At this rate, it would not take long for our elephant pair to produce a mass of descendants larger than the earth.

Obviously exponential growth cannot go on forever. In practice the number of organisms is limited by the resources available. These resources can be nutrients or places to live.

In an unperturbed system, population numbers will eventually settle into a 'steady state'. At steady state the total number of organism remains more or less constant over time.

Competition between organisms for scarce resources grows more intense under steady state conditions.

If all individuals were identical then which organisms would "win" this competition would be a matter of luck, pure and simple.

Darwin & Wallace

In 1858 Charles Darwin and Alfred Wallace noted that individual organisms of the same type are not identical - they differ from one another.

People who grow up in Mexico are likely to learn to speak Spanish, those who grow up in China are likely speak Chinese.

If you are born in stable and affluent family, your view of life as well as your behavior are likely to be different than if you were born into desperate poverty.

Soft heredity involves changes in gene expression, protein activity, cellular and supracellular organization and behavior. It plays a critical role in the formation of an organism's final form, its phenotype.

Because of its nature, soft heredity can change dramatically from one generation to the next.

While your parents may have originally learned to speak Chinese, if you are born in France, your first language is could well be be French. The way you raise your children may be quite different from the way you were raised.

Based on their own observations, and those of many generations of animal and plant breeders, Darwin and Wallace realized that part of the variation between organisms was rather different from soft heredity.

This hard heredity is encoded in a stable manner - it is passed from generation to generation, often unchanged or with only subtle modifications.

The secret of hard hereditary was solved by the discovery that it is stored in the structure of deoxyribonucleic acid or DNA.

At its base, it is this information that determines that human beings produce human beings, while dogs produce dogs and bacteria produce bacteria.

Epigenetic heredity:

There is a third type of heredity that falls between soft and hard. A trait can become established in a cell or an organism based on a change in the internal organization of its components.

Think of organization of a cellular component as having two structural states, both of which are stable but under certain conditions interconvertable.

If the component is in state A it can produce a specific "A" phenotype.

When the component is in stage B, the phenotype can change to the "B" phenotype.

Whether an organism is in the A or B phenotype is determined by which state the cellular component is in. Assuming that the transition from A to B is rare, Once an organism enters state A it, and its offspring will remain in this state.

If a transition occurs in which the component switchs states, from A to B for example, the organism's phenotype will change even though there has been no change in the organism's DNA - there is no mutation.

This type of two state system has been found in yeast and humans.

Profiting by mistakes

The replication of genetic material is never quite perfect. Complete accuracy requires a very high level of effort and energy, effort and energy that can often be better used for other tasks.

Consider proofreading one of your own term papers. If you earn an "A" whether or not your paper has a few minor grammatical errors in it, is it worth spending extra hours finding and correcting those errors?

Couldn't you be doing something better with your time?

Although it might seem unlikely, 'errors' are useful. They are the raw material for genetic variation. If all organisms were identical, then changes in the environment would effect them all in a similar manner.

Because of these errors, offspring are genetically distinct from one another and from their parents.

With each generation, a population of organisms will accumulate more and more mutations - more and more genetic variation.

The variation between organisms means that changes in the environment will effect different organisms differently.

Combined, variation and selection lead to organisms that are well adapted to their environment,

It is a common dream that we might avoid competition, that the lion will sit down with the lamb rather than attempt to eat it.

The peaceable kingdom by Edward Hicks (1780-1849).

In the 'real' world, however, there is no simple way to avoid competition between organisms.

The combination of variation and selection leads, over time, to the adaptation of organisms to their environment.

Adaptation is rarely perfect, however.

It is constrained by an organism's structure, the nature of the variation within the population, what is physically possible and the relative costs and benefits associated with 'perfection'.

Aside from the fact that it is difficult to define perfection, the 'perfect' solution may be impractical or impossible to reach by evolutionary processes.

Our ability to choke on food is a 'design flaw' that reflects the evolutionary pathway that produced us.

In practice, how good good enough is is determined by the competition.

Each organism has only a limited amount of energy at its disposal. Remember, under most conditions, the organism is competing with others for this energy.

Energy used to make something better than 'good enough' is essentially wasted, it is not available for survival or reproduction.

We see the evidence for the 'good enough' character of evolutionary processes all around us. We see it in our tendency to get backaches, the need for hip-replacement, our susceptibility to disease and aging, and the limitations of our senses.

Over time, organisms that diverge from the 'best', however imperfect, solution will be eliminated.

This process of maintaining an optimal design is known as 'conservative' selection.

Species and speciation

There are alternative ways to minimize competition, at least temporarily.

If competition for a particular food or living space is intense, an organism that utilizes an alternative may have a reproductive advantage.

Individuals with traits at the extremes of the population may, by chance, find themselves better able to exploit aspects of the environment not exploited effectively by others.

Exploiting new resources may, in turn, lead to further adaptation and the splitting of the population into distinct groups.

This process is known as 'disruptive' selection.

A species is a group of organisms that can, and normally does, interbreed successfully with one another.

As populations diverge and adapt to specific conditions, they may no longer have the opportunity to interbreed and eventually may not be able to interbreed.

One species has become two.

There are a number of mechanisms that lead to the reproductive isolation of related populations.

The establishment of reproductive isolation can itself be selected for, since it acts to preserve a population's specific adaptations.

Sometimes, the splitting of the population can be literal. Groups of organisms may find themselves on different sides of a mountain range or a river.

Once isolated from one another, these groups will evolve independently.

If 'reunited' later, they may no longer be able to interbreed - the population will have diverged to become two distinct species.

Throughout earth's long history, both gradual and catastrophic events have lead to dramatic changes in the environment.

An important part of an organism's environment are the other organisms around it, which are themselves adapting. There is a constant 'dance of adaptation'.

As the Red Queen said to Alice ...
"Here, you see, it takes all the running you can do to keep in the same place" - Though the Looking Glass by Lewis Carroll

A species once well adapted to a particular environment, may find itself maladapted as its environment changes.

Over the ages of earth's history, there have been many, many different species.

Most species exist for relatively short times, measured in millions of years, and have left no descendants today.

Others became the ancestors of whole groups of organisms