Selection and evolution
Genetic variation is caused by:
Independent assortment of chromosomes, and therefore alleles during
meiosis
Crossing over between chromatids of homologous chromosomes during
meiosis
Random mating between organisms within a species
Random fertilisation of gametes
Mutation
Discontinuous variation includes phenotypic differences such as blood group as
these are qualitative measurements:
Different alleles at a single gene locus have large effects on the phenotype
Different genes have quite different effects on the phenotype
Continuous variation includes height or mass, as there is a range of values as these
are quantitative measurements:
Different alleles at a single gene locus have small effects on the phenotype
Different genes have the same, often additive, effect on the phenotype
A large number of genes may have a combined effect on a particular
phenotypic trait, these are known as polygenes
Environmental effects on the phenotype
Factors including food, light, soil nutrients or temperature may affect the phenotype
of an organism. For example the paws, nose and tail of the Himalayan are darker in
colour, which is caused by an allele that controls the formation of dark pigment at
low temperatures.
Natural selection
All organisms have the reproductive potential to increase their population. As a
population increases there are factors that keep the population down. These are
biotic – caused by other living organisms such as through predation, competition for
food or infection by pathogens, or they may be abiotic – caused by non-living
elements of the environment such as water supply or nutrient levels.
If the pressure of environmental factors is too great then the population size will
decrease and will only rise when the population numbers have fallen significantly.
Over time the population will oscillate about a mean level.
Some organisms will have a better chance of survival than others due to variation
producing beneficial characteristics. These will have a better chance of passing on
their characteristics to their offspring so the phenotype will become more prominent
in the population.
Fitness: the capacity of an organism to survive and transmit its genotype to its
offspring.
Selection pressures increase the chance of some alleles being passed on to the next
generation and decrease the chance of others. The effect of this selection on the
frequency of alleles in a population is called natural selection.
Genetic variation is caused by:
Independent assortment of chromosomes, and therefore alleles during
meiosis
Crossing over between chromatids of homologous chromosomes during
meiosis
Random mating between organisms within a species
Random fertilisation of gametes
Mutation
Discontinuous variation includes phenotypic differences such as blood group as
these are qualitative measurements:
Different alleles at a single gene locus have large effects on the phenotype
Different genes have quite different effects on the phenotype
Continuous variation includes height or mass, as there is a range of values as these
are quantitative measurements:
Different alleles at a single gene locus have small effects on the phenotype
Different genes have the same, often additive, effect on the phenotype
A large number of genes may have a combined effect on a particular
phenotypic trait, these are known as polygenes
Environmental effects on the phenotype
Factors including food, light, soil nutrients or temperature may affect the phenotype
of an organism. For example the paws, nose and tail of the Himalayan are darker in
colour, which is caused by an allele that controls the formation of dark pigment at
low temperatures.
Natural selection
All organisms have the reproductive potential to increase their population. As a
population increases there are factors that keep the population down. These are
biotic – caused by other living organisms such as through predation, competition for
food or infection by pathogens, or they may be abiotic – caused by non-living
elements of the environment such as water supply or nutrient levels.
If the pressure of environmental factors is too great then the population size will
decrease and will only rise when the population numbers have fallen significantly.
Over time the population will oscillate about a mean level.
Some organisms will have a better chance of survival than others due to variation
producing beneficial characteristics. These will have a better chance of passing on
their characteristics to their offspring so the phenotype will become more prominent
in the population.
Fitness: the capacity of an organism to survive and transmit its genotype to its
offspring.
Selection pressures increase the chance of some alleles being passed on to the next
generation and decrease the chance of others. The effect of this selection on the
frequency of alleles in a population is called natural selection.
