The population that will be most likely to be reproductively isolated from population 2 would be population 6 because each new population has been formed by branching off from the previous one.
Reproductive isolation means when 2 groups in a species live close enough to interact with each other but yet cannot mate with each other. There are a lot of factors that can cause this reproductive isolation among species.
The factors responsible for reproductive isolation are also called barriers. They are of two types. First are the prezygotic factors that are present even before the formation of a new zygote. These can be ecological, temporal, behavioural etc
Post zygotic barriers are physical reasons by which a zygote is not formed. This means that the mating did take place but it didn't result in the formation of a new offspring. For example, the gametes of red and purple sea urchin are released in the water. They interact with each other but do not mate as they are not compatible.
In the above case, each population gives rise to new population and they are subsequently named. Population 2 arises from 1 and population 6 arises from 5.
Therefore, population 6 will be most likely to be reproductively isolated from population 2.
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