that neutral mutations accumulate at a roughly constant rate over time.

How it works:

  1. Compare homologous DNA sequences between two species
  2. Count the number of nucleotide differences
  3. Calibrate the mutation rate using fossil evidence with known dates
  4. Estimate the time since the two species shared a common ancestor

Limitations:

  • Mutation rates vary among genes, organisms, and over time
  • Natural selection can accelerate or slow sequence change
  • Calibration requires reliable fossil dates

Despite these limitations, molecular clocks are invaluable for estimating divergence times when fossils are scarce or absent.

Gregor's Tip

Gregor's tip On the AP exam, cladogram questions are common. Remember: you cannot determine the age of a species by its position left or right on a cladogram — only the branching pattern (topology) matters. Two species are most closely related if they share the most recent common ancestor (the node closest to the tips). Rotating branches around a node does not change the relationships.

Key Takeaways

  1. The biological species concept defines a species by reproductive isolation — the ability to interbreed and produce viable, fertile offspring.

  2. Prezygotic barriers (habitat, temporal, behavioral, mechanical, gametic) prevent fertilization; postzygotic barriers (hybrid inviability, infertility, breakdown) reduce hybrid success.

  3. Allopatric speciation requires geographic separation; sympatric speciation occurs without it (often via polyploidy in plants).

  4. Adaptive radiation diversifies one lineage into many species filling different niches. Convergent evolution produces similar traits in unrelated lineages under similar selection pressures.

  5. Coevolution involves reciprocal evolutionary change between interacting species (predator-prey, pollinator-flower, parasite-host).

  6. Cladistics classifies organisms by shared derived characters (synapomorphies). Cladograms display hypothesized evolutionary relationships.

  7. Molecular clocks estimate divergence times based on the rate of neutral sequence change, calibrated with fossil evidence.

AP Practice: Test Your Understanding

Question 1: Two species of frogs live in the same pond but breed at different times of year. What type of reproductive barrier is this? Is it prezygotic or postzygotic?

Answer: This is temporal isolation, a prezygotic barrier. The two species never encounter each other during their breeding seasons, so mating (and therefore fertilization) cannot occur.

Question 2: A character matrix shows that species A and B share 4 derived characters, while species A and C share only 2. Which pair is more closely related? How would this appear on a cladogram?

Answer: Species A and B are more closely related — they share more derived characters, indicating a more recent common ancestor. On a cladogram, A and B would be sister taxa (sharing a node), with C branching off earlier.

Question 3: Explain why polyploidy can lead to instant speciation in plants but is rare in animals.

Answer: Polyploidy instantly creates reproductive isolation because a polyploid individual (e.g., 4n) cannot produce viable offspring with diploid (2n) members of the parent species — the resulting triploid (3n) offspring would have unbalanced chromosome segregation during meiosis. Plants tolerate polyploidy well because they can self-fertilize or reproduce vegetatively, establishing a viable polyploid population. Animals, with obligate sexual reproduction and more complex development, rarely survive polyploidy.