New traits and variations in a population occur by

New traits and variations in a population occur by sexual reproduction.

Explanation:

Sexual reproduction is the process by which an organism reproduces by means of production and subsequent fusion of gametes. Sexual reproduction involves the production of gametes. These gametes are formed by means of meiosis.

Meiosis is a type of cell division which involves the crossing over and other processes that involves the gene mixing and subsequent variations from the parents among the offsprings. Thus newer characteristics are formed among the population, which is termed as variations. This is essential for further evolution.