Speciation

This lesson covers: 

1. What defines a species
2. How new species evolve from existing species
3. Prezygotic and postzygotic barriers that cause reproductive isolation
4. Allopatric and sympatric speciation

How new species evolve

A species is a group of individuals that are capable of breeding with one another to produce fertile offspring. Individuals within a species share the same genes but typically have different alleles. Speciation is the process by which new species are formed.

Speciation typically occurs due to:

1. Reproductive isolation of populations, which is the inability of two groups of organisms to breed with one another.
2. Genetic divergence, driven by natural selection and genetic drift.

Genetic drift refers to the random changes in allele frequencies within a population's gene pool, due to chance events, not as a result of natural selection. It is particularly influential in small, isolated populations, where it can accelerate speciation.

Adaptive radiation

Adaptive radiation is a phenomenon where organisms diversify rapidly from an ancestral species into a wide array of new forms, each adapted to a specific ecological niche.

It is more likely to occur when a change in the environment makes new resources available. This diversification is a pivotal mechanism behind speciation.

Reproductive isolation

Reproductive isolation is when populations cannot interbreed successfully to produce viable, fertile offspring.

Reproductive isolation results in genetic isolation. This is when a lack of gene flow between populations due to reproductive barriers mean that they cannot interbreed successfully. This prevents the exchange of genes between the populations, and facilitates speciation.

Prezygotic and postzygotic barriers that cause reproductive isolation

Reproductive isolation can either result from prezygotic barriers or postzygotic barriers. These barriers ensure the genetic uniqueness of diverging populations, even when they come into contact again.

Prezygotic reproductive barriers:

• These prevent fertilisation and the formation of a zygote, and act before the fertilisation of an egg cell.
• Examples of prezygotic barriers include habitat isolation, or variations in mating rituals.

Postzygotic reproductive barriers:

• These are often a result of hybridisation between different species.
• They produce infertile or non-viable hybrid offspring, reducing their reproductive potential.

Allopatric and sympatric speciation

There are two main types of speciation: allopatric speciation and sympatric speciation.

Allopatric speciation:

• Some members of a population are geographically separated from the rest by a physical barrier like mountains, rivers, or seas.
• Geographical separation exposes distinct parts of the population to different environmental pressures.
• These prezygotic reproductive barriers lead to reproductive isolation.
• Reproductive isolation interrupts gene flow, and the physical separation leads to genetic divergence.
• This causes the populations to evolve separately and form separate species.

Sympatric speciation:

• Speciation takes place within the same geographical location.
• Ecological or behavioural separation mechanisms like different habitat preference, mate selection, or chromosomal changes lead to the groups becoming reproductively isolated.
• Reproductive isolation interrupts gene flow, and the physical separation leads to genetic divergence.
• This causes the populations to evolve separately and form separate species.

Sympatric speciation is less frequent than allopatric speciation, and it is more common in plants than in animals.