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Selection and evolution (Natural selection (Types of natural selection…

- - - - Quantitative variation, a range is present between 2 extremes
        (eg: height, weight, length, absorbance, speed, rate)
    - - Qualitative variation, clearly distinguishable with no intermediates
        (eg: ABO blood group, eye colour, number of stripes)
  - - - environmental effects allow the full genetic potential height to be reached or stunted
        
        if an individual has a low food intake, it might not reach its full genetic potential height
        
        if a plant grows in a soil with fewer nutrients or is exposed to a lower light intensity, it might not reach its full genetic potential height
    - - the colouring is caused by an allele to allow the formation of the dark pigment only at low temperatures
        
        since the extremities are the coldest parts of the cat, the are darker than the rest of the body
        
        when an area somewhere else is plucked of fur and kept cold, the new fur growing will be dark
- - - - p+q=1
        where p= frequency of dominant alleles and q= frequency of recessive alleles
        p^2+2pq+q^2=1
        where q^2= frequency of homozygous recessive individuals,
        p^2= frequency of homozygous dominant individuals,
        2pq= frequency of heterozygous dominant individuals
    - - organisms are not diploid
        
        asexual production
        
        overlapping generations
        
        mating is not random
        
        small population size
        
        sex linked alleles
        
        occurrence of migration, mutation and selection
  - - - the variation in the new population is limited when a small population splinters off from the main population
        
        different selection pressures result in the splinter population evolving differently
    - - the effect of phenotypes changing due to changes in allele frequency within a small population due to random chance/migration
    - - the population is dramatically reduced usually due to a natural disaster/ disease outbreak
  - - - the norm, common and average traits are favored
        
        extreme phenotypes are selected against, intermediates are selected for
        
        environmental conditions must be stable
        
        mean number remains, but the variety of phenotypes reduces
    - - one extreme phenotype is favoured
        
        another extreme and the intermediate phenotype is selected against
        
        environmental condition s change conferring selective advantage to a different allele
        
        graph will shift to one side that is selected for
    - - both extremes are favoured and the intermediate phenotype is selected against
    - - few bacteria have mutant alleles on their plasmids that causes antibiotic resistance, these bacteria are less likely to survive in normal conditions because this trait of theirs requires them to use more energy whilst taking in more nutrients
        
        however they have a selective advantage when antibiotics are used as the use of antibiotics exerts selection pressure on bacteria
        
        the resistant bacteria are able to survive, grow and reproduce which causes its allele frequency to increase
        
        since horizontal transmission of plasmids can also occur between different species of bacteria, resistance in one species can also be passed on to another
    - - peppered moths : more abundant in areas of low pollution as they can camouflage better with lichens on tree barks
        
        dark moths: more abundant in areas where pollution is high as they can camouflage better in tree barks with no lichen growth
        
        peppered moths have a selective advantage in unpolluted areas; dark moths have a selective advantage in polluted areas as they are less prone to predation in their respective areas
    - - malaria plays a significant role in the presence of the sickle cell allele
        
        people who are homozygous HbS are selected against because they become seriously anaemic
        
        people who are homozygous for HbA are selected against as they are more likely to die from malaria
        
        people who are heterozygous HbS are selected for because they don't suffer from sickle cell anaemia and suffer less from malaria due to less red blood cells available for Plasmodium to multiply
  - - - factors
        
        sufficient food supply
        
        sufficient living space
        
        suitable mating partner
        
        absence of predators
        
        absence of competition
      - r-strategist
        
        high reproductive cycle
        
        short lifespan
        
        high number of offsprings
        
        reaches sexual maturity at a very early age
        (eg: mice)
      - k-strategist
        
        low reproductive lifespan
        
        longer lifespan
        
        usually will care for offspring over an extended period of time
        
        reaches sexual maturity at a very late age
    - - Biotic: disease, predation, competition
        Abiotic: natural disaster, weather
    - - any species would have to deal with its biotic and abiotic factor
        
        the ecosystem carrying capacity is a specific number of a particular population it can support in a stable environment
        
        any factor that attempts to kill off poorly adapted members is regarded as selection pressure
- - - - mitochondrial DNA is inherited through the female line
        
        since the mitochondrial DNA is circular and unable to undergo crossing over, changes in the nucleotide sequence only arises by mutation
        
        as different humans have different mitochondrial DNA sequence (due to a common ancestor between 150000-200000 yeas ago), the 'molecular clock hypothesis' estimated from fossil evidence indicates that here is a constant rate of mutation over time
  - - - due to geographic isolation
        
        different forms arise due to mutation
        
        different environments have different selection pressures
        
        over time, the organism's morphology, physiology and behaviour become so different that they cannot give fertile offsprings with their parent species
    - - through polyploidy, in which cells have more the=an 2 complete sets of chromosomes as meiosis goes wrong during gametogenesis
        
        tetraploids are sterile as 4 chromosomes try to pair up during meiosis I (muddle up), they can only reproduce asexually
        
        triploids due to fusion of a 2n and n gamete, are sterile because 3 sets of chromosomes are shared evenly
        
        hence speciation occurs as the tetraploid/triploid and the original plant cannot interbreed
      - allopolyploidy: results from 2 sets of chromosomes from 1 species and another 2 sets from a closely related species
        
        autopolyploidy: caused by 4 sets of chromosomes from the same species
        meiosis happens more easily in allopolyploids because their chromosomes are not identical hence they can pair up. However it is still a new species as it cannnot interbreed with its parent species
  - - - individuals don't recognise each other as potential mates
        
        don't respond to mating behaviour
        
        physically unable to mate
        
        incompatibility of pollen and stigma in plants
        
        inability of fusion between male and female gametes
    - - failure of mitosis in zygote
        
        non-viable offsprings
        
        sterile offsprings