Molecular evidence and evolution happening today

1Evidence written in every cell

Darwin worked out the tree of life from bones, shells and embryos. Today every cell offers a second, independent record: the base sequence of its DNA and the amino acid sequences of its proteins. If species share a common ancestor, their sequences should be similar, and the more recently they shared that ancestor, the more similar they should be. These predictions could have failed. They did not: molecular trees built from DNA agree closely with the trees built from anatomy and fossils. This lesson shows how molecular evidence is used, and then looks at evolution that has been watched as it happened.

2The molecules all life shares

Almost every organism uses DNA as its genetic material, the same genetic code to translate it, the same 20 amino acids in its proteins, ribosomes to make those proteins, and ATP as its energy currency. There is no chemical reason why life had to be built this way, so the simplest explanation is that all living things inherited these features from one common ancestor. This is why a human gene can work inside a bacterium (Unit 10).

3Comparing sequences

Over time, mutations accumulate in the DNA of separated lineages. Two species that separated recently have had little time to accumulate differences; species that separated long ago have many. Cytochrome c, about 104 amino acids long in vertebrates, is a classic example:

The order matches the evidence from anatomy and fossils: other primates are our closest relatives, then other mammals, then birds, fish and, much more distantly, fungi. DNA comparisons give finer detail because they include changes that do not alter the protein. For example, in the parts of their genomes that can be lined up base by base, humans and chimpanzees are about 99% identical.

Other molecular methods include DNA hybridisation (an older technique: single strands from two species are allowed to pair, and the more similar they are, the higher the temperature needed to separate them) and comparing the immune reactions to proteins of different species.

The molecular clock. If a sequence changes at a roughly steady rate, the number of differences can be turned into a time. The clock is calibrated using pairs of species whose separation is dated by fossils. It is only approximate, because different genes, and different lineages, change at different rates.

4Evolution happening today

Because evolution is a change in allele frequencies, it can be observed directly in organisms with short generation times or strong selection pressures:

• Antibiotic resistance. Within a few years of each new antibiotic coming into use, resistant strains of bacteria appear. MRSA, a strain of Staphylococcus aureus resistant to methicillin and related antibiotics, is a serious cause of infections in hospitals worldwide.
• Drug resistance in HIV. HIV makes many copying errors as it reproduces, so a patient's virus population varies. If a single drug is used, resistant variants are selected within months. Treatment therefore uses a combination of drugs: a variant would need several rare mutations at once to resist them all.
• Resistance in malaria parasites and mosquitoes. Resistance of Plasmodium falciparum to the drug chloroquine spread across Africa in the late twentieth century, and the drug had to be replaced. Many populations of Anopheles mosquitoes have evolved resistance to the insecticides used on bed nets.
• Rock pocket mice. In the deserts of the south-western United States, mice on dark lava rock are mostly dark and mice on pale sandy rock are mostly pale; owls catch mice that stand out. In one population, dark fur is caused by a few mutations in a single pigment gene, Mc1r; in another, dark mice evolved through changes in different genes.
• Evolution in a laboratory. Since 1988, Richard Lenski's team has grown 12 populations of E. coli from one ancestor, freezing samples regularly as a "fossil record". After about 31 500 generations, one population evolved the ability to use citrate, a nutrient in the medium that normal E. coli cannot use when oxygen is present. Thawing earlier samples showed that this depended on earlier mutations in that population.