Ultimate source of variation:

Mutation, sexual reproduction, and genetic drift are the three main sources of variation in a population. Each plays a unique role in introducing and shaping genetic diversity, ultimately leading to the evolution of species.

Mutation:
Mutation refers to the spontaneous changes in the DNA sequence of an organism. It is the ultimate source of all genetic variation. Mutations can occur due to errors during DNA replication, exposure to mutagenic agents (such as radiation or chemicals), or even as a result of natural biological processes. These changes can be beneficial, harmful, or have no significant effect on the organism's survival and reproduction. Mutations introduce new alleles into a population, thereby increasing genetic diversity. This variation is the basis for natural selection, allowing species to adapt to changing environments over time.

Sexual Reproduction:
Sexual reproduction involves the combination of genetic material from two parent organisms to produce offspring. This process generates new combinations of alleles through the shuffling and recombination of genes during meiosis and fertilization. Each parent contributes only half of their genetic material to the offspring, resulting in unique combinations of traits. This recombination of genetic material increases genetic diversity within a population. Sexual reproduction also allows for the elimination of harmful mutations through natural selection, as individuals with detrimental mutations are less likely to successfully reproduce.

Genetic Drift:
Genetic drift refers to the random changes in allele frequencies within a population over time. It occurs due to chance events, such as bottleneck events or founder effects, where a small subset of the population becomes isolated and undergoes genetic changes. Genetic drift is a significant source of variation, especially in small populations, as it can lead to the loss or fixation of certain alleles. The impact of genetic drift is more pronounced in populations with limited gene flow and can result in the divergence of populations and the formation of new species.

Conclusion:
In conclusion, all three factors - mutation, sexual reproduction, and genetic drift - play crucial roles in introducing and maintaining genetic variation within populations. Mutation is the ultimate source of genetic diversity, while sexual reproduction generates new combinations of alleles. Genetic drift, on the other hand, introduces random changes in allele frequencies. Together, these processes contribute to the variation essential for the survival, adaptation, and evolution of species.