Level 3 NCA Biology: Speciation Lecture

Introduction

• Lecturer: Emma Campbell, Biology Teacher at Wellington Girls College
• Session Focus: Evolutionary processes leading to speciation
• Context: Important to understand concepts within various contexts, as questions will be applied to unfamiliar species.

Key Concepts in Speciation

• Species Definition: A group of organisms that interbreed to produce fertile offspring. Complex with exceptions (e.g., hybrids like mules).
• Mechanisms of Evolution: Factors affecting allele frequency.

Mechanisms of Evolution

1. Mutation: Only source of new genetic variation.
2. Gene Flow: Movement of alleles between populations.
3. Genetic Drift: Random changes in allele frequencies, significant in small populations.
   • Bottleneck Effect: Large population reduction.
   • Founder Effect: New population establishment.
4. Natural Selection: Survival and reproduction of the fittest.
   • Types of Selection:
     • Stabilizing: Favors median traits.
     • Directional: Favors one extreme trait.
     • Disruptive: Favors extreme traits at both ends.

Speciation Types

• Allopatric Speciation: Geographic separation leads to speciation (e.g., Southern Alps).
• Sympatric Speciation: Speciation without geographic isolation.

Here are the speciation effects

• Allopatric speciation: This occurs when two populations become geographically separated, preventing gene flow between them. This can happen due to events like the rise of mountains, formation of a river, or separation of landmasses.

• Sympatric speciation: This occurs when two populations become reproductively isolated even though they live in the same geographic area. This can happen due to factors like ecological differences, temporal differences in breeding seasons, behavioral differences in courtship rituals, or gametic incompatibility.

• Polyploidy: This is a type of speciation that can happen instantly, within a single generation. It involves a change in the number of chromosomes in a cell, leading to a new species that is unable to reproduce with its parent species. There are two types of polyploidy: autopolyploidy, which occurs within a single species, and allopolyploidy, which occurs between two different species.

• Instant speciation: This refers to speciation that can occur quickly, such as through polyploidy.

• Divergent evolution: This is a pattern of evolution where species diverge from a common ancestor, leading to the formation of new species.

• Adaptive radiation: This is a type of divergent evolution where a single ancestral species rapidly diversifies into many new species, often filling a variety of ecological niches.

• Convergent evolution: This is a pattern of evolution where unrelated species evolve similar traits due to similar environmental pressures.

• Parallel evolution: This is a pattern of evolution where related species evolve similar traits independently of each other.

• Co-evolution: This is a pattern of evolution where two or more species influence each other's evolution.

Reproductive Isolating Mechanisms

• Pre-Zygotic: Prevent mating or fertilization (geographical, structural, ecological, temporal, behavioral, gametic).
• Post-Zygotic: Impact after fertilization (hybrid inviability, sterility, breakdown).

The difference between prezygotic and postzygotic

Prezygotic Isolating Mechanisms:

These mechanisms act before a zygote (the fertilized egg) is formed. They prevent mating or hinder fertilization from happening in the first place. Think of them as "pre-mating" barriers.

• Habitat Isolation: Species live in different habitats, even if they are in the same geographic area. Think of a forest frog and a meadow frog.
• Temporal Isolation: Species breed during different times of the day or year. For example, one species of flowers might release pollen in the morning, while another releases pollen in the evening.
• Behavioral Isolation: Species have different courtship rituals or signals that prevent them from recognizing each other as potential mates. Think of birds with different mating calls or dances.
• Mechanical Isolation: Physical incompatibility between reproductive structures. For instance, the shape of the reproductive organs might not match up.
• Gametic Isolation: Sperm and egg cells are incompatible even if they meet, meaning fertilization can't occur.

Postzygotic Isolating Mechanisms:

These mechanisms act after a zygote is formed. They often result in the hybrid offspring being less viable, fertile, or both. Think of them as "post-mating" barriers.

• Reduced Hybrid Viability: The hybrid offspring dies or is unable to develop properly.
• Reduced Hybrid Fertility: The hybrid offspring survives but is unable to reproduce. This is the case with the mule (donkey and horse cross).
• Hybrid Breakdown: First-generation hybrids are fertile but subsequent generations lose fertility or viability.

Why Are These Mechanisms Important?

These isolating mechanisms are crucial for speciation because they reduce or prevent gene flow between populations. Over time, as populations become increasingly isolated, they can diverge genetically and eventually become distinct species.

Exam Question Example: Eel Speciation

• Longfin and Shortfin Eels: Geographically and temporally isolated.
• Speciation Process: Natural selection, variation, no gene flow.
• Types: Allopatric and temporal reproductive isolation.

Instant Speciation

• Polyploidy: New species form in one generation.
  • Autopolyploidy: Within one species.
  • Allopolyploidy: Between two species, creating fertile hybrids.

Patterns of Evolution

1. Divergent Evolution: From one ancestor to many species.
2. Adaptive Radiation: Rapid evolution to fill various niches.
3. Convergent Evolution: Unrelated species evolve similar traits.
4. Parallel Evolution: Similar traits in related species, evolved independently.
5. Co-evolution: Reciprocal evolutionary changes between interacting species.

Evolutionary Rates

• Gradualism: Slow, consistent changes.
• Punctuated Equilibrium: Long periods of stability interrupted by rapid changes.

Evidence of Evolution

• Fossils: Historical records.
• Comparative Anatomy: Homologous and analogous structures, vestigial organs.
• Biogeography: Geographic distribution of species.
• Genetic Analysis: DNA comparisons reveal genetic relationships and variations.

Conclusion

• Study Approach: Practice applying concepts to various contexts and exam questions.
• Resources: National Geographic, scientific publications, biodiversity in New Zealand.

Additional Resources

• Study It: A website for students to ask questions and get help from expert teachers.
• National Geographic: For exploring examples of speciation and evolution.