������̳

Alfred Russel Wallace was a naturalist who independently proposed the theory of evolution by natural selection. A great admirer of Charles Darwin, Wallace produced scientific journals with Darwin in 1858, which prompted Darwin to publish On the Origin of Species the following year.

After a variety of zoological discoveries, Wallace proposed a theory of evolution which matched the unpublished ideas Darwin had kept secret for nearly 20 years. This encouraged Darwin to collect his scientific ideas and collaborate with Wallace. They published their scientific ideas jointly in 1858.

Principles of evolution by natural selection

The idea behind the theory of evolution through the process of natural selection is that all species of living things have evolved from simple life forms over a period of time. The Earth is about 4.5 billion years old, and there is scientific evidence to suggest that life on Earth began more than 3 billion years ago.

Natural selection

The accepted theory of evolution explains that it happens by natural selection. The key points are given below.

• Individuals in a species show a wide range of variation and this variation is a result of differences in their genes caused by random mutations that can be inherited.
• In every population more offspring are produced than can survive. This overproduction leads to competition, eg for food.
• Individuals with characteristics most suited to their environment are more likely to survive and reproduce. This is commonly known as survival of the fittest. The genes that allow these individuals to be successful within their environment are passed on to their offspring, which results in these specific genes becoming more common.
• Individuals that are poorly adapted to their environment are less likely to survive and reproduce. Their genes are less likely to be passed on to the next generation.
• Over a period of time, a species will gradually evolve.
• Both genes and the environment can cause variation, but only genetic variation can be passed on to the next generation.
• If two populations of one species become increasingly different in phenotype that they can no longer interbreed to form fertile offspring, this can result in the formation of two species.

Modelling natural selection

This model can be used to demonstrate how natural selection can occur based on the characteristic of camouflage in a population of prey organisms.

Method

• Use a piece of green card as a background.
• Randomly place 20 green and 20 white pieces of string on the card to represent populations of prey organisms.
• Using a forceps to represent the mouth of the predator, collect as many pieces of string as you can in 10 seconds.
• Count how many green and white pieces are left and record.
• Repeat the process twice more.

Results of the model

This model shows that even though both species of prey are predated, the species with the highest degree of camouflage can survive. In reality, these organisms would be able to breed and pass on their camouflage genes. Where there was once an even distribution of green and white organisms, now the camouflaged organisms outnumber the non-camouflaged organisms by 10 to 1.

Limitations of the model

In this model the prey do not move. What if the white organisms were much faster than the green ones?

There may also be bias in the case of the predator. The scientist selecting the prey may do so not on the basis that some are easier to see, but on the basis that they were aware of what the results should be.