5. Types of Speciation

The five evolutionary agents - genetic drift, non-random mating, gene flow, mutation and natural selection - make changes to the gene pool (microevolution). This influences the allele frequencies and numbers in a population, and therefore the success of the population.

Over time, geographically isolated populations that have no gene flow accumulate changes that can lead to reproductive isolation.

Reproductively isolated populations continue to accumulate genetic differences, which leads to speciation (macroevolution).

There are two types of speciation: 1) Allopatric and 2) Sympatric.

Allopatric (Geographic) Speciation

Allopatric speciation occurs when populations become geographically isolated, each are subjected to different selection pressures and reach a point where they are reproductively isolated.

If the two populations did happen to meet in the future, no interbreeding can occur due to reproductive isolating mechanisms that have genetically accumulated over time.

Read the supplementary notes below for the 5 stages of allopatric speciation, eventually leading to allopatry.

Allopatric speciation also occurred when populations became isolated on distant islands, such as New Zealand robins.

Allopatric speciation has been important in speciation in New Zealand, which has experienced a number of glacial and interglacial periods.

Sympatric Speciation

Sympatric speciation occurs when populations form a new species within the same geographical area as the parent species.

There are two situations where sympatric specitation is thought to occur.

1) Instant Speciation (Polyploidy)

Polyploidy occurs during cell division, when a non-disjunction event suddenly produces daughter cells with more than two sets of chromosomes (e.g. 3n, 4n, 6n, 8n etc.) It occurs mostly during meiosis, forming non-haploid gametes.

Instant speciation through polyploidy can result in a new species in a single generation due to the consequent post-zygotic reproductive barrier of hybrid sterility (except for rare cases).

There are two types of polyploidy:

Autopolyploidy (only one parent, from self-fertilisation in plants)
Allopolyploidy is the most important type as it results in hybrids from two different species, which can be advantageous should selection pressures change.

2) Speciation through Niche Differentiation / Specialisation

This can occur through ecological isolating mechanisms; a change in niche or lifestyle, where mating can only be between those who have adopted a new lifestyle.

When environments are heterogeneous (one that is not the same everywhere), there are many microhabitats within the same area.

Individuals that prefer to occupy one particular microhabitat will rarely come into contact with individuals that prefer other microhabitats.

There is a point where some individuals become so dependent on the resources offered by their particular microhabitat, that they never interact with individuals in different microhabitats.

For example: Some host-specific phytophagous insects (insects that feed on plants) prefer to ay eggs on plants identical to the species they themselves hatched on. Host-plant preference leads to isolation within the same geographical area.

Supporting Notes

B3.5 C5 Types of Speciation - Allopatric and Sympatric Speciation.pdf