What causes phenotypic variation?
- Genetic variation
- Environment
What is the primary cause of genetic variation?
Mutation
Meiosis
Random fusion of gametes
How do mutations cause genetic variation?
Mutation results in the generation of new alleles.
- The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype
- New alleles are not always seen in the individual that they first occur in
- They can remain hidden within a population for several generations before they contribute to phenotypic variation
What are other causes of genetic variation?
- Crossing over of non-sister chromatids during prophase I of meiosis
- Independent segregation of homologous chromosomes during metaphase I of meiosis
- Random fusion of gametes during fertilisation
How does crossing over cause genetic variation?
It can result in a new combination of alleles on the two chromosomes. (Genetic variation between gametes produced by an individual)
How does independent segregation cause genetic variation?
Causes different combinations of chromosomes in daughter cells. (Genetic variation between gametes produced by an individual)
How does random fertilisation cause genetic variation?
Each gamete carries substantially different alleles. (Genetic variation between zygotes and resulting individuals)
What is natural selection?
The process by which individuals with a fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations.
What is genetic drift?
A gradual change in allele frequencies in a small population due to chance and not natural selection.
What does selective advantage result in?
- Individuals with phenotypes providing selective advantages are likely to produce more offspring and pass on their favourable alleles to the next generation
- Over time, this has an effect on the allele frequencies within a gene pool
- Alleles that are favourable will increase in frequency in a gene pool
- Alleles that are not favourable will decrease in frequency in a gene pool
What are the types of selection?
- directional
- stabilising
- disruptive
what is directional selection
when phenotypes of one extreme are selected for and the other selected against causing allele frequency to shift overtime
what is stabilising selection?
acts against bot extremes in a range of phenotypes, individuals closer to the mean are favoured
- reduces variety of phenotypes (narrow spread)
Describe disruptive selection.
when the individuals contain the alleles coding for either extreme trait are more likely to survive and pass on their alleles
- middling trait becomes less frequent, over generations
- eventually no individuals will have the middling trait therefore end up with two new species
What is evolution?
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.
when does speciation occur and what is it?
speciation: the process that results in the creation of a new species
- occurs when one original population of the same species becomes reproductively isolated
–> now 2 pops of the same species that CANNOT breed together
describe sympatric speciation
- populations can become reproductively isolated due to differences in behaviour
- Sympatric speciation takes place with no geographical barrier
Seasonal changes - some individuals in a population may develop different mating or flowering seasons to the rest of the population
Mechanical changes - some individuals in a population may develop changes in their genitalia that prevent them from mating successfully with individuals of the opposite sex
Behavioural changes - some individuals in a population may develop changes in their courtship behaviours, meaning they can no longer attract individuals of the opposite sex for mating
Overtime, these individuals will accumulate different mutations and not be able to reproduce together to produce fertile offspring
What are the types of speciation?
- Allopatric (geographically)
- Sympatric (changes in reproductive mechanisms)
Describe allopatric speciation.
- populations can become separated geographically
- geographical barrier separates the original population into 2, which are now unable to reproduce
- the random mutations that occur will only be present in either populations
- overtime both populations accumulate beneficial mutations to help them survive in their environment (due to slectional pressures)
- unable to reproduce to produce fertile offspring therefore now classed as 2 different species
Explain how speciation can occur due to geographical barriers?
1) Geographic isolation / allopatric
2) Reproductive separation
3) Different selection pressures
4) Random advantageous mutation
5) Alleles are passed on over many generations
6) Allele frequency increases
7) Eventually, the 2 populations can no longer interbreed
features of licen/ saprobionts
- 2 different species co existing
- made up of algae: : can photosynthesise so creating glucose, carbohydrates which it shares w fungus
- made up of fungus: can absorb water and release extracellular enzymes onto rock which can break down rock to release minerals
- meaning theyre able to survive even though:
- little water available
- no food source
- no minerals
what causes natural selection
- competition of resources
- impact of disease
- impact of predation
Organism’s with the phenotypes that give the selective advantage will…
- survive
- produce offspring
- pass on favourable allele
- EFFECT: change in allele frequency
what is genetic drift?
the change in allele frequency within a population between generations
- substantial genetic drift can lead to evolution
