Chapter 22: The Origin of Species

Introduction

This chapter explores the processes that lead to the formation of new species (speciation) and the broader patterns of evolution above the species level (macroevolution). Understanding these concepts is fundamental to the study of biological diversity and evolutionary biology.

AP Big Ideas Relevant to Speciation

Major Themes

• Evolution (EVO): The process of evolution drives the diversity of life. Speciation may occur when two populations become reproductively isolated from one another.

• Energetics (ENE): Biological systems use energy and molecular building blocks to grow, reproduce, and maintain dynamic homeostasis. Evolution can lead to differences in timing and coordination of behavior.

• Information Storage and Transmission (IST): Living systems retrieve, transmit, and respond to information essential for life processes. Changes in modes of intercellular and intracellular signal transmission can affect speciation.

• Systems Interactions (SYI): Biological systems interact, and these interactions exhibit complex properties. Reproductive isolation can affect relationships with other species and the environment, leading to adaptation or morphological change.

Macroevolution and Speciation

Definitions and Concepts

• Macroevolution: Evolution at or above the level of species, encompassing large-scale evolutionary changes such as the origin of new species, genera, families, or higher taxa.

• Biological Species Concept: Defines a species as a group of organisms that can interbreed and produce fertile offspring, but do not produce fertile offspring with members of other such groups.

• Speciation: The process by which one species splits into two or more distinct species.

Reproductive Isolation

Barriers to Gene Flow

Reproductive isolation refers to the existence of biological factors (barriers) that impede members of two species from interbreeding and producing fertile offspring. These barriers are classified as prezygotic or postzygotic.

• Prezygotic Barriers: Prevent mating or fertilization between species.

• Postzygotic Barriers: Prevent hybrid offspring from developing into viable, fertile adults.

Types of Prezygotic Barriers

• Habitat Isolation: Species live in different habitats and do not meet.

• Temporal Isolation: Species breed at different times (day, season, or year).

• Behavioral Isolation: Differences in courtship rituals or other behaviors prevent mating.

• Mechanical Isolation: Morphological differences prevent successful mating.

• Gametic Isolation: Sperm of one species may not survive in the reproductive tract of another species, or gametes may not fuse.

Types of Postzygotic Barriers

• Reduced Hybrid Viability: Hybrids fail to develop or are frail and unlikely to survive.

• Reduced Hybrid Fertility: Hybrids are sterile or have reduced fertility.

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

Types of Speciation

Allopatric Speciation

Allopatric speciation occurs when populations are geographically isolated, preventing gene flow. Over time, genetic divergence leads to the formation of new species. Mobility and the ability to disperse influence the likelihood of allopatric speciation.

• Example: The formation of new species on islands or due to the creation of physical barriers such as the Isthmus of Panama.

• Adaptive Radiation: The rapid evolution of multiple species from a common ancestor, often as populations fill different ecological niches.

Sympatric Speciation

Sympatric speciation occurs without geographic isolation. It is less common and often involves genetic changes such as polyploidy, habitat differentiation, or sexual selection.

• Polyploidy: Especially in plants, speciation can result from errors in meiosis that lead to extra sets of chromosomes.

• Habitat Differentiation: Subpopulations exploit different resources or habitats within the same area.

• Sexual Selection: Mate choice based on specific traits (e.g., coloration) can drive speciation.

Hybrid Zones

Definition and Outcomes

A hybrid zone is a region where members of different species meet and mate, producing offspring of mixed ancestry. The success and persistence of hybrids vary.

• Reinforcement: Hybrids are less fit, and reproductive barriers are strengthened.

• Fusion: Barriers weaken, and gene pools merge, potentially resulting in a single species.

• Stability: Hybrids continue to be produced, but the zone remains narrow and stable.

Pace of Evolution

Gradualism vs. Punctuated Equilibrium

• Gradualism: Evolution occurs slowly and steadily over long periods.

• Punctuated Equilibrium: Evolution occurs in rapid bursts, followed by periods of stasis. Much of this evidence comes from the fossil record.

From Speciation to Macroevolution

Broader Patterns

Speciation is a fundamental process that contributes to macroevolution, leading to the diversity of life observed today. Macroevolution encompasses large-scale evolutionary changes, such as the origin of new taxonomic groups and major evolutionary trends.

Summary Table: Types of Reproductive Isolation

Key Equations

• Hardy-Weinberg Equation: Used to calculate allele and genotype frequencies in a population under equilibrium conditions.

Additional info: Some context and examples have been inferred and expanded for clarity and completeness.