Variation: continuous and discontinuous variation, causes of variation

What you'll learn

Variation refers to the differences that exist between individuals of the same species. Understanding variation is fundamental to Human and Social Biology as it explains why no two people are exactly alike. This topic examines the two main types of variation (continuous and discontinuous), their causes (genetic, environmental, or both), and how these concepts apply to human populations, including Caribbean communities.

Key terms and definitions

Variation — the differences in characteristics between individuals of the same species

Continuous variation — variation where characteristics show a range of values between two extremes with no distinct categories (e.g., height, weight, skin colour)

Discontinuous variation — variation where characteristics fall into distinct categories with no intermediates (e.g., blood group, ability to roll tongue)

Genetic variation — differences caused by different genes inherited from parents

Environmental variation — differences caused by the conditions in which organisms live and develop

Phenotype — the observable characteristics of an organism resulting from both genetic makeup and environmental influences

Alleles — different versions of the same gene that code for variations of a characteristic

Polygenic inheritance — characteristics controlled by two or more genes working together

Core concepts

Types of variation

Continuous variation

Continuous variation produces characteristics that can take any value within a range. These characteristics:

• Show a smooth gradation from one extreme to another
• Cannot be divided into distinct groups
• Are usually measured rather than counted
• Display a normal distribution curve when plotted on a graph (bell-shaped curve)
• Are typically controlled by multiple genes (polygenic inheritance)
• Are significantly influenced by environmental factors

Examples in humans:

• Height (can range from very short to very tall)
• Mass (body weight)
• Skin colour (ranges from very pale to very dark)
• Intelligence (measured by IQ scores)
• Hand span
• Foot length

Caribbean examples: Caribbean populations show continuous variation in characteristics like height and skin colour, reflecting diverse genetic backgrounds from African, European, Indian, Chinese, and indigenous peoples. Studies of height variation in Jamaican or Trinidadian populations would show normal distribution curves.

Discontinuous variation

Discontinuous variation produces characteristics that fall into clear-cut categories with no intermediates. These characteristics:

• Have distinct forms with nothing in between
• Can be counted rather than measured
• Are usually controlled by one or two genes only
• Show little or no environmental influence
• Display discrete categories when data is plotted

Examples in humans:

• ABO blood groups (A, B, AB, or O only)
• Rhesus factor (positive or negative)
• Ability to roll tongue (can or cannot)
• Attached or free earlobes
• Presence or absence of widow's peak
• Sex (male or female)
• Ability to taste PTC (phenylthiocarbamide) — tasters or non-tasters

Important distinction: While skin colour might appear to have categories (light, medium, dark), it is actually continuous variation because there are infinite gradations between extremes. True discontinuous variation shows no intermediates.

Causes of variation

Genetic causes

Genetic variation arises from differences in genes inherited from parents. Key mechanisms include:

Sexual reproduction:

• Gametes (sex cells) are produced by meiosis, which creates genetic variation
• Random fertilization — any sperm can fertilize any egg
• Each offspring receives a unique combination of alleles from both parents
• Siblings (except identical twins) are genetically different

Mutation:

• Random changes in DNA structure
• Can create new alleles
• Most mutations have no effect or are harmful; rarely, they are beneficial
• Spontaneous mutations occur naturally during DNA replication
• Can be caused by mutagens (radiation, certain chemicals)

Gene combinations:

• Humans have approximately 20,000-25,000 genes
• Each gene may have multiple alleles
• The number of possible combinations is enormous
• This explains why each person (except identical twins) is genetically unique

Characteristics showing purely genetic variation include:

• Blood group
• Eye colour (though multiple genes involved)
• Inherited disorders (sickle cell anaemia, which is relatively common in Caribbean populations of African descent)

Environmental causes

Environmental factors can modify how genes are expressed. Environmental variation includes influences such as:

Nutrition:

• Adequate protein and calcium intake affects height and bone development
• Malnutrition during childhood can prevent individuals from reaching their genetic potential for height
• Obesity results from excessive calorie intake relative to energy expenditure
• In Caribbean contexts, access to nutritious food affects growth patterns

