Cambridge A Levels A2 Biology Chapter 17 Selection and Evolution
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Cambridge A Levels A2 Biology
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Chapter 17 Selection and Evolution: 50 pages
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Chapter 17 Selection and Evolution
17.1 Variation
17.2 Natural and Artificial Selection
17.3 Evolution
17.1 Variation
Definitions
genetic variation:
differences between the DNA base sequences of individuals within a species
Phenotypic variation:
Differences between the observable characteristics of individuals within a species
Sources of Genetic Variation:
i. independent assortment of chromosomes, and therefore alleles, during meiosis
ii. crossing over between chromatids of homologous chromosomes during meiosis
iii. random fusion of gametes, and random mating between organisms within a species
iv. mutation
• The first four of these processes reshuffle existing alleles in the population.
Offspring have combinations of alleles which differ from those of their parents and from each other.
This genetic variation produces phenotypic variation.
• Mutation, however, does not reshuffle alleles that are already present.
Mutation can produce completely new alleles.
This may happen, for example, if a mistake occurs in DNA replication, so that a new base sequence
occurs in a gene.
The new allele is very often recessive, so it frequently does not show up in the population until some
generations after the mutation actually occurred, when by chance two descendants of organisms in
which the mutation happened mate and produce offspring.
,Both genes and the environment cause phenotypic variation.
• For example, a plant that has a genotype that allows it to grow tall will only be able to grow tall if it has
plenty of sunlight and a good supply of minerals and water in the soil.
If these are lacking, then it will not grow to the potential size determined by its genotype.
The phenotype of an organism is a result of interaction between genetic and environmental factors.
Mutations that occur in body cells often have no effects at all on the organism.
• If only one cell is affected, then the overall function of the tissue of which the cell is part continues as
normal.
• An exception is if the mutation affects the control of the mitotic cell cycle, allowing the cell to divide
uncontrollably, which can lead to the development of a tumour.
However, mutations in cells in the ovaries or testes of an animal, or in the ovaries or anthers of a plant, may be
inherited by offspring.
• If a cell containing a mutation divides to form gametes, then the gametes may also contain the mutated
allele.
• If such a gamete is one of the two which fuse to form a zygote, then the mutated allele will also be in
the zygote.
• This single cell then divides repeatedly to form a new organism, in which all the cells will contain the
mutated allele.
So genetic variation, whether caused by the reshuffling of alleles during meiosis and sexual reproduction or by
the introduction of new alleles by mutation, can be passed on by parents to their offspring, producing
differences in phenotype.
But variation caused by the environment is not passed on by parents to their offspring.
▪ Variation caused by the environment cannot be passed on by an organism to its
offspring.
▪ For example, some organisms might be larger than others because they had access to
better quality food while they were growing.
▪ This type of variation is not passed on by parents to their offspring.
,Continuous and Discontinuous Variation
Continuous Variation Discontinuous Variation
Differences between individuals of a species Differences between individuals of a species
in which each one can lie at any point in the in which each one belongs to one of a small
range between the highest and lowest number of distinct categories, with no
values. intermediates.
Quantitative differences, in a range between Qualitative differences fall into clearly
two extremes. distinguishable categories, with no
intermediates.
Eg: Blood groups Eg: Heights or masses of individuals
, Variation in Humans
The characteristics which differ between individuals of the same species constitute variation.
These variation may be:
▪ Non-inheritable (phenotypic variation arising from environmental factors)
▪ Inheritable (phenotypic variation due to genetic variation, ie differences in base
sequence of DNA)
Vp = VG + VE
Environmental factors may limit the expression of a gene or may trigger/switch on a gene)
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