When Darwin developed his theory of Evolution through Natural Selection, he didn’t know about genetics. He was unaware of DNA and the process genes play in the variations in life he observed everywhere. Today, genetics is a remarkable field that is exploding with new information on an almost daily basis. I’m sure you are familiar with DNA Testing from the news and how it is used in forensics during the investigation of a crime. Genetic researchers are working on all manner of investigative and exploratory research on genes and continue to produce jaw-dropping results. For example researchers have recently identified that a finger found in a cave is neither Neanderthal nor human but a separate and previously unknown hominid! The young girl appears to have lived during a period when Neanderthals and early humans were both alive. More on that in a moment.
How genetics play a role in the evolution of life is being continually researched. Scientists have repeatedly observed genetic changes in DNA, called mutations, occur randomly. E. Coli was kept in a lab and observed for many generations. One of the key attributes of E. Coli is its inability to metabolize citrate. There were all sorts of variations of the bacteria over the generations but one of the most amazing variations was that some of the bacteria developed the ability to metabolize citrate, effectively becoming something other than E. Coli! It’s a fascinating experiment and you can read a brief summary here.
Mutations in DNA occur regularly, often through replicating errors. Many times the errors are corrected, sometimes they aren’t. The modified DNA is now copied with its mutated changes. When you pass on genes to offspring, the genes are passed along with the new mutations. Scientists can compare the mutations in the DNA of two people to see how closely they’re related, and by calculating the number of mutations, scientists can determine how far back in time different groups split apart.
So consider the large span of time we are working with when we talk about life on Earth. Then consider the incredibly broad varieties of life, from microscopic parasites, to plants, to primates including of course, humans. How many genetic mutations might have occurred over millions of years of evolutionary time? As these genetic variations are passed along to the next generation through heredity, some of the new traits are more beneficial to reproduction of the species than other traits. Over subsequent generations those possessing those genetic traits will come to dominate their environment through their ability to reproduce at will. This is the simple, elegant and efficient model of natural selection. Life seeks to continue and wherever improvements are made in the organism that give it a competitive advantage, that organism will thrive.
The truly amazing thing about genetics to me is how genetically, species aren’t that different from each other. The difference between human DNA and that of our closest relative, the chimpanzee, is less than 2%. What I find really fascinating is that of the Great Apes (humans, chimps, gorillas and orangutans), only the human has 46 chromosomes (23 pair) vs the 48 (24 pair) the others have. It’s long been believed that because of our genetic similarities, chimps and humans likely had a common ancestor; a primate species that is now extinct of course, that led to the evolutionary rise of Homo sapiens and chimpanzees. Genetics allowed us to research this hypothesis and the result was truly extraordinary. Human beings and chimps have nearly identical DNA sequences in two chromosomes, however the human’s chromosomes are fused together, while the chimps are not. This fused chromosome, Human Chromosome 2, is the difference in our lineage and is an overwhelming piece of evidence in the evolution of species. 
Hox genes are remarkable blueprints for animal organisms in the very early stages of their development. The Hox turns on sets of other genes to guide the formation of distinct regions in the body; legs, arms, wings, etc. Humans have Hox genes too. Hox genes can foul up and create mutations. In fact, mutated flies led to the discovery of the Hox gene.
Now back to the newest find in the story of human evolution and how genetics shines the light on our ancient history. In Siberia, a finger bone was found in a cave in 2008. Scientists were able to extract DNA and sequenced the genome and found significant differences between the Neanderthal genome and the human genome! This finger belonged to a young girl of a different species variation now known as Denisovans. The DNA also showed that Denisovans bred with Homo sapiens and Neanderthals, sharing their genetic codes.
“The team compared the Denisova DNA to the genetic code from 38 present-day humans from 53 populations. More than 5 percent of the DNA of two ethnic Melanesians matched the sequences from the Denisova girl they found.” 
Evolutionary biologists continue filling in the picture of human evolution. It is now genetically apparent that three similar hominin species lived at the same time, and interbred. Only one of these species, Homo sapiens, survived. It is a remarkable tale of nature’s continuous improvement through trial and error, genetic mutation, interbreeding and adaptability.
So let’s summarize what we’ve covered so far in Part I, Part II and now Part III recalling my primary purpose in writing these posts. I want to help bring the 40% of Americans who don’t believe in evolution into the light of scientific discovery and move them to the other side of the ledger.
There are variations in all species and some of these variations are passed along through heredity to the offspring. Whenever there is limited resources, the variations that are best suited to survive long enough to reproduce will come to dominate the species over generations. If there is enough divergence between variations as they adapt to different environments, the genetic differences may modify enough over time to prevent the variations from interbreeding successfully, creating speciation or new species. Genetics play a major role in evolution which we are continuing to develop. We’ve seen mutations of genes get replicated in the lab, we’ve seen genetic evidence of interbreeding between species of hominins through the discovery of fossilized remains of our ancestors. We’ve identified Human Chromosome 2, the fused variant of two chromosomes that differentiate us from our chimpanzee cousins.
Faithful Readers, we lay people without PhD’s can thank our scientific brethren for bringing us this knowledge of the natural world. We don’t need advanced scientific degrees to see the evidence and understand how life changes over time. It’s right in front of our eyes every year as flu virus’ mutate and become resistant to our vaccines. If you recall the Swine Flu then you’ve seen nature at her evolutionary best.
I’ll try to find more basic evolutionary topics to cover in the future, but with the knowledge I’ve assembled here for you non-evo believers, unless you’re willing to shrug off the evidence of life’s glorious mission to adapt and spread, you should be able to answer Yes when asked “do you believe in evolution?”