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Year 12 Science

Speciation

Understand how new species arise through allopatric and sympatric speciation, reproductive isolation mechanisms, and the process of adaptive radiation.

Allopatric and Sympatric Speciation

Speciation is the formation of new, distinct species from a single ancestral population. It occurs when populations become reproductively isolated and diverge genetically over time. The two main modes are allopatric (geographic separation) and sympatric (same geographic area).

Modes of Speciation

Allopatric Speciation

1. Population separated by geographic barrier (river, mountain, ocean)
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2. No gene flow between populations
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3. Different selection pressures + genetic drift
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4. Reproductive isolation = new species

Sympatric Speciation

1. Population in same geographic area
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2. Reproductive isolation arises (e.g., polyploidy, habitat preference)
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3. Gene flow reduced within the population
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4. Divergence leads to new species

Key point: Allopatric speciation is the most common mode and requires a physical barrier to gene flow. Sympatric speciation occurs without geographic separation and is more common in plants (via polyploidy) than animals.

Reproductive Isolation Mechanisms

Reproductive isolation prevents gene flow between populations, allowing them to diverge into separate species. These barriers are classified as prezygotic (before fertilisation) and postzygotic (after fertilisation).

Isolation Mechanisms

Prezygotic Barriers

  • • Temporal: Different breeding seasons/times
  • • Behavioural: Different mating calls/dances
  • • Mechanical: Incompatible reproductive structures
  • • Gametic: Gametes cannot fuse
  • • Habitat: Different microhabitats within same area

Postzygotic Barriers

  • • Hybrid inviability: Hybrid embryo fails to develop
  • • Hybrid sterility: Hybrid offspring is sterile (e.g., mule)
  • • Hybrid breakdown: F2 or later generations have reduced fitness

Biological Species Concept

A species is defined as a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups.

Limitation: This concept does not apply to asexual organisms, fossils, or organisms that hybridise in nature. Alternative concepts include the phylogenetic and morphological species concepts.

Adaptive Radiation

Adaptive radiation is the rapid diversification of a single ancestral species into many new species, each adapted to different ecological niches. It typically occurs when organisms colonise new environments with many available niches and few competitors.

Classic Examples

Darwin's Finches

One ancestral finch species diversified into ~15 species with different beak shapes for different food sources on the Galapagos

Australian Marsupials

Ancestral marsupials radiated into diverse forms (kangaroos, koalas, wombats) filling niches on an isolated continent

Hawaiian Honeycreepers

A single colonising species diversified into 50+ species with varied bill shapes on the Hawaiian Islands

Conditions Favouring Adaptive Radiation

  1. 1. New or empty habitats: Colonisation of islands, post-mass extinction recovery.
  2. 2. Many available niches: Diverse food sources, habitats, and ecological roles.
  3. 3. Key innovations: A new adaptation (e.g., flight, flowers) that opens new ecological opportunities.
  4. 4. Low competition: Few existing species competing for the same resources.

Key Vocabulary

Allopatric Speciation

The formation of new species when populations are geographically separated by a physical barrier, preventing gene flow and allowing independent divergence.

Sympatric Speciation

The formation of new species within the same geographic area, often through polyploidy (plants) or habitat/behavioural differences that reduce gene flow.

Reproductive Isolation

Biological mechanisms (prezygotic or postzygotic) that prevent members of different species from producing viable, fertile offspring together.

Adaptive Radiation

The rapid diversification of a single ancestral lineage into many species, each adapted to fill different ecological niches in a new or changed environment.

Worked Examples

1

A river forms and divides a population of lizards. Over thousands of years, the two groups develop different colouration and mating displays. Identify and explain the type of speciation.

Step 1: The river is a geographic barrier that physically separates the population, preventing gene flow.

Step 2: Different environments on each side of the river exert different selection pressures, leading to genetic divergence.

Answer: This is allopatric speciation. The different colouration reflects adaptation to different environments, and the different mating displays represent a behavioural (prezygotic) reproductive isolation mechanism. Even if the river disappeared, the lizards may no longer interbreed.

2

A horse (2n = 64) and a donkey (2n = 62) can mate to produce a mule (2n = 63). Explain why the mule is sterile.

Step 1: The mule has 63 chromosomes -- an odd number that cannot be evenly divided during meiosis.

Step 2: Homologous chromosomes cannot pair properly during meiosis I because the horse and donkey chromosomes differ in number and structure.

Answer: This is hybrid sterility, a postzygotic reproductive isolation mechanism. The mule is viable but cannot produce functional gametes, confirming horses and donkeys are separate species.

3

Explain how polyploidy can lead to sympatric speciation in plants.

Step 1: Polyploidy is a mutation resulting in extra sets of chromosomes (e.g., 4n instead of 2n).

Step 2: A tetraploid (4n) plant can self-fertilise or breed with other tetraploids, but crosses with diploid (2n) plants produce triploid (3n) offspring that are usually sterile.

Answer: The tetraploid is immediately reproductively isolated from the diploid parent population (gametic isolation), creating a new species in a single generation -- without any geographic separation. This is sympatric speciation.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Allopatric speciation requires:

Question 2

Two bird species live in the same forest but breed in different months. This is an example of:

Question 3

Adaptive radiation is most likely to occur when:

Question 4

Which is a postzygotic reproductive barrier?

Question 5

Polyploidy is most commonly associated with speciation in:

Key Concepts Summary

Year 12: Evidence for Evolution Year 12: Human Evolution