Variation: causes and types – Variation and Selection (Senior 3 Biology) | Ganzaa

Variation and Selection

Lessons

Senior 3 · Biology

Variation and Selection

5 lessons

Lesson 1 of 5

Variation: causes and types

25 min

Forty pupils, two kinds of difference

Measure the height of every pupil in an S3 class and you will rarely find two exactly the same. The heights run smoothly from the shortest to the tallest, with most pupils somewhere in the middle. Now find out the same pupils' ABO blood groups. This time every pupil falls into one of only four boxes, A, B, AB or O, and nobody is "between" A and B.

Both are variation: differences between members of the same species. This lesson asks where variation comes from, and why some characteristics vary smoothly while others fall into separate groups.

Definition

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

Genetic (inherited) variation: variation caused by differences in genes (alleles). It can be passed to offspring.

Environmental (acquired) variation: variation caused by the conditions an organism lives in, such as diet, climate, light, soil or disease. It is not passed to offspring.

Causes of variation

1. Genes. Sexual reproduction mixes alleles in three ways that you met in Unit 5 and Unit 13:

Independent assortment in meiosis I: each homologous pair lines up independently, so a gamete gets a random mixture of the father's and mother's chromosomes.
Crossing over in prophase I swaps pieces between homologous chromosomes, making new combinations of alleles on one chromosome.
Random fertilisation: any sperm can fertilise any egg.

All three only shuffle alleles that already exist. Brand-new alleles arise only by mutation (Lesson 2).

2. Environment. Two organisms with the same genes can still differ. A bean plant grown in shade grows taller and paler than its twin in full sun; a child with poor nutrition may not reach the height his or her genes allow; a scar, a language spoken or a muscle built by training are all acquired in a lifetime and are not inherited.

3. Both together. Most characteristics depend on genes and environment. Identical twins have the same genotype, so differences between them, such as in body mass, must come from the environment.

Worked example

Worked example 1: how many different gametes? A human has 23 pairs of chromosomes. Considering independent assortment alone, how many different combinations of chromosomes can one person's gametes have? How many different zygotes could one couple produce?

Solution

For each pair, a gamete receives one of 2 chromosomes. For 23 independent pairs: $2^{23} = 8\,388\,608$ combinations.
Each of the father's $2^{23}$ kinds of sperm could meet each of the mother's $2^{23}$ kinds of egg: $2^{23} \times 2^{23} = 2^{46} \approx 7.04 \times 10^{13}$ .
Crossing over adds even more. So, apart from identical twins, no two children of the same parents are genetically the same.

Definition

Continuous variation: variation in which a characteristic shows a smooth range of values between two extremes, with no separate groups. Example: height, body mass, leaf length. It is usually controlled by many genes together with the environment.

Discontinuous variation: variation in which individuals fall into a few distinct groups, with no in-between values. Example: ABO blood group, sex, round or wrinkled pea seeds. It is usually controlled by one or a few genes, and the environment has little effect.

Showing the two kinds on a graph

Figure 1 shows continuous variation as a histogram: the classes are ranges of a measurement, so the bars touch. Most pupils are near the middle and few are at the extremes, giving a bell-shaped curve, often called a normal distribution.

Figure 2 shows discontinuous variation as a bar chart: each bar is a separate category, so the bars are drawn apart.

	Continuous variation	Discontinuous variation
Values	a smooth range; measured	distinct groups; counted
Genes	many genes (polygenic), each with a small effect	one gene or a few
Effect of environment	large	small or none
Graph	histogram, bars touching; often bell-shaped	bar chart, bars separate
Examples	height, mass, hand span, milk yield of cows	blood group, sex, pea seed shape

Worked example

Worked example 2: reading Figure 1. (a) Which is the modal class? (b) How many pupils are 165 cm or taller? (c) What percentage is that?

Solution

The tallest bar is 160–164 cm (10 pupils): the modal class.
The classes 165–169, 170–174 and 175–179 cm: $6 + 3 + 2 = 11$ pupils.
$\frac{11}{40} \times 100 = 27.5\%$ .

Important

Human variation is individual. Characteristics such as height and skin colour vary continuously from person to person; they do not divide people into separate groups. Most human genetic variation is found between individuals within any population, not between populations. So the idea that humans form biologically separate "races", or that any group is superior, has no basis in genetics.

Warning

Counted is not always discontinuous. The number of beans in a pod is counted in whole numbers, but it varies over a range and is affected by the plant's growing conditions, so it behaves like continuous variation. Ask: are there a few distinct types (discontinuous), or a range with most values in the middle (continuous)? Also, a histogram's bars touch; a bar chart's bars do not.

Variation, from FuseSchool. Watch for the difference between genetic and environmental causes, and for how continuous and discontinuous variation are shown on graphs.

Tip

Ruler or box? If you would measure it with a ruler or a balance, it is probably continuous. If you would sort individuals into boxes, it is discontinuous.

✦

Practice: variation

Define (a) variation, (b) discontinuous variation.
Classify each as continuous or discontinuous: body mass; ABO blood group; length of maize cobs; sex; round or wrinkled pea seeds; hand span.
Name three ways in which sexual reproduction produces genetic variation.
A farmer grows cuttings of one sweet potato plant in two fields. The plants in one field give much bigger tubers. Is this genetic or environmental variation? Explain.
Identical twins were separated at birth and raised in different homes. As adults, one is 6 kg heavier than the other. What causes this difference, and why can you be sure?
Explain why a characteristic controlled by many genes shows continuous variation, while one controlled by a single gene often shows discontinuous variation.
Using Figure 2, what percentage of the pupils are blood group O?
A pupil plots the masses of 50 bean seeds as a bar chart with gaps between the bars. Explain what is wrong and how to correct it.

Show answersHide answers

(a) The differences in characteristics between individuals of the same species. (b) Variation in which individuals fall into a few distinct groups, with no intermediates.
Continuous: body mass, maize cob length, hand span. Discontinuous: ABO blood group, sex, pea seed shape.
Independent assortment in meiosis, crossing over in meiosis, and random fertilisation (mutation also creates new alleles).
Environmental: cuttings of one plant have the same genotype, so the difference must come from the conditions, such as soil, water or fertiliser.
The environment (diet, exercise). Identical twins have the same genotype, so a difference between them cannot be genetic.
With many genes, each adding a small amount, and the environment acting too, many combinations give many slightly different values: a smooth range. With one gene there are only a few genotypes, so only a few distinct phenotypes.
$\frac{18}{40} \times 100 = 45\%$ .
Seed mass is continuous, so it should be grouped into classes (for example 0.40–0.44 g) and drawn as a histogram with touching bars.

Key points

Variation is caused by genes, by the environment, and usually by both.
Meiosis (independent assortment, crossing over) and random fertilisation shuffle alleles; only mutation makes new ones. Acquired characteristics are not inherited.
Continuous variation: a range, many genes plus environment, histogram. Discontinuous variation: distinct groups, one or a few genes, bar chart.

Sources and credits

Photo: Hydrangea macrophylla pink and blue, Gustamons, via Wikimedia Commons, CC0.
Video: Variation, FuseSchool, embedded from YouTube.

Written by Ganzaa: the text, Figures 1 and 2 (illustrative data), the table, the worked examples and the practice questions.

1Forty pupils, two kinds of difference

2Causes of variation

3Showing the two kinds on a graph

Forty pupils, two kinds of difference

Causes of variation

Showing the two kinds on a graph