Final answer:
A biologist determines the polarity of a character change by rooting the phylogenetic tree, typically with the use of an outgroup to infer ancestral versus derived traits.
Step-by-step explanation:
To determine the polarity of a character change, a biologist would need to root the phylogenetic tree, which is option d). Rooting the tree allows biologists to infer the direction of evolutionary changes, understand which traits are ancestral and which are derived, and thus establish the polarity of character state changes. Rooting is typically done using an outgroup, which is a species or group that is closely related to the group of interest but clearly diverged before members of the group. By comparing the characteristics of the outgroup with those of the ingroup (the group being studied), biologists can infer the ancestral states and thereby deduce the polarity of changes within the ingroup.
Maximum parsimony is a method used in phylogenetic analysis to minimize the total number of character-state changes, assuming that the simplest explanation or path that requires the fewest steps is preferred. This approach helps to create a phylogenetic tree that reflects the most likely sequences of ancestral and derived character states.
When constructing a phylogenetic tree from organisms with varying characteristics of their circulatory systems, the position of each organism on the tree would depend on their shared traits. Organisms with more primitive circulatory systems likely would appear towards the base, while those with more complex or derived systems might appear towards the right (extreme). The order from left to right typically reflects the series of branching events from the most ancestral to the most derived states, provided that the phylogeny is accurately rooted.