(Mods, I know this is a science topic, but please let it stay here as evolution is a recurring theme in religious discussions here).
Note: This thread is different from the other threads debating evolution; if you want to debate please post in one of those threads. This thread is educational. Genuine questions for clarification are welcome.
Did you ever notice that children don’t look exactly like their parents? There may be similarities, but they are not clones. And every child is different: some siblings are more athletic; some are a bit smarter; some have better immune systems, etc. We call this
variation. There are two main sources of variation: 1) genetic recombination (dad donates half his DNA to each child and mom donates half hers and it is a different “half” each time they have a child [except for identical twins]); and 2) mutations (random errors when the DNA is copied). Most mutations don’t seem to make any difference at all. Some mutations result in something bad, such as cystic fibrosis or sickle cell disease. But, some mutations give that child an advantage. For example, among women with a certain type of high-grade ovarian cancer, having BRCA2 genetic mutations was associated with improved overall survival and improved response to chemotherapy, compared to women with BRCA wild-type (genetic type used as a reference to compare genetic mutations), according to a study in the October 12 issue of
JAMA. Another example of a beneficial mutation is one form of genetic resistance to AIDS that is caused by a genetic defect that prevents the person’s immune cells from developing a "receptor" that lets the AIDS virus break in. If the so-called CCR5 receptor -- which scientists say is akin to a lock -- isn't there, the virus can't break into the cell and take it over.
It is amazing what a single mutation can do in affecting large scale physical changes. It only takes one single mutation to create an extra finger in those people who have 6 fingers on one hand. How is that possible? Well, the genes that contain the instructions for building a finger are already in our DNA, the mutation occurs in the part of our DNA that instructs how many times to run the “build finger instructions”. Within our DNA, we still have instructions to build a tail, but those genes are turned off in most of us, but one can see how a single mutation to turn on those genes could result in a human with a full tail.
OK, back to children. Although children are different from their parents, they also have a lot in common with them. Some traits are passed down from parents to children. We call this
inheritance. Not every child will get a copy of the gene from the parent that controls a certain trait, so not every child will have that trait. Furthermore, some traits are dominant, meaning if there is a copy of the gene in the DNA, the trait will be present. Whereas, some traits are recessive, only appearing if there are two copies of the genes (one from each parent). Now, most people are ok with the idea that the genes each of your kids get are completely due to chance.
There are two sets of DNA that never undergo genetic recombination, therefore, they are passed on unchanged from one generation to the next, unless there was a mutation. One set is known as the Y-chromosome and is passed on from father to son, generation after generation, unchanged except for the occasional mutations that are also passed on. The cool thing is, you can trace back families that way. If Man 1 had two sons, and Son 1 got a perfect copy of Man 1’s Y-chromosome, but Son 2’s copy of Man 1’s Y-chromosome had one mutation in it. Generations from now, we can tell which men are descendants of Son 1 and which from Son 2, because the men who have Son 2’s mutation are Son 2’s descendants. And these mutations add up over time as the human family branches. Scientists have been able to trace these mutations back. One starts to see concentrations of these mutations in certain parts of the world. For instance, Native American men share some mutations that are unique to them, but they also have some mutations that they share with men from Mongolia, etc, which is one piece of evidence that Native Americans are descended from people who migrated across the Bering Strait. It is also through such research that scientists believe our earliest human ancestors lived in Africa. I know I said I wasn’t going to include links, but if you want to learn more about this, go here:
https://genographic.nationalgeograph.../en/atlas.html
It is also by tracing these mutations and shared genes, we are able to see that we share some distant ancestors with other primates such as chimpanzees, etc. So, the same technology that is used to determine a child’s paternity can show we share great-great-great-grandfathers with chimpanzees, etc. The lineage of mutations shows paternity. And the great thing is we can double check our findings from Y-chromosome mutations with the other set of DNA that doesn’t undergo genetic recombination: mitochondrial DNA. Inside each woman’s eggs is mitochondria that produce the energy the cell needs to divide and do all the other work cells do. In any case, mitochondrial DNA is passed on unchanged (except for mutations) from mother to all her children, but then passed on to the next generation only through the daughters. When we trace them back, we find the same patterns of human migration that we saw from the Y-chromosome data, and again we find that we came out of Africa.
From the wiki entry on Human Evolution: "DNA comparisons also show that we share approximately 98.8-99% of our DNA with our closest cousin, the chimpanzee (i.e., we only differ in 1-1.2% of our DNA). Evidence also includes ancestral DNA evidence ("
Junk DNA") that humans share with earlier ancestors. For example, Humans have structures in their genetic make-up that were once used to produce enzymes to process vitamin C (it is called L-gulonolactone oxidase). Most other animals have this functioning DNA but at some point in our history, a mutation disabled the gene – whilst leaving behind its remnants as junk DNA. This particular junk DNA indicates a common ancestry with other species on earth.
