3.19: Mechanisms of speciation

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. An important point is that the process of speciation is continuous, there is no magic moment when one species becomes another, rather a new species emerges over time from a pre-existing species, after which the two populations evolve independently.^[150] The origin of species through evolutionary mechanisms is formally analogous to the Cell Theory; each cell is derived from a pre-existing cell. The difference is that cell division provides an unambiguous benchmark in the history of a cell. Species are populations of organisms at a moment in time, they are connected to past species, can produce new species, or can go extinct in the future.

Allopatric speciation is the simplest way to form a new species. It occurs when the original population is physically divided into isolated subpopulations that can no longer breed with one another. If the environments inhabited are distinct and different in terms of available ecological niches, climate, geographical features, predators, prey, and pathogens, the isolated subpopulations will be subject to different selection pressures leading to different phenotypes. If the physical separation between the populations persists over a sufficient period of time, the populations will diverge. Divergence will be influenced by the genetic variations (alleles) present in the founding populations and the new mutations that arise. The end result will be populations adapted to ecological niches that may well differ from the niche occupied by the parental population. For example, if the parental population was a generalist, occupying a broad range of niches, the subpopulations may occupy more specialized niches.

Consider the situation with finches (honeycreepers) found in the Hawai’ian islands.^[151] Derived from an ancestral founder population, these birds have adapted to a number of highly specialized niches. Their specializations give them a competitive edge with respect to one another when feeding off a particular type of flower. As they specialized, however, they become more dependent upon the continued existence of their host flower type (→). It is a little like the fungus that can only grow on one place on a particular beetle. The drive to occupy a particular ecological niche can lead to ecological vulnerability. If the niche disappears, species highly adapted to it may not be able to compete effectively with species adapted to other niches, leading to its extinction.^[152]

It is a sobering fact that greater than ~98% of all species that have or now live on Earth are estimated to be extinct. As an example, which of the honeycreepers illustrated above would you expect be most likely to become extinct in response to environmental changes and why?^[153] In a complementary way, the migration of a species into a new environment may produce a range of effects on the existing (native) species present.^[154] An organisms' influenceon its environment can be complex. A profound example is provided by the appearance, early in the history of life on Earth, of photosynthetic organisms that released molecular oxygen (O₂) into the atmosphere as a waste product. Because of its chemical reactivity, the accumulation of molecular oxygen led to loss of some ecological niches and the creation of new ones. The recent anthropogenic increase in atmospheric CO₂ is another example. While dramatic, similar events occur on more modest levels all the time. It turns out that extinction is a fact of life – at the same time, life has continued and diversified in an uninterrupted manner for over ~4,000,000,000 years.

Gradual or sudden environmental changes, ranging from the activity of the sun, to the drift of continents, and the impacts of meteors and comets can lead to the disappearance of existing ecological niches and the appearance of new ones. For example, the collision of the continents with one another led to the formation of mountain ranges and regions of intense volcanic activity, both of which can influence climate and the connectedness of populations. There have been periods when Earth appears to have been completely or almost completely frozen over and periods when the planet was ice free.^[155] Such geological processes are active today, with the Atlantic ocean growing (slowly) wider and the Pacific ocean shrinking, the splitting of Africa along the Great Rift Valley, and the ongoing collision of India with the rest of 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.

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

By this point you may have come 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 impact others as the selective landscape changes. A particularly obvious example is the interrelationship between predators, pathogens, and prey. Which organisms survive to reproduce will be determined in part by their ability to avoid predators or avoid or recover from infection. Certain traits may make their 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 or pathogen, these changes will impact the predator or pathogen, 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 as we go on.^[156]

---