A single pair of elephants, breeding in a unlimited manner, and living a normal life-span, would produce ~15,000,000 living descendents at the end of 500 years.

The graph displays three scenarios of human population growth, based on different levels of average fertility.  In a number of the developed countries, the birth rate already falls into the low fertility domain. 

More surprisingly, in a growing number of developing countries the fertility rate is decreasing dramatically; this decrease correlates with reductions in infant mortality and increases in the educational level of women. 

A number of economists have speculated on the effects on the global economy if the global human population begins to decline after ~2050.  

The behavior of humans appears to be rather different from that of your "average" animal, plant, or microbe.

Typically, these populations are at steady state and many more offspring are born than survive to reproduce.  If all of the offspring were identical, which ones would survive to reproduce would be a matter of luck, pure and simple.

• Why, in most cases, do populations of organisms reach a steady state level?
• The human population was "steady state" for much of its recent history; what happened that allowed it to increase so dramatically in the last 500 years?
• What factors will lead to a new steady state level in the human population?

Darwin, Wallace & the significance of genetic variation

In 1858, Charles Darwin and Alfred Wallace published their observations and proposed their idea of evolution by natural selection. 

Their ideas were based on an apparently simple observation, namely that individual organisms in a population are not identical and that part of this variation is inherited. 

Genetic variation has been exploited by plant and animal breeders for thousands of years, using a process known as artificial selection, and has been used repeatedly to generate the various forms of domesticated plants and animals.

In artificial selection, the breeders select the animals with the traits they desire - the offspring that have these traits are mated, those that do not are destroyed.

Over generations organisms with highly exaggerated forms of the desired traits are created. 

These organisms may also have other, less desirable traits, which can themselves be the focus of artificial selection (if deemed worthwhile).   Because they live in an artificial environment (a farm or as pets), these traits can be tolerated.

During the last century, the mechanism of heredity has been solved.  Genetic information is stored in molecules of DeoxyriboNucleic Acid (DNA).  The total genetic information stored in an organism's DNA is its genotype.  How this discovery was made will be examined later in the course.

This information is read out, through the processes of transcription and translation, to form macromolecules (RNAs and proteins).   These macromolecules are major building blocks of cells, and regulate cellular behavior, tissue and organ formation and organismic behavior.  

DNA also encodes a second type of information that is involved in determining where, when and how much a gene is "expressed", that is, transcribed and then turned into an active form.

Changes in genotype can lead to changes in phenotype, the appearance or behavior of an organism. 

Mutations and inherited variation

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

Consider proofreading one of your own term papers. If you earn an "A" even if 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, mutational "errors" are useful; mutations are the raw material that produces genetic variation.  If all organisms were identical, then changes in the environment would effect them all in a similar manner, which survived to reproduce would be a matter of chance.

Because of mutations (and sex) offspring are genetically distinct from one another and from their parents.  With each generation, a population of organisms accumulates more and more different mutations - more and more genetic variation. 

These organisms come to have different genotypes, and so different phenotypes.  Sometimes the phenotypic differences are subtle, other times dramatic. 

• The products of artificial selection rarely compete well with "native" species; why is that do you suppose?
• Explain how variations in the myostatin gene could lead to variations in a population.
  
• Analyze the possible benefits and disadvantages of having an altered myostatin protein? 
• What limits the extent to which a trait can be selected?

Evolution by Natural Selection:

Organisms with different phenotypes often differ in their ability to survive and reproduce in a particular environment.    Over time, the genotypes of organisms that are the most successful at reproduction within a particular environment will become more common within the population. 

In a word, these genotypes will be selected for based on the phenotypes they produce, and the effects of these phenotypes on the organism's reproductive success.  Genotypes that decrease an organism's chances of reproductive success will be selected against and may disappear altogether.

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.

In the real world there is almost constant pressure due to the presence of predators, pathogens, and the search for food, mates and places to live.

These combine to produce a "natural selection" for those organisms that reproduce most successfully. 

Over generations, this process leads to adaptation of organism to their particular environment or to their extinction. 

What is an organism's environment? This is a complex question.  The answer is determined by the organism's life-style.  

Which parts of the environment matter, in terms of survival and reproduction, can be quite specific.  Organisms can, and often do, avoid competition with other organisms by adapting to very specific environments and life-styles, often referred to as an ecological niche.

There is a principle in evolutionary biology, known as the competitive exclusion principle or Gause's law, that two species cannot (stably) occupy the same ecological niche.

"some organisms are amazingly specialized. Perhaps the narrowest ecologic niche of all is that of a species of the fungus family Laboulbeniaceae, which grows exclusively on the rear portion of the elytra of the beetle Aphenops cronei, which is found only in some limestone caves in southern France. Larvae of the fly Psilopa petrolei develop in seepages of crude oil in California oilfields; as far as is known they occur nowhere else." – Th. Dobzhansky.

• Why is it rare / unstable that two species occupy exactly the same ecological niche? 
  

It is possible that the costs of a particular "imperfect" design are offset by other advantages.  For example, the small but significant possibility of death by choking may, in an evolutionary sense, be worth the ability to make complex sounds (speech).

As a general rule, evolutionary processes generate structures and behaviors that are as good as they need to be for an organism to effectively exploit a specific set of environmental resources and to compete effectively with its neighbors.

If being better than "good enough" does not enhance reproductive success, it will not be selected for, and variations in that direction will be lost, particularly if they come at the expense of other important processes or abilities. 

We see the evidence for the "good enough" character of evolutionary processes all around us.   It explains the limitations of our senses, as well as our tendency to get backaches, need hip-replacements, and our susceptibility to diseases and aging. 

For example, the design of our eyes leaves a blind spot.  We have adapted to this blind spot through the use of saccadic movements - why, because this is an evolutionarily easier fix to the problem than rebuilding the eye (which is essentially impossible). 

Complex eyes have arisen a number of times , apparently independently, and not all have a blind spot.   An intelligently designed human eye would presumably not have such an obvious design flaw. 

Over time, organisms that diverge from the optimal rather than the best, however imperfect, solution will be at a selective disadvantage.  This process of maintaining an already optimal design is known as "conservative" selection.

• Consider a specific trait; what limits its perfectibility?
• How would you describe the difference between optimal and perfect?
• Consider human behavior and the behavior of our ancestors; why is our eyesight not as acute as that of an eagle.
  

The theory of evolution has been controversial since its inception because it deals with issues of human behavior and origins, our place in the universe, life and its meaning.

Logically, evolution should be no more offensive to the religious than any other science. 

All sciences assume that observable events can be explained in purely natural (that is observable, measurable, and coherent), as opposed to supernatural, terms.

Yet there are rarely, except in The Onion, demonstrations against the laws of thermodynamics!

Some people are confused and assume that the goal of science is to provide TRUTH.   This is a unwarranted assumption – the TRUTH about the universe may well be unknowable or even incomprehensible to us. 

Science, based on the simple assumption of naturalism, has proven to be quite effective at dealing with a wide array of practical problems, from weather prediction to curing disease.  It provides a method to understand, and often manipulate the world to our benefit. 

What does that mean in practice – it means that children who would otherwise die of bacterial infections can be cured; that crop yields can be increased and pesticide use reduced through genetic engineering; that people can communicate with one another through satellite-transmitted signals ; and that much of what happens in the world around us can be understood in a logical and ever more complete and accurate manner. 

• If it were possible, should biologists be called upon to correct bad biological designs, or should we just live with them?