How do reproductive and geographic isolation lead to the formation of new species?
Explain how speciation occurs and the effects of reproductive isolation and geographic isolation on the formation of new species (Virginia 2018 Biology SOL BIO.7.c, BIO.7.d, and BIO.7.e).
A SOL-level answer on speciation for the Virginia Biology EOC: what a species is, how geographic isolation and then reproductive isolation split one population into two, the difference between prezygotic and postzygotic barriers, and how allele frequencies diverge until interbreeding is no longer possible.
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What this topic is asking
Virginia Biology SOL standard BIO.7 covers how populations change through time, and substandards BIO.7.c, BIO.7.d, and BIO.7.e ask how speciation occurs and the effects of reproductive isolation and geographic isolation on speciation. The EOC expects you to define a species, explain how one population can split into two, and tell the difference between the barrier that separates groups (often geographic) and the outcome that makes them separate species (reproductive isolation). Items often describe a scenario (a new river, a mountain range, an island) and ask you to identify or explain the process.
What a species is
Defining a species by interbreeding is what makes reproductive isolation the bar for speciation: when two populations can no longer interbreed to make fertile offspring, they have become two species.
Geographic isolation starts the split
Once separated, the two groups are on independent evolutionary paths. New mutations arise in each group, and natural selection acts on each one according to its own environment, which may differ in climate, food, or predators. Random changes in allele frequency (genetic drift) also accumulate. Over many generations the two gene pools diverge.
Reproductive isolation completes it
Divergence eventually produces reproductive isolation, the inability of two groups to interbreed and produce fertile offspring. This is the defining condition for separate species. Reproductive isolation can arise even without a geographic barrier, but in the most common EOC scenario it is the result of long geographic separation.
Two kinds of barrier are worth knowing:
- Prezygotic barriers act before fertilization, preventing mating or fertilization. Examples include different breeding seasons or times of day, different mating behaviors or signals, and incompatible reproductive structures.
- Postzygotic barriers act after fertilization, so that any offspring are inviable or sterile (as with the sterile mule).
When either kind of barrier keeps two populations from successfully interbreeding, they are reproductively isolated, and speciation is complete.
Putting the steps together
The usual sequence the EOC tests is: one population to geographic isolation to divergence (mutation plus natural selection in each environment) to reproductive isolation to two species. The barrier that begins the process is usually geographic; the change that finishes it is reproductive. Keeping those two roles straight is the most common point tested.
Try this
Q1. State the difference between geographic isolation and reproductive isolation. [2]
- Cue. Geographic isolation is separation by a physical barrier that stops interbreeding; reproductive isolation is the inability to interbreed and produce fertile offspring even if reunited. Reproductive isolation defines separate species.
Q2. A horse and a donkey can mate, but their offspring (a mule) is sterile. Are the horse and donkey the same species? Explain. [2]
- Cue. No. A species must interbreed to produce fertile offspring; because the mule is sterile, the horse and donkey are reproductively isolated and so are different species.
Exam-style practice questions
Practice questions written in the style of VDOE exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
VA Biology SOL (2023 released style)1 marksA river changes course and splits one population of lizards into two groups that can no longer reach each other. Over many generations each group adapts to its own side. This separation by a physical barrier is (A) reproductive isolation. (B) geographic isolation. (C) natural selection. (D) genetic drift.Show worked answer →
A 1-point multiple-choice item on the cause of isolation.
The correct answer is B. A physical barrier (the river) that keeps two groups apart is geographic isolation. Over time geographic isolation can lead to reproductive isolation (A) if the groups become so different that they could no longer interbreed even if reunited, but the separation itself is geographic. C and D are mechanisms of change within a population, not the barrier described.
VA Biology SOL (2024 released style)2 marksTwo populations of the same species are separated by a mountain range for a long time. (a) Explain how this geographic isolation can lead to the formation of two species. (b) State what must be true for the two populations to be considered separate species.Show worked answer →
A 2-point item on the steps of speciation.
(a) 1 point: separated by the mountains, the two populations no longer interbreed, so different mutations arise and natural selection acts on each one differently in its own environment; over many generations their allele frequencies diverge and they become genetically different.
(b) 1 point: they are separate species when they are reproductively isolated, that is, they can no longer interbreed to produce fertile offspring even if brought back together.
Markers reward describing divergence over time and defining species by the ability to interbreed and produce fertile offspring.
Related dot points
- Explain how the role of variation and mutations drives natural selection, producing adaptation and changing the heritable traits of a population over generations (Virginia 2018 Biology SOL BIO.7.b).
A SOL-level answer on natural selection for the Virginia Biology EOC: variation and mutations as the raw material, overproduction and competition, differential survival and reproduction (fitness), and how selection produces adaptation and shifts allele frequencies, with antibiotic resistance as the worked example.
- Describe the evidence supporting the theory of evolution by natural selection, including the fossil record, comparative anatomy, embryology, molecular evidence, and biogeography (Virginia 2018 Biology SOL BIO.7.a).
A SOL-level answer on the evidence for evolution for the Virginia Biology EOC: the fossil record, comparative anatomy (homologous, analogous, and vestigial structures), comparative embryology, molecular and DNA evidence, and biogeography, and why independent lines that agree make the theory strong.
- Explain the basis of the modern classification system, compare the domains and kingdoms, use dichotomous keys, and analyze relationships using phylogenetic trees and cladograms (Virginia 2018 Biology SOL BIO.6.a, BIO.6.b, BIO.6.c, and BIO.6.d).
A SOL-level answer on classification for the Virginia Biology EOC: the basis of the modern system, the three domains and the kingdoms, binomial nomenclature and the taxonomic hierarchy, using a dichotomous key, and reading phylogenetic trees and cladograms as evidence of common ancestry.
- Explain that a mutation is a change in the DNA base sequence with harmful, beneficial, or neutral effects, and that genetic variation (from mutation and sexual reproduction) is important to the survival of a species (Virginia 2018 Biology SOL BIO.5.c).
A SOL-level answer on mutations for the Virginia Biology EOC: what a mutation is, its harmful, beneficial, or neutral effects, the difference between body-cell and gamete mutations, and why genetic variation matters for survival.
- Describe meiosis as the division that produces gametes with half the chromosome number, and explain how crossing over, independent assortment, and fertilization create genetic variation (Virginia 2018 Biology SOL BIO.3.d, supporting BIO.5).
A SOL-level answer on meiosis for the Virginia Biology EOC: producing haploid gametes, the contrast with mitosis, and how crossing over, independent assortment, and fertilization generate genetic variation.
Sources & how we know this
- 2018 Science Standards of Learning (Biology) — Virginia Department of Education (2018)
- SOL Practice Items (All Subjects) — Virginia Department of Education (2024)