Genetic and Environmental Variation
This topic explains the two fundamental sources of differences we see among individuals in a species: inherited genetic factors and external environmental influences. Understanding this distinction is crucial for grasping how natural selection and evolution operate.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Variation refers to the differences in characteristics between individuals within a population.
- Genetic variation is inherited. It arises from different versions of genes (alleles), which are created by mutations, and results in a range of observable traits (phenotypes).
- Environmental variation is caused by external factors like diet, climate, or lifestyle. These changes affect the phenotype but are not passed on to offspring.
- Many phenotypes, such as human height or plant growth, are the result of a complex interaction between an organism's genetic makeup and its environment.
- Only genetic variation provides the raw material for natural selection, as these are the only traits that can be inherited by the next generation.
Definitions
- Variation
- The differences in physical traits (phenotypes) among individuals within a species.
- Phenotype
- The observable characteristics of an organism, determined by its genes and influenced by its environment.
- Genetic Variation
- Differences among individuals in the composition of their genes or other DNA sequences.
- Environmental Variation
- Differences among individuals caused by environmental factors such as food, light, or temperature, rather than by genetic differences.
Worked example
Two cuttings are taken from the same parent geranium plant and are therefore genetically identical. One is grown in a sunny spot with nutrient-rich soil. The other is grown in a shaded area with poor soil. After a month, the first plant is large with many flowers, while the second is small with none. What is the source of this variation, and is it heritable?
- 1
Identify the genetic relationship between the plants.
As they are cuttings from the same parent, they are clones with identical genotypes.
- 2
Analyse the differences in their growing conditions.
The conditions differ in light intensity and soil nutrients.
- 3
Connect the conditions to the observed phenotypes.
The differences in growth (size, flowers) directly correspond to the differences in their environment.
- 4
Determine the source of variation.
Since the genotype is constant, the variation must be environmental.
- 5
Determine heritability.
Environmental variation is not passed on genetically.
Seeds from either plant would not inherit the 'large' or 'small' trait caused by the conditions.
Answer: The source of the variation is environmental. It is not heritable.
Common mistakes
- ×Confusing the two sources of variation. A common mistake is to assume that characteristics acquired from the environment, like increased muscle mass from exercise, can be passed on to offspring.
- ×Failing to consider the interaction between genes and environment. Many traits are not simply 'genetic' or 'environmental' but a product of both. For example, an individual might have genes for being tall, but poor nutrition (environment) could limit their final height.
- ×Assuming all variation within a family is purely genetic. While siblings share genes, they also experience different environmental factors that contribute to their unique phenotypes.
No-calculator tips
- ✓Use the 'identical twins' or 'plant cuttings' thought experiment to isolate variables. If the genetics are identical, any differences observed must be due to the environment.
- ✓When evaluating a trait, ask the simple question: 'If this organism reproduces, will its offspring inherit this specific trait?'. A scar is not inherited (environmental), but eye colour is (genetic).