A obvious example is the interrelationship between predators, pathogens and prey.

Predation & evolution

If we assume that the earliest forms of life derived their energy from their non-living environment, the origins of predators and pathogens can be seen as an major adaptive breakthrough.

Other organisms represent a new and unexploited source of nutrition and habitat.

Once predators and pathogens appear, the organisms they prey upon must either adapt or become dinner.

There are many ways to respond to predation: get larger, get faster, get poisonous, get craftier, get pricklier or get tougher.

As we look at the fossil record, we see abundant evidence for all of these strategies.

In a world of inter-evolving organisms, there is very little that stays the same forever.

There is another type of selection, known as sexual selection.

The importance of sexual selection was first recognized by Darwin in his book "The Descent of Man and Selection in Relation to Sex".

Sexual organisms do not, as a general rule, mate randomly.

Typically, females either choose their mating partners or males battle with one another for access to females.

Why is this? From the female's perspective, generating offspring involves a serious commitment of resources. Consider human pregnancy as an example.

The more robust the mate, the more likely that the offspring will survive and reproduce. The more the female gains an evolutionary 'return' on her investment.

In many species the male's 'investment" in the next generation is much less substantial. Male's can often maximize their reproductive success by mating with multiple females.

Both males and females "know" these rules and sometimes they can actually play tricks on one another.

A male may appear more robust that he actually is, a female may mate with multiple males in order to select the most robust sperm.

In turn, males may respond by attempting to monopolize the female. There are many weird variations to be found in the natural world.

Sexual selection can lead to seemingly maladaptive traits. Consider the peacock's tail.

It takes energy to generate a large and colorful tail.

On the other hand, the presence of a gaudy tail implies that its owner is able to successfully compete with others for resources, and so is likely to be genetically "well endowed".

Once females notice tail size as a marker of success, they select as mates those males with the largest and most attractive tails.

This can lead to 'runaway' selection, the more prominent the male's tail the more likely he will find a mate.

So even though a large tail may have negative effects, such as making avoiding predators more difficult, it will still be selected for.

In the end there is a balance between the positive effects (access to females) and the negative effects (increased vulnerability).

Totally non-adaptative evolutionary mechanisms.

You might think that all evolutionary processes are by their very nature adaptive, but that is not the case.

Often accidents can 'select' a small set of organisms from the larger population.

Founder effects and evolutionary bottlenecks occur when a new population is based on a small, randomly selected group of individuals.

The founder effect applies when a small group of individuals first colonizes a new and isolated territory, such as an island.

An evolutionary bottleneck occurs when some disaster or disease reduces a once large population to a small one very quickly.

There is strong evidence for an evolutionary bottleneck during the course of human evolution.

The end result of both founder effects and evolutionary bottlenecks is that certain traits can be over-represented by chance - an effect known as sampling error.

These traits may or not be adaptive in the new environment.

A related process is genetic drift. In small populations, chance events lead to the survival or predominance of genotypes even though they are not particularly adaptive.

This is one reason that it can be difficult to determine whether a particular trait is adaptive or not. It really depends upon the history of the population.