Allopatric and Sympatric Speciation
Environment & Ecology › Population Ecology and Species Interactions · Topic 3.9
Speciation is the evolutionary process by which one species splits into two or more new species. It depends on reproductive isolation, a barrier to gene flow between populations that allows them to diverge by mutation, natural selection and genetic drift. In allopatric speciation, populations are separated by a geographical barrier such as a mountain, river, sea or desert; this is the most common mode and was proposed by Ernst Mayr. A special case is peripatric speciation, in which a small founder population is isolated at the edge of a range. In sympatric speciation, new species arise within the same geographic area without physical separation, through polyploidy in plants, shifts to a new host or food, or sexual selection. Parapatric speciation occurs in adjoining areas along a gradient. Darwin's finches of the Galapagos are a classic example of adaptive radiation.
5 min read · UPSC Prelims · Environment & Ecology · Population Ecology and Species Interactions
At a Glance
- Allopatric Geographic barrier
- Sympatric Same area
- Peripatric Small edge population
- Plant route Polyploidy
- Example Darwin’s finches
- Exam link UPSC Prelims, Environment & Ecology: Population Ecology and Species Interactions
Where Does It Fit?
This is the last topic of the macro on population ecology and species interactions. It explains how new species form, which connects ecology with evolution and with the origin of biodiversity. Previous topic: Endemic Species, Keystone Species, Indicator Species, Invasive Species. Next topic: Terrestrial Ecosystems: Forest Ecosystem in India, Grasslands, Tundra, Deserts, Mountains.
What Is Speciation and How Does Allopatric Speciation Work?
| Mode | Process and example |
|---|---|
| Speciation | Formation of new species from an existing one. Needs reproductive isolation so that gene pools separate |
| Allopatric speciation | Geographic isolation by a mountain range, river, land bridge, island formation or climate change splits a population. The separated groups face different conditions, diverge, and become unable to interbreed even if rejoined. Example: Darwin’s finches on different Galapagos islands; snapping shrimps on either side of the Isthmus of Panama; Himalayan birds |
| Vicariance | A barrier splits a population in two, like a river changing course |
| Dispersal | A few individuals cross a barrier and colonise a new area, such as birds reaching an island |
| Peripatric speciation | A small founder group at the edge of a range changes rapidly through genetic drift and strong selection |
| Founder effect | Loss of genetic variety in a small colonising group |
| Reproductive isolation | Prezygotic barriers stop mating or fertilisation (habitat, timing, behaviour, mechanical); postzygotic barriers cause hybrid sterility or low survival, like mule |
What Are Sympatric and Parapatric Speciation?
- Sympatric speciation: species form without geographic separation, inside the same area; less common and harder to prove
- Polyploidy: in plants, a mistake in cell division doubles the chromosome sets in one generation, giving an instant new species; important in wheat, cotton and many flowering plants
- Host shift: insects specialising on a new host plant, as in the apple maggot fly Rhagoletis
- Disruptive selection: extremes are favoured over intermediates, splitting a population
- Sexual selection: mate choice differences, as in cichlid fishes in African and Central American lakes
- Parapatric speciation: adjoining populations along an environmental gradient diverge while still exchanging some genes, as in grasses on mine-contaminated soil
- Adaptive radiation: rapid diversification of one ancestor into many forms filling many niches, as in Darwin’s finches, Hawaiian honeycreepers and Australian marsupials
- Hybrid zones: where closely related species meet and may interbreed
What Else Should You Remember?
- Mayr: Ernst Mayr gave the biological species concept and supported allopatric speciation
- Darwin’s finches: about 14 species with differently shaped beaks on the Galapagos Islands
- Genetic drift: random change in gene frequencies in small populations
- Natural selection: differential survival and reproduction
- Mutation: source of new variation
- Gene flow: exchange of genes between populations that prevents divergence
- Isthmus of Panama: rose about 3 million years ago and split marine populations
- Western Ghats: isolation and rainfall gradients have produced many endemic frogs and plants
- Himalayan speciation: valleys isolate birds and plants
- Island speciation: Andaman and Nicobar islands have many endemic forms
- Lake cichlids: hundreds of species in Lake Victoria, Malawi and Tanganyika
- Wheat evolution: bread wheat arose through hybridisation and polyploidy
- Time: speciation can take thousands to millions of years, but polyploidy takes one generation
- Extinction versus speciation: biodiversity is the balance of the two
- Conservation use: isolated populations may be separate evolutionary units
- Link: types of species are in Types of Species
- Link to endemism: speciation in isolation produces endemics, as in Endemic Species, Keystone Species, Indicator Species, Invasive Species
- Caution: sympatric speciation examples are debated, as geographic isolation may have played a part
- Ring species: a chain of populations around a barrier where neighbours interbreed but the ends do not, as in greenish warblers around the Tibetan plateau
- Reinforcement: selection against hybrids strengthens mating barriers
- Hybrid sterility: mules from a horse and donkey are sterile, an example of a postzygotic barrier
Why Does It Matter for the Exam?
