Chapter 17 Variation
1. The causes of variation
2. Continuous and discontinuous variation
3. Using means and standard deviations to quantify variation
Causes of variation
Variation refers to the differences observed among individuals within any given
population. Despite the high degree of similarity, especially in genetically identical
organisms such as twins, each individual is distinct.
Variations can arise due to variety of factors.
Genetic factors that cause variation
Genetic variation is variation due to the genes and alleles an individual possesses.
Variations in alleles can lead to different phenotypes, such as the diversity seen in
human blood groups.
Sources of genetic variation:
1. Mutations - Changes to genes and chromosomes that may be passed on to the
next generation.
2. Meiosis - New combinations of alleles are present in the gametes formed,
produced by independent assortment of chromosomes and crossing over
between chromatids.
3. Random fertilisation - Random fertilisation of gametes produces new
combinations of alleles in a zygote.
4. Random mating
Environmental factors that cause variation
Environmental variation is variation caused by the environment in which an organism
lives.
Differences in environmental factors like climate or diet can cause variations among
individuals, such as variations in accents or the presence of pierced ears.
, Environmental factors that can cause variation include:
1. Light
2. Nutrient and food availability
3. Temperature
4. Rainfall
5. Soil conditions
6. pH
Environmental influences can also affect the way an organism's genes are expressed.
The combined effect of genetic and environmental factors on variation
While genetic makeup sets the potential for variation, environmental conditions can
influence the actual outcome, such as with human height.
It is hard to distinguish between the effects of genetic and environmental influences on
variation as many combine to produce differences between individuals.
Polygenes:
These are different genes at different loci that all contribute to a particular aspect
of phenotype.
Their individual effects on a phenotype are too small to observe, but they can act
together to produce observable variation.
This combined effect of multiple genes is common in continuous variation.
Continuous and discontinuous variation
Variation may either be continuous or discontinuous.
Continuous variation:
This is when there are a range of values between two extremes without distinct
categories, which produce a spectrum of phenotypes.
An example is the variation in milk yield among cows.
Typically affected by both genes and environment.
Discontinuous variation:
This features clear, distinct categories with no intermediates.
For instance, human blood types can be classified as A, B, AB, or O.
1. The causes of variation
2. Continuous and discontinuous variation
3. Using means and standard deviations to quantify variation
Causes of variation
Variation refers to the differences observed among individuals within any given
population. Despite the high degree of similarity, especially in genetically identical
organisms such as twins, each individual is distinct.
Variations can arise due to variety of factors.
Genetic factors that cause variation
Genetic variation is variation due to the genes and alleles an individual possesses.
Variations in alleles can lead to different phenotypes, such as the diversity seen in
human blood groups.
Sources of genetic variation:
1. Mutations - Changes to genes and chromosomes that may be passed on to the
next generation.
2. Meiosis - New combinations of alleles are present in the gametes formed,
produced by independent assortment of chromosomes and crossing over
between chromatids.
3. Random fertilisation - Random fertilisation of gametes produces new
combinations of alleles in a zygote.
4. Random mating
Environmental factors that cause variation
Environmental variation is variation caused by the environment in which an organism
lives.
Differences in environmental factors like climate or diet can cause variations among
individuals, such as variations in accents or the presence of pierced ears.
, Environmental factors that can cause variation include:
1. Light
2. Nutrient and food availability
3. Temperature
4. Rainfall
5. Soil conditions
6. pH
Environmental influences can also affect the way an organism's genes are expressed.
The combined effect of genetic and environmental factors on variation
While genetic makeup sets the potential for variation, environmental conditions can
influence the actual outcome, such as with human height.
It is hard to distinguish between the effects of genetic and environmental influences on
variation as many combine to produce differences between individuals.
Polygenes:
These are different genes at different loci that all contribute to a particular aspect
of phenotype.
Their individual effects on a phenotype are too small to observe, but they can act
together to produce observable variation.
This combined effect of multiple genes is common in continuous variation.
Continuous and discontinuous variation
Variation may either be continuous or discontinuous.
Continuous variation:
This is when there are a range of values between two extremes without distinct
categories, which produce a spectrum of phenotypes.
An example is the variation in milk yield among cows.
Typically affected by both genes and environment.
Discontinuous variation:
This features clear, distinct categories with no intermediates.
For instance, human blood types can be classified as A, B, AB, or O.
