Difficulty: Intermediate | Prerequisites: Basic understanding of natural selection, population genetics, and the biological species concept.
Speciation sits at the heart of evolutionary biology: it is the process that generates new species. This case study follows a real-time speciation event observed by Peter and Rosemary Grant on Daphne Major, a tiny island in the Galapagos archipelago. It connects population genetics (gene flow, genetic drift, founder effect) to reproductive isolation mechanisms (prezygotic barriers, behavioural isolation) in a living, field-documented example.
If you are comfortable with natural selection and can distinguish allopatric from sympatric speciation in the abstract, this material shows you what those concepts look like when they actually happen in the wild. If those terms are not yet solid, review them first.
In 1981 a hybrid finch landed on Daphne Major and bred with the native Geospiza fortis population. After a drought bottleneck, two surviving siblings bred with each other, founding a small lineage that diverged in song and beak size from the resident finches. The lineage became behaviourally and morphologically isolated, representing an incipient new species forming within the same geographic area (sympatric speciation) in real time.
Speciation
The process by which one species splits into two or more reproductively isolated lineages.
In simple terms, it is how new species form.
Allopatric speciation
Speciation that occurs when a population is split by a geographic barrier (a mountain range, a body of water, a new island colonisation), preventing gene flow between the two groups.
Think of it as: populations that cannot physically reach each other evolve apart.
Sympatric speciation
Speciation that occurs within the same geographic area, without a physical barrier. Reproductive isolation arises through ecological, behavioural, or genetic differences.
Think of it as: living side by side but no longer interbreeding.
Reproductive isolation
The inability of two populations to interbreed and produce fertile offspring, even if they overlap geographically.
In simple terms, this means something prevents them from mating successfully.
Prezygotic barrier
A reproductive barrier that prevents mating or fertilisation from occurring in the first place (before a zygote forms).
Think of it as: the barrier stops the egg from ever being fertilised.
Postzygotic barrier
A reproductive barrier that acts after fertilisation, producing offspring that are inviable, infertile, or otherwise unfit.
Think of it as: mating happens, but the offspring do not thrive or cannot reproduce.
Behavioural isolation
A prezygotic barrier where differences in courtship behaviour, mating calls, or (in this case) song prevent interbreeding between populations.
In simple terms, they do not recognise each other as potential mates.
Gene flow
The transfer of genetic material from one population to another through migration and interbreeding.
Think of it as: genes travelling between populations when individuals move and mate.
Founder effect
A form of genetic drift that occurs when a small number of individuals establish a new population, carrying only a subset of the original population's genetic variation.
In simple terms, a small founding group does not carry the full genetic diversity of its source population.
Genetic drift
Random changes in allele frequency within a population, more pronounced in small populations.
Think of it as: chance events shaping which genes survive, especially when the group is small.
Endogamy
Mating exclusively within a defined group.
In simple terms, breeding only with members of your own lineage or population.
Biological species concept
The definition of a species as a group of organisms that can interbreed freely in nature and produce viable, fertile offspring.
Think of it as: if they can mate and have healthy babies that also reproduce, they are the same species.
Incipient species
A population in the early stages of divergence that is on its way to becoming a separate species but may not yet meet all formal criteria.
In simple terms, a species "in progress", not quite there yet.
In 1981, a population of Geospiza fortis (medium ground finch) lived on the Galapagos island of Daphne Major.
An immigrant finch arrived from another island. It was a hybrid: 66% G. fortis and 34% G. scandens (cactus finch).
The immigrant was unusually large, had a disproportionately large beak, and sang an atypical song.
Over four generations, the immigrant's descendants interbred with native G. fortis.
A severe drought then killed all but two of these descendants: one brother and one sister.
The bottleneck and founding event
The surviving siblings bred with each other (endogamy), founding a new lineage.
Their offspring were morphologically and behaviourally distinct from the native G. fortis population.
This is a textbook example of the founder effect: a tiny subset of the gene pool (two individuals) established a new population with reduced genetic diversity.
The evolutionary force at work when an individual from one population arrives and breeds with another is gene flow.
The force that randomly selects a small number of individuals from a larger gene pool to start a new population is genetic drift, specifically the founder effect.
Types of speciation illustrated
The original immigrant being geographically displaced from its home population represents allopatric speciation (geographic separation).
