In-Depth Notes on Biological Species Concept and Speciation

Key Concepts of Biological Species Concept

• The Biological Species Concept evaluates species based on reproduction and gene flow.
  • It emphasizes the absence of gene flow between species, yet gene flow can occur between distinct ones.

Limitations of Biological Species Concept

• Fossils: Cannot apply to extinct organisms or fossils.
• Case of Neanderthals and Humans:
  • Homo sapiens sapiens (modern humans) vs. Homo neanderthalensis (Neanderthals) can interbreed, creating hybrids, indicating shared ancestry but also differences in classifications.
  • Genetic testing shows a percentage of modern human DNA is from Neanderthals, hinting at gene flow.

Other Species Concepts

• Morphological Species Concept:

  • Defines species by structural features. It applies to sexual and asexual species but is somewhat subjective due to the ambiguity in choosing key traits.

• Ecological Species Concept:

  • Focuses on the ecological niche and disruptive selection, where the average trait is selected against, pushing populations to extremes, leading to speciationover time.

• Phylogenetic Species Concept:

  • Defines a species as the smallest group on a phylogenetic tree.

Types of Speciation

• Allopatric Speciation:
  • Occurs due to geographic separation leading to reproductive isolation.
  • Example: Flightless cormorant evolved due to isolation.
• Sympatric Speciation:
  • Occurs without geographic barriers, often due to behavioral differences or niche adaptation.
  • Example: Fish in the same lake evolving into separate species due to differing habitats.

Mechanisms of Speciation

• **Allopatric Mechanisms:
  • Separation through geological barriers like mountains or canyons results in independent evolution.
• Examples of barriers:
  • Isthmus of Panama: Sparked division of species that evolved separately for millions of years.
  • Grand Canyon: Presents barriers leading to new species of squirrels.

Factors Leading to Reproductive Isolation

• Natural Selection, Genetic Drift, and Mutations can change separated populations, isolating them through genetic differences, resulting in speciation.
• Hybridization does not always result in infertility or lack of viability, allowing for complex species relationships, such as polar and brown bears producing hybrids under certain environmental conditions.

Sympatric and Polyploidy

• Polyploidy is common in plants, resulting in different chromosome numbers and leading to speciation without geographical barriers, unlike animals where polyploidy can often result in defects.
• Example: The yellow delicious apple is a clone as it does not reproduce sexually, being sterile.

Reinforcement, Fusion, and Stability in Speciation

• Reinforcement: Strengthening barriers between species, ensuring hybrids are less fit than parents, further separating species.
• Fusion: Weak reproductive barriers can lead to hybridization and merging of species.
• Stability: Hybrid individuals continue to form without significant genetic divergence or reproductive isolation, allowing for ongoing interbreeding.

Time Course of Speciation

• Speciation can occur rapidly or slowly, influenced by genetic changes across few or many genes.
• Punctuated Equilibrium: Suggests sudden appearances of new species over short geological time frames, followed by long periods of stability.