Variation and Evolution

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  • Variation is the differences that exist between individuals. Even clones show some variation.
  • Variation can occur within and between species.
  • Continuous - The individuals in a population vary within a range, there are no distinct categories. An example of this would be height.
  • Discontinuous - There are two or more distinct categories, and each individual only falls into one category. An example of this would be blood group.
  • Variation can be genetic or environmental.
  • The genes an organism has make up its genotype. The differences in genotype make up the phenotype, the characteristics displayed by an individual.
  • Inherited characteristics that show continuous variation are usually influenced by many genes - these characteristics are polygenic.
  • Inherited characteristics that show discontinuous variation are usually influenced by only one gene. Characteristics controlled by one gene are monogenic.
  • Differences in the environment can also cause variation.
  • Genetic factors determine genotype and the characteristics an individual's born with, but environmental factors can influence how some characteristics develop.
  • Most phenotypic variation is caused by the combination of genotype and environmental factors.
  • Phenotypic variation influenced by both usually shows continuous variation.
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  • Evolution is caused by a change in allele frequency.
  • The complete range of alleles present in a population is called the gene pool.
  • New alleles are usually generated by a mutation in genes.
  • How often an allele occurs in a population is called the allele frequency.
  • The frequency of alleles changes over time - this is evolution.
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Natural Selection

  • Individuals within a population vary because they have different alleles.
  • Selection pressures such as disease, predation, and competition create a struggle for survival.
  • Because individuals vary, some are better adapted to the selection pressures than others.
  • Individuals that have an allele that increases their chance of survival are more liekly to survive, reproduce, and pass on the beneficial allele, than individuals with different alleles.
  • This means that a greater proportion of the next generation inherit the beneficial allele.
  • They are also more likely to survive, reproduce, and pass on their genes.
  • So the frequency of the beneficial allele increases from generation to generation.
  • This proces is called natural selection.
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Common Characteristics

  • Whether the environment is stable or changing affects which characteristics are selected for by natural selection.
  • When the environment isn't changing much, individuals with characteristics towards the middle of the range are more likely to survive and reproduce.
  • This is called stabilising selection and it reduces the range of possible phenotypes.
  • When there's a change in the environment, individuals with a characteristic of an extreme type are more likely to survive and reproduce.
  • This is called directional selection.
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Genetic Drift

  • Evolution can also occur due to genetic drift - instead of environmental factors affecting which individuals survive, breed, and pass on their alleles, chance dictates which alleles are passed on.
  • Individuals within their population show variation in their genotypes.
  • By chance, the allele for one genotype is passed on to the offspring more often than the others.
  • The number of individuals with the allele increases.
  • if by chance the same allele is passed on more often again and again, it can lead to evolution as the allele becomes more common in the population.
  • Natural selection and genetic drift can work alongside each other to drive evolution, but one process can drive evolution more than the other depending on population size.
  • Evolution by genetic drift usually has a greater effect in smaller populations where chance has a greater influence. in larger populations, any chance factors tend to even out across the whole population.
  • Evolution by genetic drift also has a greater effect if there's a natural bottleneck - when a large population becomes smaller because of a natural disaster.
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Hardy-Weinberg Principle

  • The Hardy-Weinberg principle predicts that frequencies of alleles in populations wont change from one generation to the next.
  • This prediction is only true under certain conditions - it has to be a large population without immigration, emmigration, mutation or natural selection. There also needs to be random mating - all possible genotypes can breed with all others.
  • The Hardy Weinberg equations are based on this principle. They can be used to estimate the frequency of particular alleles and genotypes within a population.
  • If the allele frequencies do not change between generations in a large population, then immigration, emmigration, mutation or natural selection must have occured.
    • p + q = 1
    • p = The frequency of the dominant allele
    • q = The frequency of the recessive allele
    • p^2 + 2pq + q^2 = 1
    • p^2 = The frequency of the homozygous dominant genotype
    • q^2 = The frequency of the homozygous recessive genotype
    • 2pq = The frequency of the heterozygous genotype.
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Natural and Artificial Selection


  • Both change the allele frequencies in the next generation - the alleles that code for the beneficial/desireable characteristic wil become more common in the next generation.
  • Both may make use of random mutations when they occur - if random mutation produces an allele that gives a desireable/beneficial phenotype, it will be selected for the next generation.


  • In natural selection, the organisms that reproduce are selected by the environment, but in artificial selection this is carried out in humans.
  • Artificial selection aims for a predetermined result, but in natural selection the result is unpredictable.
  • Natural selection makes the species better adapted to the environment, but artificial selection makes the organism more useful for humans.
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  • Speciation is the development of a new species.
  • A species is defined as a group of similar organisms that can reproduce to form fertile offspring.
  • It occurs when populations of the same species become reproductively isolated - changes in allele frequencies cause changes in phenotype that mean they can no longer breed together to form fertile offspring.
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Geographical Isolation

  • Geographical isolation and natural selection lead to speciation.
  • Geographical isolation occurs when a physical barrier divides a population of a species - floods, volcanic eruptions, and earthquakes can all cause barriers that isolate some individuals from the main population.
  • Conditions on either side of the barrier will be slightly different.
  • Because the environment is different on each side, different characteristics wil be more common due to natural selection and different selection pressures.
    • Because different characteristics will be advantageous on each side, the allele frequencies will change in a population.
    • Mutations will take place independantly in each population, changing the allele frequencies.
    • The changes in allele frequencies will lead to changes in phenotypic frequencies - different phenotypes will become more common on each side.
  • Eventually, individuals from different populations will have changed so much that they won't be able to breed with one another to produce fertile offspring - they'll have become reproductively isolated.
  • The two groups will become separate species.
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Reproductive Isolation

  • Reproductive isolation occurs because the changes in the alleles and the phenotypes of the two populations prevents that from successfully breeding together.
  • The changes include:
    • Seasonal changes - Individuals from the same population develop different flowering or mating seasons, or become sexually active at different times of the year.
    • Mechanical changes - Changes in genitalia prevent successful mating.
    • Behavioural changes - A group of individuals develop courtship rituals that arent attractive to the main population.
  • A population doesn't have to be geographically isolated to become reproductively isolated.
  • Random mutations can occur within a population, resulting in changes, preventing members of that population breeding with other members of the species.
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Classifying Species

  • The traditional definition of a species is a group of similar organisms that can reproduce to give fertile offspring. This is the Biological Species Concept.
  • Scientists have issues with this definition - problems deciding which species an organism belongs to or if it's a new, distinct species.
  • Reproductive behaviour can't always be studied;
    • Animals may be extinct, so their mating behaviour can't be studied.
    •  They might reproduce asexually - they never reproduce together so they might be the same species.
    • There might be practical or ethical issues involved.
  • Because of these issues, scientists sometimes use the phylogenetic species concept to classify organisms.
  • Phylogenetics is the study of the evolutionary history of groups of organisms.
  • All organisms have evolved from shared common ancestors. The more closely related two species are, the more recent their common ancestor.
  • Scientists can use phylogenetics to decide which species an organism belongs to or if it's a new species - if it's closely related to members of another species then it's probably the same species.
  • However, there's no cut off telling how closely related two organisms have to be.
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