No Two Organisms Are Identical In All Their Traits
Definition
Variation
Variation refers to differences in traits among individuals within a population, encompassing physical characteristics, behaviors, and genetic sequences.
- Variation can manifest as differences in:
- Morphological traits: e.g., fur color in cats, height in plants.
- Behavioral traits: e.g., bird mating displays, migration patterns.
- Biochemical traits: e.g., enzyme variants, antibiotic resistance in bacteria.
Even closely related organisms, such as siblings, exhibit variation because each inherits a unique combination of alleles during reproduction.
Causes of Variation
1. Genetic Causes
- Every individual of a species has a unique combination of alleles, except in the case of identical twins or clones.
- Genetic variation is introduced by:
- Mutation: Permanent changes in DNA sequences create new alleles.
- Meiosis: Processes such as crossing over (exchange of DNA between homologous chromosomes) and independent assortment of chromosomes create novel genetic combinations in gametes.
- Fertilization: Random fusion of gametes during sexual reproduction ensures that each zygote is genetically unique.
- A mutation in a gene coding for melanin can result in different skin pigmentation.
2. Environmental Causes
- Even genetically identical organisms show variation due to environmental influence.
- Environmental factors can switch genes on or off or affect the degree to which they are expressed.
- Nutrient availability, temperature, light, oxygen concentration, and pH all affect phenotype.
- Genes act like a blueprint, while the environment is the construction site.
- A perfect blueprint may result in different buildings if the construction conditions differ.
- Monozygotic twins (identical twins) are an excellent example of how variation arises.
- These twins start with identical genetic material, but as they grow, environmental differences and random mutations cause them to diverge.
- They may differ in height or weight because one had greater access to nutrition during development.
