Natural Selection is a theory which suggests the occurrence of evolution among organisms which help the future generations survive in the world. Evolution has been discusses and observed over the ages, since the time of the ancient Greeks and was first witnessed in the 20th century. The theory was first proposed by the British biologist Charles Darwin and is accepted even today even though he was unaware of chromosomes, gene and DNA or even allele!
Discontinuous variation is a type of genetic variation. It describes the variation in genes in particular characteristics or catagories. My previous posts have been about discontinuous variation such as blood type, height, flower color, leguminous or non legume plant etc. To make this elaborate, we can take a particular population and set it up in a bar graph and show how the population shows a genetic variation in a particular feature such as blood group.
Continuous variation shows us the variation in Geno-type of a particular population. The continuous variation is limited to show the variation to a low number of categories, such as height or color . The bar graph can be used for the continuous variation.
Natural selection explains the process of evolution. This has been observed for ages. Evolution was most focused in the 20th century and the theory of natural selection was given by the famous biologist Charles Darwin. He made 3 observations and 2 deductions from these observations. These observations were:
- The populations across the planet reproduce rapidly and produce more offspring than needed to replace them.
- No matter how much over-population occurs, the amount of population is always stable i.e; the land still has a lot of space and does not fill up with people and animals.
- All species show genetic variation
After observing these three points he gave 2 deductions that:
- The population of different species need to struggle for obtaining food and shelter to survive. . He also deduced that an organism has to compete others for survival. They need to compete for food and for shelter. Those who survive get to reproduce in large amounts. Their next generation of organisms have a different habit and features as opposed to their parents which helps them to survive while those who do not survive will reproduce in small amounts and would soon perish. This gave way to the phrase “Struggle for Existence”
- They need to adapt to their environment. They need to adapt to the temperature of the environment, the structure of the environment, and have to live in many different living conditions. Basically, they have to fit into an environment to survive and reproduce. This gives way to the phrase “Survival of the Fittest”. (“Fittest” does not mean strongest. It means those organisms who are more “fit” to live in an environment.)
These deductions were made when Charles Darwin was completely unaware of DNA structures and alleles. Although evolution is slow it is most effected by selection pressures. Selection pressures are environmental factors which lead to evolution. Some of these factors are:
- food supply
- competition for nest
- competition for finding a mate
Some examples of natural selection are the use of insecticides on insects and the use of drugs on microorganisms.
Microorganisms like Penicilium are used to produce the antibiotic penicillin. When this was first used the results were spectacular and it was a great form of protection against bacterial diseases. Soon the bacteria’s genes were mutated. These mutated bacteria survived the effects of antibiotic and reproduced forming new bacteria species who have immunity against such antibiotics. Though evolution is a process of decades, the bacteria evolved rather quickly because the bacteria reproduces very quickly. Same is the case with Insects.
Insecticides like the DDT were used to kill pest and insects to protect the crops and produce higher crop yield. Soon the insects were mutated and they reproduced very quickly. The new generation produced had immunity against such insecticides.
Sickle Cell Anemia
Although the alleles for abnormal hemoglobin Hb^S Hb^S lead to death before the patient reaches maturity, people around the world are still affected by it, meaning this does not have selective disadvantage. It should be noticed that people who have sickle cell anemia are mainly found around the regions where malaria is common. Having a sickle cell allele produces protection against malaria. This has a selective advantage for those who are heterozygous with this allele.However, there are people in America, where Malaria is not common, with the disease mainly because they are of African descent.