Genetic causes of variation
Genetic variation arises from several processes:
Mutation is a random change in the base sequence of DNA. It is the ultimate source of all new alleles. Most mutations are neutral or harmful, but occasionally a mutation produces a beneficial allele that gives a survival advantage.
Meiosis produces gametes with different combinations of alleles through independent assortment of chromosomes and crossing over between homologous chromosomes. This means each gamete is genetically unique.
Random fertilisation means any sperm can fuse with any egg, producing offspring with unique combinations of alleles from both parents. Together, meiosis and random fertilisation ensure that sexually reproducing organisms produce genetically varied offspring.
Environmental causes of variation
The environment can alter the phenotype without changing the genotype. For example, a plant grown in poor light may be taller and thinner than an identical plant grown in good light, because it is stretching toward the light source. The genes have not changed, but the expressed characteristic has.
Other environmental causes include diet (affecting height and mass in animals), soil mineral content (affecting plant growth), temperature (affecting fur thickness in some animals) and exercise (affecting muscle development). These changes are not inherited because they do not alter the DNA.
Gene-environment interaction
Most characteristics are influenced by both genes and environment. Height has a strong genetic component (tall parents tend to have tall children) but is also affected by nutrition during growth. Skin colour is influenced by genes that determine melanin production and by sun exposure.
To distinguish genetic from environmental effects, scientists study identical twins raised in different environments. Any differences between them must be environmental because their genotypes are identical. Any similarities despite different environments suggest a strong genetic influence.