7.10 Speciation

Speciation is how one species splits into two or more. It happens when populations become reproductively isolated, so they can no longer interbreed and produce viable, fertile offspring. For AP Biology, focus on the biological species concept, allopatric versus sympatric speciation, prezygotic versus postzygotic barriers, and the difference between gradualism and punctuated equilibrium.

Speciation Summary

Speciation is the formation of new species when populations become reproductively isolated from each other. Once gene flow stops, mutation, genetic drift, natural selection, and sexual selection can push populations in different directions until they can no longer produce viable, fertile offspring together.

For AP Biology, focus on the mechanism that blocks gene flow. Allopatric speciation uses geographic isolation, sympatric speciation happens with geographic overlap, prezygotic barriers prevent fertilization, and postzygotic barriers prevent viable, fertile offspring.

more resources to help you study

practice multiple choiceFRQ practice & scoringcheatsheetsscore calculatorkey terms

Why This Matters for the AP Biology Exam

Speciation pulls together a lot of Unit 7 in one place, so it shows up in both multiple-choice questions and free-response explanations. You may be asked to describe the conditions that produce new species, explain how a specific barrier blocks gene flow, or interpret a scenario and decide whether speciation is allopatric or sympatric. A common scoring mistake is mixing up the types of reproductive isolation, so being precise about prezygotic versus postzygotic mechanisms gives you an edge. You may also connect speciation to phylogenetic trees, where branching points (nodes) represent speciation events.

Key Takeaways

• Speciation happens when two populations become reproductively isolated and gene flow between them stops.
• The biological species concept defines a species as a group that can interbreed and produce viable, fertile offspring.
• Allopatric speciation involves a geographic barrier; sympatric speciation happens with geographic overlap.
• Reproductive isolation is either prezygotic (blocks fertilization) or postzygotic (blocks viable, fertile offspring).
• Evolution can follow gradualism (slow, steady change) or punctuated equilibrium (rapid change after long stasis).
• Divergent evolution and adaptive radiation increase speciation rates; convergent evolution makes unrelated species look similar.

How New Species Arise

Speciation is the process by which new species evolve from existing ones. It happens when two populations become reproductively isolated, meaning they can no longer interbreed and exchange genetic information. Once gene flow between the populations stops, they can evolve in different directions until they become separate species.

The biological species concept is the most common way to define a species for sexually reproducing organisms. It defines a species as a group capable of interbreeding and exchanging genetic information to produce viable, fertile offspring. The key words are viable and fertile: the offspring must survive and be able to reproduce.

A horse and a donkey can breed and produce a mule, but a mule is sterile and cannot produce its own offspring. Because the cross does not yield fertile offspring, horses and donkeys are classified as separate species.

Processes That Drive Speciation

Several factors can push two populations apart until they become different species:

1. Gene flow interruption: When gene flow stops, separated populations evolve independently.
2. Genetic drift: Random changes in allele frequencies cause divergence, especially in small populations.
3. Different selective pressures: Varying environments favor different adaptations.
4. Mutation accumulation: Unique mutations build up independently in each population.
5. Sexual selection: Different mating preferences create behavioral barriers to reproduction.

Rates of Evolution

The pace of evolution and speciation can vary with ecological conditions. Two well-known models describe this pace.

Punctuated equilibrium describes evolution that happens rapidly after long periods of stasis (little or no change). In this model, species stay mostly the same for long stretches, then change quickly in short bursts.

Gradualism describes evolution as a slow, steady process that occurs over hundreds of thousands or millions of years. New species form through the gradual accumulation of small changes.

Divergent Evolution and Adaptive Radiation

Divergent evolution occurs when populations adapt to new habitats, leading to phenotypic diversification. When a population faces new selective pressures, it can evolve traits that help it survive and reproduce in that environment.

Speciation rates can be especially rapid during adaptive radiation, when a lineage diversifies into many new forms as new habitats become available. This can follow events like the formation of a new island or a mass extinction that opens up empty niches.

The Hawaiian Drosophila are an illustrative example of divergent evolution. From a small number of ancestral species, many endemic fruit fly species evolved across the Hawaiian islands, with distinct mating behaviors and ecological specializations on different islands.

