Variation within a population primarily results from genetic diversity and environmental influences.

Genetic Diversity
Genetic diversity is a crucial factor contributing to variation in a population. This diversity emerges from the process of sexual reproduction, where genes from two parents combine to create offspring. Each offspring is genetically unique, inheriting a distinct mix of genes from both parents. This mixing leads to a broad array of genetic combinations, which in turn results in variability in traits such as height, eye color, and susceptibility to certain diseases.

Mutations—changes in the DNA sequence—also enhance genetic diversity. These mutations can occur spontaneously due to errors during DNA replication or as a consequence of exposure to environmental factors like radiation or chemicals. While many mutations are neutral or detrimental, some may confer advantages that improve an organism’s chances of survival and reproduction. Over time, these advantageous mutations can proliferate within a population, further increasing genetic variation.

Environmental Influences
Environmental factors significantly impact variation, even among genetically identical organisms. For instance, the growth, health, and flowering of a plant can be affected by environmental conditions such as sunlight, soil quality, and water availability. Similarly, in animals, traits like body size and behavior can be influenced by diet, stress levels, and exposure to diseases.

Migration and Gene Flow
Migration also plays a vital role in enhancing genetic variation. When individuals from one population migrate to a different area and interbreed with the local population, they introduce new genetic material. This process, known as gene flow, can introduce novel traits and augment the genetic diversity within the population.

Conclusion
In conclusion, variation within a population arises from a combination of genetic diversity—driven by sexual reproduction and mutations—and environmental influences. Additionally, migration contributes to this variation by bringing in new genetic material, thereby enriching the population’s genetic landscape.