Speciation and the Biological Species Concept

Speciation

Introduction to Speciation

Speciation is the evolutionary process by which new biological species arise. It involves the splitting of a single ancestral lineage into two or more genetically independent lineages. This process is fundamental to the diversity of life on Earth.

• Definition: Speciation is the formation of new and distinct species in the course of evolution.

• Key Requirement: The cessation of gene flow between populations is essential for speciation to occur.

Major Steps in the Formation of Species

1. A single population of a species becomes isolated, and all gene flow stops.

2. A barrier develops, completely separating the groups. The separated groups are now said to be allopatric (geographically isolated).

3. The two populations, now experiencing different environments, begin to differentiate as a result of natural selection until they can no longer interbreed.

4. The barrier disappears and the populations overlap geographically (now sympatric), but they are genetically incompatible and cannot interbreed.

Key Terms

• Allopatric: Populations that are geographically isolated from each other.

• Sympatric: Populations that occupy the same geographic area but are reproductively isolated.

• Gene Flow: The transfer of genetic material between populations.

Biological Species Concept

Definition and Application

The biological species concept defines a species as a population or group of populations whose members are actually interbreeding or have the potential to interbreed and produce fertile offspring. Gene flow occurs between populations within a species, maintaining genetic cohesion.

• Definition: A species is a group of organisms that can interbreed and produce fertile offspring in nature.

• Gene Flow: The exchange of genes between populations, which maintains species integrity.

• Clear Application: This concept works well for sexually reproducing, extant (living), and non-geographically isolated populations.

Problems with the Biological Species Concept

While the biological species concept is widely used, it has several limitations:

• Extinct Species: The interbreeding criterion cannot be applied to extinct species, as it is impossible to determine if two individuals could interbreed.

• Geographically Isolated Populations: Populations may look identical but may not have the opportunity to interbreed in nature, making it difficult to determine if they are the same species.

• Asexually Reproducing Organisms: The concept is not applicable to organisms that reproduce asexually, as they do not interbreed but produce genetically identical lineages.

• Gene Flow in Asexuals: Asexual organisms have little to no gene flow between lineages and are assigned to species based on appearance and biochemical features.

Therefore, the biological species concept is most useful for populations that:

• Reproduce sexually

• Are not extinct

• Are not geographically isolated

However, even in these cases, the concept may not always be unambiguously applicable, as seen in certain populations such as deer mice in the Rocky Mountains.

Example: Deer Mice Populations

Deer mice populations in the Rocky Mountains may overlap geographically, and some interbreeding occurs in overlap zones. However, gene pools are not completely shared, and some subspecies may not interbreed at all, challenging the application of the biological species concept.

Additional info: The biological species concept is one of several species concepts in biology. Others include the morphological species concept (based on physical traits) and the phylogenetic species concept (based on evolutionary history).