Variation & Selection

Variation

• Variation: differences between individuals of the same species
• Phenotypic variation is caused by both genetic and environmental factors
• Continuous variation is influenced by genes and environment, resulting in a range of phenotypes between two extremes, e.g. height in humans
• Discontinuous variation is caused by genes alone and results in a limited number of distinct phenotypes (e.g. you are either blood group O, A, B or AB, nothing else)
• Mutation: genetic change
• Gene mutation: a change in the base sequence of DNA
• Mutation is the way in which new alleles are formed
• Ionising radiation and some chemicals increase the rate of mutation
• Mutation is a source of variation e.g. in Down’s syndrome, where a parent’s chromosomes are unevenly distributed in meiosis. In fertilisation, a zygote with a number of chromosomes that is not 46 is created (e.g. 23 + 24). Characteristics: broad forehead, short neck, downward-sloping eyes, short nose and mental retardation.

 

Sickle Cell Anaemia

• Disease in which the red blood cell has a sickle shape instead of a round biconcave shape, controlled by a recessive allele, which causes weakness, aching joints and poor circulation.
• The fact that it is recessive means that a heterozygous person can be a carrier: they have the allele but it is not expressed.
• Being a carrier of sickle cell anaemia makes you resistant to malaria
• In equatorial Africa, having sickle cell anaemic causes death. Malaria also causes death. But the carriers have immunity to malaria and have some symptoms of anaemia, in severe cases, they are very weak

 

Adaptive Features

• Adaptive feature:
  • inherited feature that helps an organism to survive and reproduce in its environment
  • the inherited functional features of an organism that increase its fitness
• Fitness: the probability of an organism surviving and reproducing in the environment in which it is found
• Xerophytes: live in deserts where water is scarce and evaporation is rapid, or in windy habitats. Their features are:
  • Deep roots to reach water far underground
  • Leaves reduced spines with minimum surface area for transpiration
  • Shallow spreading roots to collect occasional rain
  • Rolled leaves, leaf hairs and stomata sunk in pits to trap moist air
  • Waxy leaf cuticle, impermeable water
  • Stomata opening at night and closed at midday when evaporation is highest
  • E.g. cactus and marram grass

 

• Hydrophytes: live in wholly or partly submerged in water. Their features are:
  • Leaves are highly divided to create large surface area for absorption and photosynthesis
  • Very little cuticle formation
  • Lack of xylem tubes, no stomata underside of leaves
  • Stomata is in the upper surface, has a thick waxy layer to repel water and to keep the stomata open and clear
  • Roots are often reduced and root hairs are often absent

 

Natural Selection

• The greater chance of passing on of genes by the best adapted organisms.
• Variation is natural or random changes in all living organisms.
• Variation leads to survival of the fittest since the variations in certain organisms allow that organism to have an advantage over the others in its species in that area
• The surviving organisms reproduce, since they don’t get eaten up, so variation has caused the species to evolve.
• Evolution is caused by natural selection which is caused by a change in the environment

 

Artificial Selection

• Is breeding organisms with valued characteristics together in order to try to produce offspring which shares those useful characteristics (selective breeding).
• Can be used to produce organisms which are more economically valued
• For example: cows that produce more milk, wheat that is easier to separate from grain, dogs which have better appearance

 

• Selective breeding:
  • Selecting by humans of individuals with desirable features
  • Crossing three individuals to produce the next generation

 

Resistant Antibiotic

• Strains of antibiotic-resistant bacteria are developing as the use of antibiotics is increasing.
• In a group of many bacteria, one might mutate to be resistant to the antibiotic, as a result it reproduces and the others die making a new strain of bacteria, which is resistant to antibiotics.
• The susceptible (weak) bacteria are killed first
• Only resistant (strong) bacteria are left