Question 1

What is the difference between homologous and analogous structures?

Accepted Answer

Homologous structures are similar because they evolved from a common ancestor, even if they have different functions. For example, the pentadactyl limb in mammals (human arm, whale flipper, bat wing) has the same bone arrangement. Analogous structures have similar functions but different evolutionary origins, like the wings of birds and insects—they both allow flight but evolved independently (convergent evolution).

Question 2

How do scientists use molecular evidence to classify organisms?

Accepted Answer

Scientists compare DNA sequences or amino acid sequences of proteins (e.g., cytochrome c, rRNA). The more similar the sequences, the more closely related the organisms are. This is because mutations accumulate over time; species that diverged recently have fewer differences. Molecular data can also be used to build phylogenetic trees and estimate when species split (molecular clock).

Question 3

What is the three-domain system and why was it introduced?

Accepted Answer

The three-domain system, proposed by Carl Woese in 1990, divides life into Bacteria, Archaea, and Eukarya. It was introduced after molecular analysis (16S rRNA sequencing) showed that Archaea are genetically distinct from Bacteria, despite both being prokaryotes. Archaea have unique cell membrane lipids and ribosome structures, and many live in extreme environments. This system replaced the older five-kingdom system.

Question 4

How does natural selection lead to evolution?

Accepted Answer

Natural selection is the process where individuals with advantageous traits are more likely to survive and reproduce, passing those traits to offspring. Over generations, this increases the frequency of beneficial alleles in a population. If populations become isolated, they may diverge enough to form new species (speciation). Key steps: variation, overproduction, competition, differential survival, and reproduction.

Question 5

What is a phylogenetic tree and how do you read it?

Accepted Answer

A phylogenetic tree (cladogram) is a diagram showing evolutionary relationships among species. The branching points (nodes) represent common ancestors. Species that share a node are more closely related. The length of branches can indicate time or genetic change. To read it, start at the root (common ancestor of all) and follow branches to the tips (extant species). Sister groups are two lineages that split from the same node.

Question 6

Why is classification important in biology?

Accepted Answer

Classification organises the diversity of life, making it easier to study and communicate about organisms. It helps predict characteristics (e.g., if a new species is placed in a known genus, it likely shares traits with other members). It also aids in understanding evolutionary relationships, conservation (identifying endangered species), and medicine (tracking pathogens). Without classification, biology would be chaotic.

Classification and evolution

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Related Topics in OCR A-Level Biology

Analysis

Biodiversity

Biological membranes

Biological molecules

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