Speciation and the Concept of Species

Speciation and the Concept of Species

Introduction

This study guide explores how new species originate and how biologists define and distinguish species. It covers the Biological Species Concept, reproductive barriers, and alternative species concepts, providing a foundation for understanding speciation and evolutionary theory.

Endemic Species

Definition and Significance

• Endemic species are species found nowhere else on Earth.

• Many endemic species are found on islands, such as the Galápagos, which fascinated Darwin and contributed to his theory of natural selection.

• Speciation—the origin of new species—is a central focus of evolutionary theory.

• Example: Galápagos tortoise and unique bird species of the Galápagos Islands.

Microevolution vs. Macroevolution

Levels of Evolutionary Change

• Microevolution: Changes in allele frequency within a population over time.

• Macroevolution: Broad patterns of evolutionary change above the species level, such as the emergence of new groups of organisms.

• Example: Changes in flower color within a population (microevolution) vs. the evolution of tetrapods (macroevolution).

What Constitutes a Species?

Defining Species

• Species definitions are debated and can be based on:

  • Morphology

  • Physiology

  • Biochemistry

  • DNA sequences

• The Biological Species Concept (BSC) is widely used and emphasizes reproductive isolation.

• BSC Definition: A group of organisms that can interbreed and produce viable, fertile offspring, but are reproductively isolated from other such groups.

Reproductive Isolation

Barriers to Gene Flow

• Reproductive isolation: Biological factors that prevent two species from producing viable, fertile offspring.

• Not limited to physical separation; can include genetic, behavioral, or physiological barriers.

• Hybrids: Offspring from crosses between different species, often infertile (e.g., mule = horse × donkey).

• Reproductive isolation can be classified as prezygotic (before fertilization) or postzygotic (after fertilization).

Exploring Reproductive Barriers

Types of Barriers

Barrier Type	Prezygotic	Postzygotic
Habitat Isolation	✔
Temporal Isolation	✔
Behavioral Isolation	✔
Mechanical Isolation	✔
Gametic Isolation	✔
Reduced Hybrid Viability		✔
Reduced Hybrid Fertility		✔
Hybrid Breakdown		✔

Additional info: Prezygotic barriers prevent fertilization; postzygotic barriers affect hybrid offspring viability or fertility.

Prezygotic Barriers

• Habitat Isolation: Species occupy different habitats and rarely encounter each other (e.g., aquatic vs. terrestrial garter snakes).

• Temporal Isolation: Species breed at different times (e.g., Atlantic salmon spawn in fall, rainbow trout in spring).

• Behavioral Isolation: Unique courtship behaviors prevent mating (e.g., blue-footed booby courtship dance).

• Mechanical Isolation: Physical differences prevent successful mating (e.g., incompatible genitalia in bushbabies).

• Gametic Isolation: Sperm of one species may not fertilize eggs of another.

Postzygotic Barriers

• Reduced Hybrid Viability: Hybrid offspring fail to develop or survive.

• Reduced Hybrid Fertility: Hybrids are sterile (e.g., mules).

• Hybrid Breakdown: Hybrids are fertile, but their offspring are inviable or sterile due to genetic incompatibilities.

Limitations of the Biological Species Concept

• Cannot be applied to fossils or asexual organisms (including prokaryotes).

• Gene flow can occur between distinct species (e.g., grizzly bears and polar bears can produce fertile offspring).

• Some "species" may not be reproductively isolated in nature.

Other Definitions of Species

Morphological Species Concept

• Defines species by structural features.

• Applicable to both sexual and asexual organisms, but relies on subjective criteria.

• Example: Different butterfly species identified by wing patterns.

Phylogenetic Species Concept

• Defines a species as the smallest group of individuals sharing a common ancestor on a phylogenetic tree.

• Can be applied to sexual and asexual species.

• Often uses genetic, molecular, and morphological data.

• Dominant in modern biology due to advances in molecular techniques.

• Example: Species defined by unique DNA sequences or branching patterns on a phylogenetic tree.

Lecture Recap

• Defining a species is complex and context-dependent.

• The Biological Species Concept is useful but limited, especially for asexual organisms and cases with gene flow between groups.

• Morphological and phylogenetic species concepts provide alternative frameworks, with the latter being especially important in modern biology.