- Allopatric: geographic barrier
- Sympatric: same area, polyploidy
- Examples: Darwin’s finches, cichlids
- Current affairs link: new species discoveries in the Ghats
- Common errors: calling allopatric speciation sympatric or saying polyploidy is common in animals
As an analytical point, speciation produces biodiversity, but it is slow compared with the present pace of extinction. Protecting isolated habitats such as islands and mountain ranges conserves the engines of future speciation.
Why Is This Topic Important?
- Modes Four
- Isolation Barrier needed
- Examples Finches, cichlids
What Came Next?
The next macro covers terrestrial and aquatic ecosystems, beginning with terrestrial ecosystems. Next topic: Terrestrial Ecosystems: Forest Ecosystem in India, Grasslands, Tundra, Deserts, Mountains.
Key People and Terms
- Speciation Formation of new species
- Allopatric In different places
- Sympatric In the same place
- Polyploidy Extra chromosome sets
- Reproductive isolation No interbreeding
- Adaptive radiation Rapid diversification
Exam Corner
Points to Remember
- Allopatric needs a geographic barrier.
- Sympatric in plants: polyploidy.
- Darwin’s finches: adaptive radiation.
- Biological species concept: Mayr.
- Mule is sterile (postzygotic barrier).
Do Not Confuse With
- Allopatric and sympatric Separate places versus same place
- Prezygotic and postzygotic barrier Before versus after fertilisation
- Speciation and adaptive radiation One split versus many forms from one ancestor
Memory Hook
Allo is apart by a barrier, sym is same room by chromosome doubling; finches radiate.
Mains Angle
Questions ask for modes and examples. These points help.
- Modes Types
- Barriers Isolation
- Value Biodiversity origin
A Question You May Face
An original practice question, not a past paper question.
“Differentiate between allopatric and sympatric speciation with examples and explain the role of reproductive isolation.”
How to Answer
- Define Speciation.
- Allopatric Barrier.
- Sympatric Polyploidy.
- Isolation Barriers.
GS Relevance
Frequently Asked Questions
What is speciation?
Speciation is the evolutionary process by which one species splits into two or more distinct species. It happens when populations become reproductively isolated, so that genes no longer flow between them and they diverge through mutation, natural selection and genetic drift until they cannot interbreed.
What is allopatric speciation?
Allopatric speciation is the formation of new species when a population is divided by a geographical barrier, such as a mountain, river, sea or desert. The separated populations evolve independently in different environments and eventually become reproductively isolated, as in Darwin's finches on different Galapagos islands.
What is sympatric speciation?
Sympatric speciation is the formation of new species within the same geographic area without any physical barrier. It can occur by polyploidy in plants, by specialisation on a new host or food, or by sexual selection, as in cichlid fishes of African lakes and the apple maggot fly. It is less common than allopatric speciation.
How does polyploidy cause speciation?
Polyploidy is a condition with more than two sets of chromosomes, caused by an error in cell division. A polyploid plant often cannot breed with its diploid parents, so it forms a new species in a single generation. It is common in flowering plants such as wheat, cotton and potato.
What is adaptive radiation?
Adaptive radiation is the rapid diversification of a single ancestral species into many species that occupy different ecological niches, often after reaching a new area with few competitors. Darwin's finches on the Galapagos, Hawaiian honeycreepers and cichlid fishes of the African Great Lakes are classic examples.
PYQ Practice — Statement Analysis
1 Allopatric speciation involves geographic isolation.
Such as a mountain or sea.
2 Polyploidy is a common cause of sympatric speciation in plants.
It can give a new species in one generation.
3 Sympatric speciation requires a physical barrier.
It occurs without one.
4 Darwin's finches are an example of adaptive radiation.
On the Galapagos.
Past Mains Questions on This Topic
This topic comes under Population Ecology and Species Interactions. UPSC Mains questions asked on the matching syllabus topic, year-wise with marks:
Related Topics
Browse the full Environment & Ecology index