Simplify the following text for a Grade 9 native English-speaking person.
Original: Under natural selection , a gene encoding a trait that enhances the fitness of each individual carrying it should increase in frequency within the population ; and conversely , a gene that lowers the individual fitness of its carriers should be eliminated .
Simplified: Under natural selection , a gene which improves the fitness of individuals will increase in frequency . A gene which lowers the fitness of individuals will become rare .