TASK DEFINITION: In this task, we ask you to rewrite a sentence in simple English without changing its general meaning. Essentially, you want to make the sentence easier to read by using simpler words, utilizing more straightforward sentence structures, and omitting non-essential information etc.
PROBLEM: 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 .

SOLUTION: The Swedish botanist and cytogeneticist Gunnar O stergren in 1945 noted how chromosomes may spread in a population because of their own " parasitic " nature .

PROBLEM: Keith Richards ( born 18 December 1943 ) is an English musician , singer , and songwriter , best known as the co-founder , guitarist , secondary vocalist , and co-principal songwriter of the Rolling Stones .

SOLUTION: Keith Richards ( born 18 December 1943 ) is an English guitarist best known as a founder member of the Rolling Stones .

PROBLEM: In humans , the pigmentation of the iris varies from light brown to black , depending on the concentration of melanin in the iris pigment epithelium ( located on the back of the iris ) , the melanin content within the iris stroma ( located at the front of the iris ) , and the cellular density of the stroma .

SOLUTION:
In humans , the pigment of the iris varies from light brown to black .