BIODIVERSITY & EVOLUTION NOTES

SPECIES CONCEPTS

THE BIOLOGICAL SPECIES CONCEPT

  • Definition of a biological species according to Ernst Mayr (1942):

    • "Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups."

    • Key components include:

    • Mating among members of a species produces fertile offspring.

    • Reproductive isolation prevents mating or production of fertile offspring with other groups.

PROBLEMS WITH THE BIOLOGICAL SPECIES CONCEPT

  • Cannot be applied to fossils, as reproductive isolation cannot be tested.

  • The criteria do not fit asexual organisms as asexual reproduction is more common.

  • Ambiguity in defining reproductive isolation:

    • Example: Individuals from different populations producing partially sterile hybrids.

    • Hybrids in plants and fungi often viable and fertile, complicating definitions.

ALTERNATIVE SPECIES CONCEPTS

  • Many alternative concepts have been proposed including (selection of notable ones):

    • Agamospecies Concept

    • Cohesion Species Concept

    • Ecological Species Concept

    • Phylogenetic Species Concept (two versions: Diagnosable and Monophyly)

    • Recognition Species Concept

    • Evolutionary Species Concept: Describes species as lineages of populations maintaining identity and evolutionary tendencies.

KEY FEATURES OF AN OPERATIONAL SPECIES CONCEPT

  • Reproductive cohesion within species.

  • Reproductive isolation from other groups.

  • Recognition of species as dynamic evolutionary lineages, not static types.

  • Noted by Charles Darwin: no clear line between species and sub-species, allowing for incipient species (well-marked varieties).

SPECIATION

DEFINITION AND PROCESS

  • Speciation is the process by which a single genetically cohesive population splits into two or more reproductively isolated populations.

  • According to the Dobzhansky-Muller model:

    • An ancestral population is divided by a barrier to gene flow.

    • Groups evolve independently, with new alleles becoming fixed at different loci.

    • Genetic incompatibility emerges, meaning combinations of certain alleles can be inferior or lethal.

REPRODUCTIVE ISOLATION AND GENETIC DIVERGENCE

  • Reproductive isolation increases as genetic divergence increases:

    • More genetically different Drosophila species show greater reproductive isolation.

MECHANISMS OF SPECIATION

  1. Divergence: Speciation cannot occur within a population.

  2. Speciation mechanisms include:

    • Allopatry: Geographic isolation.

    • Sympatry: Different feeding/mating grounds or seasonal differences.

    • Can be reinforced by post-introduction mate preferences.

    • Extinction: A better adapted species may lead to the extinction of a predecessor.

MODES OF SPECIATION

  • Allopatric: Populations are geographically isolated.

  • Peripatric: Small populations at the edge of a larger population.

  • Parapatric: Continuously distributed population where a barrier may lead to speciation.

  • Sympatric: Occurs within the ancestral population's range without physical separation.

EXAMPLES OF SPECIATION

ALLOPATRIC SPECIATION: STICKLEBACK EVOLUTION

  • Changes in sea level trap marine sticklebacks in freshwater lakes:

    • Marine three-spined sticklebacks develop pelvic spines.

    • Freshwater counterparts show reduced pelvis due to selective pressure from different predators.

ALLOPATRIC SPECIATION MECHANISMS

  • Environmental differences can lead to different selective pressures and evolve different populations.

  • Genetic drift and mutations contribute to speciation events.

PARAPATRIC SPECIATION: RING SPECIES

  • Expansion around a barrier such as in the greenish warbler can result in reproductive barriers upon re-contact.

SYMPATRIC SPECIATION

  • Speciation without physical isolation, often linked to phenotypic preferences for various microhabitats influenced by disruptive selection.

  • Can result from polyploidy, where chromosomal duplication leads to reproductive isolation (e.g., plants).

REINFORCEMENT OF REPRODUCTIVE ISOLATION

  • Mechanisms strengthen reproductive isolation:

    • Genetic differences reduce successful mating probabilities.

    • Prezygotic isolating mechanisms: Prevent hybridization before fertilization (mechanical, temporal, behavioral, habitat, gametic).

    • Postzygotic isolating mechanisms: Reduce hybrid offspring fitness.

EXAMPLE: BEHAVIORAL ISOLATION

  • Female frogs ignore calls from unrelated males, reinforcing isolation intensively in sympatric conditions.

CONTINUUM OF SPECIATION

  • Speciation is a process, not a one-time event. The transition from one species to another can occur incrementally over time.

MACROEVOLUTION vs. MICROEVOLUTION

MICROEVOLUTION

  • Evolutionary processes observable at the population level:

    • Populations adapt to environmental changes.

    • Breeders select favorable traits.

MACROEVOLUTION

  • Involves development of species over millions of years, primarily initiated through speciation.

  • Major changes include body plan alterations and extinction events.

PACE OF EVOLUTION

  • Two prevailing views:

    • Gradualism: Small changes accumulate, with divergence mainly post-speciation.

    • Punctuated Equilibrium: Rapid changes during speciation, interspersed with long periods of stasis.

ADAPTIVE RADIATION

  • Rapid emergence of new species due to adaptation to environmental opportunities (e.g., after mass extinctions or colonization events).

  • Notable example includes the finches of the Galápagos Islands.

CHARACTER DISPLACEMENT

  • Differences among species help reduce competition by allowing occupation of different ecological niches through disruptive selection.

MASS EXTINCTIONS AND ADAPTIVE RADIATION

  • Five major mass extinctions facilitated rapid evolution of new species, filling available ecological niches.

  • Example includes the diversification of mammals and birds post-Cretaceous extinction.

SUMMARY

  • Definitions and problems of the Biological Species Concept, a pragmatic approach to species delimitation, and an understanding of speciation as an ongoing process influenced by geographical or ecological factors. Additionally, discusses the broad scope of macroevolutionary processes contrasted with microevolutionary mechanisms.