Species and Evolution by Natural Selection

Introduction

This study guide covers foundational concepts in General Biology related to the definition of species, major species concepts, and the process of evolution by natural selection. Understanding these topics is essential for exploring biodiversity, evolutionary relationships, and mechanisms of speciation.

Species Concepts

Overview of Species Concepts

The concept of a species is central to biology, but its definition can vary depending on the context. Biologists use several species concepts to classify and understand the diversity of life.

• Biological Species Concept: Defines species based on the ability to interbreed and produce viable offspring. Emphasizes reproductive isolation.

• Evolutionary Species Concept: Focuses on lineages that maintain their identity over time and have their own evolutionary trajectory.

• Phylogenetic Species Concept: Identifies species as the smallest diagnosable group with a unique derived character state.

Biological Species Concept

The Biological Species Concept (Mayr, 1942) states that species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups.

• Key Points:

  • Emphasizes genetic relatedness and reproductive compatibility.

  • Reproductive isolation prevents gene flow between species.

• Problems:

  • Cannot be applied to asexual organisms.

  • Hybridization can occur between distinct species.

  • Fossil species cannot be tested for reproductive isolation.

• Example: Panthera leo (lion) and Panthera tigris (tiger) are reproductively isolated in nature.

Evolutionary Species Concept

The Evolutionary Species Concept (Simpson, 1950; Wiley, 1978) defines a species as a single lineage of ancestral-to-descendant populations that maintains its identity from other lineages and has its own evolutionary tendencies and historical fate.

• Key Points:

  • Focuses on the continuity and identity of lineages over time.

  • Applicable to both sexual and asexual organisms.

• Example: Fossil lineages can be classified as species based on morphological continuity.

Phylogenetic Species Concept

The Phylogenetic Species Concept (Cracraft, 1983) identifies species as the smallest group of organisms that share a unique, derived character state distinguishing them from other groups.

• Key Points:

  • Uses phylogenetic analysis and shared derived traits (synapomorphies).

  • Can be applied to all organisms, including those known only from genetic data.

• Example: DNA sequence differences used to distinguish cryptic species.

Evolution by Natural Selection

Defining Evolution and Natural Selection

Evolution is the change in the heritable characteristics of biological populations over successive generations. Natural selection is a mechanism of evolution where individuals with advantageous traits are more likely to survive and reproduce.

• Requirements for Natural Selection:

  1. Phenotypic Variation: Individuals in a population vary in their traits (e.g., coat color in mice).

  2. Heritability: Most phenotypic traits have a genetic basis and can be passed to offspring.

  3. Differential Survival and Reproduction: Some individuals are favored by agents of selection (e.g., predators, environment), leading to higher relative fitness.

• Relative Fitness: The reproductive success of an individual compared to others in the population.

Agents of Selection

Agents of selection are environmental factors that influence which individuals survive and reproduce. Examples include predators, climate, and competition.

• Example: Falcons preying on mice with conspicuous coat colors, favoring those with camouflage.

Acclimation vs Adaptation

It is important to distinguish between acclimation and adaptation:

• Acclimation (Acclimatization): Short-term physiological changes in an individual in response to environmental conditions.

• Adaptation: Long-term genetic changes in a population resulting from natural selection.

Darwin's Idea: Origin of Species

Speciation and Evolutionary Relationships

Darwin proposed that new species arise from ancestral species through branching evolutionary processes. This is depicted as a tree, with nodes representing speciation events and branches representing lineages.

• Root: Common ancestor of all species in the tree.

• Node: Point of speciation where one lineage splits into two.

• Branch: Represents the evolutionary path of a species or lineage.

Summary Table: Major Species Concepts

Key Equations

• Relative Fitness ():

Additional info: Some content was inferred and expanded for clarity and completeness, including definitions, examples, and the summary table.