Selection and evolution
Charles Darwin and Alfred Russel Wallace proposed a theory of natural selection to account for the
evolution of species in 1858. A year later, Darwin published On the Origin of Species providing
evidence
for the way in which aspects of the environment act as agents of selection and determine which variants
survive and which do not. The individuals best adapted to the prevailing conditions succeed in the
‘struggle for existence’.
Variation
The variation that exists within a species is categorised as continuous and discontinuous.
The environment has considerable influence on the expression of features that show continuous (or
quantitative) variation..
genetic variation is caused by:
1. Independent assortment of chromosomes and therefore alleles during meiosis
2. Crossing over between chromatids of homologous chromosomes during meiosis
3. Random mating between organisms within a species
4. Random fertilisation of gametes
5. Mutation
continuous Discontinuous
Significantly affected by the Usually unaffected by the environment
environment
Tend to be quantitative (can be Tend to be qualitative (cannot be
measured) measured)
Vary over a range of values Controlled by a few genes usually with
Controlled by many genes multiple alleles.
Range of characteristics. Fit into distinct categories
- Different alleles at a single gene - different alleles on single gene
locus have small effects on locus have large effects on the
phenotype phenotype.
- Different genes have the same, - Different genes have quite
often additive, effect on different effect on the phenotype
phenotype
- Large number of genes may have
combined effect on particular
phenotypic trait. (Polygenes)
polygenic inheritance
Eg: height and mass Eg: eye colour, or genetically inherited
diseases.
In discontinuous ch- squared testing appropriate.
Continuous potted on histogram might show typical bell shaped curve.
Charles Darwin and Alfred Russel Wallace proposed a theory of natural selection to account for the
evolution of species in 1858. A year later, Darwin published On the Origin of Species providing
evidence
for the way in which aspects of the environment act as agents of selection and determine which variants
survive and which do not. The individuals best adapted to the prevailing conditions succeed in the
‘struggle for existence’.
Variation
The variation that exists within a species is categorised as continuous and discontinuous.
The environment has considerable influence on the expression of features that show continuous (or
quantitative) variation..
genetic variation is caused by:
1. Independent assortment of chromosomes and therefore alleles during meiosis
2. Crossing over between chromatids of homologous chromosomes during meiosis
3. Random mating between organisms within a species
4. Random fertilisation of gametes
5. Mutation
continuous Discontinuous
Significantly affected by the Usually unaffected by the environment
environment
Tend to be quantitative (can be Tend to be qualitative (cannot be
measured) measured)
Vary over a range of values Controlled by a few genes usually with
Controlled by many genes multiple alleles.
Range of characteristics. Fit into distinct categories
- Different alleles at a single gene - different alleles on single gene
locus have small effects on locus have large effects on the
phenotype phenotype.
- Different genes have the same, - Different genes have quite
often additive, effect on different effect on the phenotype
phenotype
- Large number of genes may have
combined effect on particular
phenotypic trait. (Polygenes)
polygenic inheritance
Eg: height and mass Eg: eye colour, or genetically inherited
diseases.
In discontinuous ch- squared testing appropriate.
Continuous potted on histogram might show typical bell shaped curve.
