Consider the recent phylogenetic tree of five different organisms.

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old classification systems based on physical characteristics would most likely disagree with the relationship between which two animals in the tree?

crocodiles and insects
bats and humans
birds and bats
humans and crocodiles

The right answer is C. birds and bats.

There are two types of classification, which present some differences, so they classify the animals differently:

Linnaean classification:

The species (biologically defined by inter-fertility) is the basis of the classification. It is stored in a system of boxes more or less predefined (at least when the outline of the classification are well established).

Reign comprising different branches themselves composed of Classes which group Orders formed of Families composed of Genres in which the species are placed.

If we find a new species (current or fossil), we try to place it in an existing box, taking into account all of its characters. If this is not possible, we create one (or a series of new boxes - new genre - new family ...).

The outline of the classification was drawn before the ideas of evolution, using states of character that a subsequent study recognized as derivatives, but also ancestral states. Groups of the same level (Class for example) can drift from each other.

Cladistic phylogenetic classification:

It is based on the phylogenetic tree. The defined groups are monophyletic: they include a hypothetical ancestor (a node of the tree) and the complete set of his descendants. If one group gives birth to another, it belongs to it and forms a subset.

The advantage is obvious: a perfect match with the most probable scenario of the evolution of the studied group. The disadvantage is the multiplication of nesting levels, already 10 for the vertebrate tree below which is however greatly simplified.