So now we consider the various mechanisms that can lead a species to give rise to one or more new species. Remembering that species, at least species that reproduce sexually, are defined by the fact that they can and do interbreed to produce fertile offspring, you might already be able to propose a few plausible scenarios. An important point is that the process of speciation is continuous, there is generally no magic moment when one species changes into another, rather a new species emerges over time from a pre-existing species 98. Of course the situation is more complex in organisms that reproduce asexually, but we will ignore that for the moment. More generally, species are populations of organisms at a moment in time, they are connected to past species and can produce new species.
Perhaps the simplest way that a new species can form is if the original population is physically divided into isolated subpopulations. This is termed allopatric speciation. By isolated, we mean that individuals of the two subpopulations no longer mingle with one another, they are restricted to specific geographical areas. That also means that they no longer interbreed with one another. If we assume that the environments inhabited by the subpopulations are distinct, and that they represent distinct sets of occupied and available ecological niches, distinct climate and geographical features, and distinct predators, prey, and pathogens, then these isolated subpopulations will be subject to different selection pressures, different phenotypes (and the genotypes associated with them) will have differential reproductive success. Assuming the physical separation between the populations is stable, and persists over a significantly long period of time, the populations will diverge. Both selective and non-selective processes will drive this divergence, and will be influenced by exactly what new mutations arise and give rise to alleles. The end result will be populations adapted to specific ecological niches, which may well be different from the niche of the parental population. For example, it is possible that while the parental population was more a generalist, occupying a broad niche, the subpopulations may be more specialized to a specific niche. Consider the situation with various finches (honeycreepers) found in the Hawai’ian islands99. Derived from an ancestral population, these organisms have adapted to a number of highly specialized niches. These specializations give them a competitive edge with respect to one another in feeding off particular types of flowers. As they specialize, however, they become more dependent upon the continued existence of their host flower or flower type. It is a little like a fungus that can only grow on one particular place on a particular type of beetle, as we discussed earlier. We begin to understand why the drive to occupy a particular ecological niche also leads to vulnerability, if the niche disappears for some reason, the species adapted to it may not be able to cope and effectively and competitively exploit the remaining niches, leading to its extinction. It is a sobering thought that current estimates are that greater that ~98% of all species that have or now live on Earth are extinct, presumably due in large measure in changes in, or the disappearance of, their niche. You might speculate (and provide a logical argument to support your speculation) as to which of the honeycreepers illustrated above would be most likely to become extinct in response to environmental changes100. In a complementary way, the migration of organisms into a new environment can produce a range of effects as the competition for existing ecological niches get resolved. If an organism influences its environment, the effects can be complex. As noted before, a profound and global example is provided by the appearance of photosynthetic organisms that released molecular oxygen (O2) as a waste product early in the history of life on Earth. Because of its chemical reactivity, the accumulation of molecular oxygen led to loss of some ecological niches and the creation of new ones. While dramatic, similar events occur on more modest levels all of the time, particularly in the microbial world. It turns out that extinction is a fact of life.
Gradual or sudden environmental changes, ranging from the activity of the sun, to the drift of continents and the impacts of meteors and comets, leads to the disappearance of existing ecological niches and appearance of new ones. For example, the collision of continents with one another leads to the formation of mountain ranges and regions of intense volcanic activity, both of which can influence climate. There have been periods when Earth appears to have been completely or almost completely frozen over. One such snowball Earth period has been suggested as playing an important role in the emergence of macroscopic multicellular life. These geological processes continue to be active today, with the Atlantic ocean growing wider and the Pacific ocean shrinking, the splitting of Africa along the Great Rift Valley, and the collision of India with Asia. As continents move and sea levels change, organisms that evolved on one continent may be able to migrate into another. All of these processes combine to lead to extinctions, which open ecological niches for new organisms, and so it goes.
At this point you should be able to appreciate the fact that evolution never actually stops. Aside from various environmental factors, each species is part of the environment of other species. Changes in one species can have dramatic impacts on others as the selective landscape changes. An obvious example is the interrelationship between predators, pathogens, and prey. Which organisms survive to reproduce will be determined in large part by their ability to avoid predators or recover from infection. Certain traits may make the prey more or less likely to avoid, elude, repulse, discourage, or escape a predator's attack. As the prey population evolves in response to a specific predator, these changes will impact the predator, which will also have to adapt. This situation is often call the Red Queen hypothesis, and it has been invoked as a major driver for the evolution of sexual reproduction, which we will consider in greater detail in the next chapter (follow the footnote to a video).101
As the Red Queen said to Alice ... "Here, you see, it takes all the running you can do to keep in the same place"
-Lewis Carroll, Though the Looking Glass