Phylogeny, Speciation, and Life

Overview of Phylogeny and Speciation

• Life originated on Earth over 3.5 billion years ago.

• Current estimate of approximately 10 million species on Earth.

• Key questions:

  • How do we define species and how do they evolve?

  • How can we understand the evolutionary history of Earth?

• Understanding achieved through:

  1. Nested patterns of similarities among extant species using phylogenies.

  2. Historical patterns of evolution seen through fossils.

Chapter Interrelationship

• Approaching Chapters 24 and 25 simultaneously:

  • Chapter 24: Discusses how species arise.

  • Chapter 25: Describes and explains evolutionary relationships between species.

  • Understanding one chapter requires knowledge of the other.

• Preparatory reading for the upcoming class: Finish Chapter 24 and read Bioskills 13; review Chapter 25.1 if time permits.

Speciation and Biological Classification

Chapter 24: Speciation

Opening Roadmap

• Learning objectives:

  • Understanding how speciation creates new branches in the tree of life.

  • Addressing the following key questions:

  • How is a species defined?

  • How do isolation and divergence of populations produce new species?

    • Allopatry (24.1)

    • Sympatry (24.2)

    • Outcomes when isolated populations make contact (24.4)

Binomial Nomenclature

• Definition:

  • The system for naming species, consisting of

  • Genus name and specific epithet.

  • Example: Odocoileus virginianus (Virginia deer).

Linnaean System of Classification

• Founded by Carl Linnaeus (1707-1778).

• Originally phenetic.

• System structure bonds life into a nested hierarchical system:

  1. Domain

  2. Kingdom

  3. Phylum

  4. Class

  5. Order

  6. Family

  7. Genus

  8. Species

• Mnemonic devices can assist in remembering categories.

Limitations of the Linnaean System

• Ranks are categorized arbitrarily – subjective nature of taxon breadth.

• Not based on evolutionary history unlike cladistics:

  • Example: Reptilia (reptiles) and Aves (birds) are classified separately under traditional Linnaean taxonomy; however, evolutionary relationships show that birds are indeed reptiles.

Defining Species

Characteristics of Species

• Closed gene pool; alleles shared among members but not usually with other species.

• Species can become extinct through genetic divergence lead to new species development.

• Local extinction is a concept worth exploring.

Speciation

• Definition: Splitting event leading to distinct species stemming from an ancestral species.

  • Can occur rapidly or gradually.

  • Involves:

  • Genetic isolation: Barriers prevent gene flow between populations.

  • Genetic divergence: Mutation, natural selection, and genetic drift affect each isolated population.

Defining a Species

• Taxonomy Questions: Is species definition based on arbitrary naming, or ability to exchange genetic material? The debate includes sexuality as a central aspect.

• Biological Species Concept (BSC):

  • Definition: Groups of interbreeding populations that are reproductively isolated from others.

  • Notable mechanism of reproductive isolation includes:

  1. Prezygotic Isolation: Preventing mating or fertilization.

  2. Postzygotic Isolation: Hybrid offspring do not survive or are sterile.

Mechanisms of Reproductive Isolation

BSC Complaints

• Challenges in testing BSC.

• Not applicable to asexual organisms or extinct species.

• Issues arise with ring species and hybridization.

Morphospecies Concept

• Definition: Species distinguished by morphological features.

• Strengths and Applicability:

  • Useful in absence of gene flow data.

  • Applicable to sexual, asexual, and fossil species.

• Limitations:

  • Polymorphisms and cryptic species complicate identification.

  • Subjectivity in morphological materials leading to inconsistencies.

Phylogenetic Species Concept

• Definition: Species identified by evolutionary history; smallest group with a common ancestor.

• Key points:

  • Monophyletic groups include an origin and all its descendants.

  • Applicable to all population types, including fossils and asexual populations.

  • May recognize more species compared to other concepts, reflecting life’s diversity.

Application of Species Concepts

• Researchers employ all three species concepts to define evolutionarily distinct populations.

• Conflicts may arise from differing perspectives on classification.

Additional Definitions of Species

• Ecological Species Concept (ESC): Based on ecological niche and resource usage.

Summary Table: Common Species Concepts

Speciation Process

• Definition: Development of reproductively isolated populations from genetic divergence.

• Mechanisms leading to divergence:

  • Mutations lead to genetic differences over time.

• Speciation occurs when populations can no longer produce viable offspring.

Types of Speciation

• Allopatric Speciation: Occurs through geographic separation.

• Sympatric Speciation: Occurs without geographic separation.

• Mechanisms:

  • External (e.g., disruptive selection based on ecology).

  • Internal (e.g., chromosomal mutations).

Allopatric Speciation

• Processes:

  • Dispersal: individuals migrating.

  • Vicariance: physical barriers such as mountains or rivers.

• Example: Genetic divergence through physical separation and habitat change (e.g., Amazon basin's geological division).

Sympatric Speciation

• Populations can interbreed but are economically impacted by niche differentiation or species-specific preferences leading to divergence.

• Instantaneous speciation through hybridization can lead to distinct reproductive isolation.

• Polyploidy, particularly in plants, plays a significant role in sympatric speciation.

Outcomes of Secondary Contact

• Processes Upon Fusion of Isolated Populations:

  • Fusion leading to interbreeding.

  • Extinction of one species.

  • Reinforcement of divergence through natural selection against hybrids.

  • Hybrid zone formations with geographic movements.

  • Formation of new species leading to new ecological niches.

Large-Scale Patterns in Life’s History

Adaptive Radiation

• Definition: Rapid emergence of many distinct species from a single ancestor due to diverse adaptations.

• Examples:

  • Hawaiian silverswords evolved from tarweed, showing monophyly, rapid speciation, and ecological diversity.

  • Analysis of factors prompting adaptive radiation includes extrinsic and intrinsic triggers.

Factors Triggering Adaptive Radiations

• Extrinsic Factors: Favorable new environmental conditions.

• Intrinsic Factors: Evolution of crucial morphological, physiological, or behavioral traits (such as the development of flowers or birds' flight adaptations).

Ecological Opportunity and its Role

• Defined as the availability of various resources for exploitation and diversification.

• As observed in Hawaiian islands where flowering plants were few, leading to ecological diversification by silverswords.

Conclusion: The Case of Galapagos Finches

• Overview of natural selection's role in species development among Galapagos finches, with multiple case studies illustrating allopatric speciation and adaptive radiation principles culminating in notable evolutionary patterns and adaptations.