Having looked at the evidence over the years, I believe things can change given time, so while I don't believe in the Darwinian mechanics of evolution, I believe things can change given time. For example, we can breed a species of cat that has a fair chance of having green eyes. This, of course, proves that things can change given time.
First off, this “Darwinian mechanics” is actually called “Natural Selection”, not “Darwinian mechanics”.
And second, your example described “sexual selection” and basic genetics, not “Natural Selection”.
Both sexual selection and Natural Selection required genetics, but in evolution, Natural Selection deal with changes at population-level changes, and unless it concern bacteria, the changes are lot slower, in term of many thousands of generations.
Sexual selection come from breeding program, and nothing to do with adapting/survival to changing environments.
For example, people breed horses for either speed or stamina or the ability to pull heavier carts or wagons. This is sexual selection, not natural selection. It has nothing to do with the horse survival.
And in Natural Selection, you are forgetting that the “environmental” factors that drive evolution.
Meaning that external forces as factors for changes, like the combination of changes to climate, terrain, the availability of food supply (changes in diet), etc, these can cause life (eg animals) to adapt to new environment so future generations can survive and thrive better than ancestors, or go extinct.
Changing the colours of eyes have nothing to do with Natural Selection. There are no reason to think or to believe that changing a breed of cats having green eyes will assist in cats adapting to new environment and ensure cats with green eyes in survival in generations to come.
To give you example, I want to focus on one family and genus - the bears.
Before the most recent Ice Age or a series of glaciation periods (known as the Quaternary Ice Age or Quaternary Glacial Cycle), the brown bears predominantly thrive in the North America and the Eurasian continents, with no polar bears.
When the first of glaciation periods arrived some regions in Europe, Asia and America were covered in ice sheets, but some parts weren’t.
Below, is the maximum extent of the Quaternary Ice Age, as you can see, the ice sheets didn’t cover the whole continents:
Those places that weren’t covered in ice sheets, continued to have annual summer and winter seasons. The brown bears that live in regions with no ice sheets, as a species, remained unchanged; the brown bear continued to find food, eating meat (eg seals, fishes) and sometimes fruits air leaves, and come winters, they would find shelter and hibernate through the coldest time of the year.
But those brown bears that found themselves living in regions, covered in ice sheets, there would be no summer seasons for thousands or even tens of thousands of years.
So these brown bears have to adapt in Arctic-like region during the glacial periods, where they have to adapt physically, and change the way they lived, or else go extinct.
The brown bears that do survive, began to change slowly, adapting better and better with each new generations. And at some point in time, the polar bears exist, and they are only species that can survive better than their ancestors, the brown bears. The divergence from the brown bear species occurred somewhere between 150 and 110 thousands years ago; the oldest polar bear remains are about 130,000 years old.
What they (brown bears) used to eat, like fishes that catch in rivers or near the bank of lake shore, are not enough to survive in regions with no summer in their lifetime.
So their diets have to change, to something with higher fat contents, like from sea seals and sea lions. Eating a lot of fat, will allow bears to retain fat in their bodies, to insulate themselves better from the icy winds and icy water.
The higher fat contents also give them better buoyancy in icy sea water. All large species of bears can swim, but for polar bears they can stayed in water for days. Since sea seals swim and rest on some icebergs, in order to hunt and feed, the polar bears often spent days in sea water to reach those icebergs.
Other changes occurred, such as the polar bears’ fur. It is not just the whiteness of the fur, that allow them to hide and hunt among the ice, but the texture of the fur, make it more windproof and more waterproof.
Their paws are also larger than brown bears, allowing to swim better with more control in the sea. And their claws are different in size and shapes that give them better grip on ice.
And lastly, since newer species of bears might not experience summer and warmer climate during their lifetime, the polar bears don’t hibernate in winter, like their southern cousins, the brown bears.
Even when the ice sheets retreated to the now, Arctic region, the polar bears continued to live in much regions, because their bodies have fully adapted to Arctic climate and terrains.
The polar bears have to pass on the necessary genes and genetic traits to new generations of polar bears for their continued survival. And the polar bears have over hundred thousand years to perfected their adaption in such climate and terrain.
Do you now understand the difference between natural selection and sexual selection?
Sexual selection don’t require species to change due to changes in climate, terrain or diet (any combination of the three). Natural selection do.