Natural Selection & Evolution
- Species do not stay the same over time; the species that we see around us today have developed over millions of years
- This process of species change is known as evolution
- Evolution can be defined as:
Changes in the heritable characteristics of organisms over generations
- Scientist Charles Darwin came up with the theory of evolution by natural selection after a five-year expedition around the world during which he observed an incredible variety of organisms
- Natural selection is the process that drives evolution. It can be defined as:
- The process by which organisms that are better adapted to their environment survive, reproduce, and pass on their advantageous alleles, causing advantageous characteristics to increase in frequency within a population
On the Origin of Species
- Darwin published his book "On the Origin of Species" in 1859
- It contained the following statements and deductions
- The increased survival chances of individuals with advantageous alleles mean that advantageous characteristics are more likely to be passed down through the generations because those individuals reproduce more
- The number of individuals in a population with a particular favourable characteristic will increase over time; the characteristic is said to increase in frequency
- Eventually, this favourable characteristic will become the most common of its kind in the population; the population can be said to have adapted to its environment through the process of natural selection
- While favourable characteristics increase in frequency by natural selection, unfavourable characteristics decrease in frequency by the same process
- Individuals with unfavourable characteristics are less likely to survive, reproduce, and pass on the alleles for their characteristics, so unfavourable characteristics are eventually lost from the population
NOS: Darwin’s theory provided a convincing mechanism and replaced Lamarckism. Students should understand the meaning of the term “paradigm shift”
- Darwin's theory was seen as very controversial at the time, it is said to have caused a paradigm shift
- Paradigm shifts occur when scientific research contradicts previous assumptions
- Darwin's theory replaced Lamarckism
- This was the idea that an organism could pass on physical characteristics they acquired during their lifetime to their offspring
- Nearly 200 years of genetic research backs up Darwin's theory of evolution by natural selection
An example of natural selection in rabbits
- Variation in fur colour exists within a rabbit population
- One allele codes for brown fur and another for white fur
- Rabbits have natural predators such as foxes, which act as a selection pressure
- The brown rabbits are more likely to survive and reproduce due to having more effective camouflage
- When the brown rabbits reproduce they pass on their alleles to their offspring
- The frequency of brown fur alleles in the population will increase
- Over many generations, the frequency of brown fur will increase and the frequency of white fur will decrease
Selection pressures in a rabbit population diagram
Selection pressures act on a rabbit population for one generation; predation by foxes causes the frequency of brown fur in rabbits to increase and the frequency of white fur in rabbits to decrease