Introduction:
Crossing over is a key process that occurs during meiosis, specifically in the prophase I stage. It plays a crucial role in generating genetic variation within a population. This genetic variation is essential for the survival and evolution of species.

Crossing Over:
During crossing over, homologous chromosomes pair up and exchange segments of genetic material. This occurs at specific points called chiasmata, where the chromosomes physically exchange genetic material. The result is an exchange of DNA between maternal and paternal chromosomes.

Contributions to Genetic Variation:
Crossing over contributes to genetic variation in several ways:

1. Recombination of Alleles:
Crossing over allows for the recombination of alleles between homologous chromosomes. Alleles are alternative forms of a gene that occupy the same position on homologous chromosomes. Through crossing over, alleles from the maternal and paternal chromosomes can be exchanged, leading to new combinations of alleles in the resulting gametes.

2. Creation of Novel Alleles:
Crossing over can lead to the creation of novel alleles through the exchange of genetic material. This occurs when genetic segments from different chromosomes are combined, resulting in new combinations of genes. These new alleles may provide individuals with advantageous traits or characteristics that can be selected for in evolution.

3. Shuffling of Genetic Material:
Crossing over shuffles genetic material between homologous chromosomes, resulting in a mixing of genetic information. This shuffling can lead to the creation of new combinations of genes that were not present in either parent. It increases the potential for genetic diversity within a population.

4. Reduction of Linkage Disequilibrium:
Crossing over disrupts the linkage disequilibrium between genes that are located close to each other on the same chromosome. Linkage disequilibrium refers to the non-random association of alleles at different loci. By breaking up the physical connection between genes, crossing over allows for independent assortment and greater genetic variation.

Conclusion:
Crossing over is a crucial process that contributes to genetic variation by allowing for the recombination of alleles, creation of novel alleles, shuffling of genetic material, and reduction of linkage disequilibrium. These mechanisms ensure genetic diversity within a population, providing the basis for adaptation and evolution.