Climate and sun exposure:

• Sun exposure causes increased melanin production (tanning)
• People with the genetic potential to tan will vary in actual skin colour based on UV exposure
• Caribbean residents typically experience greater sun exposure than those in temperate regions

Exercise and physical activity:

• Muscle development depends on use
• Athletes develop characteristics through training
• Manual labour (e.g., in Caribbean agriculture or construction) builds muscle mass

Disease and injury:

• Childhood diseases can stunt growth
• Infections may affect development
• Scarring from injury
• In Caribbean populations, diseases like dengue fever or the after-effects of malnutrition can affect physical development

Lifestyle factors:

• Smoking can stunt growth and affect skin appearance
• Alcohol consumption affects liver function and overall health
• Drug use impacts physical and mental development

Social and economic factors:

• Access to healthcare
• Quality of housing
• Educational opportunities
• Economic status affecting nutrition and healthcare access — significant consideration across Caribbean nations with varying levels of development

Combined genetic and environmental causes

Most human characteristics result from both genetic and environmental factors working together. The phenotype (observable characteristic) is determined by the genotype (genetic makeup) and the environment.

Height is the classic example:

• Genetic component: Controlled by multiple genes (polygenic), inherited from parents
• Environmental component: Adequate nutrition, especially during childhood and adolescence, allows genetic potential to be reached
• A person may have genes for tall stature but remain relatively short if malnourished during growth years
• Conversely, good nutrition cannot make someone with genes for short stature become very tall

Body mass:

• Genetic component: Genes influence metabolic rate, body build, and tendency to store fat
• Environmental component: Diet (calorie intake) and exercise (energy expenditure) significantly affect actual body mass
• Some individuals are genetically predisposed to obesity but maintain healthy weight through lifestyle choices

Skin colour:

• Genetic component: Number and type of melanin-producing cells genetically determined
• Environmental component: UV exposure triggers melanin production (tanning)
• Caribbean populations show wide genetic variation in skin colour; sun exposure then causes further variation within each individual's genetic range

Intelligence:

• Genetic component: Genes influence brain structure and function
• Environmental component: Education, stimulation, nutrition, and social environment significantly affect intellectual development
• Access to quality education varies across Caribbean territories, affecting how genetic potential is realized

Representing variation graphically

Bar charts for discontinuous variation

Discontinuous data is represented using bar charts (or bar graphs) because the categories are separate and distinct.

Features:

• Bars are separated (not touching)
• X-axis shows distinct categories
• Y-axis shows frequency (number of individuals)
• Each bar represents one category

Example: Blood group distribution in a Caribbean school

• Categories: A, B, AB, O
• Each student counted once
• No students "between" categories

Line graphs and histograms for continuous variation

Continuous data is typically shown as a histogram or frequency curve.

Histogram features:

• Bars touch each other (showing continuous nature)
• X-axis shows measured characteristic (e.g., height in cm)
• Y-axis shows frequency
• Usually shows normal distribution (bell-shaped curve)

Normal distribution curve characteristics:

• Symmetrical bell shape
• Most individuals cluster around the mean (average)
• Fewer individuals at the extremes
• Examples: height, mass, hand span in a population

Worked examples

Example 1: Identifying types of variation

Question: Study the following characteristics and state whether each shows continuous or discontinuous variation:

(a) Ability to roll tongue [1 mark] (b) Mass in kg [1 mark] (c) ABO blood group [1 mark] (d) Height in cm [1 mark]

Mark scheme answers:

(a) Discontinuous variation — individuals can either roll their tongue or cannot; no intermediate forms exist

(b) Continuous variation — mass can take any value within a range and shows gradation between extremes

(c) Discontinuous variation — individuals belong to one of four distinct blood groups (A, B, AB, or O) with no intermediates

(d) Continuous variation — height can take any value within a range and is measured on a continuous scale

Example 2: Explaining causes of variation

Question: A study compared the heights of 100 Jamaican adults with the heights of their biological parents.

