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IB DP Biology Study Notes

5.2.1 Speciation

Speciation is the evolutionary process by which populations evolve to become distinct species. This process includes genetic isolation, genetic divergence, and the eventual formation of new species, with various modes such as allopatric and sympatric speciation.

Genetic Isolation

Definition and Importance

  • Definition: Genetic isolation occurs when two populations become separated, preventing gene flow between them.
  • Importance: This isolation allows populations to evolve independently, leading to differences that may culminate in speciation.

Causes of Genetic Isolation

  • Geographical Barriers: Mountains, rivers, deserts, or oceans that prevent breeding between populations. Such barriers can develop gradually or suddenly due to geological events.
  • Ecological Isolation: Differences in habitat preferences within the same geographical area, leading to reduced mating opportunities.
  • Behavioural Differences: Mating calls, dances, or rituals that are specific to a population, leading to mate selection within that population.
  • Temporal Isolation: Breeding at different times of the day or different seasons.

Understanding how species and reproductive isolation are interconnected provides deeper insights into genetic isolation's role in speciation.

Genetic Divergence

Definition and Importance

  • Definition: Genetic divergence refers to the process where isolated populations evolve independently, accumulating significant genetic differences.
  • Importance: Genetic divergence may lead to reproductive barriers, preventing interbreeding and solidifying the formation of new species.

Factors Contributing to Genetic Divergence

  • Mutation: Spontaneous changes in DNA can create new alleles and increase genetic variation.
  • Natural Selection: Different environments exert different selective pressures, favouring distinct traits in isolated populations.
  • Genetic Drift: In small populations, allele frequencies can change randomly, leading to genetic divergence.
  • Sexual Selection: Preferences in mating partners can also drive divergence, especially if those preferences are linked to genetic or physical traits.
  • Gene Flow: Lack of gene flow reinforces the genetic differences between the populations.

The semiconservative model of DNA replication plays a critical role in understanding genetic changes that fuel divergence.

Formation of New Species

Climax of Divergence

  • New Species Formation: If genetic divergence reaches a point where interbreeding becomes impossible, a new species is officially formed.
  • Reproductive Isolation: Mechanisms that prevent successful breeding between the new species and the original population, such as hybrid infertility. Mechanisms like polyspermy prevention highlight the complexity of reproductive isolation.

Modes of Speciation

Allopatric Speciation

  • Definition: Occurs when a physical barrier divides a population, leading to genetic isolation and divergence.
  • Example: Different squirrel species evolving on either rim of the Grand Canyon.
  • Factors Influencing Allopatric Speciation: Size of the barrier, length of isolation, environmental differences across the barrier.

Sympatric Speciation

  • Definition: Occurs within the same geographical area, often due to ecological, behavioural, or chromosomal differences.
  • Example: Apple maggot flies evolving preferences for different types of apples.
  • Mechanisms of Sympatric Speciation: Polyploidy, especially in plants; niche differentiation; sexual selection.

Other Modes

  • Parapatric Speciation: Populations are adjacent but not overlapping, with a gradient of genetic change across the boundary.
  • Peripatric Speciation: A small subgroup becomes isolated (e.g., by distance or ecological niche) and evolves into a separate species.
  • Artificial Speciation: Human-induced speciation through selective breeding, as seen in domesticated plants and animals.

To further explore the evolutionary processes leading to the diversity of life forms, examining evidence of evolution can provide a broader understanding of how species have changed over time. Additionally, modern techniques such as DNA profiling have significantly enhanced our ability to study genetic differences contributing to speciation.


Yes, artificial selection can lead to speciation. By selecting and breeding organisms for specific traits, humans can create strong selective pressures. If artificial selection is applied differently in separate populations, it can cause them to diverge significantly. Over many generations, this divergence might become substantial enough to create new species.

A hybrid zone is an area where two closely related species interbreed, producing hybrid offspring. The relationship to speciation lies in the study of these zones to understand how reproductive barriers form. If hybrids are less fit, reinforcement occurs, strengthening pre-zygotic barriers and driving speciation. Conversely, if hybrids are fit, the species may merge.

Chromosomal changes, such as polyploidy, can lead to instant reproductive isolation in sympatric speciation. If an organism inherits extra sets of chromosomes and others in the population do not, it can no longer interbreed with the normal chromosomal counterparts. Polyploidy is particularly common in plants, where it can lead to the immediate formation of a new species.

Sexual selection can drive speciation by favouring specific mating preferences or secondary sexual characteristics. If different populations evolve differing preferences or traits, they may no longer find each other attractive or compatible for mating. This reproductive barrier can lead to speciation as the populations diverge in their sexual characteristics.

A peripatric speciation is a form of allopatric speciation where a small subgroup of a population becomes isolated at the periphery of the larger population's habitat. Unlike general allopatric speciation, where any barrier may lead to isolation, peripatric speciation specifically involves a small group breaking away. This small population may experience more intense genetic drift and selective pressures, leading to rapid divergence.

Practice Questions

Explain the differences between allopatric and sympatric speciation, providing an example for each.

Allopatric speciation occurs when a physical barrier divides a population, leading to genetic isolation and divergence. For example, different squirrel species have evolved on either rim of the Grand Canyon due to the canyon acting as a physical barrier. Sympatric speciation, on the other hand, occurs within the same geographical area, often due to ecological, behavioural, or chromosomal differences. An example of sympatric speciation is apple maggot flies that have evolved preferences for different types of apples. Whereas allopatric speciation relies on geographic isolation, sympatric speciation involves reproductive isolation without geographical barriers.

Describe the role of genetic divergence in the speciation process and identify at least two factors that contribute to genetic divergence.

Genetic divergence plays a critical role in the speciation process by allowing isolated populations to evolve independently, accumulating significant genetic differences that may lead to reproductive barriers. These barriers can ultimately prevent interbreeding between different populations, solidifying the formation of new species. Factors contributing to genetic divergence include mutation, which introduces new alleles, increasing genetic variation, and natural selection, where different environments exert distinct selective pressures, favouring specific traits in isolated populations. Both of these factors drive the evolutionary changes that lead to divergence, reinforcing the genetic differences between the populations, and possibly culminating in speciation.

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Written by: Dr Shubhi Khandelwal
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