Introduction to Phylogenetics

Phylogenetic Tree

  • A phylogenetic tree is a diagram that illustrates the evolutionary relationships between species (or groups like genes or populations).
    • It serves a similar purpose as a family tree but focuses on species and their common ancestry.

Key Terms

  • Node: A junction where two lineages diverge, representing a common ancestor.
  • Branch: A line that indicates the passage of genetic change over time in evolutionary history.
  • Tip (Leaf): The end of a branch that signifies a living or extinct species.
  • Root: The common ancestor of all species depicted in the tree.
  • Clade: A group encompassing an ancestor and all its descendants; also termed a monophyletic group.
  • Sister Taxa: Two species or groups with a shared immediate common ancestor.
  • Outgroup: A species or group outside the principal group, utilized for comparative analysis.

How to Read a Phylogenetic Tree

  • Time progresses from the root towards the tips (older to newer).
    • Species sharing a more recent common ancestor are considered more closely related.
    • The order of branching is crucial for indicating relationships rather than the arrangement of tips.
  • Important reminder: the branching pattern conveys genetic relationships; placement of the tips is not as significant.
  • All forms of life can be traced back to a common origin.

Methodology in Phylogenetics

  • Features (traits) of current organisms help researchers infer evolutionary connections:
    • DNA sequences
    • Physical structures: Such as bones, flowers, limbs.
    • Biochemical pathways
    • Developmental patterns

Key Assumption in Phylogenetics

  • The assumption underpinning phylogenetics posits that if two species share a trait, it is likely they inherited that trait from a common ancestor, indicating that this trait arose before their divergence.
    • The more traits that are shared between two species, the more closely related they are.
    • If species possess many derived (new) characteristics, they likely diverged more recently; if they share ancient traits, their common ancestor lived further back in time.

Building a Phylogenetic Tree

  1. Descent with Modification
    • This is the foundational principle of evolution.
      • Descent: All species have common ancestors.
      • Modification: Changes in traits over time (mutations) lead to the formation of new species with distinct features.
  2. Descendants as Identical Copies of Ancestors
    • Descendants inherit traits from ancestors and may modify those traits.
      • Over time, this leads to divergence, or new species evolving from a shared origin.
  3. Modification Equals Change in a Character
    • A character is any observable trait (e.g., limb count, flower type, DNA sequence).
    • A derived character state is the evolved version of a trait found in a descendant; the original version is termed the ancestral character state.
  4. Mosaics of Traits
    • Since traits evolve at different rates and times, species often exhibit a combination of:
      • Ancestral traits: Characteristics unchanged from the ancestor.
      • Derived traits: Traits modified from the ancestral state.
    • Example: A bird possesses a vertebrate backbone (ancestral) alongside feathers (derived).
  5. Focusing on Derived Traits
    • To construct a phylogenetic tree, scientists:
      • Identify derived character states.
      • Determine which species possess those traits.
    • These shared derived traits, known as synapomorphies, help group species that share a common ancestor.
  6. Step-by-Step Process in Tree Construction
    1. Select species for comparison.
    2. List traits (both ancestral and derived).
    3. Define trait polarity:
      • Identify which traits are ancestral (shared with the outgroup).
      • Determine which traits are derived (specific to certain lineages).
    4. Group species based on their shared derived traits.
    5. Create a branching diagram (the tree):
      • Each node signifies a common ancestor.
      • Each branch represents a lineage with a distinct set of traits.

Morphological Phylogeny

  • Morphological phylogeny focuses on reconstructing evolutionary relationships using physical traits.
Traits in Morphological Phylogeny
  • These traits can include:
    • Body structures: Such as limbs, bones, teeth, flowers.
    • Developmental patterns.
    • Presence or absence of specific organs.
    • Shape and size of different parts.
Advantages of Morphological Phylogeny
  • Especially beneficial when DNA data is unavailable (e.g., in fossil studies).
  • Facilitates the investigation of extinct species by comparing physical remnants.
  • Helps scientists visualize evolutionary trends (like the progression from aquatic to terrestrial organisms).
Limitations of Morphological Phylogeny
  • Convergent evolution can produce misleading details:
    • Example: The wings of birds and bats evolved independently.
  • Homoplasy (similarity not due to shared ancestry) may confuse analyses.
  • Morphological traits might be influenced by environmental factors, potentially obscuring true ancestry.

Identifying Homologous Characters

  • Homologous characters (or traits) are features shared by multiple species due to inheritance from a common ancestor.
Potential Problems in Identifying Homologous Characters
  • Homoplasy: Referred to as "false similarity"—similarity that appears to be homology but arises from factors other than shared ancestry.
    • Causes of homoplasy include:
    1. Convergent evolution.
    2. Parallel evolution.
    3. Evolutionary reversals.
Types of Characters
  • Apomorphy: A new or derived character state that has evolved from the ancestral form; represents a modification from the ancestral condition and can be unique to a species or shared across several.
  • Synapomorphy: A shared derived character, defined as an apomorphy found in multiple taxa inherited from their most recent common ancestor; a key instrument for constructing phylogenetic trees.
  • Plesiomorphy: An ancestral character state; a trait that existed in the common ancestor of a group. Its utility is limited in determining recent relationships.
  • Symplesiomorphy: A shared ancestral character state present in multiple taxa; it is a plesiomorphy found across taxa due to ancient trait inheritance rather than recent common ancestry.