(a) Explain why children of tall parents tend to be taller than average. [3 marks]

(b) Explain why some children of tall parents are shorter than their parents. [3 marks]

Mark scheme answers:

(a)

• Height is partly controlled by genes/inherited/genetic (1 mark)
• Children inherit genes/alleles from their parents (1 mark)
• Tall parents possess genes/alleles for tallness which they pass to offspring (1 mark)

(b)

• Height is also influenced by environmental factors (1 mark)
• Such as nutrition/diet during growth/childhood or disease/illness during development (1 mark)
• If nutrition is inadequate or disease occurs, the person may not reach their genetic potential for height (1 mark)

Alternative valid point: Genes are inherited in new combinations, so children may not inherit all the "tall" alleles from their parents (1 mark)

Example 3: Applying knowledge to Caribbean context

Question: A nutritionist studied children in two Caribbean communities. Community A had regular access to fresh fruits, vegetables, fish, and meat. Community B experienced food insecurity with limited access to protein-rich foods.

(a) Identify the type of variation that would be shown if the heights of children in Community A were measured and plotted on a graph. [1 mark]

(b) Suggest why average height might differ between the two communities even though the children are genetically similar. [3 marks]

Mark scheme answers:

(a) Continuous variation (1 mark)

(b)

• Height is affected by both genetic and environmental factors (1 mark)
• Nutrition/diet is an environmental factor that influences growth/height (1 mark)
• Children in Community B may lack adequate protein, calcium, or other nutrients needed for growth, preventing them from reaching their genetic potential for height (1 mark)

Common mistakes and how to avoid them

• Confusing continuous and discontinuous variation: Remember that continuous variation can be measured and shows a range of values (height, mass), while discontinuous variation falls into distinct categories (blood group, tongue rolling). If you can find intermediates, it's continuous.

• Stating that discontinuous characteristics are affected by environment: Characteristics showing discontinuous variation are controlled by one or two genes and are not significantly influenced by environment. You cannot change your blood group through diet or exercise.

• Claiming purely environmental causes for height or intelligence: Always recognize that major characteristics like height, body mass, and intelligence result from both genetic and environmental factors. Examiners expect you to discuss both components.

• Using vague environmental factors: Be specific. Instead of writing "affected by environment," state precisely which environmental factors (nutrition, disease, exercise, climate) and explain how they influence the characteristic.

• Forgetting that variation is essential for evolution: While not always directly tested in this topic, remember that variation provides the raw material for natural selection and adaptation.

• Mixing up phenotype and genotype: The phenotype is what you observe; the genotype is the genetic makeup. For characteristics influenced by both genes and environment, identical genotypes can produce different phenotypes.

Exam technique for variation questions

• Command word "State" or "Name": Requires brief answers, usually one word or short phrase. For example, "State the type of variation shown by height" needs only "continuous variation" for the mark.

• Command word "Explain": Requires reasoning. You must say what happens and why. For example, explaining genetic variation should mention genes/alleles, inheritance from parents, and why this causes differences (unique combinations).

• Drawing graphs: For continuous variation, bars should touch (histogram) or use a line graph. For discontinuous variation, bars must be separated. Always label axes with quantities and units. The x-axis should show the characteristic, y-axis should show frequency.

• Mark allocation guides detail needed: A 1-mark question needs one clear point. A 3-mark question typically requires three distinct points or one point explained with supporting detail across three marking points.

Quick revision summary

Variation refers to differences between individuals. Continuous variation shows a range of values between extremes (height, mass, skin colour) and is typically controlled by many genes plus environmental factors, displaying a normal distribution. Discontinuous variation shows distinct categories with no intermediates (blood group, tongue rolling) and is controlled by one or two genes with minimal environmental influence. Genetic variation arises from sexual reproduction and mutation. Environmental variation results from nutrition, climate, disease, and lifestyle. Most human characteristics (height, mass, intelligence) result from both genetic and environmental factors working together. Understanding these concepts helps explain diversity in Caribbean populations and human populations worldwide.