Variation: meaning, causes, advantages and disadvantages – Variation and Selection (Senior 6 Biology) | Ganzaa

Variation and Selection

Lessons

Senior 6 · Biology

Variation and Selection

5 lessons

Lesson 1 of 5

Variation: meaning, causes, advantages and disadvantages

30 min

A field of copies

Many farmers plant bananas from suckers cut from a parent plant, so every plant in a plot can be a genetic copy of every other. The plants still differ: those in deep, well-watered soil grow taller and bear heavier bunches than those on a dry slope. Yet a field of copies has a hidden weakness. In the middle of the twentieth century the export banana variety of the time, Gros Michel, was almost wiped out of commercial plantations by a soil fungus that causes Panama disease (Fusarium wilt): when one plant was susceptible, they all were. Variation is not just a curiosity. It decides how organisms respond to their surroundings and whether a population can survive a new threat. In Senior 3 you sorted variation into types; this lesson looks at where it comes from, at A-level depth, and at what it costs and gives.

Definition

Variation: the differences in characteristics between individuals of the same species.

Genotype: the alleles an organism carries. Phenotype: its observable characteristics, which result from its genotype and its environment acting together.

Genetic (heritable) variation: differences caused by differences in genotype, which can be passed to offspring.

Environmental variation: differences caused by the surroundings (food, light, temperature, disease, lifestyle), which are not inherited.

Mutation: a change in the base sequence of DNA or in the structure or number of chromosomes; the only source of new alleles.

Polymorphism: the existence of two or more distinct, inherited forms of a characteristic in the same population, each too common to be maintained by mutation alone.

Phenotype = genotype + environment

Every phenotype depends on both. Genes set the range of what is possible; the environment decides where in that range an individual ends up. Three examples show the balance shifting:

Mostly genetic. A person's ABO blood group is fixed by their alleles; diet or climate cannot change it.
Genes acting through the environment. Himalayan rabbits and Siamese cats carry an allele for a heat-sensitive form of an enzyme that makes the dark pigment. It works only below about body temperature, so fur grows dark only on the cooler extremities: ears, nose, feet and tail. If a patch of white fur on the back is shaved and kept cold with an ice pack, the new fur grows dark.
Genetic disorder, environmental outcome. Children with phenylketonuria (PKU) inherit two faulty alleles for the enzyme that breaks down the amino acid phenylalanine. On a normal diet, phenylalanine builds up and damages the developing brain; on a low-phenylalanine diet started soon after birth, development can be normal. The genotype is the same; the phenotype depends on diet.

Worked example 1: what twins can tell us

Illustrative data, in the pattern of classic twin studies. Identical (monozygotic) twins have the same genotype; non-identical (dizygotic) twins share about half their alleles, like ordinary brothers and sisters. The table gives the mean difference between the two twins of each pair.

Pairs of twins	Mean difference in height / cm	Mean difference in body mass / kg
Identical, raised together	1.7	1.9
Identical, raised apart	1.8	4.5
Non-identical, raised together	4.4	4.5

Which characteristic is influenced more by the environment? Explain.

Height. Identical twins differ by about the same amount whether raised together or apart (1.7 and 1.8 cm), but non-identical twins raised together differ much more (4.4 cm). Different genotypes cause the difference, so height here is mainly genetic.
Body mass. Identical twins raised apart differ by 4.5 kg, as much as non-identical twins, although their genotypes are the same. Different environments (diet, activity) cause that difference.
Conclusion. Body mass is influenced more by the environment than height is.

Caution. Twins raised "apart" often grow up in similar homes, and both studies use small samples, so such data show the relative importance of genes and environment in one population, not a fixed property of the characteristic.

Where genetic variation comes from

There are four sources. Only the first makes new alleles; the other three shuffle existing alleles into new combinations.

Mutation. Gene mutations (substitution, insertion or deletion of bases) create new alleles; chromosome mutations change the structure or number of chromosomes. Mutations happen at random and are rare, but over a whole population and many generations they supply all the alleles there are. Only mutations in cells that make gametes are passed on.
Crossing over in prophase I of meiosis: non-sister chromatids of a homologous pair exchange sections at chiasmata, producing chromatids with new combinations of alleles of linked genes.
Independent assortment in metaphase I: each homologous pair lines up on the equator independently of the others, so the maternal and paternal chromosomes are shuffled into gametes in different combinations. With $n$ pairs there are $2^{n}$ combinations.
Random fertilisation: any one of a male's genetically different gametes may fuse with any one of a female's.

Worked example 2: how many genotypes?

A characteristic is controlled by several genes, each with two alleles, on different chromosomes. (a) How many different genotypes are possible for one gene? (b) How many for 10 such genes together? (c) What does this suggest about the chance that two people who are not identical twins have the same genotype?

(a) For alleles A and a: AA, Aa and aa, which is 3 genotypes.
(b) Each gene's genotype is independent of the others, so multiply: $3^{10} = 59\,049$ genotypes.
(c) Humans have about 20 000 genes, many with several alleles, and crossing over makes new combinations even of linked genes. The number of possible genotypes is astronomically larger than the number of people who have ever lived, so apart from identical twins, every person's genotype is unique.

