The trap: NEET questions on speciation routinely offer "geographic isolation" as a distractor for sympatric speciation scenarios — or vice versa. The distinction turns on one detail: was there a physical barrier, or wasn't there?
Natural selection is the differential survival and reproduction of organisms with heritable traits better suited to their environment. Darwin proposed this after observing variation in Galápagos finches — different beak shapes correlated with different food sources (NCERT Class 12 Biology Chapter 6, page 121). Selection acts on phenotypic variation; favourable traits increase in frequency over generations. It is directional (shifts mean toward one extreme), stabilising (narrows variance around the mean), or disruptive (favours both extremes over intermediates).
Genetic drift is the random change in allele frequencies in small populations, independent of selective advantage. Two special cases matter for NEET: the founder effect (a small group colonises a new area, carrying only a subset of alleles) and the bottleneck effect (population crashes, surviving alleles are a random sample). Drift is strongest when population size is small — large populations buffer against random fluctuation.
Speciation is the formation of new species from existing ones. Allopatric speciation requires geographic isolation — a mountain range, river, or ocean physically separates populations, and independent mutation + selection + drift cause divergence until reproductive isolation develops. Sympatric speciation occurs without geographic barriers — polyploidy in plants is the textbook example, where a chromosome-doubling event instantly creates reproductive isolation within the same habitat.
Watch-out: When a NEET stem describes a population splitting due to a river changing course, that is allopatric (geographic barrier present). When it describes polyploidy or host-shift in the same habitat, that is sympatric (no barrier). Read the stem for the barrier, not the outcome.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Which of the following is NOT a type of natural selection?
Answer: D. D is correct. Reductional selection is not a recognised type of natural selection. The three types are directional, stabilising, and disruptive (NCERT Class 12 Biology Chapter 6, page 120).
Why A is wrong: A is a valid type — directional selection shifts the population mean toward one phenotypic extreme.
Why B is wrong: B is a valid type — stabilising selection reduces phenotypic variance by favouring intermediates.
Why C is wrong: C is a valid type — disruptive selection favours both extremes of a trait distribution.
Founder effect is a special case of:
Answer: C. C is correct. The founder effect occurs when a small group colonises a new area, carrying a non-representative sample of alleles — this is a form of genetic drift, not selection (NCERT Class 12 Biology Chapter 6, page 121).
Why A is wrong: A is wrong — natural selection acts on fitness differences, not random sampling of alleles by a migrating subgroup.
Why B is wrong: B is wrong — gene flow involves movement of alleles between existing populations, not the founding of a new population from a small sample.
Why D is wrong: D is wrong — mutation creates new alleles; the founder effect redistributes existing alleles by random sampling.
Sympatric speciation differs from allopatric speciation in that sympatric speciation:
Answer: B. B is correct. Sympatric speciation occurs within the same geographic area without a physical barrier — for example, polyploidy in plants creates instant reproductive isolation without spatial separation (NCERT Class 12 Biology Chapter 6, page 121).
Why A is wrong: A is wrong — geographic isolation defines allopatric speciation, not sympatric. This is the core confusion the question tests (trap: sympatric vs allopatric).
Why C is wrong: C is wrong — sympatric speciation is well-documented in plants (polyploidy) and also reported in some animal populations (host-shift in insects).
Why D is wrong: D is wrong — sympatric speciation can be driven by polyploidy or host-shift, not exclusively by natural selection.
A river changes its course and permanently divides a population of frogs into two isolated groups. Over thousands of years, the two groups can no longer interbreed. This is an example of:
Answer: C. C is correct. A physical barrier (river) geographically isolates the population — this is the defining feature of allopatric speciation (NCERT Class 12 Biology Chapter 6, page 121).
Why A is wrong: A is wrong — sympatric speciation occurs without geographic barriers; the river explicitly creates a physical barrier (trap: sympatric vs allopatric confusion).
Why B is wrong: B is wrong — parapatric speciation involves partially overlapping ranges with a contact zone, not complete physical separation by a river.
Why D is wrong: D is wrong — peripatric speciation involves a small peripheral population budding off, not a major geographic event splitting a population roughly in half.
An autotetraploid plant arises in a diploid population. It can self-fertilise and produce fertile offspring, but cannot cross with the original diploid plants. This is an example of:
Answer: D. D is correct. Polyploidy creates instant reproductive isolation within the same habitat — no geographic barrier is involved. This is the textbook example of sympatric speciation (NCERT Class 12 Biology Chapter 6, page 119).
