We have been discussing changes that occur in populations through time using the mechanisms of evolution. We are also interested in seeing how entirely new species can evolve, the process of speciation.

Let's first review our definition of a species: A species is one or more populations whose members interbreed under natural conditions, produce fertile offspring, and are reproductively isolated (can not interbreed) with other populations.

As your textbook notes, such definitions are not perfect. Non-sexually reproducing organisms and fossils can not be given species names objectively using this definition. None the less, it's a good working foundation.

In order to have speciation two things must happen:

• Populations of a species must be isolated from each other (in the sense that their gene pools are separate)
  
• Genetic divergence must occur.

• We know that selection pressures result in gene frequency changes in populations through time.
  
• We know, too, that mechanisms exist which isolate gene pools.
  
• As a result of these isolation mechanisms, gene pools can become distinct from each other (genetic divergence) so that interbreeding is no longer possible, hence, new species.

To do so, we must search for isolating mechanisms, both reproductive isolations and isolations that separate species ecologically, behaviorally or in time. We shall look at a variety of these in this section.

Allopatric (Geographic) Speciation
Most commonly, populations are isolated in space or geography, so that gene flow is minimized. When there is little gene flow, changes that occur tend to isolate the gene pools. If something occurs which shuts off gene flow (some barrier), then adaptation to different environments or genetic drift or some other selection mechanism can result in divergent selection. A result of divergent selection can be separate species, if reproductive isolation ultimately results.

It is reasonably easy for us to determine speciation characteristics when we look at divergent speciation. Biologists also look at fossil records and changes that show up in fossils. Although there may be gradual changes, when fossils from one era are compared to fossils from distant eras, they can be very different. When these differences appear significant, we call the extremes different species, and some use the term, phyletic speciation. In other words, one species evolves into a new, different, species through time.

Some geographic (or allopatric) isolation barriers:

• Rivers and changing river patterns
  
• Mountain ranges
  
• Lakes which dry up
  
• Volcanic eruptions
  
• Climatic changes, such as the ice ages
  
• Loss of land bridges

Some ecological sympatric examples are:

• Organisms may have differences in food preferences, such as fruit flies which feed and live on only hawthorn fruits or only apple fruits
  
• Organisms may have differences in type of shelter sites
  
• In plants, a flower color or pattern may affect pollinator visits.

Polyploidy: A Genetic sympatric example
In plants it is not unusual for a fertilized egg to have DNA replication but not divide. The organism now has twice as many chromosomes, forming a tetraploid. Most tetraploids are healthy and fertile.

Meiosis will produce diploid eggs or sperm. They can fertilize haploid gametes forming triploid offspring. The triploids are sterile, so that only tetraploids that reproduce with other tetraploids will survive - forming an instant new species.

Polyploidy is not uncommon in plants, but rare in animals. It may be that plants are more capable of self-fertilization so an individual that makes diploid eggs and sperm has other suitable gametes nearby.

Anatomical Change and Speciation
Speciation can occur with little or no anatomical change in organisms. If the organisms do not reproduce, they are biologically different species. Similarly, change in appearance within a species is a natural process and may not lead to speciation in the absence of any isolating factors. Such isolation factors can be random events or the result of natural selection.

Maintaining Reproductive Isolation
Ultimately in order for new species to form, the separation of gene pools by geography, ecology, genetics or behavior, must be accompanied by or followed by reproductive barrier so that interbreeding is not possible, even if the gene pools were to be mixed again. Remember that species are defined by reproductive isolation.

There are any number of such reproductive isolating mechanisms. Some prevent successful fertilization (pre-zygotic), and some prevent successful development (at least some stage), called post-zygotic. And broadly speaking, the isolation mechanisms of geography, time and behavior are also pre-zygotic reproductive mechanisms.

Pre-Mating Reproductive Barriers
Geographic Isolation
The individuals are geographically isolated so they never come in contact with each other, such as North American elk and European reindeer. Such geographic isolation is more important in forming new species than in maintaining separate species. Geographic Isolation
Mating occurs in different habitat types so the location where mating occurs is different. Potential viable mates do not look in that area, or go to that area, even though they are geographically together.

Temporal Isolation
Fertility occurs at different times. This can happen because of different hormonal signals, or because adults mature at different times of the year.

Behavioral Isolation
Mating behavior, as we have discussed is crucial to successful reproduction in a number of species. Individuals do not recognize courtship patterns or signals from members of populations different from their own.
Examples:

• Visual clues -- patterns and physical movements
  
• Audio clues
  
• Chemical clues, such as pheromones
  
• Pollination attractants

Mechanical Isolation (Mechanical Incompatibility)
There may be physical differences that prevent copulation between members of different populations.

• Differences in structure or function of the reproductive organs
  
• The genitalia don't fit (or in plants the pollinators don't work)

Post-Mating Reproductive Barriers
Gametic Isolation Mating (copulation) may occur but fertilization is not successful

• The gametes can not fuse or are not chemically attracted
  
• The sperm can not survive in the female reproductive tract
  
• The zygote can not develop if fusion does occur

Non-viable Hybrid
Even if gametes fuse, the offspring can be weakened, and may not survive.