Advantages and disadvantages of variation

Advantages	Disadvantages
It is the raw material for natural selection: when the environment changes, some individuals are likely to have alleles that let them survive, so the population can adapt.	Some variants are harmful: mutations can cause genetic disorders or reduce fitness, and many offspring with new combinations are less well suited than their parents.
A varied population is less likely to be wiped out by one disease or pest, as the Gros Michel banana was.	Producing variation by sexual reproduction is costly: it needs two parents, mate-finding and meiosis, and it breaks up combinations of alleles that already work well.
Variation lets breeders select better crops and livestock (lesson 4).	Farmers and industries often want uniform crops that ripen together and give a predictable product, which is easier with genetically identical plants.
Over long periods, variation allows new species to evolve (Unit 12).	Environmental variation can hide genetic differences, making selection by breeders less efficient.

Warning

Common mistakes

"Crossing over makes new alleles." It makes new combinations of existing alleles. Only mutation makes new alleles.
"Characteristics are either genetic or environmental." Almost all are both; the question is how much each contributes, and this can differ between populations.
"Acquired characteristics are inherited." Muscles built by exercise, or a scar, change the phenotype but not the alleles in the gametes, so they are not passed on.
"Mutations are always harmful." Most are neutral, some are harmful and a few are beneficial in a particular environment.

Amoeba Sisters: "Meiosis". Watch for crossing over in prophase I and the way homologous pairs line up independently in metaphase I: the two sources of variation in Figure 1.

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Practice: variation and its causes

Define (a) phenotype, (b) polymorphism.
Name the only source of new alleles, and explain why mutations in skin cells are not inherited.
Describe how (a) crossing over and (b) independent assortment produce genetic variation.
A plant has 2n = 14. Ignoring crossing over, how many genetically different gametes can one plant produce?
Explain why a Himalayan rabbit has dark ears but a white back.
Using PKU as an example, explain why a genetic disorder's phenotype can depend on the environment.
Give two advantages and two disadvantages of genetic variation in a population.
A study finds that identical twins raised apart differ as much in a test score as non-identical twins raised together. Evaluate what this suggests about the causes of variation in the score.

Show answersHide answers

(a) The observable characteristics of an organism, the result of its genotype and environment. (b) Two or more distinct inherited forms of a characteristic in the same population, too common to be kept only by mutation.
Mutation. Skin cells are somatic cells; only mutations in cells that produce gametes can be passed to offspring.
(a) Non-sister chromatids of homologous chromosomes exchange sections at chiasmata in prophase I, making new combinations of alleles on each chromatid. (b) Homologous pairs line up at the equator independently in metaphase I, so maternal and paternal chromosomes are distributed to gametes in many combinations ( $2^{n}$ ).
$n = 7$ , so $2^{7} = 128$ kinds of gamete.
Its pigment enzyme is heat-sensitive and works only at lower temperatures; the ears are cooler than the body, so pigment forms there but not on the warm back.
People with PKU have the same genotype whatever they eat, but a low-phenylalanine diet prevents the build-up that damages the brain; the phenotype (brain development) depends on diet.
Advantages, any two: allows adaptation by natural selection; protects against total loss to one disease; allows selective breeding; leads to new species. Disadvantages, any two: harmful mutations and disorders; less well-adapted offspring; costs of sexual reproduction; non-uniform crops.
Identical twins have the same genotype, so differences between them reared apart are environmental. If these are as large as differences between non-identical twins reared together, the environment strongly influences the score. But the samples may be small, "apart" environments may be similar, and the result applies only to that population, so the conclusion is tentative.

Key points

Phenotype results from genotype and environment acting together; the balance differs between characteristics (blood group, Himalayan rabbit fur, PKU).
Genetic variation comes from mutation (new alleles), crossing over, independent assortment ( $2^{n}$ combinations) and random fertilisation.
Environmental variation is not inherited.
Twin studies compare identical and non-identical twins to estimate the relative importance of genes and environment.
Variation allows adaptation and protects populations, but some variants are harmful and uniformity is sometimes wanted.

Sources and credits

19.1 Population Evolution and 11.1 The Process of Meiosis, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax (Rice University), licensed under CC BY 4.0. Used for the sources of genetic variation. Adapted by Ganzaa.
Photo: Grove snail Cepaea nemoralis, showing colour and banding polymorphism, by Angus Davison, via Wikimedia Commons, licensed under CC BY-SA 4.0.
Video: "Meiosis", Amoeba Sisters, embedded from YouTube.

Figure 1, the tables, the worked examples (with illustrative data) and the practice questions were written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A field of copies

2Phenotype = genotype + environment

3Where genetic variation comes from

4Advantages and disadvantages of variation

A field of copies

Phenotype = genotype + environment

Where genetic variation comes from

Advantages and disadvantages of variation