How are various complex structures such as wings -for example- developed, when -until they become functional wings- they are essentially a waste of appendages?

To some extent the evolution of the wing can be seen as an adaptation of appendages to variable degrees of an animal's ability to glide.

But what of other complex organs, like the eye, which can be seen in various forms throughout the animal kingdom, how could such a complex organ be developed from scratch?

Evolution of the Eye (movie)

Even with such an appreciation of similarities and differences -along with potential variables of genetic differences within given populations- the fundamental concepts that I have been trying to get across to you assume that any given species is essentially a relatively stable pool of inherent variability (alleles) that is able to acquires new trait from gene flow and loses or gains other traits (more often than not) through random loss/acquisition......which we call...?

While this may seem complex enough, it really is an overly simplistic appreciation of "species" and "populations".   There are many, many more additional layers of complexity, some subtle and some not so subtle.

Very often they compete for resources and/or are interconnected with other species in a variety of different ways, the most easy to understand being the interaction of prey and predator.

Predator–prey interactions interactions, that can even provide change over evolutionary time
Predators are agents of natural selection and mortality since they do not capture prey individuals randomly. Therefore, prey species have evolved many adaptations to make them more difficult to capture and consume.

Toxic hairs, tough spines, and noxious chemicals are some examples of defensive adaptations.

Camouflage, or even Mimicry are both evolved defense mechanisms, ultimately brought about to thwart the actions of predators:

Batesian mimicry occurs when a non-aggressive species mimics the look of a more aggressive and dangerous one, or a more palatable species mimics an unpalatable or noxious one.

Directional selection then this potentially causes unpalatable species to evolve away from their mimics. This happens because unpalatable individuals that look least like the "average" predator and the mimic will be preyed on more than the onees the mimic more closely resembles.

Predators may in turn evolve more effective means of overcoming prey defences.

One, rather unusual interaction between predator and prey, illustrates just how interwoven the predator prey relationship can get: Toxic Newts. (movie)

Species that mutually influence one another's evolution are said to have co-evolved.
In diffuse evolution, species traits are influenced by interactions with a wide variety of predators, parasites, prey, or mutualists.

Populations are, therefore, dependent upon their environment -their Ecology, but there are aso inherent variables within a given population that we have not yet addressed.

Other factors also effect evolutionary chang, such as:

a) population density (Individuals in a population may have uniform, random, or clumped distributions)

b) Births and Deaths: Births, deaths, immigration, and emigration drive changes in population density and distribution.

N = N0 + B - D + I + E.

 

We can go one step further. We can analyze a cohort (or a sub group of a population), defined by some determination of Age.

c) Age distribution within a population, which reveals the recent history of births and deaths. Timing of these may influence age distributions for many years.

Furthermore, ecologists, also analyze a category called survivorship, to appreciate how the various individuals within a cohort survive in the environment. For some, survival is uniform throughout the the =different ages within the population.  For other populations it is far from it.

d) Population growth rate  "r" = average number of births / average number of deaths x N (the number in the population).

Theoretically, the trend for a population growth -unfettered by the environment is logarithmic, but when constrained by the environment population growth can vary depending upon many inherent factors; the type of organism and it's lifestyle under investigation.

However, other external factors must come in to play, giving rise to the more sigmoidal curve, which defines a maxima,"K" or "carrying capacity" for that particular population.

When the number in the population = maximum or the "carring capacity", the population growth will stop.

Population numbers are essentially regulated in response to population density, for a variety of reasons such as predation, food supply and or potential spread od diseases.

Consequently, even though in the short term, catastophes may effect a populations size....in the long term (assuming the population survives), the population's density ought to recover towards, what is termed a density equilibrium.

Because we humans have opted out of the selective process our population curve is still exploding in an exponential manner.