Hybrid Sterility

• Offspring survive but they are sterile, since they lack homologous chromosomes essential for meiosis
  
• Hybrid may lack appropriate mating signals (behavioral or morphological)

New Species by Chromosomal Isolation
As discussed, polyploidy results in instantaneous new species. Polyploids are common in plants, and very useful in agriculture and floriculture.

Adaptive Radiation as a Speciation Mechanism
It's fairly easy, too, to distinguish patterns of speciation that involve adaptive radiation. Much of Darwin's early work was the result of his observations of the finch species on the Galapagos Islands. Darwin found fourteen species of similar birds on the islands, each feeding on separate food items, and each with a different beak shape. He speculated that the available food on the different islands "selected" for a specific beak shape. The distance between islands meant that only those birds in one area interbred, which resulted in the separation of species, and divergent evolution. Such multiple divergence is called adaptive radiation.

Genetic Models and Rates of Speciation
How does all of this go together to give a model or models for evolution? As we have seen and read, the process of evolution needs the presence of inheritable variation, and variations that can be beneficial within one's immediate surroundings. Adaptations of value in one habitat may be quite negative in others, or even within the same area if conditions change.

Darwin's model for evolution, which stresses the process of natural selection gradually changing populations, is one way for evolution to proceed, and works well for the changes we see within species through time.

The gradualist model of evolution can be difficult to document in nature (although not in laboratory situations), since we observe the current state, not the past or the future. There is also evidence that suggests that gradualism is not always the way that populations change. Many fossils, for example, "suddenly" appear in sediments, and many organisms remain unchanged for thousands or even millions of years. Even Darwin proposed that changes probably occur in relatively short periods of times, perhaps alternating with longer, more stable periods.

Genetically, the method for such evolution would be a series of incremental mutations, each which can change the organism bit by bit, until the accumulation is significant.

As an alternative, some biologists propose that change occurs through non-frequent major alterations in genes, in particular certain regulatory genes that can control many developmental events. When these changes affected reproduction, new species would evolve.

There is also evidence for times when species have changed rapidly, when the stresses of the surroundings (environment) exert strong pressures for particular variants. The finches that Darwin described in the Galapagos Islands exhibit adaptive radiation. Each island has a different habitat and food supplies. To survive, the finches had to be specialized for their specific habitats. Those with variants less able to forage, did not pass on their genes.

Punctuated Equilibrium
Another model for the process of evolution maintains that such rapid changes in response to intense selective pressures are followed by periods when the populations seem more constant. This model, which offers an alternative to the gradualist model, is known as punctuated equilibrium (This is pretty easy to do in laboratory situations, too). Much of what we see as diversifying selection follows the principles of punctuated equilibrium. Punctuated equilibrium also explains why fossils seem to just appear in layers and not always change. There are some species that have been evolutionarily stable for thousands of years. For punctuated equilibrium, morphology is crucial to change, and good morphology remains constant until some pressure makes that morphology less advantageous. For punctuated equilibrium, chance may have just as an important role in species selection as natural selection has acting on individuals of the species.

Both gradualism and punctuated equilibrium have merit. Evolution can, and does, result from selection pressures. Evolution also takes place in "jumps" consistent with punctuated equilibrium and random events. The differences are in the interpretation of the forces that shape evolution, not in process of evolution as the underlying foundation of biology.

Extinction
Before we leave the subject of life, just as we have new species, we have species that decline in numbers and eventually die out altogether. We discussed a several examples of successful adaptations in this section. What about species that lack adaptations in their environment, and lose to those who are better adapted? Adaptations can affect not just populations of one species, but natural selection can also lead to the loss of a species by extinction.

When we look at extinction, the most significant cause of extinction is change in habitat. Today we often discuss loss of habitat, but this loss is generally not a physical loss of geography, but a change in that habitat which results in an area no longer suitable for the individuals who originally lived there. Humans have done much in the past few centuries to alter habitat, to the detriment of thousands of species.

Species that have narrow gene pools (are highly specialized) and/or have restricted distribution are more vulnerable to changing environments. Those who are large and have greater resource demands are more vulnerable.

Natural ecological species interactions such as competition and predation impact ability to survive. Superior competitors may deprive resources to the point where the less competitive species can no longer survive.

Geographic changes and accompanying climate changes have also been responsible for loss of populations. Catastrophic geological events such as volcanic eruptions or massive earthquakes cause major environmental alterations. It is speculated that a meteor may be responsible for the extinction of dinosaurs, along with numbers of other organisms that lived in that era.

However, the overwhelming rate of extinction we see today on earth is the result of habitat destruction caused by human activities such as deforestation and pollution of water sources. Tropical rainforest loss each year exceeds the area of the state of Washington. 20 billion pairs of disposable chopsticks are made each year from trees that were once part of forests. The number of acres of trees felled for Sunday newspapers boggles the imagination.

Clearing of trees to provide new buildings and parking lots at BCC has resulted in the loss of 90% of the vegetation that once graced this campus, along with the other organisms that depended on this habitat.