Evolution

Two scientists are credited with independently proposing the theory of evolution. 

A recap of GCSE evolution

1. Alfred Russel Wallace 

2. Charles Darwin

Darwin AND Wallace

Darwins observations

1. Offspring are similar to their parents but all are different to each other. 

2. Organisms have many offspring but the population usually stays the same size. 

Darwins observations led to these explanations

When many young are produced, there will be pressure on the supplies of food and resources. The offspring that are better adapted to survive (reach and eat the food, make use of the resources, avoid difficulties) are the offspring that survive and reproduce. The genes that lead to the beneficial adaptations are passed onto their offspring, and this second generation also has variety. The best adapted of these will survive to pass their genes on until there some alleles are lost from a population as they are not favoured and passed on. 

Natural Selection

Evidence for Evolution

Fossils

Darwin was a keen geologist and noticed that the fossils left behind from long dead and extinct species, were very similar to species living in his time. 

He proposed that fossil species died out and were replaced by a new modern species. These modern species were better adapted to the environment. 

Biological molecules

Molecules that are essential to the survival of an organism are unlikely to change very much from generation to generation. Species that have evolved from another species are likely to have very similar molecules to the original ancestor. If the two species we are comparing have had a very recent common ancestor, these molecules could be identical. A good example of this is the protein cytochrome c, an essential enzyme in respiration. 

Below. the red boxes indicate some essential amino acids that are the same in all of the organisms. These amino acids cannot be changed. You can also see that the closer the species are to each other, the more amino acids they have in common. 

How many do Pigs and humans have?

How many do humans and dogfish have?

DNA 

DNA sequences can also be lined up next to each other and the differences and similarities counted.