Creatures change with time. You might believe that God controls, guides, or designs those changes or you may have the opinion that random acts in the environment create mutations which change a species. The former idea is accepted by the majority of Christians while the latter is the basis of modern evolutionary biology. You might even combine these ideas (as many people do) and believe that God intelligently engineers genetic changes through radiation and chemicals, which mutates creatures and causes evolution. Either way, you are with the majority that agrees that species change with time.
Part of the idea of the evolution of species is that similar creatures had common ancestors at some point in the past. The date of common ancestry can be calculated by looking at the differences in the DNA between two species and counting the number of bases (T, A, G, C) that are different. Through controlled experimentation, biologists have found that out of each set of 30 million DNA base pairs, roughly one mutates each generation. (At this rate, humans accumulate a total of 200 to 300 mutated base pairs each generation.) Most of these mutations occur in bits of DNA material that (as far as biologists can determine) have no function. They help create RNA which builds 3-base codons that do not produce any proteins – they are like the white space on a printed page.
Other mutations in a base pair may change the RNA’s codon into one which makes a protein identical to the non-mutated codon. This is really common, too, because many of the triple-groups of base nucleotides (codons) create the same protein. For example, both CCA and CCG are codons that make the same protein, proline. So if a mutation causes the final ‘A’ in that string to be replaced by a ‘G’, the resulting protein would nevertheless be the same. That particular mutation would have no effect on the offspring. But other mutations may be detrimental. They result in new codons that make a protein that may cause a nervous system to lack connections or a stomach to lack a lining, for example. Such killer mutations end up in non-viable offspring, believed to cause roughly one-half of spontaneous miscarriages in mammals. If a creature survives with a disadvantaged mutation, it likely will have few (if any) offspring and the bad mutation may die out.
This leaves a small number of mutations that occur in important stretches of DNA where a change can result in a permanent positive difference in the offspring. In recent decades, scientists have learned how to accelerate such changes by irradiating millions of seeds and then germinating them, observing the mutated plants that result (many will never germinate or will die early, due to detrimental mutations). The genetic engineers will occasionally find some plants with desired new traits – pest or drought resistance, for example. They save seeds from those offspring. The seeds of the mutated offspring continue to carry the mutation. It is now a permanent part of the DNA sequence of all future generations. In this way, new colours of begonias were developed and frost-resistant tomatoes have been created.
There is more to this story and the development of genetic diversity than I have just covered. I haven’t mentioned how the detrimental mutations are washed from the present DNA data set nor have I indicated the ways in which mutations may be accelerated in nature. A great starting point for a better explanation of how scientists have calculated the rate of species diversification is found in Genomic clocks and evolutionary timescales by S. Blair Hedges and Sudhir Kumar, a highly readable article available free in PDF format.
The theory that genetic alteration occurs at a predictable rate has been tested and observed thousands of times. Mutations cause permanent changes in offspring and those changes are occasionally helpful to offspring survival. By examining the DNA of any two creatures – let’s say European honey bees (Apis mellifera) and bumblebees (Bombus distinguendus) – we can count the number of differences in a bit of DNA and calculate how long it would have taken the two species to have diverged from a common ancestor. In the case of a bumblebee and a honey bee, the two had a common ancestor about 108 million years ago. This is according to results of observations by 5 different research teams at 5 different labs, doing DNA examinations between 2007 and 2011. They did not all get the exact same result – Chenoweth, et al. calculated 90 million years while Litman et al. found it took 121 million years for the observed mutations to occur. The other three research teams had intermediate results. The consensus based on all five is 108 million years (the median of the 5 studies is 93 million years). This tells us a couple of things. It took an awful long time for the changes to add up. And there must be a heck of a lot of differences between these two insects. Surprisingly, there are more observed DNA differences between bumblebees and honey bees than between dogs and cats. (Dogs and cats had a common ancestor 55 million years ago – they have only half the genetic changes in their DNA as honey bees and bumblebees.)
In the past decade, there have been thousands of independent calculations made for various species divergences. I recommend the website The Time Tree of Life which has categorized 2,000 studies and 50,000 different creatures. They operate an interesting and informative site with an easy to use interface where you can enter your favourite creatures and find out the amount of time that has passed since those creatures had a common ancestor. Using data from their site, here are a few other species and the calculated time (in millions of years) of their last common ancestry:
Last Common Ancestry between Honey Bees and various other Species
|common name||species||million years ago|
|Asian honey bee||Apis cerana||18.5|
|great yellow bumblebee||Bombus distinguendus||108|
|leaf cutter bee||Megachile rotundata||122|
|house fly||Musca domestica||348|
|European bee-eater bird||Merops apiaster||847|
|alfalfa||Medicago sativa||1,515 (1.5 billion years)|
|AFB (American foul brood)||Paenibacillus larvae||4,290 (4.3 billion years)|