The subsequent reproductive isolation occurring within the same island, without a physical barrier, represents sympatric speciation (behavioural and ecological divergence within one area).
In Geospiza finches, two main traits prevent interbreeding: song and body shape.
Male finches learn their song during a brief sensitive period early in life. G. fortis males sing one species-typical song. Females do not sing but select mates based on song.
The immigrant lineage learned songs incorrectly across generations. Sonograph data show the immigrant's song (and those of his descendants) diverged progressively from the standard G. fortis song pattern.
Females within the immigrant lineage recognised and accepted the "wrong" song. Native G. fortis females did not, and would not mate with males singing it.
Why this matters for isolation
Song recognition is a prezygotic barrier: it prevents mating from occurring in the first place, before any zygote is formed.
The specific mechanism is behavioural isolation: differences in a learned courtship signal (song) prevent interbreeding.
A male with an incorrect song cannot attract native G. fortis females, reducing his fitness (reproductive success). He can only breed within his own lineage, reinforcing the isolation.
This is a case where a culturally transmitted trait (learned song), not a purely genetic one, drives reproductive isolation.
Beak size and shape are key adaptive traits for Galapagos finches. They determine which seeds a bird can exploit and serve as visual cues for mate recognition.
The immigrant lineage had noticeably larger beaks than native G. fortis. Researchers dubbed them the "Big Bird" lineage.
Beak width distribution data show the native G. fortis population (n = 280) had a broad, roughly normal distribution of beak widths (7.5 to 11.5 mm). The immigrant lineage (n = 20) clustered tightly at the larger end (around 10 to 10.5 mm).
Small gene pool and low beak-size variation
With only 20 individuals descended from a severely bottlenecked founding pair, the immigrant lineage has far less genetic diversity than the native population.
Low genetic diversity means low phenotypic variation, including in beak size. This tight clustering is a direct consequence of the founder effect and genetic drift in a tiny population.
Beak shape differences reinforce isolation: native finches can visually distinguish the immigrant lineage from their own, reducing the chance of cross-lineage mating.
Ecological vulnerability
Low diversity in beak sizes makes the immigrant population vulnerable to environmental change. If seed availability shifts (for example, if only small seeds remain after a drought), a population with uniformly large beaks cannot exploit them efficiently.
The native population, with a wider range of beak sizes, has more individuals capable of surviving such a shift.
Nest-site maps from 1993 and 2007 show the immigrant lineage's nesting locations shifted away from the original mixed area to a distinct part of the island.
The brother and sister mated at nest site 16833, surrounded by native G. fortis nesting sites. Their descendants subsequently moved to separate nesting areas.
The lineage likely moved because behavioural isolation (song rejection by native females) reduced mating opportunities in the original territory. Relocating to an area where they could breed among themselves increased reproductive success.
Why this is sympatric speciation
There is no physical barrier separating the immigrant lineage from the native G. fortis. They live on the same small island.
Isolation is driven entirely by behavioural (song) and morphological (beak size, body shape) differences, not geography.
The Grant researchers describe the lineage as an "incipient species," noting it has been reproductively isolated from sympatric G. fortis for at least three generations of exclusively within-group mating.
The biological species concept and the Big Bird lineage
Under the biological species concept, organisms of the same species must breed freely in nature and produce viable, fertile offspring.
The Big Bird lineage is genetically compatible with G. fortis (they could theoretically produce viable offspring), but behaviourally they do not interbreed.
This raises a genuinely open question in taxonomy: how many generations of exclusive within-group mating are needed before a population is formally recognised as a separate species? The Grants note there is no non-arbitrary answer.
For exam purposes, understand that the Big Bird lineage satisfies the behavioural component of reproductive isolation even though the genetic barrier is absent. The biological species concept hinges on what happens "in nature," not what is possible in a laboratory.
Students often confuse allopatric and sympatric speciation in this case. The original immigrant arriving from another island involved allopatric separation. The subsequent isolation on Daphne Major, where both populations share the same island, is sympatric. Both types feature in this one case study.
Students assume speciation requires a genetic incompatibility. It does not. Behavioural isolation alone (here, song differences) can prevent interbreeding even when the organisms are genetically capable of producing viable offspring.
Students sometimes classify song recognition as a postzygotic barrier. Song recognition acts before mating occurs, making it prezygotic. Postzygotic barriers involve problems after fertilisation (hybrid inviability, infertility).