Why A is wrong: A is wrong — no geographic isolation is described; the polyploid arose within the same population (trap: sympatric vs allopatric confusion).
Why B is wrong: B is wrong — genetic drift is random change in allele frequency in small populations; polyploidy is a chromosomal event creating reproductive isolation, not a stochastic frequency shift.
Why C is wrong: C is wrong — stabilising selection reduces phenotypic variance around the mean; it does not create new species through chromosome doubling.
After a volcanic eruption destroys most of a beetle population, the surviving beetles show a markedly different allele frequency from the original population. This change is due to:
Answer: A. A is correct. A drastic population crash (volcanic eruption) randomly reduces allele diversity — this is the bottleneck effect, a form of genetic drift (NCERT Class 12 Biology Chapter 6, page 121).
Why B is wrong: B is wrong — the allele frequency change is due to random survival during the catastrophe, not differential fitness. Natural selection requires heritable fitness differences driving the change.
Why C is wrong: C is wrong — gene flow involves movement of alleles between populations; no migration between populations is described here.
Why D is wrong: D is wrong — adaptive radiation is diversification of a lineage into multiple ecological niches, not a population crash reducing allele diversity.
In a population of birds, individuals with medium-sized beaks survive and reproduce better than those with very large or very small beaks. This is an example of:
Answer: A. A is correct. Stabilising selection favours intermediate phenotypes and reduces variance — medium beaks are favoured over extremes (NCERT Class 12 Biology Chapter 6, page 118).
Why B is wrong: B is wrong — disruptive selection favours both extremes over intermediates, which is the opposite of what is described here.
Why C is wrong: C is wrong — directional selection shifts the mean toward one extreme (e.g. only large beaks favoured), not toward the middle of the distribution.
Why D is wrong: D is wrong — artificial selection is human-directed breeding for desired traits, not a natural process acting on wild bird populations.
Genetic drift has maximum effect on allele frequencies in:
Answer: B. B is correct. Genetic drift is inversely related to population size — small populations experience large random fluctuations in allele frequency, while large populations buffer against drift (NCERT Class 12 Biology Chapter 6, page 121).
Why A is wrong: A is wrong — large populations have a buffering effect against random fluctuations; drift is negligible when N is large.
Why C is wrong: C is wrong — strong directional selection is a deterministic force overriding random drift; drift dominates when selection is weak or absent.
Why D is wrong: D is wrong — high gene flow introduces alleles from other populations, counteracting drift rather than amplifying it.
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Given
A population of grasshoppers inhabits a valley. A landslide creates a permanent ridge dividing the valley into two halves. After many generations, the two subpopulations can no longer interbreed when brought together.
Required
Identify the type of speciation and the key factor responsible.
Concept
Speciation is classified by whether geographic isolation is present. Allopatric speciation requires a physical barrier; sympatric speciation occurs without one.
Formula / Rule
Allopatric = geographic barrier → reproductive isolation over time.
Sympatric = no geographic barrier → reproductive isolation via mechanisms like polyploidy.
Substitution / Application
The landslide ridge is a geographic barrier. The two subpopulations are physically separated. Over generations, independent mutation, drift, and selection cause divergence until reproductive isolation develops.
Calculation / Reasoning
Physical barrier present → geographic isolation → independent evolution → reproductive isolation. This matches the definition of allopatric speciation exactly.
Final answer
This is allopatric speciation, driven by geographic isolation caused by the landslide.
Common trap
Aspirants may select "sympatric speciation" because the grasshoppers are still in the same valley. The key distinction is not whether the organisms are nearby, but whether a physical barrier prevents gene flow. The ridge does exactly that — it is allopatric.
Similar NEET-style question
"Two populations of snails on either side of a mountain range have diverged to the point of reproductive isolation. What type of speciation does this illustrate?" → Allopatric (mountain range = geographic barrier).
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Allopatric (geographic isolation) vs sympatric (no geographic barrier). Founder effect (small founding population), bottleneck (reduced population). Reproductive isolation: pre/postzygotic barriers.
-- NCERT Class 12 Biology, Ch. 6, p. 121These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
Allopatric: geographic isolation. Sympatric: same area, no physical barrier (e.g. polyploidy in plants, host-shift).
Question gives speciation scenario and asks which type.
If geographic barrier mentioned → allopatric. If population overlaps → sympatric.
More in Genetics and Evolution: 20 exam traps and mistakes · 4 formulas · 1 question pattern from its other lessons.
4 questions from NEET 2020, 2021, 2022, 2026. Answers verified against NTA official keys.
The factor that leads to Founder effect in a population is :
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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