Final answer:
An out-group is a group that an individual or a set of organisms does not belong to, often used as a reference point in evolutionary biology to establish the evolutionary relationships among different species.
Therefore, the correct answer is: option "a reference group related to but not part of a group under study a population"
Step-by-step explanation:
An out-group is essential for establishing the polarity of character changes when assessing phylogenetic relationships between different species or groups of organisms.
It is best described as a group in which an individual does not belong, and may often be in competition with. In terms of evolutionary biology, an out-group is a group of organisms that diverged before the lineage containing the group(s) of interest, known as the in-group.
This is critical in studies of evolutionary relationships, as comparisons between out-groups and in-groups can help determine which characteristics are evolutionary modifications. For instance, when studying the evolutionary relationships (cladistics) within a group of organisms (such as mammals), a non-mammalian species like a reptile could serve as a suitable out-group because it split from the lineage leading to mammals earlier in evolutionary history.
By providing a reference point, biologists can infer the directionality of evolutionary changes and identify shared derived traits that signify evolutionary branch points among the organisms within a clade or monophyletic group.