Variation in Species
Explain variation within and between species, distinguishing continuous variation (e.g. height) from discontinuous variation (e.g. ABO blood group), and genetic causes from environmental causes
What a learner can do afterwards
- Gives examples of continuous and discontinuous variation in humans
- Distinguishes between genetic causes of variation (inherited differences) and environmental causes (diet, sunlight, etc.)
- Explains why sexual reproduction usually produces more genetic variation among offspring than asexual reproduction
- Interprets a frequency graph showing continuous variation
1 · Read
Variation means differences in traits. Members of one species, such as humans, can differ in height or hair texture. Species also differ from one another, such as cats with fur and birds with feathers. Continuous variation includes intermediate measurements across a range, like height, mass or arm length. Discontinuous variation has separate categories, like ABO blood group.
In the ABO system, each person has blood group A, B, AB or O. AB is a separate category, not a halfway measurement between A and B. The example chart counts people in each group.
Frequency means how many observations fall in a group or range. Compare the numbers above the bars to find the largest frequency. Height ranges must not overlap: 145 to <155 cm means at least 145 cm but less than 155 cm.
Invented ABO blood groups for 31 people. Each person is counted in one category.
Genes inherited from parents influence ABO blood group, eye colour and natural hair texture. Diet, sunlight, health and exercise are environmental factors. Extra sunlight can cause a tan, although genes affect skin colour and tanning too. Height depends on both genes and environment. Plants with the same genes can grow differently in different soils. Family resemblance can support an inherited explanation, but does not prove the cause.
Sexual reproduction joins genetic material from egg and sperm cells, usually from two parents. Some plants can self-fertilise. Offspring from sexual reproduction usually have more genetic variation than asexual offspring from one parent because they inherit different combinations of genes. Cuttings usually copy a plant's genes. Mutations are changes in DNA and can create genetic differences in either group. Growing conditions can affect both groups.
Variation can be continuous or discontinuous, and it comes from genes, environment, or both.
Lesson, questions and flashcards written for this dataset. OpenStax records were studied for scientific background.
Readings written for this dataset. Videos 'Variation in a species' and 'Phenotype plasticity' are by Khan Academy. 'Genetic variation, gene flow, and new species' is by California Academy of Sciences.
More to read
Different genes and different growing conditions
A gardener can grow new plants from cuttings of one parent. These plants usually copy that parent's genes. A mutation, which is a change in DNA, can introduce a genetic difference. Having matching genes does not mean every plant will grow to the same size. Water, light and soil conditions can affect growth.
Seeds made by crossing two genetically different parents receive genetic information from both. Sexual reproduction produces different combinations of inherited genes. Across many offspring, this usually gives more genetic variation than making cuttings from one parent. Some plants can also reproduce sexually by self-fertilising, so sexual reproduction does not always involve two separate plants.
Keep the comparison clear. Genetic variation concerns differences in inherited genetic information. Differences in appearance can also result from growing conditions. Two plants may look different without having different inherited genes, and two plants with different genes may share a measured trait.
Describing a pattern and explaining its cause
Calling variation continuous or discontinuous describes how a trait is recorded. Height is a measurement with intermediate values. ABO blood group uses separate categories: A, B, AB and O. AB is a category of its own, not a measurement halfway between A and B.
Explaining the cause is a different task. Genes influence height, and nutrition and health can affect growth too. Extra sun exposure can cause tanning, while inherited differences also influence natural skin colour and the response to sunlight. Describing an environmental effect does not mean genes have no influence on the trait.
A frequency chart counts observations. Measurements can be grouped into ranges, but the boundaries must be clear. A range from 150 to less than 160 cm includes 150 cm and excludes 160 cm. Each measurement then has one place. The largest frequency identifies the range containing the most observations. A middle-peaked chart alone does not establish whether a trait is continuous; consider what was measured and whether intermediate values are possible.
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