Key Components

• Relatedness: refers to how recently a certain species split from a common ancestor
• Recombination: the mixing of alleles as chromosomes cross over in meiosis

Molecular Clock

Overtime, the DNA of different species accumulates changes due to mutations. These changes in DNA result in changes in the proteins produced. Because these changes accumulate in at a steady rate, by calculating the rate at which they occur, the differences between the proteins or DNA sequence of two related species can be used to measure the length of time since they shared a common ancestor

DNA Hybridisation

DNA hybridisation is a technique that provides a measure of the similarity of the genetic material of two species and gives an estimate of the genetic distance between them

• Two DNA fragments from two species are heated to 94^oC, separating the polynucleotide strands
• The single strands of DNA are then cooled to 55^oC to allow the DNA strands to anneal
• The more similar the strands are, the more hydrogen bonds will form between complementary nucleotides
• The hybrid DNA is then heated again and the temperature that the two strands separate at is measured

The higher the temperature to which the hybrid DNA can be heated before the strands separate, the higher the degree of hybridisation and the more closely related the species are. This is because strands of DNA are held together by hydrogen bonds between complementary nucleotides. Therefore, the less complementary base pairs there are, the less hydrogen bonds there are. And the less hydrogen bonds here, the less heat required to separate the strands

DNA Hybridisation

Mitochondrial DNA

mtDNA is present in large quantities, long lasting, only inherited from the mother, does not undergo recombination and is not repaired by repair enzymes, so mutations occur rather rapidly in the hypervariable regions. mtDNA sequencing is ideal for comparing organisms within a species or those who have diverged recently. This is achieved by measuring the number of mutations and comparing it to the expected mutation rate.

Other applications of mtDNA is that it allows us to determine relationships between modern human populations, migration patterns of modern humans and possible models of human evolution