A species is a population of organisms that can, and normally does, interbreed successfully with one another but not with members of other populations (other species).

Over geological time, a species can have one of two fates --  it can continue, and perhaps give rise to other species, or it can become extinct. 

The idea of extinction is a relatively new one.  In most non-scientific views of the world, the world was created largely as is – organisms do not become extinct nor do new types of organisms appear.

Fossils of various types have been known for quite some time, but were originally thought to be either "jokes of nature" (lusus naturae) or animals that still existed some where on earth.

This latter idea of fossils was plausible because much of the earth remained to be explored (or better put, widely known to scientists) – it was easy to imagine that strange organisms lived elsewhere (we might refer to this as the "lost world" hypothesis).

With the voyages of discovery in the 15th through 18th centuries, the places where strange organisms could be hidden decreased drastically.

George Cuvier (1769-1832), played a key role in establishing the science of paleontology and argued persuasively that past ages had seen organisms that were quite distinct from those now in existence. 

We now appreciate that the fossil record provides unambiguous evidence that, at different times in the past, quite different organisms existed.

The most famous of these extinct organisms were the dinosaurs.   Dinosaurs, and their extinct reptilian relatives, were found both on land and in the sea.   Flying reptiles, the pterosaurs, ranged in size from that of small modern birds to large airplanes.

 The ancestors of this family of vertebrates first appeared in the fossil record ~250 million years ago; they were a dominant from of life from ~230 to 65 million years ago. 

During the Ages of Dinosaurs (Triassic – 248 to 206 million years ago, Jurassic – 206 to 144 million years ago, and Cretaceous –144 to 65 million years ago), there were many different species of dinosaurs, each characteristic of a particular age. 

Species of dinosaurs appeared and disappeared; what led to these changes in dinosaur species?   This leads to the broader question, why do species appear and disappear? 

There are two general mechanisms.  The first are more or less gradual responses to new and changing environments. 

For example, there is an on-going process of continental drift, which is responsible for earthquakes and volcanoes. 

About 200 million years ago, the supercontinent Pangaea broke up to form the continents we know today. 

This led to changes in climate and the physical isolation of many types of plants and animals.

There have been other periods when the Earth appears to have completely frozen over.  

One such snowball earth period may have been involved in the emergence of macroscopic multicellular life.

Sometimes, populations that had evolved on one continent were introduced into another, for example through the land bridge between Asia and the Americas.

There can also be more drastic and acute events that lead to essentially instant and unanticipated environmental changes, such as massive meteorite impact . 

It has been hypothesized that such events are responsible for some mass extinctions. 

The most catastrophic of these mass extinction events was the Permian extinction (~245 million years ago), during which it appears that ~95% of marine organisms and ~75% of land species died off.    

Another source of environmental change has been the appearance of new species, new pathogens or predators. 

It is widely held that the appearance of modern humans contributed to the extinction of various Pleistocene megafauna which occurred ~8000-10,000 years ago.

In the event of changes, whether drastic or gradual, a species is faced with new challenges.  Depending on how specialized it has become to a particular ecological niche, even a minor change may leave a species without the genetic resources (variations) needed for successful adaptation.

• How do we know that a species is a species if we do not directly observe whether it can interbreed with other organisms? 
• If one population is the ancestor to another, how do we decide if they are more than one species? 
• Does extinction imply mistakes by an "intelligent designer"?
• How, in terms of Gause's law, does a catastrophic event influence subsequent evolutionary events.
• Which (and why) are more likely to become extinct, a highly specialized species extremely well adapted to a particular niche, or a less specialized generalist?

Species formation: New species are the result of divergences between pre-existing populations. 

Once organisms have diverged to such an extent that they can no longer interbreed successfully, two species have emerged.

How does speciation occur?  The most common mechanism, thought to drive most speciation, is known as allopatric speciation.   

In allopatric speciation, a single population is divided into two or more physically separate populations.

The size of the physical barrier involved clearly depends upon the organism.  Some organisms have only a limited ability to move about; they interbreed only with organisms in their immediate neighborhood.

If a species is spread over a large physical range, it may consist of many distinct subpopulations that live in distinct environments and have varying levels of reproductive interactions with one another.

Consider the black bear, Ursus americanus. Originally distributed across North America roll over, the distribution is now much more fragmented. 

Isolated bear populations are now free to adapt to their own particular environments.  Clearly the environment in Florida is different from that in Mexico or Alaska or Newfoundland.   Different environments are likely to favor different adaptations. 

If, over time, these populations were to come back in contact with one another, they may or may not be able to interbreed successfully. 

If they are reproductively isolated, two (or more) species will have been formed. 

This bifurcating process, taking place over hundreds of millions of years, has produced many different species. 

In this figure
~3000 species are arranged with regards to their apparent familial relationships.

There are a number of adaptations that can lead to, and reinforce reproductive isolation.

Over time two daughter species may be "reunited", but be unable to interbreed - reproductive isolation has occurred and one species has become two.   

Sympatric speciation:  A second type of speciation is more controversial, it is known as sympatric speciation.   In sympatric speciation, a single population of organisms splits into two reproductively isolated communities. 

The problem with sympatric speciation has been, what stops organisms of the two nascent species from interbreeding - how are these populations isolated so that different traits can emerge? 

Recently a number of plausible mechanisms have been identified.  One involves sexual selection, while another involves host selection. 

Here animals (such as insects) that feed off specific hosts may reproduce in distinct zones associated with their hosts - in a process very much like allopatric specification, this can lead to differential adaptation, divergence and eventual reproductive isolation, that is, speciation. 

Most species exist for relatively short periods of time, measured in millions of years, and have left no descendants among organisms alive today.

A few species are the ancestors of many (most) current species.

Some species have persisted, superficially unchanged, for long periods of time. These are sometimes referred to as living fossils.

• How might you determine whether organisms that reproduce in a strictly asexual manner are part of the same or different species?
• Horses and donkeys sometimes interbreed; are they one or two species?
• How can the types of changes that lead to reproductive isolation be beneficial?