No Two Organisms Are Identical In All Their Traits
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.
Patterns of Variation
1. Continuous Variation
- Traits exhibit a range of values without distinct categories.
- Characteristics:
- Measured on a continuous scale.
- Represented by a bell-shaped curve (e.g., height distribution).
- Often influenced by polygenic inheritance and environmental factors.
Human height, leaf length in plants, fur color in animals.
TipTraits showing continuous variation are controlled by multiple genes working together.
2. Discontinuous Variation
- Traits fall into distinct categories with no intermediates.
- Characteristics:
- Categorical traits (e.g., yes/no).
- Represented by bar graphs.
- Typically controlled by a single gene or a few genes, with minimal environmental influence.
Blood type in humans (A, B, AB, O) or flower petal presence.
TipContinuous traits show a range (e.g., height), while discontinuous traits fall into specific categories (e.g., blood type).
Variation Between Species vs. Within Species
- Between species: Structural differences allow classification.
- Within species: Members of a species share a common genetic basis but differ due to allelic combinations.
- Between species: presence of chloroplasts in plants vs. absence in animals.
- Within species: humans all have the same genes for blood type, but different alleles (A, B, O) produce different phenotypes.
How does our understanding of variation challenge the idea of fixed categories in nature? For example, consider the ethical implications of classifying human populations based on genetic differences. Are these classifications scientific, or do they risk reinforcing stereotypes?
Self review- Can you explain the difference between continuous and discontinuous variation?
- Why is variation considered a defining feature of life?
- List three genetic and three environmental causes of variation in organisms.
- Explain why monozygotic twins are not completely identical, despite sharing the same genome.
