Concept & Uses of Classification Systems - 2 | Biology for GCSE/IGCSE - Year 11 PDF Download

Reflecting Evolutionary Relationships: Extended

• Classification systems aim to reflect the evolutionary relationships between species.
• Traditional biological classification grouped organisms based on shared features.
  • Organisms with more similar features were considered more closely related.
• Scientists have faced challenges in determining evolutionary relationships using this method.
• Relying on physical features (such as color, shape, size) has limitations and can lead to incorrect classification of species.

Using DNA to Classify Organisms: Extended

• Organisms share features because they descend from a common ancestor.
• Example: All mammals have bodies covered in hair, feed their young from mammary glands, and have external ears (pinnas).
• Initially, organisms were classified using morphology (overall form and shape, e.g., wings or legs) and anatomy (detailed body structure determined by dissection).
• With technological advances, microscopes, biochemistry knowledge, and DNA sequencing allowed for a more scientific approach to classification.
• Studies of DNA sequences show that more similar base sequences indicate closer relationships and a more recent common ancestor.
• Thus, the DNA base sequences in mammals are more similar to those in other mammals than to those in other vertebrate groups.

DNA sequences can show how closely related different species are

• The sequences show that Brachinus armiger and Brachinus hirsutus are more closely related than any other species in the list, as their DNA sequences are identical except for the second-to-last base (B. armiger has a T in that position, whereas B. hirsutus has an A).
• Since DNA base sequences code for amino acid sequences in proteins, similarities in amino acid sequences can also be used to determine how closely related organisms are.