Macroevolution

1. What is Macroevolution?

Macroevolution refers to large-scale evolutionary changes that occur over long periods of time, resulting in the emergence of new species, genera, families, and other higher taxa.

📌 Key Difference from Microevolution

• Microevolution = Small genetic changes within a species (e.g., allele frequency shifts).

• Macroevolution = Evolutionary processes that lead to speciation and large evolutionary transitions.

2. Modern Biological Nomenclature

• Linnaean Classification System (1758)

  • Established a hierarchy for organizing species.

  • Still used today.

• Rules for Species Naming

  • Genus (Capitalized, Latin noun) → Unique within each kingdom.

  • Species (Lowercase) → No longer required to be a Latin adjective.

  • You cannot name a species after yourself (ICZN rules).

• Examples of Species Naming

  • Canis familiaris (Dog)

  • Aptostichus stephencolbertii (Spider named after Stephen Colbert)

  • Carmenelectra shechisme (Beetle named after Carmen Electra)

3. What is a Species?

There are multiple definitions of what constitutes a species, each useful in different contexts.

1. Morphological Species Concept (MSC)

• Definition: A species is a group of individuals that are morphologically distinct from others.

• Process of Classification:

  1. Collect many specimens to capture variation.

  2. Identify distinct, consistent traits.

  3. Group similar individuals based on shared characteristics.

  4. Name and formally describe the species.

✅ Strengths

• Can be applied to living, extinct, and asexual organisms.
  ❌ Weaknesses

• Arbitrary trait selection.

• May fail to identify cryptic species.

• May split or lump species incorrectly.

2. Biological Species Concept (BSC)

• Definition: A species consists of interbreeding populations that produce fertile offspring and are reproductively isolated from others.

• Introduced by Ernst Mayr (1942).

• Example:

  • Horse + Donkey = Mule (sterile) → Not a species.

  • Lion + Tiger = Liger

    • Female Liger + Male Lion = Fertile Offspring

    • Female Liger + Male Tiger = Sterile

    • Is this a species? 🤔

✅ Strengths

• Focuses on genetic relationships.
  ❌ Weaknesses

• Doesn’t apply to asexual organisms.

• Difficult to test in fossils.

3. Recognition Species Concept (RSC)

• Definition: A species is a group of organisms that recognize each other as mates.

• Example:

  • Crickets → Each species has a unique mating song.

  • Females only mate with males singing their species’ song.

4. Ecological Species Concept (ESC)

• Definition: A species is a group of organisms that exploit a single ecological niche.

• Niche = Role of a species in its environment (e.g., food, habitat, predators).

• Example:

  • Ascaris lumbricoides (Human roundworm) vs.

  • Ascaris suum (Pig roundworm)

  • Morphologically similar, but occupy different hosts.

📌 Key Takeaway: Different species definitions can lead to different classifications!

• Example: The plant Cotoneaster has 70–300 species depending on the definition used.

4. Evolutionary Classification

1. Evolutionary Systematics vs. Cladistics

2. Homologies vs. Analogies

• Homologies: Traits inherited from a common ancestor.

  • Example: Stereoscopic vision in primates.

• Analogies: Similar traits due to convergent evolution (not common ancestry).

  • Example: Wings in bats and birds.

• Homoplasy: Independent evolution of similar traits.

5. Phylogenetic Trees & Cladograms

• Phylogenetic Tree: Shows evolutionary relationships based on ancestor-descendant lineage.

• Cladogram:

  • Based only on shared derived traits.

  • No time component or assumption of ancestry.

• Types of Clades:

  • Monophyletic: Includes all descendants of a common ancestor.

  • Paraphyletic: Includes some, but not all descendants.

  • Polyphyletic: Includes organisms from different ancestors.

6. The Process of Speciation

For new species to evolve, gene flow must be disrupted.

Three Steps of Speciation:

1. Differentiation:

   • Individuals start to diverge in survival & reproduction traits.

2. Reproductive Isolation:

   • Something prevents mating between different groups.

3. Reinforcement:

   • Mechanisms strengthen isolation to prevent interbreeding.

7. Modes of Speciation

8. Adaptive Radiation

• Definition: Rapid diversification of species to fill new ecological niches.

• Often occurs after mass extinctions.

• Example: Mammals diversified after dinosaurs went extinct.

• Generalized traits → Specialized traits.

9. Gradualism vs. Punctuated Equilibrium

• Phyletic Gradualism: Slow, continuous evolution.

• Punctuated Equilibrium: Evolution happens in rapid bursts followed by long stability.
  📌 Fossil discoveries can shift perceptions of these models.

10. Summary: Key Concepts of Macroevolution