How Long Does It Take To Evolve A New Trait?

Fundies often claim that there is not enough time for significant evolution to take place. This is a bit strange as some of the same fundies talk about “kinds” being taken onboard the ark to get round the problem of the number of species Noah would have to gather. This however mean that all dog or cat species etc would have to have evolved from these “kinds” in a few thousand years – something they say cannot happen.

Anyway, how long would it take for a simple new characteristic to appear? Well, that is going to depend on genome size, population size, generation time and how much of an advantage the new characteristic is.

I will use the example of the rock pocket mouse (Chaetodipus intermedius). These are found in the south western USA and Mexico. They occur in two forms; a sandy coloured form and a dark form. The difference in colour is due to mutations in the Melanocortin 1- Receptor gene (Mc1r). It known that the mutation rate in the mouse genome is in the order of 2 mutations per billion bases (a mouse genome contains about 5 billion bases). Furthermore, there are 10 different mutations of the Mc1r genes that cause the dark coat colour. Therefore the possibility of a dark coat mutation is 10 (mutations) X 2 (copies of the gene) X mutation rate (2 per billion). This means that a relevant mutation will occur in 40 times per billion mice. That is 1 in 25 million mice. The local population sizes of this species is in the order of 10 000 to 100 000 individuals. This means about half this number are female. The average number of pups a female has per year is 5, so taking the lower estimate of 10 000, this means that 25 000 pups are born each year (or 250 000 for the upper estimate). Again, using the lower estimate, if we multiply this number by the probability of a relevant mutation occurring (1 in 25 million) we get a black mouse arising every 1000 years (or every 100 years for the upper estimate of population size). This is because only one mutant copy is necessary to darken coat colour. These calculations apply to producing just about any particular mutant that involves a simple change in a codon, and it should be remembered that populations contain many mutations and evolutionary change does not happen one step at a time. Many characteristics can be selected a once, so there is no reason that coat colour and hair length or foot size can not all be subject to selection at the same time.

So, what about the selectability of the mutant? What I haven’t mentioned is that these mice occur in an area that has a sandy substrate, interspersed with areas of dark basaltic lava, so sandy mice on dark rock are at a disadvantage as they are more visible to predators. So, the greater the advantage, the faster the spread of the gene through out the population will be. This is proportional to the selection coefficient (s). The equation linking number of generations (t) to s and the breeding population (Ne) is:

T = (2/s) natural log (2Ne) generations

The estimated s value for this mutation is around 0.01. This means that once a black mutation arises, most mice in the population will be black after 1981 generations. (less than 1981 years).
The lava flows these mice live on are around 1.7 million years old. This means that a black mutation has occurred independently 1 700 times (lower population estimate) or 17 000 times (upper population estimate). Then it is left to natural selection.
One final prediction is that if this scenario is true, we should find different mutations in the population. This is indeed what we find. Creationism is clearly a dishonest intellectual black hole.