Variation and Natural Selection


Variation is defined as differences between individuals within the same species. Variation can be caused by various factors. The most important ones that you need to be aware of are: 

  • Sexual reproduction – The mixing of genes from the father & mother to produce offspring with a completely unique genetic make-up causes variation
  • Mutation – Random changes of the DNA of an organism
    • Often happen for no apparent reason
      • Can be caused due to ionizing radiation
      • Can be caused by mutagens – chemicals that affect the copying of DNA such as lead, mercury and their compounds
    • A type of mutation is when the X chromosomes in a female do not separate and one egg gets both while another gets none. The egg with none dies and if the egg with both is fertilized, the baby has Down’s Syndrome.

Genetic v/s phenotypic variation

  • Genetic variation – The differences in genetics among individuals within the same species (mainly due to sexual reproduction & mutation)
  • Phenotypic variation – The variability of phenotypes within a population (influenced by both the genetic & environmental factors)

Continuous v/s discontinuous variation

  • Continuous variation – Results in a range of phenotypes between two extremes i.e. height/weight/etc. is caused by both genes and the environment
  • Discontinuous variation – Results in limited number of set phenotypes with no intermediates i.e. tongue rolling and is caused by genes alone

Adaptive Features

Adaptive features are inherited functional features of an organism that increases in fitness. Fitness is defined as the probability of an organism surviving and reproducing in the environment in which it is found. 

Adaptive features of xerophytes – Xerophytes are plants that survive with very little water i.e. cactus. These plants have adaptive features which allow it to maximize water uptake and minimize water loss. 

  • Thick cuticle to minimize evaporation
  • Small leaves to reduce surface area for evaporation 
  • Less stomata to reduce the surface area for diffusion
  • Sunken stomata to maintain humidity around stomata (and therefore reduce diffusion of water out of the plant) 
  • Rolled leaves to maintain humidity around the stomata 
  • Extensive roots to maximize water uptake 

Adaptive features of hydrophytes – Hydrophytes are plants that grow on or in water.

  • Wide flat leaves to increase surface area for floatation and to be exposed to as much light as possible 
  • Reduced plant structure as there is less need for a rigid structural support 
  • Thin waxy cuticle as water preservation isn’t as important 
  • Small roots as water can be obtained directly through the leaf and stem 
  • Stomata are open on the upper side of the leaf in order to maximize gas exchange without worrying about too much water

Natural Selection and Evolution

Natural selection – Describes the concept that for a given environment, organisms with the most beneficial features are ‘selected’ to survive and pass on their genes to the next generation. 

The steps are as follows: 

  • There is variation within a population
  • Many offspring are produced 
  • There is competition for resources among individuals within the population 
  • There is struggle for survival
  • The most ‘fit’ individuals that are more adapted to the environment will survive and reproduce 
  • Fitter individuals pass on their genes/alleles to their offspring 

Evolution – Natural selection ensures that only the most favourable genes get passed onto the next generation. Over time, populations therefore become more suited to their environment. This is known as evolution. 

Antibiotic resistance – Upon the use of antibiotics, most bacteria of a certain strain will die. However, due to pure chance, there may be some bacteria that have had a genetic mutation with gives them resistance to the antibiotics used. 

These resistant bacteria survive and reproduce rapidly. This produces many more bacteria that also have the resistant gene. Eventually, the antibiotic will be ineffective due to the abundance of resistant bacteria. 

Sickle Cell Anaemia and Malaria

Sickle cell anaemia is caused by the mutation in a gene that codes for haemoglobin. The abnormal base sequence of the haemoglobin gene causes sickle-shaped red blood cells. 

Sickle celled RBCs are less efficient at carrying oxygen, and more likely to become stuck in capillaries preventing blood flow. 

The faulty Hb gene is dominated by the normal Hb gene but it can still have an effect in the heterozygous genotype. 

Consider the three possible genotypes (N = normal, n = abnormal) 

  • HN HN – 100% normal Hb
  • HN Hn – Mostly normal Hb, but some abnormal Hb found i.e. non-life-threatening sickle cell trait
  • Hn Hn – 100% abnormal Hb, life-threatening condition

The symptoms of sickle-cell anaemia are as follows: 

  • Fatigue 
  • Pain crises
  • Bacterial infections
  • Lung & heart injury 
  • Leg ulcers 
  • Eye damage 

Malaria is a serious and sometimes fatal disease caused by a parasite that commonly infects a certain type of mosquito which feeds on humans.

Sickle-cell anaemia provides protection against malaria, because the parasites cannot penetrate sickle-celled RBCs.

Therefore, sickle-celled genotypes are favourable in countries where malaria is highly prevalent. 

  • A person with sickle cell anaemia (Hn Hn) will be fully protected against malaria, but likely to die from sickle-cell disease
  • A person that has no sickle cell anaemia (HN HN) has no protection from malaria, and is therefore likely to contract the disease
  • A person that is a sickle-cell carrier (HN Hn) gains moderate malarial protection, and will not die from sickle cell disease

Artificial Selection

Artificial selection is the intentional reproduction of animals and plants by humans that have desirable traits. 

By using this method, the organisms can pass down favourable traits to their offspring to produce more organisms with the characteristics that we want. 

Natural selection is natural because the environment provides selective pressure for organisms that live within the environment. Artificial selection is artificial because it is not the environment which provides the pressure, it is human manipulation instead. 

Selective breeding

  • Selection by humans of individuals with desirable traits 
  • Crossing these individuals to produce next generation
  • Selection of offspring showing the desirable characteristics 
  • Over many generations, this can improve the quality of crop plants and domesticated animals