"
So, I’ve now talked about variation, genetic recombination, mutations – both harmful and beneficial, heritability, and tracing lineage through inherited mutations, even across species. Now, let’s talk about something that thankfully isn’t common among us humans today, but was more prevalent in the past and is still prevalent now among other animals: high mortality rates. Lots of kids died; some from disease, some from predators, some from lack of enough food, or lack of proper nutrition, etc. Those kids who were best suited for their environment lived; those who were not died. So, if you were lucky enough through genetic recombination or mutation to have a stronger immune system, or you could run faster to get away from predators, or smart enough to outwit your predators or prey or enemies, stored calories better, etc, etc, then you survived whereas your brothers or sisters without those traits didn’t. So, you passed on your traits to the next generation, and as a species your species became better fit to survive and reproduce in its environment.
Now, suppose, your brother leads a small band of followers across the mountains down to the sea. So, he and his band are now in a different environment with different predators, different prey, different diseases, etc. So, his descendants become better and better fit for their unique environment. After a very long time and many generations, his descendants and your descendants are very different, both optimized for their own environment. They are so different that were they to ever meet and try to have children, they can’t. By definition, they are now different species. There is fossil evidence (which I haven’t talked about yet) that there have been other “human” species in the past living at the same time as our ancestors, but our human species was the only one that survived.
From one generation to the next, the struggle for existence will favor individuals with some variations over others and thereby change the frequency of traits within the population. This process is
natural selection. The traits that confer an advantage to those individuals who leave more offspring are called
adaptations.
In order for natural selection to operate on a trait, the trait must possess heritable variation and must confer an advantage in the competition for resources, survivability, and reproduction. If one of these requirements does not occur, then the trait does not experience natural selection.
An example of evolution by human selection we all are familiar with is the human selective breeding of dogs. The domestic dog is a domesticated form of the grey wolf. From the wiki entry on dogs: “There is conclusive evidence dogs genetically diverged from their wolf ancestors at least 15,000 years ago…The bulk of the scientific evidence for the evolution of the domestic dog stems from archaeological findings and mitochondrial DNA studies”. While all dogs are genetically very similar, selective breeding has reinforced certain characteristics in certain populations of dogs, giving rise to dog types and dog breeds.
OK, as for evidence of evolution by natural selection, we have talked about our daily observations that children inherit some traits but also are not identical to their parents. It makes sense that some of these natural differences between kids would give kids an advantage over their peers in acquiring resources, surviving and reproducing. This would naturally increase in the population through heritability the presence of those traits in the next generation. These advantageous traits could add up over time. But, this just shows the morphing of an entire generation over time. It makes sense that if a population becomes separated through migration, etc, the cumulative changes in each group could cause the groups to diverge in traits enough that they become separate species.
We have three branches of evidence. 1) Looking at the similarities/differences between living species in physical features (i.e., we look more closely related to the Great Apes than to cats), 2) the DNA evidence of which species we share genes with and following the lineage of mutations, and 3) the fossil record (to be discussed right now).
We all know from our experience what happens to most animals bones after they die in the woods. They get spread around by scavengers, possibly crushed by some animal’s jaws, and slowly turn into dirt. Those bones obviously won’t become fossils. It takes special circumstances for fossils to be made and they are rare in location and in time. Plus, it is difficult to find the fossils that do exist. So, our fossil record is spotty.
The following ideas were part of the intellectual climate of Darwin's time and are copied from Evolution and Natural Selection .
- No one knew how old the earth was, but geologists were beginning to make estimates that the earth was considerably older than explained by biblical creation. Geologists were learning more about strata, or layers formed by successive periods of the deposition of sediments. This suggested a time sequence, with younger strata overlying older strata.
- A concept called uniformitarianism (http://www.geog.ouc.bc.ca/physgeog/contents/10c.html - broken link), due largely to the influential geologist Charles Lyell, undertook to decipher earth history under the working hypothesis that present conditions and processes are the key to the past, by investigating ongoing, observable processes such as erosion and the deposition of sediments.
- Discoveries of fossils were accumulating during the 18th and 19th centuries. At first naturalists thought they were finding remains of unknown but still living species. As fossil finds continued, however, it became apparent that nothing like giant dinosaurs was known from anywhere on the planet. Furthermore, as early as 1800, Cuvier pointed out that the deeper the strata, the less similar fossils were to existing species.
Since that time, geologists have become good at determining the age of rocks in different strata through various dating techniques that we can discuss if you want to. The fossils found in the rock of a certain strata are assumed to be the same age as the rock of that strata. By comparing ages of fossils, locations, and the development of different physical features in these fossils through time, scientists have been able to piece together part of the evolutional lineage of various classes of organisms. We do have a number of fossils of what appear to be human ancestors that are not quite human due to the size and shape of the skull and other features.
I think that lays out the basics. I welcome input from those more familiar with the theory of evolution than I am to add or correct anything you wish. For those who are not familiar with what I have just laid out, I’d be happy to go into more detail of any part you have questions or interests about. If you think you spot any holes or unreasonable assumptions, please point them out so that I can fill them in.
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