The phylogenetic tree depicts the relationship between 5 species of snakes(labelled A to E) and provides information about their habitat specialization. Given the principle of parsimony (least number of evolutionary changes required)and that ancestor Y was terrestrial, which one of the options given is correct?
Analysis using the Principle of Parsimony
The principle of parsimony requires determining the habitat state for the internal node X that minimizes the total number of evolutionary changes (habitat switches) across the tree, given the habitats of the tips and the root ancestor Y (Terrestrial).
Given Habitats:
- Tips: A (Aquatic), B (Terrestrial), C (Aquatic), D (Aquatic), E (Terrestrial)
- Root: Y (Terrestrial)
Step 1: State of Node Ancestor(C, D)
Since C and D are both Aquatic, their common ancestor must be Aquatic to require 0 change in that branch.
Step 2: State of Node Ancestor(A, B)
A (Aquatic) and B (Terrestrial) differ. The ancestor requires 1 change regardless of its state (T requires T $\to$ A change for A; A requires A $\to$ T change for B). Minimum changes = 1.
Step 3: Compare Hypotheses for Node X
Node X is the ancestor of E, and the ancestral states leading to A/B and C/D. We evaluate the total changes required for the entire tree, starting from Y (Terrestrial).
Hypothesis 1: X is Terrestrial (T)
Changes needed:
- Y (T) $\to$ X (T): 0 change.
- X (T) $\to$ Ancestor(C, D) (A): 1 change (T $\to$ A).
- Changes within Clade (A, B): 1 change (minimum).
- Y (T) $\to$ E (T): 0 change (E branches off Y below X).
Total Minimum Changes = $0 + 1 + 1 + 0 = 2$
Hypothesis 2: X is Aquatic (A)
Changes needed:
- Y (T) $\to$ X (A): 1 change (T $\to$ A).
- X (A) $\to$ Ancestor(C, D) (A): 0 changes.
- Changes within Clade (A, B): 1 change (minimum).
- Y (T) $\to$ E (T): 0 change.
Total Minimum Changes = $1 + 0 + 1 + 0 = 2$
Since both hypothetical states for Node X yield the same minimum number of required evolutionary changes (2 changes), the principle of parsimony cannot distinguish between the two states.
Conclusion
X was equally likely to be aquatic or terrestrial.
The correct option is Option 3.
Important Questions from Phylogenetic trees
- Newts produce tetrodotoxin (TTX), a poison that targets sodium ion channels, for defense against their predators, garter snakes. Several species of garter snakes are resistant to newt TTX due to specific known amino acid substitutions in garter snake sodium channels. Which one of the following is best-suited to identify whether this adaptation in garter snakes has a single origin or multiple independent origins?
A phylogenetic tree representing the evolutionary relationship between various vertebrates is shown below.
Given this tree topology, which one or more of the following statements is/are correct?A phylogenetic tree for the evolution of two pigmentation traits in species of fish is shown for clades X, Y and Z. Genes A and/or B, if mutated, can cause dark pigmentation in the body.
Which one or more of the following statements is/are correct?- Whales and dolphins are hypothesized to have evolved along the northern shore of the Tethys Sea, prior to the Indian plate's collision with the Eurasian plate. To which one of the following animals are these aquatic mammals most closely related?
In the phylogenetic trees shown, the tips represent different species of geckos(labeled A to E) and the areas to which they belong. Which one of these is most consistent with the hypothesis that the geckos colonized the Western Ghats from Northeast India through the Eastern Ghats?
