Darwin’s theory had two major tenets, one was that all living things (organisms) are related through common descent and the other was that natural selection was the main mechanism by which diversity in form and function among organisms although related, occurred.
In Darwin’s own words from the Origin of the species, published in 1859
“No one ought to feel surprise at much remaining as yet unexplained in regard to the origin of species and varieties, if he makes due allowance for our profound ignorance in regard to the mutual relations of all the beings which live around us. Who can explain why one species ranges widely and is very numerous, and why another allied species has a narrow range and is rare? Yet these relations are of the highest importance, for they determine the present welfare, and, as I believe, the future success and modification of every inhabitant of this world. Still less do we know of the mutual relations of the innumerable inhabitants of the world during the many past geological epochs in its history. Although much remains obscure, and will long remain obscure, I can entertain no doubt, after the most deliberate study and dispassionate judgment of which I am capable, that the view which most naturalists entertain, and which I formerly entertained -- namely, that each species has been independently created -- is erroneous. I am fully convinced that species are not immutable; but that those belonging to what are called the same genera are lineal descendants of some other and generally extinct species, in the same manner as the acknowledged varieties of any one species are the descendants of that species. Furthermore, I am convinced that Natural Selection has been the main but not exclusive means of modification.”
We will deal in the next few classes with the mechanisms of evolution, focusing of course on the major mechanism of evolution as proposed by Darwin, natural selection.
You can see the importance Darwin placed on this mechanism by noting that the extended title of the Origin of Species includes this mechanism.
The Origin of species by Means of Natural Selection of The Preservation of Favoured Races in the Struggle for Life First edition. Charles Darwin. http://www.talkorigins.org/faqs/origin.html
Darwin recognized genetic drift as a valid mechanism but did not think it was a major mechanism of change. A growing appreciation for a larger relative role of genetic drift in generating adaptation is one of the major refinements by today’s evolutionists to Darwin’s theory. What the role is of genetic drift in shaping diversity (descent modification) is still being debated today.
This is due to the fact that now we recognize that there is a lot of variation that may not be subject to strong selection in populations. How and why we will talk about later. There is also new evidence that there are a lot of different solutions to the same problem. https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment
This phenomenon is observed in a host of different species and situations.
https://faculty.washington.edu/hueyrb/pdfs/GilchristEAGenetica.pdf
"Parallel latitudinal clines across species and continents provide dramatic evidence of the efficacy of natural selec- tion, however little is known about the dynamics involved in cline formation. For example, several drosophilids and other ectotherms increase in body and wing size at higher latitudes. Here we compare evolution in an ancestral European and a recently introduced (North America) cline in wing size and shape in Drosophila subobscura .We show that clinal variation in wing size, spanning more than 15 degrees of latitude, has evolved in less than two decades. In females from Europe and North America, the clines are statistically indistinguishable however the cline for North American males is significantly shallower than that for European males. We document that while overall patterns of wing size are similar on two continents, the European cline is obtained largely through changing the proximal portion of the wing, whereas the North American cline is largely in the distal portion. We use data from sites collected in 1986/1988 (Pegueroles et al. 1995) and our 1997 collections to compare synchronic (divergence between contemporary populations that share a common ancestor) and allochronic (changes over time within a population) estimates of the rates of evolution. "
Why were there increases in wing size? The current hypothesis is that larger wings allowed the generation of relatively greater lift at low wing beat frequencies, hence would be selectively advantageous under cool ambient conditions.
So because of these we will be looking at mainly two forces of change: genetic drift and natural selection.
Review of the classical view of change and evolutionary forces.
https://evolution.berkeley.edu/evolibrary/article/evo_16
(mutation and migration for the future)