A new study seems to have invalidated the long-held notion that evolution can only be witnessed over an extremely long period of time. Scientists have discovered that in a species of chicken, the rate of mutation in mitochondrial DNA is 15 times faster than previously thought.
Mitochondrial DNA (mtDNA), sometimes referred to as our "second genome," is a type of DNA that is found outside our chromosomes. It is contained in structures called mitochondria, the “powerhouses” of the cells where sugars are converted into energy. In humans, it contains just 37 genes, an extremely small figure compared to the 20,000-25,000 in the nuclear DNA wound into our chromosomes. MtDNA is also almost exclusively inherited from the mother.
The study focused on a well-documented 50-year pedigree population of White Plymouth Rock chickens developed at Virginia Tech by Professor Paul Siegel. The original population comes from seven partially interbred lines, and since 1957 scientists have been using a selective mating approach. This led to chickens that are 10 times the size of the standard White Plymouth Rock chickens at 56 days old.
The team collected blood samples from 12 chickens of the same generation and analyzed them using the most distantly related maternal lines; in this way they could reconstruct how mtDNA is inherited in the population.
Using fossil-based estimates, earlier work determined that the rate of change in a mitochondrial genome is around 2 percent per million years. Over a 50-year period, no mutations should have therefore been observed, but the study actually found two mutations in the mtDNA.
Senior author Professor Larson, from Oxford University, said in a statement: "Our observations reveal that evolution is always moving quickly but we tend not to see it because we typically measure it over longer time periods."
The study also showed the first direct evidence of paternal transmission of mtDNA in birds. "The one thing everyone knew about mitochondria is that it is almost exclusively passed down the maternal line, but we identified chicks who inherited their mitochondria from their father, meaning so-called 'paternal leakage' can happen in avian populations," study lead author Dr Michelle Alexander, from the University of York, said in a statement. "Both of these findings demonstrate the speed and dynamism of evolution when observed over short time periods."
The research has been published in Biology Letters.