What is Speciation?

Speciation occurs when a group within a species separates from other members of its species and develops its own unique characteristics. The key step is reproductive isolation – the inability of the separated groups to interbreed and produce fertile offspring.

There are several modes of speciation, primarily:

  • Allopatric Speciation: Occurs when populations are geographically separated (e.g., by mountains, rivers, oceans), preventing gene flow. Over time, they accumulate different mutations and adapt to different environments, leading to divergence.
  • Sympatric Speciation: Occurs within the same geographic area. Less common in animals, it can happen through mechanisms like polyploidy (changes in chromosome number, common in plants), habitat differentiation, or sexual selection.
  • Parapatric Speciation: Occurs when populations are adjacent but mate mostly within their own group, leading to divergence despite some gene flow.
  • Peripatric Speciation: A subtype of allopatric speciation where a small group colonizes a new, isolated niche.

Key Concept: Speciation isn't a single event but a process. Observing intermediate stages (populations that can interbreed but produce less fertile offspring, or distinct populations that rarely interbreed) provides strong evidence.

Examples in Animals

Speciation has been observed or strongly inferred in numerous animal groups.

Three-Spined Sticklebacks (Gasterosteus aculeatus)

After the last ice age, marine sticklebacks colonized numerous freshwater lakes and streams in the Northern Hemisphere. Isolated populations adapted rapidly to different conditions (e.g., deep vs. shallow water, different predators).

  • In several Canadian lakes (e.g., Paxton Lake), distinct "benthic" (bottom-dwelling) and "limnetic" (open-water) forms evolved sympatrically (or possibly through double invasion).
  • They differ in size, shape, armor, and feeding structures.
  • They primarily mate with their own type, showing strong reproductive isolation. This is considered a classic example of ecological speciation.

Cichlid Fish in African Lakes

Lakes like Victoria, Malawi, and Tanganyika host hundreds of endemic cichlid species that evolved relatively recently from a few ancestors. This is a prime example of adaptive radiation.

  • Speciation was driven by factors like habitat specialization (rock vs. sand dwellers) and sexual selection (females preferring males of specific colors, which vary with water depth/clarity).
  • Genetic studies confirm rapid divergence and reproductive isolation between closely related species.

Anolis Lizards in the Caribbean

Islands like Hispaniola and Puerto Rico show repeated evolution of similar "ecomorphs" (species adapted to specific niches like twigs, canopy, trunk-ground) on different islands.

  • Colonization of new islands (geographic isolation - allopatry) followed by adaptation to different structural habitats led to speciation.
  • Different ecomorphs show reproductive isolation.

Fruit Flies (Drosophila)

Laboratory experiments have induced speciation. For example, Diane Dodd raised fruit fly populations on different food sources (starch vs. maltose).

  • After many generations, flies showed a strong preference for mating with others raised on the same food source, demonstrating incipient sympatric or habitat-based isolation.

Other Examples

Include Galapagos finches (beak adaptations to different food sources on different islands), greenish warblers around the Tibetan plateau (a "ring species" showing divergence along a geographic loop), and London Underground mosquitoes (diverged from surface populations).

Examples in Plants

Speciation is particularly common in plants, often involving hybridization and polyploidy (having more than two sets of chromosomes).

Polyploidy

An error in meiosis can produce gametes with a full diploid set of chromosomes instead of a haploid set. If two such gametes fuse, or if a diploid gamete fuses with a normal haploid gamete, offspring with extra chromosome sets (e.g., triploid, tetraploid) can result.

  • Polyploid individuals are often instantly reproductively isolated from their diploid parents because hybrid offspring (e.g., triploids from diploid x tetraploid cross) are usually sterile due to problems pairing chromosomes during meiosis.
  • Many important crop plants are polyploids (e.g., wheat, cotton, potatoes, tobacco).

Goatsbeards (Tragopogon)

A classic example of recent speciation via hybridization and polyploidy occurred in the early 20th century in the Palouse region of Washington and Idaho, USA.

  • Three European species were introduced: T. dubius, T. porrifolius, and T. pratensis.
  • Within decades, two new tetraploid species arose through hybridization and chromosome doubling:
    • T. mirus (from T. dubius x T. porrifolius)
    • T. miscellus (from T. dubius x T. pratensis)
  • These new species are reproductively isolated from their parent species and thrive in the region.

Speciation in Goatsbeards (Tragopogon) involved hybridization between introduced species followed by chromosome doubling, creating new, reproductively isolated species within decades.

Other Plant Examples

Include various species of sunflowers (Helianthus), ferns, and orchids where hybridization and polyploidy have played significant roles in generating new species adapted to different environments.

Examples in Microorganisms

While defining species is complex in asexual organisms, experiments show divergence and adaptation leading to distinct lineages.

Lenski's E. coli Experiment

Richard Lenski's Long-Term Evolution Experiment (LTEE) has tracked 12 populations of E. coli since 1988.

  • All populations adapted to the glucose-limited medium, increasing in fitness.
  • One population famously evolved the ability to metabolize citrate in the presence of oxygen, a defining characteristic not typical of E. coli. This required several potentiating mutations before the final breakthrough mutation occurred.
  • This new ability allowed the population to access a previously unusable resource, leading to a massive population boom and ecological divergence from other E. coli in the same flask.
  • While not speciation by the biological species concept (which relies on sexual reproduction), it demonstrates the evolution of novel traits and ecological divergence driven by natural selection on mutations.

Significance: The LTEE provides a real-time view of evolution, showing adaptation, divergence, and the origin of key innovations over tens of thousands of generations.

Conclusion: Evolution in Action

The examples above, from fish and lizards to plants and bacteria, demonstrate that speciation is not just a theoretical concept but an observable process.

  • Populations become reproductively isolated through various mechanisms (geography, polyploidy, ecological adaptation, sexual selection).
  • Natural selection and genetic drift drive divergence between isolated populations.
  • We can observe different stages of the speciation process, from populations with slight mating preferences to fully distinct species unable to interbreed.

The observation of speciation provides direct evidence for evolution and its ability to generate the biodiversity we see today.