A3.2.4 Clades as groups of organisms with common ancestry and shared characteristics (HL) Notes

1. A clade is a group of organisms that includes:
   1. A common ancestor.
   2. All its descendants (living and extinct).
2. Clades can vary in size, from small groups with few species to vast ones encompassing millions.

How Are Clades Identified?

1. Molecular Evidence: The Genetic Blueprint

1. The strongest evidence comes from DNA base sequences and amino acid sequences in proteins.
2. The more similar the sequences, the more recent the common ancestor.
3. As time passes, mutations accumulate, leading to differences in sequences.

2. Morphological Evidence: Traits and Structures

1. In fossils or extinct organisms, DNA may not be available, so scientists rely on morphological traits.
2. Similarities in bone structure, reproductive organs, or other physical features may indicate shared ancestry.
3. While useful, this approach has limitations:
   1. Convergent Evolution: Similar traits evolve independently (e.g., wings in bats and birds).
   2. Trait Loss: Evolution can erase traits, obscuring relationships (e.g., whales lost hind limbs but retain vestigial pelvic bones).

3. Nested Clades: A Hierarchy of Relationships

1. Clades are hierarchical, with smaller groups nested inside larger ones.
2. This nested structure mirrors the branching process of evolution, where each split represents divergence from a common ancestor.

Importance of Clades in Classification

1. Clades reflect evolutionary history, making classification more accurate than traditional taxonomy.
2. Traits within a clade can be used to predict features of newly discovered organisms.
3. Provides a framework for studying divergence, speciation, and biodiversity.