SELECTION AND EVOLUTION
1. Variation
Variation refers to the differences that exist between individuals of a species
Phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and
environmental factors
o Genetic factors: crossing over, independent assortment, random fusion of gametes, mutation in
the DNA base sequence of alleles
o Environmental factors: Length of sunlight hours, supply of food, availability of water,
temperature range, oxygen levels
Discontinuous variation is caused by qualitative differences (only genotype affects)
o Different genes have different effects on the phenotype as only few gene control a characteristic
o Different alleles at a single gene locus have a large effect on the phenotype
o Not affected by environment
Continuous variation is caused by quantitative differences (genotype and environment affects)
o Different genes can have the additive effect on the phenotype as many genes control a
characteristic. controlled by many genes. Normal distribution
o Different alleles at a single locus have a small effect on the phenotype
o Environment factors affect gene expression
The t-test can be used to compare the means of two sets of data and determine whether they
are significantly different or not (continuous date, data has a normal distribution, less than 30 samples,
standard deviation approximately same)
Allele frequency changes due to natural selection, genetic drift, mutation. This can cause a reduction
in gene pool.
, 2. Natural selection
Environmental factors or variation in phenotype of a
species affect the chance of survival – act as selection
pressure
Individuals with a fitter phenotype who are
advantageous are more likely to survive and pass on
their alleles to their offspring - alleles frequency change
over time and generations
Types of natural selection
Stabilising - keeps allele frequencies
constant over generations
Disruptive - maintains high frequencies of two
different sets of alleles
Directional - produces a gradual change in allele
frequencies over several generations
2. Genetic drift
Gradual change in allele frequencies in a population due to chance and not natural selection.
1. Variation
Variation refers to the differences that exist between individuals of a species
Phenotypic variation is due to genetic factors or environmental factors or a combination of genetic and
environmental factors
o Genetic factors: crossing over, independent assortment, random fusion of gametes, mutation in
the DNA base sequence of alleles
o Environmental factors: Length of sunlight hours, supply of food, availability of water,
temperature range, oxygen levels
Discontinuous variation is caused by qualitative differences (only genotype affects)
o Different genes have different effects on the phenotype as only few gene control a characteristic
o Different alleles at a single gene locus have a large effect on the phenotype
o Not affected by environment
Continuous variation is caused by quantitative differences (genotype and environment affects)
o Different genes can have the additive effect on the phenotype as many genes control a
characteristic. controlled by many genes. Normal distribution
o Different alleles at a single locus have a small effect on the phenotype
o Environment factors affect gene expression
The t-test can be used to compare the means of two sets of data and determine whether they
are significantly different or not (continuous date, data has a normal distribution, less than 30 samples,
standard deviation approximately same)
Allele frequency changes due to natural selection, genetic drift, mutation. This can cause a reduction
in gene pool.
, 2. Natural selection
Environmental factors or variation in phenotype of a
species affect the chance of survival – act as selection
pressure
Individuals with a fitter phenotype who are
advantageous are more likely to survive and pass on
their alleles to their offspring - alleles frequency change
over time and generations
Types of natural selection
Stabilising - keeps allele frequencies
constant over generations
Disruptive - maintains high frequencies of two
different sets of alleles
Directional - produces a gradual change in allele
frequencies over several generations
2. Genetic drift
Gradual change in allele frequencies in a population due to chance and not natural selection.
