Origin of species

The Origin of Species

Chapter 24: Speciation

Speciation

• Definition: Speciation is the evolutionary process by which populations evolve to become distinct species. It acts as a bridge between microevolution (changes within a species) and macroevolution (formation of new species and larger taxonomic groups).

Biological Species Concept

• Definition: The biological species concept defines a species as a group of populations that can interbreed in nature and produce viable and fertile offspring, maintaining genetic cohesion.

• Reproductive Isolation: Different species are separated by reproductive isolation, which encompasses various biological barriers that prevent interbreeding and the successful production of offspring.

Prezygotic and Postzygotic Barriers

• Prezygotic Barriers: These hinder fertilization from occurring. Types include:

  • Habitat Isolation: Populations live in different habitats (e.g., terrestrial vs. aquatic)

  • Behavioral Isolation: Unique courtship behaviors attract mates within a species but not others.

  • Temporal Isolation: Species breed at different times, preventing interbreeding (e.g., seasonal or daily variations).

  • Mechanical Isolation: Species are anatomically incompatible for successful mating (e.g., snail shell coil differences).

  • Gametic Isolation: Incompatibilities occur at the gamete level, preventing fertilization even if mating occurs.

• Postzygotic Barriers: These barriers occur after fertilization, affecting the viability or fertility of hybrids. Types include:

  • Reduced Hybrid Viability: Genetic incompatibility may lead to an aborted development of hybrids.

  • Reduced Hybrid Fertility: Hybrids may be vigorous but sterile (e.g., mules).

  • Hybrid Breakdown: First-generation hybrids are viable and fertile, but subsequent generations may be feeble or sterile.

Reproductive Barriers Summary

• Reproductive barriers are essential for maintaining species integrity. When populations cannot interbreed, they tend to diverge over time, potentially leading to the emergence of new species.

Alternative Species Concepts

• Limitations of the Biological Species Concept: It cannot be applied to extinct organisms or those that reproduce asexually.

• Ecological Species Concept: Defines species by their ecological niche, which includes their interactions with both living and nonliving elements of the environment.

• Phylogenetic Species Concept: Uses shared ancestry to define a species, considering species as branches in the tree of life, utilizing morphology and molecular data to trace evolutionary histories.

Modes of Speciation

• Allopatric Speciation: Occurs when geographic barriers lead to reproductive isolation. Fragmented populations evolve independently over time into new species.

• Sympatric Speciation: Speciation occurs without geographic isolation, commonly through:

  • Polyploidy: Involves errors in cell division resulting in extra chromosome sets, prevalent in plants.

  • Natural Selection: Favoring extremes can split a population into two distinct species.

  • Sexual Selection: Mate preferences based on traits can isolate gene pools.

Tempos of Speciation

• Gradualism: Suggests a slow, steady pace of evolutionary change.

• Punctuated Equilibrium: Proposes that species experience long periods of stasis interrupted by brief, rapid changes, resulting in new species in shorter timescales.

Patterns of Evolution

• Divergent Evolution: New species evolve in response to varying environments after being geographically isolated.

• Convergent Evolution: Different species develop similar characteristics due to adaptation to similar environments, resulting in analogous structures (e.g., wings of birds and butterflies).

• Parallel Evolution: Related species evolve similarly after diverging from a common ancestor.

• Coevolution: Interdependent evolution of two species that influence each other’s adaptations, such as milkweed and monarch butterflies, where both exhibit toxicity and feeding adaptations that affect their survival and reproduction.