Speciation and Evolution

The Origin of Species: Isolation Types and Speciation

Isolation Types that can Lead to Speciation

• Geographic Isolation

  • Example: AmoebaSisters quote: "If you REALLY loved me, you'd swim across."

• Behavioral Isolation

  • Indicator: Disinterest in interaction, e.g., "Ugh…country."

• Temporal Isolation

  • Cited Scenario: "TED! Wake up! It's time to release pollen!" followed by "Ughhh. give me a few more months."

Outline of Key Concepts

• What is a Species?

  • Definitions and concepts surrounding species classification.

• Biological Definition

  • Pre- vs post-zygotic barriers

• Morphological Definition

  • Body structure and features.

• Ecological Definition

  • Relation to ecological niches.

• Allopatric and Sympatric Speciation

• Phylogenetic Definition

  • Evolutionary relationships among species.

• Tempo of Speciation

  • How environmental conditions influence speciation.

Microevolution vs Macro-evolution

• Microevolution

  • Definition: Change in allele frequency in a population over time (across generations).

• Macroevolution

  • Definition: The origination of new taxonomic groups, representing the development of new species.

Timeline of Evolutionary Changes

• Microevolution

  • Example of epochs and periods:

  • Holocene: 0 - 0.01 million years ago (Ma)

  • Quaternary: Pleistocene, 0.2 - 2 Ma

  • Neogene: 0.5 - 8 Ma (in relation to various Homo species such as H. sapiens, H. neanderthalensis, H. erectus).

Key Evolutionary Events:

• Common Ancestor

• Timeline features various hominids:

  • H. rudolfensis

  • Kenyanthropus platyops

  • H. antecessor

  • H. ergaster

Speciation Process

• Definition of Speciation

  • Process by which one species splits into two or more species.

  • Outcomes include shared characteristics due to descent with modification.

  • Links microevolutionary processes with macroevolutionary outcomes.

Defining a Species

• Concepts of Species

  1. Biological Species Concept

  2. Morphological Species Concept

  3. Ecological Species Concept

  4. Phylogenetic Species Concept

Biological Species Concept (BSC)

• Definition

  • A species is defined as a group of populations whose members can:

  • Interbreed in nature

  • Produce viable and fertile offspring

  • Have reproductive isolation from other groups.

Reproductive Isolation: BSC

• Definition

  • Biological barriers that block successful interbreeding between species.

• Types of Barriers:

  • Prezygotic Barriers

  • Prevent fertilization from occurring.

  • Types include:

    • Habitat Isolation: Different habitats prevent encounters.

    • Temporal Isolation: Different breeding times.

    • Behavioral Isolation: Different courtship practices.

    • Mechanical Isolation: Incompatible reproductive structures.

    • Gametic Isolation: Sperm and egg incompatibility.

  • Postzygotic Barriers

  • Fertilization occurs but with reduced offspring fitness.

  • Types include:

    • Reduced Hybrid Viability: Hybrids do not develop properly.

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

    • Hybrid Breakdown: Offspring of hybrids are less viable or sterile.

Take-home for Biological Species Concept

• Speciation is linked to the evolution of reproductive isolation mechanisms.

Limitations of BSC

• Challenges in evaluating reproductive isolation in fossils.

• Inapplicability to asexual organisms (like prokaryotes).

• Certain distinct species can have gene flow despite differences in morphology and ecology.

Morphological Species Concept

• Definition

  • Distinguishes species based on body shape and other structural features.

• Advantages

  • Applicable to both sexual and asexual organisms.

• Disadvantages

  • Criteria for what defines structural features are subjective.

Ecological Species Concept

• Definition

  • Defines a species based on its ecological niche.

• Advantages

  • Emphasizes the role of environmental selection.

• Disadvantages

  • Ignores gene flow between species in different environments.

Phylogenetic Species Concept

• Definition

  • Defines a species as the smallest group that shares a common ancestor.

• Advantages

  • Utilizes morphological and molecular data for classification.

• Disadvantages

  • Determining the degree of difference required to classify species can be subjective.

Ecological Speciation

• Concept: Discussed in relation to the metaphor comparing species to amoebae, emphasizing reproduction and division: "Species are like amoebae - they multiply by splitting" (E.O. Wilson).

Types of Speciation

• Allopatric Speciation

  • Geographically isolated populations split into subpopulations.

  • Vicariance: Subpopulations formed through physical disruptions (e.g., volcanic activity, tectonic shifts).

  • Colonization/Dispersal: Subpopulations emerge when groups occupy previously uninhabited regions.

• Sympatric Speciation

  • Occurs when speciation happens within populations that share the same geographic area.

Tempo and Mode of Speciation

• Pace of Speciation

  • Can occur either rapidly or slowly.

• Punctuated Equilibrium

  • Characterized by rapid, dramatic changes in relatively short time spans, followed by long periods of stability.

• Phyletic Gradualism

  • Changes occur steadily and gradually over extended time periods.