IB Biology Topic A3.2.4: Clades as Groups of Organisms with Common Ancestry and Shared Characteristics Notes

A Clade Is an Ancestor Plus All Its Descendants

1. A clade is a group that includes a common ancestor and all of its descendants, living and extinct.
2. Clades vary in size, from a few species to millions.

How Clades Are Identified

Molecular Evidence

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.

Morphological Evidence

1. For fossils or extinct organisms, DNA may be unavailable, so scientists use morphological traits.
2. Similarities in bone structure or other features can indicate shared ancestry.
3. This approach has limits:
   1. Convergent evolution: similar traits evolve independently, such as wings in bats and birds.
   2. Trait loss: evolution can erase traits, such as whales losing their hind limbs.

Nested Clades

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

Why Clades Matter in Classification

1. Clades reflect evolutionary history, making classification more accurate than traditional taxonomy.
2. They give a framework for studying divergence, speciation, and biodiversity.