SS3 First Term Biology Lesson Note – Variation and Evolution

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WEEK: 8

TOPIC: VARIATION AND EVOLUTION

CONTENT:

1. Morphological variations
2. Physiological variation
3. Application of variation

INTRODUCTION

Evolution is the cumulative changes in the characteristics of population or organisms occurring in the course of successive generations related by descent. Variations are differences in traits or characteristics between individuals of the same species.

Variation can be:

In Discontinuous variation, individuals fall into distinct categories e.g. Pea plant with either red flowers or white flowers. There are no intermediate forms between these traits. They are easily distinguishable and are not affected by environmental conditions.

Examples of such traits are

1. The ability to roll the tongue
2. Taste phenylthiocarbamide (PTB),
3. ABO blood groups in man and
4. Normal and vestigial wings in Drosophila.

Such discontinuous variation is brought about by one or a few genes.

In Continuous variation, there is a complete range of measurements from one extreme to another. In other words individuals do not fall into discrete categories,

Examples includes:

1. The colour of skin
2. Height etc

Man is not just dark or fair, nor is he either tall or short. There are many intermediate skin colours and heights. Such characteristics show a continuous variation from one extreme to the other. A continuous variation is brought about by the combined (or additive) effects of many genes.

There may be genes for dark skin and gene for fair skin. The more dark-skin genes a person has, the darker will his skin colour be. Continuous variations are also affected by the environmental conditions for example; a greater exposure to sunlight may cause a person to have a darker skin colour. This is also known as acquired variation. Other examples of continuous variation in man are intelligence and weight.

Variation found in or among organizations could be physical (morphological) or behavioural (physiological).

SUB-TOPIC 1: MORPHOLOGICAL VARIATION

Variations in physical traits or outward appearances in organizations are known as morphological variations.

Examples include:

• Height,
• weight,
• colour of the eye, hair or coat of animals,
• facial features
• finger print patterns.

1. Variation in Height: if you measure the heights of all the students in your class that are of the same age, there is likely to be a steady graduation from the students who are very short to those who are very tall.
2. Variations in weight can be due to genetic heredity or certain environmental factor such as i. lifestyle, diet or whether one is affected by an illness.

Weight of individual organisms in a population can vary greatly from newborn babies to teenagers and mature adults there is a continuous variation form one extreme to another (underweight to overweight).

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