Based on the above phylogeny and the distribution of red and blue character states among the extant species, and using the principle of parsimony, which of the following is the correct inference about plumage colour of the ancestor at the root P?
(1) Ancestral state at P is blue.
(2) Ancestral state at P is red.
(3) Ancestral state at P is more likely to be red than blue
(4) Ancestral state at P is equally likely to be red or blue.

Using Parsimony to Infer Ancestral Plumage Color in Birds: Red or Blue at the Root?

Understanding the evolutionary origins of traits, such as plumage color in birds, is a central goal in phylogenetics. When faced with a phylogenetic tree displaying the distribution of red and blue plumage among current bird species, scientists use the principle of parsimony to infer the most likely ancestral state at the root of the tree.

What Is the Principle of Parsimony?

The parsimony principle in evolutionary biology states that the simplest explanation requiring the fewest evolutionary changes is preferred when reconstructing ancestral traits or relationships. In phylogenetic analysis, this means that the scenario with the least number of trait gains or losses is considered the most likely.

Applying Parsimony to Plumage Color

Given a phylogenetic tree with nine extant bird species, each displaying either red or blue plumage, the goal is to deduce the most likely color of the ancestor at the root (P).

• If red and blue are distributed such that grouping all red species together or all blue species together would require fewer changes, parsimony favors that color as ancestral.

• If both red and blue are scattered in such a way that either state would require the same minimum number of changes, then both are equally likely under parsimony.

When Are Ancestral States Equally Likely?

According to the parsimony principle, if the distribution of character states (red and blue) among the species is such that assigning either color to the root requires the same number of evolutionary transitions (from red to blue or vice versa), then parsimony cannot favor one state over the other. This situation often arises when the trait is distributed in a balanced or alternating pattern among the tree’s branches.

Conclusion: What Does Parsimony Say About the Ancestral State?

Based on the principle of parsimony, if the number of evolutionary changes required is the same whether the root is red or blue, the ancestral state at P is equally likely to be red or blue. This is a direct application of the parsimony criterion, which seeks the scenario with the fewest changes but does not arbitrarily prefer one trait over another when both require the same number of steps.

Correct answer: (4) Ancestral state at P is equally likely to be red or blue.