Task: Text Simplification
Input (English): The earliest clear statement of how chromosomes may spread in a population not because of their positive fitness effects on the individual organism , but because of their own " parasitic " nature came from the Swedish botanist and cytogeneticist Gunnar O ̈ stergren in 1945 .
Output (English): The Swedish botanist and cytogeneticist Gunnar O ̈ stergren in 1945 noted how chromosomes may spread in a population because of their own ” parasitic ” nature .