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  • Variation is all the difference between individuals of the same species
  • Those variations that can be inherited are determined by genes.They are genetic variations
  • Phenotypic variations may be brought about by genes, but can also be caused by the environment, or a combination of both genes and the environment
  • There are two kinds of variations: continuous and discontinuous
  • Continuous variation results in a range of phenotypes between two extremes, e.g. height in humans
  • Discontinuous variation results in a limited number of phenotypes with no intermediates, e.g. tongue rolling
  • Discontinuous variation is mostly caused by genes alone, e.g. A, B, AB, and O blood groups in humans


  • Gene mutation is a change in the base sequence of DNA
  • Mutation is the way in which new alleles are formed
  • Ionizing radiation and some chemicals increase the rate of mutation

Sickle Cell Anemia and Its Incidence to that of Malaria

  • Sickle cell anemia is caused by a mutation in the blood pigment hemoglobin
  • When the faulty hemoglobin is present in a red blood cell, it causes the cell to deform and become sickleshaped, especially when oxygen levels in the blood become low
  • In this state, the sickled red blood cells are less efficient at transporting oxygen and more likely to become stuck in a capillary, preventing blood flow
  • The faulty allele is dominated by the allele for normal hemoglobin, but still has some effect in a heterozygous genotype
  • The possible genotypes are:
    • HNHN normal hemoglobin; no anemia
    • HNHn, some abnormal hemoglobin; sickle cell trait (not life-threatening)
    • HnHn, abnormal hemoglobin; sickle cell anemia (life-threatening)
  • Malaria is a life-threatening disease caused by a parasite that invads red blood cells
  • The parasite is carried by some species of mosquito
  • A person who is heterozygous (HNHn) for sickle cell anemia has protection from malaria, because the malaria parasite is unable to invade and reproduce in the sickle cells
  • A person who is homozygous for sickle cell anemia (HnHn) also has protection, but is at high risk of dying from sickle cell anemia
  • A person with normal hemoglobin (HNHN) in a malaria country is at high risk of contracting malaria
  • When the distribution of malaria and sickle cell anemia are shown on a map of the world, it is found that the two coincide in tropical areas because of the selective advantage of the Hn allele in providing protection against malaria

    Photo Credit: Saburchill.com

Adaptive Features

  • Adaptive feature are the inherited functional features of an organism that increases its fitness
  • Fitness is the probability of an organism surviving and reproducing in the environment in which it is found

Adaptive Features of Xerophytes and Hydropytes

The Adaptive Features of Hydrophytes and Xerophytes to their Environments


  • Natural selection is the process by which plants and animals that can adapt to changes in their environment are able to survive and reproduce while those that cannot adapt do not survive
  • It gives the greater chance of passing on of genes by the best adapted organisms
  • Evolution is the change in adaptive features of a population over time as the result of natural selection
  • The process of adaptation is the process, resulting from natural selection, by which populations become more sullied to their environment over many generations

The Development of Strains of Antibiotic Resistant Bacteria as an Example of Evolution by Natural Selection

  • Slow changes in the environment results in adaptation in a population, to cope with the change
  • Failure to adapt could result in the species becoming extinct
  • This gradual change in the species through natural selection over time, in response to changes in the environment, is a possible mechanism for evolution
  • Bacteria reproduces rapidly – a new generation can be produced every 20 minutes by binary fusion
  • Antibiotics are used to treat bacterial infections; an antibiotic is a chemical that kills bacteria by preventing bacterial cell wall formation
  • Mutations occur during reproduction, which produce some variation in the population of bacteria
  • Individual bacteria with the most favorable features are most likely to survive and reproduce
  • A mutation occurs that enables a bacterium to resist being killed by antibiotic treatment, while the rest of the population is killed when treated
  • This bacterium would survive the treatment and breed, passing on the antibiotic-resistant gene to its offspring
  • Future treatment of this population of bacteria using the antibiotic would be ineffective

    Photo Credit: NIH.gov

Selective Breeding

  • Artificial selection is a method used by humans to produce varieties of animals and plants which have an increased economic importance
  • People use selective breeding to produce new varieties of a species, so that certain desirable traits are represented in successive generations
  • A variety is a type of a particular species that is different in some clear way from other varieties of that species
  • The different breeds of domestic dogs and large ears of maize corn are products of artificial selection

The Differences between Natural and Artificial Selection

  • Natural selection occurs in groups of living organism through the passing on of genes to the next generation by the best adapted organisms, without human inteference
  • Those with genes in environmental conditions, for example, are more likely to survive, while others die before they can breed and pass on their genes.
  • However, variation withing the population remains
  • Artificial selection is used by humans to produce varieties of animals and plants that have an increased economic importance
  • It is considered a safe way of developing new strains of organisms, compared with genetic engineering, and is a much faster process than natural selection
  • However, artificial selection removes variation from a population, leaving it susceptible to disease and unable to cope with changes in environmental conditions
  • Potentially, therefore, artificial selection puts a species at risk of extinction
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