What Is Evolution? Heritable Change Over Time
Evolution is not just a biological concept , it is the central organizing principle of all life sciences. At its core, evolution involves heritable changes in populations over time. These changes originate from mutations in DNA, which can alter the information encoded in genes and, ultimately, the traits expressed by organisms.
Because DNA encodes proteins, evolutionary changes leave a record in two key molecular archives:
- Base sequences of DNA and RNA , the nucleotide order in genes
- Amino acid sequences of proteins , the products of gene expression
By comparing these molecular sequences across species, scientists can infer how closely related organisms are and reconstruct their shared evolutionary history. This field is sometimes called molecular phylogenetics.
Natural selection is the primary mechanism driving evolutionary change. It works through four key components:
- Variation , individuals in a population differ in their heritable traits
- Heritability , traits are passed from parents to offspring via DNA
- Differential reproductive success , individuals with traits better suited to their environment survive and reproduce more
- Change in allele frequency , over generations, advantageous traits become more common in the population
All living organisms share the same genetic code and many conserved genes, which is strong molecular evidence that all life on Earth is connected through common descent.
DNA Sequence Comparisons as Evidence for Evolution
DNA sequencing allows scientists to compare the order of nucleotides (A, T, C, G) in specific genes across species. The underlying logic is simple: species that share a more recent common ancestor will have had less time to accumulate mutations, so their DNA sequences will be more similar.
- Closely related species (e.g., humans and chimpanzees) show very high sequence similarity
- Distantly related species (e.g., humans and fish) show greater sequence differences
- Conserved genes , those essential for survival, such as genes involved in respiration , change very slowly and are especially valuable for comparing organisms separated by hundreds of millions of years
Comparing DNA sequences:
- Humans vs. Chimpanzees: When comparing aligned single-nucleotide positions in coding regions, humans and chimpanzees share approximately 98–99% identity. When the full genome including insertions, deletions, and copy number variants is considered, the overall similarity is closer to 95–96%. Either way, these figures reflect a common ancestor approximately 5–7 million years ago.
- Humans vs. Fruit Flies: Approximately 60% of human genes known to be involved in disease have a recognisable functional counterpart (homologue) in Drosophila. This is a measure of gene-function conservation, not overall genome sequence identity , the actual nucleotide sequence identity is far lower. This still demonstrates shared ancestry over hundreds of millions of years.
The greater the DNA similarity, the more recently the two species diverged from a shared ancestor.
DNA can sometimes be extracted and sequenced from preserved remains of extinct organisms, such as Neanderthals. This allows direct molecular comparisons between extinct and living species , a powerful extension of the molecular evidence toolkit.