EVOLUTION

Phenotypic variation refers to differences in observable traits among individuals of the same species

An organism’s phenotype is its observable characteristics.

-Genetic factors -Environmental factors -Both genetic and environmental factors

by the genetic variation within the population and the interaction of the environment on the individuals: Phenotypic variation = genetic variation + environmental influence

Genetic variation is transferred from one generation to the next, and it generates phenotypic variation within a species population

This variation arises mainly from mutations, which create new alleles These alleles may be beneficial, harmful, or have no effect Some may remain unexpressed for generations This generates new allele combinations in offspring, increasing diversity within a population

-Crossing over of non-sister chromatids during prophase I of meiosis -Independent assortment of chromosomes during metaphase I of meiosis -Random fertilisation of gametes during fertilisation This generates new allele combinations in offspring, increasing diversity within a population

-Random alignment of chromosomes results in different combinations of chromosomes and different allele combinations in each gamete -Genetic variation between gametes produced by an individual

-Exchange of genetic material between non-sister chromatids leads to new combinations of alleles on chromosomes. It can also break the linkage between genes -Genetic variation between gametes produced by an individual

-Any male gamete can fuse with any female gamete (random mating in a species population) -Genetic variation between zygotes and resulting individuals

-Random change in the DNA base sequence results in the generation of a new allele. A mutation must occur in gametes to be passed on -Genetic variation between individuals within a species population

genetic variation directly affects the DNA of the gametes, but variation in phenotype caused by the environment does not.

Environmental factors affect an individual’s chance of survival

In any species population, genetic variation means some individuals will have phenotypes that make them better adapted to survive

Those with a selective advantage are likely to produce more offspring and pass on their favourable alleles to the next generation

Over generations, differential reproductive success and natural selection lead to an increase in advantageous allele (and phenotype) frequencies When allele frequencies change significantly over time, the characteristics of the population change — if the changes are great enough, a new species may form

The formation of new species from pre-existing species over time, as a result of changes to gene pools and allele frequencies from generation to generation

stabilising selection directional selection disruptive selection

Stabilising selection favours the average phenotype and removes extremes, maintaining average values, e.g. human birth weigh

Directional selection favours individuals with a phenotype at one extreme, causing a shift in a population’s traits over time, e.g. antibiotic resistance in bacteria

Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles In other words, individuals with intermediate phenotypes or alleles are selected against

Disruptive selection causes polymorphism: the continued existence of two or more distinct phenotypes in a species

For evolution to occur, the new species population must be genetically and reproductively separated from the pre-existing species population When this happens, there can no longer be an exchange of genes between the two populations

-Reproductive isolation occurs when changes in alleles or phenotypes prevent certain individuals from breeding successfully with others in the same population -These changes reduce gene flow and can lead to speciation