This isn't just a principle of a party but a natural law. This is displayed in liberal politics also. Life is a fight using powers as a utility, hard and soft powers. Liberals tend to bank on using the powers of certain ideologies, many as vague as conservative ideologies, in order for those ideologies to survive. Some use the soft power of love which may prove sometimes to be fittest when that power gathers enough political support, some use the hard powers of war and competition which may seem fittest in some occasions. And of course, love and warfare is seen and used in both/all parties even when they don't seem to realize it, sometimes hypocritically on the surface but often when you look deep enough, you see the forces of natural law shining through and it's not just love and war against each other, it's just organisms which may be ideological, biological, etc competing in what some would view as a thermodynamic machine racing towards entropic neutrality.

But to say only conservatives employ 'survival of the fittest' is such a vast simplification.