"The varieties of man seem to act on each other in the same way as different species of animals - the stronger always extirpating the weaker. " Charles Darwin, Voyage of the Beagle.
Not a very cheery passage. Would be interesting to see what he means by weaker and stronger. Scientists don't typically think weaker or stronger but more fit (able to produce relatively more young) and less fit and this may have nothing to do with weakness or strength but something like disease resistance, a pheromone to attract mates, ability to metabolize a particular food, and so on.