Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
A clade is a group of organisms that have evolved from a common ancestor, comprising the ancestor and all its descendants. Clades are determined using base sequences of a gene ( or ) or the amino acid sequence of a protein.
The 'molecular clock' hypothesis states that mutations in sequences occur at a relatively constant rate. Therefore, the number of differences in the sequence of a particular gene between two species can be used to estimate the time since they diverged from a common ancestor.
Traits are classified as either homologous or analogous. Homologous structures (e.g., the pentadactyl limb) are similar because of common ancestry, whereas analogous structures (e.g., wings in birds vs. insects) are similar because of convergent evolution to similar selection pressures.
Cladograms are tree diagrams that show the most probable sequence of divergence in clades. They are typically constructed using the principle of parsimony, which favors the simplest explanation with the fewest evolutionary changes.
Cladistics has led to the reclassification of several groups. For example, the Figwort family (Scrophulariaceae) was found to be polyphyletic through sequence analysis and was subsequently split into five different clades.
In a cladogram, a node represents a speciation event where a common ancestor splits into two or more descendant species.
📐Formulae
where is the number of nucleotide differences and is the total sequence length.
💡Examples
Problem 1:
Two species of primates, Species A and Species B, show a difference in a specific non-coding sequence. If the known mutation rate for this sequence is every million years, calculate the estimated time since these two species shared a common ancestor.
Solution:
The species have a total difference. This means each lineage has accumulated change since the point of divergence. Using the rate of per million years: million years. Alternatively, using the total divergence: years.
Explanation:
Because the mutation rate applies to both lineages simultaneously after they split, the total genetic distance is the sum of mutations in both branches. Therefore, we divide the total difference by the rate relative to the shared time .
Problem 2:
Identify whether the following is homologous or analogous: The streamlined body shape of a Shark (fish) and a Dolphin (mammal).
Solution:
Analogous structures resulting from convergent evolution.
Explanation:
Sharks and dolphins do not share a recent common ancestor with a streamlined shape; rather, they evolved similar shapes independently to reduce drag while swimming in an aquatic environment ( levels and water density provide similar selection pressures).