Students confuse gene flow with genetic drift. The immigrant arriving and breeding with the native population is gene flow (movement of alleles between populations). The random reduction of genetic diversity when the population crashed to two individuals is genetic drift (specifically the founder effect/bottleneck).
⚠️ Be able to identify the evolutionary forces at play: gene flow (immigrant breeding with natives), genetic drift/founder effect (population crash to two individuals), and natural selection (beak size and ecological adaptation).
⚠️ Know the distinction between prezygotic and postzygotic barriers, and be ready to classify specific examples. Song recognition = prezygotic, behavioural isolation.
⚠️ This case study is commonly tested because it illustrates both allopatric and sympatric speciation within a single, well-documented example.
⚠️ Understand that the biological species concept is defined by what happens in nature ("breed freely in nature"), not by what is theoretically possible in controlled conditions.
⚠️ The concept of an "incipient species" may appear in essay questions asking you to evaluate whether the Big Bird lineage qualifies as a new species.
True or False: The immigrant finch on Daphne Major was a pure G. fortis individual. (False, it was a hybrid: 66% G. fortis, 34% G. scandens.)
Fill in the blank: Song recognition in finches is a ______ barrier because it prevents mating before fertilisation occurs. (prezygotic)
True or False: Sympatric speciation requires a physical geographic barrier between populations. (False, it occurs without geographic separation.)
Fill in the blank: The random loss of genetic diversity when a population is founded by a very small number of individuals is called the ______ . (founder effect)
True or False: Under the biological species concept, the Big Bird lineage is considered a separate species because it is genetically incompatible with G. fortis. (False, the isolation is behavioural, not genetic.)
Q: What kind of speciation do we associate with members of one population being geographically displaced to a new, smaller population?
A: Allopatric speciation. Geographic separation prevents gene flow, allowing the two populations to diverge independently.
Q: What kind of speciation do we associate with members of a population becoming ecologically, genetically, or behaviourally distinct within that population such that they become reproductively isolated?
A: Sympatric speciation. Reproductive isolation develops without a geographic barrier, driven by ecological, behavioural, or genetic divergence.
Q: Which force of evolution is seen when a hybrid bird lands on an island that is not its home and breeds with the indigenous population?
A: Gene flow. The immigrant introduces alleles from its source population into the resident population's gene pool through interbreeding.
Q: Which force of evolution randomly selects an individual from a larger gene pool to form a new, smaller population with less genetic variety?
A: Genetic drift, specifically the founder effect. A small number of colonising individuals carry only a fraction of the original population's genetic variation.
Q: Is song recognition a prezygotic or postzygotic barrier? Explain.
A: Prezygotic. Song recognition operates before mating takes place. If a female does not recognise a male's song, she will not mate with him, so no zygote is ever formed.
Q: What type of isolating mechanism does song recognition represent?
A: Behavioural isolation (a subcategory of prezygotic barriers). Differences in learned courtship signals prevent individuals from different populations from mating.
Q: How could low diversity of beak sizes place the immigrant population at risk if ecological conditions change?
A: With little variation in beak size, the population lacks individuals adapted to alternative food sources. If, for example, a drought eliminated large seeds and only small seeds remained, the uniformly large-beaked immigrant population could not exploit them efficiently, risking starvation and population collapse.
Q: Under the biological species concept, does the Big Bird lineage qualify as a separate species even though the birds could genetically produce offspring with G. fortis?
A: The biological species concept defines species by whether organisms breed freely in nature, not by genetic compatibility alone. The Big Bird lineage does not interbreed with G. fortis in nature because of behavioural isolation (song and mate recognition). By that criterion, it is behaving as a separate species, even though hybridisation remains theoretically possible. The Grants treat it as an incipient species.
This material connects directly to population genetics: the forces of evolution (natural selection, genetic drift, gene flow, mutation) that you study in the abstract are all operating here in a single, observable system.
It also connects to adaptive radiation, the broader pattern seen across Darwin's finches. The Galapagos finches are the classic example of one ancestral species diversifying into many species to fill different ecological niches. The Daphne Major case is one small chapter in that larger story.
Finally, the role of learned behaviour (song) in driving reproductive isolation connects to animal behaviour and ethology. Cultural transmission of traits is not limited to humans; in finches, a socially learned song can be as powerful a barrier to gene flow as a mountain range.
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