Which of the following is a species?

a. a monophyletic group of beetles who share a common ancestor and can be distinguished from the most closely related group of beetles

b. a group of birds who are interbreeding with each other but are reproductively isolated from other groups

c. a metapopulation of salamanders who are linked by gene flow

d. a lineage of microbes adapted to a particular ecological niche

e. all of the above are considered species under at least one species concept

e. All of the above are considered species under at least one species concept.

Explanation:

All of the abovementioned conditions satisfy at least one condition of species definition. Further arguments are given below to support each statement,

a. A species is the monophyletic group of individuals who has a common ancestor which could be distinguished from the closest phylogenetic relative  based on conserved genes (16S in bacteria and 18S in eukaryotes). In the statement above, beetles satisfy this condition.

b. A species is the group of individuals that are capable of exchanging genes or can interbreed. In the above statement, birds who are interbreeding with each other must be a single species.

c. A metapopulation is generically defined as the group of populations who are separated by space but are the same species according to phylogenetic analysis. Thus, the metapopulation of salamanders who are linked by gene flow (gene migration) should be treated as one species.

d. The word "lineage" already conveys the message that these bacteria belong share sample place in the phylogenetic tree and they are capable to adapt the same environmental niche. Therefore, they should be considered as one species. This can be easily tested via 16S rRNA sequencing.

layer 3 is younger than layer 6.