speciation.complete

Speciation: Overview

  • Speciation refers to the patterns and processes that allow the universal common ancestor to diversify into over 10 million species.

Defining Species

  • Conceptual Definition: Species are viewed as "evolutionary units" where individuals within a species evolve together, whereas different species evolve independently.

  • Moving from a conceptual to a practical definition poses challenges due to life's complexity.

1. Biological Species Concept

  • Definition: Biological species are defined by reproductive isolation.

  • Different biological species may:

    • Not breed in nature.

    • Breed but produce no offspring.

    • Produce inviable offspring (fail to develop to adulthood).

    • Produce sterile offspring (cannot reproduce).

Mechanisms of Isolation

  • Prezygotic Isolation: Mechanisms preventing successful mating.

  • Postzygotic Isolation: Mechanisms preventing offspring from reproducing.

  • "Zygote" refers to the cell formed by the fusion of sperm and egg.

Mechanism Examples

  • Pre- or Post-Zygotic Mechanisms?

    • Different malaria parasites breed in different hosts (prezygotic).

    • Some dove species nest together but eggs fail to hatch (postzygotic).

    • The offspring of horses and donkeys are healthy but infertile (postzygotic).

    • Sea urchin eggs are inaccessible to sperm from other species (prezygotic).

    • Pine tree species release pollen at different times (prezygotic).

Pre- vs. Post-Zygotic Mechanisms: Adaptive Favorability

  • Prezygotic mechanisms reduce wasted reproductive efforts, thus are generally favored.

  • Natural selection favors prezygotic isolation to avoid the costly production of sterile hybrids, like mules.

Disadvantages of Biological Species Concept

  • Not applicable to asexual species.

  • Inapplicable for extinct species.

  • Difficult to evaluate due to geographic distance among populations.

1.2 Morphological Species Concept

  • Definition: Differences in physical characteristics define morphological species.

  • Useful for fossils and diverse groups (e.g., insects).

  • Dependent on expert interpretation; subjective and can lead to disagreements.

1.3 Ecological Species Concept

  • Definition: Groups of organisms sharing the same ecological niche and resources.

  • Commonly applied to small organisms like bacteria and archaea.

1.4 Phylogenetic Species Concept

  • Definition: A monophyletic group of populations, defined by a single common ancestor.

  • Provides clarity as long as a population's definition is known.

Advantages of Phylogenetic Species Concept

  • Clearly defined and broadly applicable.

Disadvantages of Phylogenetic Species Concept

  • Difficulties in estimating phylogenies and requiring extensive population data, leading to practical limitations.

Defining Species

  • Formally defining species is challenging without a universally accepted method.

  • Controversies arise over the classification of closely-related species, like elephants, while larger distinctions, such as rhinoceroses, are clearer.

Generating Species

  • New species evolve from existing species through gradual evolution or divergence.

  • Divergence involves genetic isolation followed by genetic divergence, typically initiated by isolation.

2. Species Divergence in Allopatry

  • Allopatry: Populations living apart are expected to diverge due to:

    • Genetic drift.

    • Natural selection responding to different environments.

2.1 Dispersal

  • Occurs when a few individuals colonize a new area, leading to population isolation and potential divergence.

2.2 Vicariance

  • Split in populations due to geographical or ecological barriers, known as vicariance events, creating opportunities for divergence.

  • Examples include the isolation of ratites due to the drifting of Gondwanaland.

3. Species Divergence in Sympatry

  • Sympatry: Populations living in the same geographic area typically face challenges in diverging due to gene flow and competition.

3.1 Disruptive Selection

  • Divergence may occur through habitat partitioning.

  • Preference by individuals for the same host plant facilitates reproductive isolation, as in the case of hawthorn flies feeding on apples.

3.2 Genetic Incompatibility

  • Divergence can emerge from mutations causing reproductive isolation, with potential competition leading to extinction of one population.

Hybridization and Polyploidy

  • Hybrid species can arise from successful crosses between two species, sometimes leading to new adaptations that support their survival.

  • Polyploidy can create instant reproductive barriers and foster genetic innovations.

4. Reuniting Isolated Populations

  • Contact between previously isolated populations can provide insights into speciation processes.

Fusion and Reinforcement

  • Populations may fuse, or reinforcement may occur through selection against hybrids with low fitness (e.g., the eastern and western meadowlarks).

Conclusion

  • The diversity observed in modern ecosystems largely results from speciation events, encompassing the splitting of species that generally includes processes of isolation and divergence.