Types and Factors of Variation

12.1

Types and Factors of Variation

Variation refers to the differences in characteristics found within the same population or species

Necessity of variation:

Enables natural environment to continue selecting beneficial characteristics and remove the non-suitable one
Enables survival of the species when the environment changes
Allows cross breeding among species to from new species

Continuous variation	Discontinuous variation
The differences in a character are not distinctive	The differences in a character are distinctive
Characters are quantitative; can be measured	Characters are qualitative; cannot be measured
Shows a normal distribution; exhibits intermediate characters	Exhibits distinctive characters with no intermediates
Influenced by environmental factors and controlled by two or more genes for the same character	Not influenced by environmental factors and controlled by a single gene that determines the differences between characters
The phenotype controlled by many pairs of alleles	The phenotype controlled by a pair of allele
Example: height, body weight, skin colour	Example: blood group, fingerprints

Similiraties between continuos variation and discontinuos variation :

The differences between continuos variation and discontinuos variation:

Continuos variation	Discontinuos variation
Presence of intermediate characteristics	No intermediate characteristics
No obvious differences in characteristics	Obvious and distinct differences in characteristics
Graph with normal distribution	Graph with discrete bars
Characteristic is controlled by many genes	Caharacteristic is controlled by one single gene
Influenced by environmental factors	Not influenced by environmental factors
Can be measured (quantitive)	Cannot be measured (qualitative)

Causes of variation:

Organisms of the same species differ in terms of morphology,physiology and genetics

Continuos variation	Discontinuos variation
Temperature Light pH	Crossing over Independent assortment of chromosomes Random fertilisation Mutation

Genetics factors:

Cross -linking during meiosis, independent selection during meiosis and random fertilization.
Gene mutations (permanent changes in the nucleotide sequence) caused by physical/chemical mutating agents.
Chromosome mutations: an increase or decrease in the number of chromosomes or changes to the structure of chromosomes.
Types of chromosomal mutations: truncation, translocation, duplication and inversion.
May cause genetic disorders; Turner syndrome, Down syndrome, trisomy X and Klinefelter syndrome.

Environmental factors:

Includes abiotic factors; temperature, light intensity, humidity, nutrition and soil fertility.
However, environmental factors cannot change the phenotype more than what has been predetermined by the genotype.
Examples:

The flowering plant (Hydrangea sp.); produces blue flowers when the soil is acidic but pink flowers in alkaline soil.
Well-nourished one identical twin becomes heavier and bigger, while poorly nourished twin becomes smaller and lighter.

Interactions between genetic and environmental factors:

Before industrialisation and the environment is not polluted, the grey-coloured moth population is higher than the other one because they can be camouflaged by the lichen-covered tree trunk.
Meanwhile, dark melanic moth dies easily, which eaten by predators because they cannot camouflage.
After industrialisation, the grey-coloured moth dies because they cannot camouflage as the pollution kills the lichen-covered three trunks.
Meanwhile, dark melanic population increases as they can be camouflaged by the polluted environment.
In such a case, the phenotype of the organism affects its chances of survival.