Understanding Phylogenetic Trees and Speciation

Posted on Oct 28, 2024 in Biology

Phylogenetic Trees and Speciation

Understanding Phylogenetic Trees

A phylogenetic tree visually represents evolutionary relationships. Key components include:

  • X-axis: Represents time.
  • Red dots: Indicate new emerging traits within a species.
  • Node: Marks the branch point where two species diverge (a splitting event).
  • Lineage (orange): Shows a series of ancestor and descendant populations.
  • Taxon: Any named group of organisms.
  • Clade: A taxon including all evolutionary descendants of a common ancestor.

Types of Groups

  • Paraphyletic: A group that doesn’t include all descendants of a common ancestor.
  • Polyphyletic: A group that doesn’t include its common ancestor.
  • Monophyletic (Clade): Includes an ancestor and all its descendants. A true monophyletic group can be removed from a phylogenetic tree with a single “cut,” representing a speciation event (for a tree of species) or a gene duplication/divergence event (for a tree of genes).

Character and Trait Evolution

  • Character: A general structure or feature with different forms between species.
  • Character State: The specific condition of a character, used to analyze evolutionary relationships.
  • Ancestral Trait: The older version of a character.
  • Derived Trait: The changed form of a character over time.
  • Synapomorphies: Shared derived traits, evidence of shared ancestry (often homologous structures).
  • Homologous Structures: Physical features shared by two or more species due to common ancestry (all homologous structures are synapomorphies).

Evolutionary Processes

  • Convergent Evolution: Similar traits arise due to similar environmental pressures, not shared ancestry.
  • Evolutionary Reversal: A derived trait reverts to an ancestral state.

The strongest hypothesis minimizes homoplasies (convergent evolution and reversals) and maximizes synapomorphies.

Adaptive Radiation

Adaptive radiation involves rapid speciation driven by diverse environments and new adaptations.

Reproductive Isolation and Hybridization

Reproductive isolation reinforces species separation. Incomplete isolation can lead to hybridization, where two different species reproduce. Hybridization is rare in the wild, even between closely related species.

Mechanisms Preventing Hybridization

  • Prezygotic (before zygote formation):
    • Mechanical Isolation: Differences in reproductive organ shapes prevent mating.
    • Temporal Isolation: Species breed at different times.
    • Behavioral Isolation: Mating behaviors differ between species.
  • Postzygotic (after zygote formation): Prevent hybrid viability or fertility.

Species Concepts

  • Morphological: Based on physical appearance. Limitations include intraspecific polymorphism (variation within a species) and cryptic species (similar-looking but distinct species).
  • Biological: Based on interbreeding capability. Limitations include exclusion of asexual organisms and its focus on a single point in time.
  • Lineage: Based on shared ancestry. Limitation: Life constantly evolves, and speciation can be slow.

Speciation

Species: Groups of organisms sharing genetic and morphological traits, producing fertile offspring, and reproductively isolated from other species.

Speciation: Divergence of a lineage into reproductively isolated lineages.

Dobzhansky-Muller Model

Explains how genetic incompatibility can lead to speciation:

  1. A population splits.
  2. Each group evolves different, incompatible alleles.
  3. These incompatible alleles prevent successful reproduction between the groups.

Types of Speciation

  • Allopatric: Physical barriers (e.g., glaciers, continents drifting) separate populations, leading to speciation.
  • Sympatric: Speciation occurs without physical isolation, often due to disruptive selection or polyploidy (genome duplication).

Examples of Speciation

  • Allopatric: Glacier formation separating species.
  • Sympatric: Disruptive selection or polyploidy in plants.

Other Isolation Mechanisms

  • Habitat Isolation: Species live in different habitats.
  • Gametic Isolation: Sperm and egg from different species are incompatible (common in aquatic species).
  • Postzygotic Isolation: Hybrids are less viable or infertile.

Natural selection favors prezygotic mechanisms to prevent the costly production of less fit hybrids.