Evidence for Evolution

Biology: Cambridge International A Level, topic 17.3.2

The Evidence for Evolution by Natural Selection

The theory of evolution by natural selection has three main evidence sources:

Fossils are the remains or traces of ancient life.
Fossils are found in rocks and have been preserved by natural processes.
Fossils show the gradual changes in organisms over vast periods of time.
They can help us to see physical similarities in organisms, but not all organisms leave a fossil behind when they die. This means there are big gaps in the fossil record that we have.

Genomes of different species can be sequenced and compared to see how similar they are to each other.
Even distantly related organisms share a surprising amount of the same DNA, implying that organisms shared a common ancestor.
The more similar the genomes are to each other, the more closely related they are; this often confirms what fossil evidence tells us.
In eukaryotes, we can also sequence mitochondrial DNA, which has been used to examine human evolutionary relationships through the maternal line.

Molecular evidence includes any evidence from biological molecules, including DNA, but also comparisons of RNA, amino acid sequences, and overall protein structure.
Some proteins, like cytochrome c in mitochondria (used in respiration), are highly conserved across organisms. By looking at small changes in highly conserved proteins, we can work out relatedness of different taxa.

Sometimes, organisms we think are closely related because of their physical characteristics have more differences in their genome sequence than we might expect.
This is why it is important to use a variety of evidence to confirm evolutionary relationships.