• Identify structural and molecular evidence for relatedness between species.
• Interpret phylogenetic trees to understand evolutionary relationships.

Introduction

Scientists determine how closely species are related by comparing their physical features and analysing genetic information. These lines of evidence can be used to reconstruct evolutionary pathways and build phylogenetic trees that show how species have diverged from common ancestors over time.

Key Concepts

Structural Morphology

• Homologous structures – body parts that have a similar underlying structure due to shared ancestry, even if they have different functions (e.g., human arm, bat wing, and whale flipper).

• Vestigial structures – reduced or non-functional remnants of organs or structures that were functional in ancestral species (e.g., human appendix or pelvic bones in whales).

Molecular Homology

• DNA sequences – closely related species have a higher percentage of similar nucleotide sequences in their DNA.

• Amino acid sequences – similarities in protein structure (amino acid sequences) reflect the level of relatedness and can be used to compare evolutionary distance.

Phylogenetic Trees

• Diagrams that show inferred evolutionary relationships between species or groups of species.

• Can be constructed using morphological data, molecular data, or a combination of both.

• Interpreting phylogenetic trees:

o Nodes represent a common ancestor where a lineage splits.

o Branches represent evolutionary lineages over time.

o Branch length may represent time passed or amount of genetic change (depending on the tree).