Types of Speciation

Allopatric Speciation

Allopatric speciation occurs in populations that are geographically isolated. A physical barrier, like a mountain range or river, separates a population so that the groups can no longer interbreed. Over time, the two populations evolve separately until they can no longer reproduce together even if reunited.

Sympatric Speciation

Sympatric speciation occurs in populations that have geographic overlap. Instead of a physical barrier, factors like disruptive selection or mating preferences drive divergence. Disruptive selection favors two extreme traits over the intermediate, splitting the population.

The apple maggot fly (Rhagoletis pomonella) is an illustrative example. Originally laying eggs only on hawthorn trees, some flies shifted to apple trees, which led to different emergence times and host preferences. This reduced gene flow and caused genetic divergence without geographic isolation.

Reproductive Isolation Mechanisms

Reproductive isolation mechanisms are either prezygotic or postzygotic. Prezygotic mechanisms prevent fertilization from happening in the first place. Postzygotic mechanisms allow fertilization but prevent a viable, fertile offspring, like the sterile mule.

Prezygotic Barriers (prevent fertilization)

1. Habitat isolation: Species live in different habitats and rarely meet.
2. Temporal isolation: Species breed at different times.
3. Behavioral isolation: Different courtship behaviors prevent mating. The Caribbean Anolis lizards are an illustrative example, with distinct dewlap colors and display behaviors that keep species from interbreeding.
4. Mechanical isolation: Reproductive structures are physically incompatible.
5. Gametic isolation: Sperm and egg are chemically incompatible.

Postzygotic Barriers (after fertilization)

1. Hybrid inviability: Embryos fail to develop.
2. Hybrid sterility: Offspring survive but are sterile (for example, mules).
3. Hybrid breakdown: Offspring are fertile, but later generations have reduced fitness.

Convergent Evolution

Convergent evolution occurs when similar selective pressures produce similar phenotypic adaptations in different populations or species. Unrelated organisms independently evolve comparable solutions to the same environmental challenge. A few examples:

• Flight: Birds, bats, and insects independently evolved wings.
• Aquatic streamlining: Dolphins, sharks, and ichthyosaurs share similar body shapes.
• Vision: Vertebrates and cephalopods independently evolved camera-style eyes.
• Desert survival: Cacti in the Americas and euphorbias in Africa evolved similar water-storing forms.

Keep in mind that convergent evolution makes species look similar without being closely related, which is the opposite of divergent evolution.

How to Use This on the AP Biology Exam

MCQ

• Read scenarios carefully to decide if speciation is allopatric (geographic separation) or sympatric (geographic overlap).
• Match isolation examples to the correct type. Different mating times point to temporal isolation; sterile hybrids point to postzygotic.
• Watch for questions that test the biological species concept by describing offspring that are viable but sterile.

Free Response

• When describing speciation, state clearly that populations became reproductively isolated and that gene flow stopped.
• Name the specific barrier and explain how it blocks gene flow rather than just listing the term.
• Be precise: prezygotic barriers prevent fertilization, while postzygotic barriers prevent viable, fertile offspring.
• If you reference a phylogenetic tree, remember that nodes mark speciation events where lineages split.

Common Trap

• Do not write Lamarckian statements. Populations do not change traits because they "need" them; variation already exists and isolation plus selection or drift does the rest.

Common Misconceptions

• Speciation does not require geographic separation. Sympatric speciation happens in overlapping populations through factors like disruptive selection or mating preferences.
• Hybrids being born does not mean two organisms are the same species. If the hybrid is sterile or its offspring have reduced fitness, the parents are still separate species.
• Prezygotic and postzygotic are not interchangeable. Prezygotic barriers act before fertilization; postzygotic barriers act after. Mixing these up is a frequent scoring error.
• Convergent evolution does not mean common ancestry. Similar traits in unrelated species come from similar selective pressures, not a recent shared ancestor.
• Individuals do not evolve. Natural selection acts on individuals, but it is the population that evolves over generations.
• Punctuated equilibrium does not mean evolution skips steps. It still involves the same mechanisms, just compressed into shorter bursts after long stasis.

Related AP Biology Guides

• 7.1 Introduction to Natural Selection
• Unit 7 Overview: Natural Selection
• 7.2 Natural Selection
• 7.4 Population Genetics
• 7.3 Artificial Selection
• 7.6 Evidence of Evolution