Proportional representation is not necessary in a country where a two-party system has developed. This can be explained as follows:

Two-party system
A two-party system refers to a political system where two major political parties dominate the political landscape and win most of the votes and seats in the parliament. In such a system, the parties compete for the majority of votes and seats, and the party with the majority forms the government. This means that there is no need for proportional representation, as the winning party gets the majority of seats and forms the government.

First-past-post system
The first-past-post system is a voting system where voters cast their votes for a candidate, and the candidate with the most votes wins the election. This system is used in countries like the United States and the United Kingdom. In such a system, there is no need for proportional representation, as the candidate with the most votes wins the election.

Reserved constituencies
Reserved constituencies are constituencies that are reserved for a particular group, such as women or minorities. In such constituencies, the voters belonging to the reserved group vote for their representative. In such a system, proportional representation may be necessary to ensure that the representation of the reserved group is proportional to their population.

Fusion of presidential and parliamentary forms of government
In a fusion of presidential and parliamentary forms of government, the president is both the head of state and the head of government. The president is elected by the people, while the parliament is elected separately. In such a system, proportional representation may be necessary to ensure that the parliament is representative of the people.

Conclusion
In conclusion, proportional representation is not necessary in a country where a two-party system has developed. In such a system, the winning party gets the majority of seats and forms the government. However, in other systems, such as reserved constituencies or a fusion of presidential and parliamentary forms of government, proportional representation may be necessary to ensure that the representation is fair and proportional.