How do new species form from existing ones?
Topic 7.10 Speciation: explain how reproductive isolation leads to speciation, including allopatric and sympatric speciation.
A focused answer to AP Biology Topic 7.10, covering the biological species concept, reproductive isolation (prezygotic and postzygotic barriers), allopatric and sympatric speciation, and rates of speciation, with a worked example.
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What this topic is asking
The College Board (Topic 7.10) wants you to explain how reproductive isolation leads to speciation, including allopatric and sympatric speciation, and the barriers that keep species reproductively separate.
Species and reproductive isolation
Allopatric and sympatric speciation
Reproductive barriers
Speciation can also occur at different rates. In gradualism, differences accumulate slowly over long times; in punctuated equilibrium, long periods of little change are interrupted by bursts of rapid speciation, often after a disturbance opens up new niches.
Try this
Q1. State the difference between allopatric and sympatric speciation. [2 points]
- Cue. Allopatric speciation involves a geographic barrier separating populations; sympatric speciation occurs within the same area without a physical barrier.
Q2. Explain why reproductive isolation is necessary for speciation. [2 points]
- Cue. Without isolation, gene flow keeps the populations' gene pools mixed; isolation stops gene flow, letting the populations diverge until they become separate species.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2019 (style)4 marksSection II (long FRQ excerpt). A river changes course and splits a population of lizards into two groups that can no longer interbreed. Over many generations the two groups diverge. (a) Identify the type of speciation and the role of reproductive isolation. (b) Explain how the two populations could become separate species.Show worked answer →
A 4-point identify-and-explain FRQ on speciation.
(a) Identify (2 points): (1 point) allopatric speciation, because a geographic barrier (the river) physically separated the populations; (1 point) the barrier causes reproductive isolation: gene flow between the groups stops.
(b) Explain (2 points): (1 point) with no gene flow, the two populations accumulate different mutations and experience different selection and drift, so they diverge genetically; (1 point) eventually the differences are large enough that even if reunited they could not interbreed successfully, so they are separate species.
Markers reward identifying allopatric speciation, the loss of gene flow, and the divergence by selection, drift and mutation leading to reproductive isolation.
AP 2018 (style)1 marksSection I (multiple choice). Speciation that occurs without a geographic barrier, within the same area, is called: (A) allopatric speciation. (B) sympatric speciation. (C) convergent evolution. (D) gene flow.Show worked answer →
The correct answer is (B).
Sympatric speciation occurs within the same geographic area, without physical separation, often through mechanisms such as polyploidy, habitat differentiation or mate choice. Allopatric speciation (A) requires a geographic barrier; (C) and (D) are not speciation modes.
Related dot points
- Topic 7.2 Natural Selection: explain how directional, stabilizing and disruptive selection change the distribution of phenotypes in a population.
A focused answer to AP Biology Topic 7.2, covering directional, stabilizing and disruptive selection, sexual selection, and how each changes a phenotype distribution, with a worked interpretation of selection on a trait.
- Topic 7.4 Population Genetics: explain how natural selection, mutation, gene flow, genetic drift and non-random mating change allele frequencies.
A focused answer to AP Biology Topic 7.4, covering the gene pool, allele frequencies, and the five mechanisms of microevolution (selection, mutation, gene flow, genetic drift, non-random mating), including bottleneck and founder effects, with a worked allele-frequency calculation.
- Topic 7.9 Phylogeny: interpret and construct phylogenetic trees and cladograms from shared characters and molecular data.
A focused answer to AP Biology Topic 7.9, covering phylogenetic trees and cladograms, shared derived characters, nodes and common ancestors, out-groups, and reading relatedness from a tree, with a worked tree interpretation.
- Topic 7.11 Extinction: explain the causes of extinction, including mass extinctions, and its role in shaping biodiversity.
A focused answer to AP Biology Topic 7.11, covering the causes of extinction, background versus mass extinction, the five mass extinctions, adaptive radiation after extinction, and the current human-driven loss, with a worked example.
- Topic 7.12 Variations in Populations: explain why genetic variation within a population is important for survival and the response to environmental change.
A focused answer to AP Biology Topic 7.12, covering the sources and importance of genetic diversity, how variation buffers populations against change, the risks of low diversity, and the role of variation in evolution, with a worked example.
Sources & how we know this
- AP Biology Course and Exam Description — College Board (2020)