Why does DNA similarity decrease over time after two species diverge?

Once two populations are reproductively isolated, they accumulate independent mutations. Over time, these unique genetic changes build up, leading to a decrease in overall sequence identity.

What is the primary difference between using physical features and molecular data to determine evolutionary relationships?

Physical features can be misleading due to convergent evolution (analogous structures), whereas molecular data (DNA/proteins) provides a more objective and quantitative measure of genetic divergence.

How does the amount of precipitate in an immunological test relate to evolutionary distance?

A larger amount of precipitate indicates a higher degree of protein similarity and thus a closer evolutionary relationship. Less precipitate means the antibodies found fewer matching sites, suggesting a more distant common ancestor.

Why might amino acid sequences be more useful than DNA sequences for comparing very distantly related species?

Amino acid sequences are often more conserved because changes in the protein can be lethal, whereas DNA sequences change more rapidly due to the degenerate nature of the genetic code and mutations in non-coding regions.

What is a common error when interpreting a phylogenetic tree's branch lengths?

A common error is assuming that the horizontal order of species at the tips indicates their relationship. Relationships are actually determined by the most recent common ancestor (the nodes), not the proximity of the names at the top.

Why is it incorrect to assume that two species with similar habitats are closely related?

Similar habitats can lead to convergent evolution, where unrelated species evolve similar physical adaptations (analogy) to survive, masking their true evolutionary distance.

What mistake is made when using a rapidly mutating gene to study the relationship between two different kingdoms?

Rapidly mutating genes accumulate too many changes over long periods, making it impossible to align the sequences accurately. For deep evolutionary time, highly conserved genes (like rRNA) must be used.

Define 'Genome Sequencing' in the context of phylogeny.

It is the process of determining the exact order of nucleotides in an organism's DNA, which allows for direct comparison of genetic codes between species to identify evolutionary divergence.

What is Cytochrome c and why is it significant in evolutionary biology?

Cytochrome c is a highly conserved protein involved in the electron transport chain of respiration. Because it is found in almost all eukaryotes, it serves as a universal standard for comparing evolutionary relationships.

Define a 'Phylogenetic Tree'.

A branching diagram that represents a hypothesis about the evolutionary relationships among various biological species based upon similarities and differences in their physical or genetic characteristics.

Evolutionary Relationships | AQA AS-Level Biology

Genetics, Variation & Interdependence

Evolutionary Relationships

Summary

Evolutionary relationships describe the ancestral connections between different species, determined by the time elapsed since they shared a common ancestor. Modern biology utilizes molecular evidence, such as DNA sequencing and immunological testing, to provide a more accurate map of these relationships than physical characteristics alone.

1. Definition & Core Concepts

Phylogeny is the study of the evolutionary history and relationships among individuals or groups of organisms. It seeks to determine how species have diverged from common ancestors over geological time.
Common Ancestry refers to the principle that different species descended from a single ancestral population. The more recently two species shared a common ancestor, the more closely related they are considered to be.
Divergence occurs when populations of a single species become isolated and accumulate enough genetic differences to become distinct species. This process is the foundation of the 'tree of life' model.

A phylogenetic tree showing the divergence of three species from common ancestors over time.

2. Underlying Principles

The Molecular Clock hypothesis suggests that mutations in DNA and proteins accumulate at a relatively constant rate over time. By counting these differences, scientists can estimate the time since two species diverged.
Genetic Similarity is directly proportional to evolutionary relatedness. Species with highly similar DNA sequences are assumed to have separated more recently, leaving less time for independent mutations to occur.
Conserved Proteins are essential proteins that change very little over millions of years. Cytochrome c, a protein involved in cellular respiration, is a primary example used for comparison because it is present in almost all eukaryotic organisms.

3. Methods & Techniques

Genome Sequencing

DNA Comparison: Scientists sequence specific regions of the genome to identify substitutions, deletions, or insertions. High similarity in the order of bases indicates a close relationship.
Amino Acid Sequencing: Comparing the primary structure of proteins is often more stable than DNA comparison. Because the genetic code is degenerate, multiple DNA sequences can code for the same protein, making protein sequences a 'filtered' view of essential changes.

Immunological Comparison

Antibody-Antigen Interaction: This method involves injecting a protein (like albumin) from Species A into a rabbit to produce specific antibodies. These antibodies are then mixed with the same protein from Species B.
Precipitate Formation: The antibodies will bind to the protein of Species B if its structure is similar to Species A. The amount of precipitate (solid complex) formed is a quantitative measure of similarity; more precipitate equals closer relatedness.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions