4.2.2(f) - Types and causes of variation
Variation means differences in characteristics. Those differences matter because biologists use them to describe biodiversity, compare organisms, collect data and explain how phenotypes arise. In this lesson you will separate variation within and between species, compare continuous and discontinuous variation, and connect each pattern to genetic and environmental causes.
Within And Between Species
A characteristic is a measurable or observable feature of an organism, such as height, blood group, leaf length, enzyme activity, colony colour or antibiotic resistance. A phenotype is the observable expression of characteristics in an organism.
Variation
Differences in characteristics between organisms.
The first classification asks whether the organisms being compared belong to the same species.
Intraspecific variation
Variation between individuals of the same species.
Interspecific variation
Variation between individuals of different species.
Intraspecific variation is seen when different individuals of one species do not have exactly the same phenotype. Humans vary in height, mass, blood group and skin pigmentation. Oak trees of the same species can vary in leaf length. Colonies of one bacterial species can vary in diameter if they grow under slightly different conditions.
Interspecific variation is seen when different species have different characteristics. A human and a chimpanzee differ in many anatomical and biochemical features. An oak tree and a grass plant differ in leaf form and growth habit. A yeast species and a bacterial species differ in cell organisation.
The key point is the comparison being made:
| Comparison | Type of variation | Example |
|---|---|---|
| Two plants of the same species have different leaf lengths | Intraspecific | Same species, different individuals |
| A plant species has vascular tissue but a bacterial species does not | Interspecific | Different species |
| Two colonies of the same bacterial species have different diameters | Intraspecific | Same species, different colonies |
Do not use intraspecific and interspecific as vague words for "small difference" and "large difference". They are about whether the comparison is within one species or between different species.
Continuous Or Discontinuous
The second classification asks what kind of values the characteristic can take. Some characteristics are measured on a range. Others fall into separate categories.
Continuous variation
Variation in which a characteristic can take many values within a range, usually measured quantitatively.
Examples of continuous variation include plant height, leaf length, human height, human mass, bacterial colony diameter and enzyme activity. These characteristics can be measured, and many intermediate values are possible.
Continuous variation often produces a frequency distribution. If many individuals are measured, most values may cluster around the mean, with fewer values at the extremes. The exact shape depends on the characteristic and the sample, so do not claim every continuous trait must form a perfect bell-shaped curve.
Discontinuous variation
Variation in which a characteristic falls into separate categories, with no continuous range of intermediate values.
Examples of discontinuous variation include ABO blood group in humans, flower colour categories in some plants, and whether a microbial colony has or lacks a particular pigment. These are categories rather than measurements along a smooth scale.
| Feature | Continuous variation | Discontinuous variation |
|---|---|---|
| Data type | Quantitative measurements | Qualitative categories |
| Values | Many values across a range | Distinct groups |
| Typical graph | Histogram or frequency distribution | Bar chart |
| Common genetic pattern | Often many genes with small additive effects | Often one or a small number of genes |
| Environmental influence | Often important | Often small or absent |
A table may show categorical groups and continuous measurements together. Male and female groups in a mammal population are categories, while mass within either group still takes a range of values. A difference between group mean masses does not make every mass measurement discontinuous. Comparing two species concerns interspecific variation; comparing individuals of one species concerns intraspecific variation. These distinctions answer different questions.
The words often and usually matter. Continuous traits are commonly polygenic and environmentally influenced, but a measured trait is not automatically continuous just because the environment affects it. Discontinuous traits are often controlled by one or a few genes, but they do not have to be controlled by only one allele pair.
If asked to distinguish continuous and discontinuous variation, compare both the phenotype pattern and the data type: continuous variation is measured across a range, whereas discontinuous variation is grouped into categories.
Genetic And Environmental Causes
Observed variation is variation in phenotype. A phenotype can be affected by genetic factors, environmental factors, or both.
phenotype = genotype + environmental influence
This is not a calculation formula. It is a reminder that the observable characteristic may depend on the organism's genotype and the conditions in which the organism develops or lives.
Genetic causes of variation are differences in DNA, genes, alleles or combinations of alleles. Mutation creates new alleles; sexual reproduction combines alleles in different ways through meiosis and fertilisation. A change in the genes controlling pigment synthesis can switch pigment production on or off, whereas many alleles with small additive effects can produce a range of heights. Genetically identical clones grown under different conditions help reveal environmental effects; different genotypes grown in the same controlled conditions help reveal genetic effects.
Examples include:
| Example | Type of characteristic | Genetic point |
|---|---|---|
| ABO blood group in humans | Discontinuous | Different alleles give distinct blood group categories |
| Flower colour category in some plants | Discontinuous | Allele differences can produce different pigment phenotypes |
| Human height | Continuous | Many genes can each contribute a small effect |
| Bacterial pigment production | Discontinuous | A genetic difference may switch pigment production on or off |
Environmental causes of variation are differences in external or internal conditions that affect phenotype without necessarily changing the organism's DNA sequence. Examples include light intensity affecting plant growth, mineral availability affecting leaf colour, diet affecting animal mass, and temperature or nutrient concentration affecting microbial growth rate.
Many real characteristics involve both. Plant height may depend on genotype, but also on light, water and mineral ions. Human body mass may depend on genotype, diet and activity. Microbial colony diameter may depend on genotype and on temperature, oxygen and nutrient supply.
A common error is to write "environmental variation is inherited". Environmental conditions can change the phenotype of an individual, but the environmental change itself is not passed on as an allele. Only genetic differences can be inherited in the usual genetic sense.
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