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Speciation
The Origin of New Species

Explore how populations diverge to form new species through geographic isolation, reproductive barriers, and natural selection.

AP Biology

What Is a Species?

Before we can understand how new species form, we need to define what a species actually is. The most widely used definition in biology is the Biological Species Concept.

This concept was articulated by Ernst Mayr and emphasizes reproductive isolation as the key criterion separating one species from another.

What Is Speciation?

Speciation is the evolutionary process by which populations evolve to become distinct species. It is the mechanism responsible for the incredible biodiversity of life on Earth.

Ancestral Population Barrier Species A Species B Time

Why Does Speciation Matter?

Biodiversity
Speciation is the engine that generates all species diversity on Earth: an estimated 8.7 million species alive today.
Adaptation
New species arise when populations adapt to different environments, niches, or ecological pressures over time.
Conservation
Understanding speciation helps biologists protect endangered species and predict how populations respond to habitat fragmentation.

Two Main Modes of Speciation

Allopatric Population Geographic Barrier Sp. A Sp. B Sympatric Population Niche / Behavioral / Polyploidy Divergence Sp. A Sp. B Same geographic area

Allopatric Speciation

Allopatric speciation (from Greek allos = "other" + patra = "homeland") occurs when a population is geographically divided. Each subpopulation then evolves independently.

1
Geographic barrier appears: a mountain range, river, glacier, or ocean separates the population.
2
Gene flow ceases: the separated groups can no longer exchange alleles through interbreeding.
3
Independent evolution: mutation, natural selection, and genetic drift act differently in each group.
4
Reproductive isolation: over time, the groups diverge enough that they can no longer interbreed, even if reunited.

Sympatric Speciation

Sympatric speciation (from Greek sym = "together") occurs without geographic separation. It is driven by factors that reduce gene flow within a single population.

Polyploidy
An error in cell division creates organisms with extra chromosome sets. Common in plants: an autopolyploid cannot breed with the diploid parent.
Habitat Differentiation
Subpopulations exploit different resources or microhabitats within the same area, reducing gene flow between them.
Sexual Selection
Mate preferences diverge within a population, leading to assortative mating and eventual reproductive isolation.

Reproductive Barriers

Reproductive isolation is maintained by prezygotic and postzygotic barriers. These barriers prevent gene flow between species.

Prezygotic Barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Postzygotic Barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Before fertilization ← → After fertilization

Allopatric vs Sympatric

Allopatric Sympatric
Geographic separation Yes: physical barrier No: same area
Primary mechanism Natural selection + drift Polyploidy, niche shifts
Prevalence Most common mode Less common; plants > animals
Gene flow disrupted by Physical barrier Behavioral or genetic change
Classic example Darwin's finches Apple maggot fly
Speed Typically gradual Can be rapid (polyploidy)

Geographic context changes gene-flow opportunities

Birds occupy a mountain valley and forest-to-grassland landscape beside three exact population schematics.Geographic barriers, shared habitat and adjacent ranges create different opportunities for gene flow. Reproductive isolation must be tested separately from geographic separation.

Geography changes gene flow

Geography changes gene flow. Geographic barriers, shared habitat and adjacent ranges create different opportunities for gene flow. Reproductive isolation must be tested separately from geographic separation.Geographic barriers, shared habitat and adjacent ranges create different opportunities for gene flow. Reproductive isolation must be tested separately from geographic separation.

Hybrid Zones and Reinforcement

When partially diverged populations come back into contact, a hybrid zone may form. Three outcomes are possible:

Reinforcement
Hybrids are less fit, so natural selection strengthens prezygotic barriers. The populations become fully separate species.
Fusion
Hybrids are equally fit. Gene flow resumes and the two populations merge back into a single species.
Stability
Hybrids persist in a narrow zone indefinitely. The hybrid zone is maintained by a balance of selection and gene flow.

How to Identify the Type

1
Check geography: Is there a physical barrier separating the populations? If yes, consider allopatric speciation.
2
Look for polyploidy: Does the organism have extra chromosome sets? This is a hallmark of sympatric speciation, especially in plants.
3
Identify barriers: Determine whether reproductive isolation is prezygotic (before mating) or postzygotic (after fertilization).
4
Assess gene flow: Has gene flow stopped completely? Partial gene flow may indicate an incomplete speciation event or a hybrid zone.
5
Consider time scale: Gradual divergence suggests allopatric; rapid isolation (one generation) suggests polyploidy-driven sympatric.

Example: Darwin's Finches

Mainland Ancestor Colonize islands Island 1 Island 2 Island 3 Large ground finch Crushing beak: seeds Warbler finch Thin beak: insects Cactus finch Probing beak: cactus Adaptive Radiation

Example: Apple Maggot Fly

The apple maggot fly (Rhagoletis pomonella) is a classic case of sympatric speciation in progress in North America.

1
Original host: historically fed and mated exclusively on native hawthorn fruit.
2
New host: when European settlers introduced apples, some flies shifted to apples as their host fruit.
3
Temporal isolation: apples fruit earlier than hawthorns, so the two fly populations now mate at different times.
4
Genetic divergence: the two "races" show measurable allele frequency differences, though they still occupy the same geographic range.

Example: Polyploidy in Plants

Species A
\(2n = 14\)
 Species B
\(2n = 14\)
 Hybridize Hybrid
\(n=7+7\) (sterile)
 Chromosome doubling New Species!
Allopolyploid: \(4n = 28\)
 Fertile: can self-pollinate Reproductively isolated from both parents
Exact chromosome pairing model
Exact chromosome pairing model
Detailed illustrative plant tissue with flowers and nuclei; an authored companion shows an AB hybrid and a doubled AABB cell.
Illustrative plant tissue introduces a chromosome-doubling model; counted chromosome pairing is shown in the authored companion.

Rates of Speciation

Gradualism
Species diverge slowly and steadily through the gradual accumulation of small differences over long periods of time.
Punctuated Equilibrium
Species remain stable for long periods (stasis), then undergo rapid bursts of change associated with speciation events.
Gradualism Time Morphological change Punctuated Equilibrium Time Morphological change

Common Mistakes

✕ Confusing modes
Students often mix up allopatric and sympatric. Remember: allo = other place (geographic barrier); sym = same place (no barrier).
✕ Ignoring gene flow
Speciation requires reduction or elimination of gene flow. Two populations can look different but are not separate species if they still interbreed freely.
✕ Pre vs Post barriers
Prezygotic barriers prevent mating or fertilization. Postzygotic barriers act after a zygote forms. A mule's sterility is postzygotic, not prezygotic.

Question 1 of 5

A river forms and divides a population of lizards into two groups. Over thousands of years, the groups evolve different body sizes and mating calls. What type of speciation is this?

Question 2 of 5

A plant species undergoes an error in meiosis, producing offspring with four sets of chromosomes (\(4n\)) instead of two (\(2n\)). These \(4n\) offspring can reproduce with each other but not with \(2n\) plants. This is an example of:

Question 3 of 5

Two closely related bird species live in the same forest. Species A feeds in the canopy and sings at dawn, while Species B feeds on the ground and sings at dusk. They do not interbreed. Which prezygotic barriers are maintaining their reproductive isolation?

Question 4 of 5

A horse and a donkey can mate to produce a mule, but mules are sterile. What type of reproductive barrier does this represent?

Question 5 of 5

Two partially diverged populations of frogs come back into contact. Hybrids between them have lower survival rates than non-hybrids. Over time, the frogs evolve stronger mating call preferences, reducing interbreeding. This process is called: