Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
Biology (9700) notes A-Level - Selection and Evolution (CIE)
Variation
● Phenotypic variation: Difference in phenotypes between
organisms of the same species.
● Phenotypic variation is due to the genetic factors/
environmental factors/combination of genetic and
environmental factors.
A2
● Phenotype = genotype + environment
● Continuous variation: Difference between individuals of species
in which each one can lie at any point in the rage between
highest and lowest values (quantitative).
s
● Discontinuous variation: Difference between individuals of
am
species in which each one belongs to one of a small number of
distinct categories, with no intermediates (qualitative).
● Genetic basis of continuous variation: Interaction between
Ex
genetics and environment. At genetic level; different alleles at
a single locus have small effect on the phenotype, different
genes can have the same effect on phenotype and these add
ck
together to have an additive effect. If a large number of
genes have a combined effect on the phenotype they are known
as polygenes.
ra
● Genetic basis of discontinuous variation: This type of variation
occurs solely due to genetic factors. Different genes have
C
different effect on phenotype. Different alleles at a single
locus have a large effect on the phenotype.
Natural and artificial selection
● Natural selection occurs because populations have the capacity
to produce many offspring that compete for resources; in the
CRACK EXAMS A2 1
, Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
‘struggle of existence’ individuals that are best adapted are
most likely to survive and reproduce and pass on their alleles to
next generations.
● Environmental factors that affect the chance of survival of an
organism are selection pressures. The selection pressures can
have different effects on the allele frequency of a population
through natural selection.
● Stabilizing force: That keeps allele frequencies relatively
A2
constant over generations.
s
am
Ex
Picture from Biology A-Level CourseBook
● Directional force: Gradual change in allele frequency over
several generations.
ck
ra
C
Picture from Biology A-Level CourseBook
● Disruptive force: That maintains high frequencies of two
different sets of alleles.
CRACK EXAMS A2 2
Biology (9700) notes A-Level - Selection and Evolution (CIE)
Variation
● Phenotypic variation: Difference in phenotypes between
organisms of the same species.
● Phenotypic variation is due to the genetic factors/
environmental factors/combination of genetic and
environmental factors.
A2
● Phenotype = genotype + environment
● Continuous variation: Difference between individuals of species
in which each one can lie at any point in the rage between
highest and lowest values (quantitative).
s
● Discontinuous variation: Difference between individuals of
am
species in which each one belongs to one of a small number of
distinct categories, with no intermediates (qualitative).
● Genetic basis of continuous variation: Interaction between
Ex
genetics and environment. At genetic level; different alleles at
a single locus have small effect on the phenotype, different
genes can have the same effect on phenotype and these add
ck
together to have an additive effect. If a large number of
genes have a combined effect on the phenotype they are known
as polygenes.
ra
● Genetic basis of discontinuous variation: This type of variation
occurs solely due to genetic factors. Different genes have
C
different effect on phenotype. Different alleles at a single
locus have a large effect on the phenotype.
Natural and artificial selection
● Natural selection occurs because populations have the capacity
to produce many offspring that compete for resources; in the
CRACK EXAMS A2 1
, Complete notes for A-Levels BiologyCIE according to syllabus objectives - updated 2025
‘struggle of existence’ individuals that are best adapted are
most likely to survive and reproduce and pass on their alleles to
next generations.
● Environmental factors that affect the chance of survival of an
organism are selection pressures. The selection pressures can
have different effects on the allele frequency of a population
through natural selection.
● Stabilizing force: That keeps allele frequencies relatively
A2
constant over generations.
s
am
Ex
Picture from Biology A-Level CourseBook
● Directional force: Gradual change in allele frequency over
several generations.
ck
ra
C
Picture from Biology A-Level CourseBook
● Disruptive force: That maintains high frequencies of two
different sets of alleles.
CRACK EXAMS A2 2
