Insights from: "Ecology" from Michael Begon, Colin R. Townsend and John L. Harper

Darwin's Natural Selection Theory rests on a series of propositions:

1) The individuals that make up a population of species are not identical: they vary, although sometimes only slightly, in size, rate of development, response to temperature, and so on.
2) Some, at least, of this variation is heritable. In other words, characteristics of an individual are determined to some extent by its genetic make-up.
3) Different ancestors live different number of descendants.
4) The number of descendants that an individual leaves depends, not entirely but crucially, on the interaction between the characteristics of the individual and its environment.

-> The fittest individuals in a population are those that leave the greatest number of descendants.

Important take outs derived from Darwin's theory:

a)Environmental Drivers forces the characteristics of populations to diverge (speciation) from each other only if: (i) there is sufficient heritable variation on which selection can act, (ii) the forces favoring divergence are strong enough to counteract the mixing and hybridization of individuals from different sites (two population won’t diverge completely if their individuals are continually migrating from one to another and mixing their genes.

b)There are specializations within species. Since the natural world is not composed of a continuum of types of organism each grading into the next we can recognize boundaries between one organism and the other. Nevertheless, within what we recognize as species, there is often considerable variation, and some of this is heritable.

c)Natural selection can force populations of plants and animals to change their character – to evolve.
What, then, justifies naming two populations as different species?
Ecological Speciation: It is the speciation driven by divergent natural selection in distinct subpopulations

It would be wrong; however, to imagine that all examples of speciation conform to this truly orthodox picture, there are different levels of speciation and speciation mechanisms.
The most celebrated example of speciation on islands is the case of Darwin’s finches in the Galapagos archipelago. The Galapagos are volcanic islands isolated in the Pacific Ocean. http://en.wikipedia.org/wiki/Darwin's_finches
Fourteen species of finch are found on the islands. The Galapagos finches, despite being closed related, have radiated into a variety of species with contrasting ecologies, occupying ecological niches that elsewhere are filled by quite unrelated species.

Isolation, both the archipelago itself and of individual islands within it has led to an original evolutionary line radiating into a series of species, each matching its own environment.
We will see repeatedly that the isolation of islands –and not just islands in sea water (http://en.wikipedia.org/wiki/Island_biogeography) - can have a profound effect on the ecology of the populations and communities living there. Such isolation provides arguably the most favorable environment for populations to diverge into distinct species.
Many of the species on islands are either subtly or profoundly different from those on the nearest comparable area of mainland. Put simply there are two reasons for this:
1. The animals and plants on an island are limited to those types having ancestors that managed to disperse there, although the extent of this limitation depends on the isolation of the island and the intrinsic dispersal ability of the animal or plant in question.
2. Because of this isolation, the rate of evolutionary change on an island may often be fast enough to outweigh the effects of the exchange of genetic material between the island population and the related populations elsewhere.
The diversity of matches within communities
Although a particular type of organism is often characteristic of a particular ecological situation, it will almost inevitably be only part of a diverse community of species. A satisfactory account, therefore, must do more than identify the similarities between organisms that allow them to live in the same environment – it must also try to explain why species that live in the same environment are often profoundly different.

Environments are Heterogeneous
There are no homogeneous environments in nature. Even a continuously stirred culture of microorganisms is heterogeneous because it has a boundary –the walls of the culture vessel- and cultured microorganisms often subdivide into two forms: one that sticks to the walls and the other that remains free in the medium.
The extent to which an environment is heterogeneous depends on the scale of the organism that senses it. To a mustard seed, a grain of soil is a mountain; and to a caterpillar, a single leaf may represent a lifetime’s diet. There may be also gradients in space (e.g. altitude) or gradients in time, and the latter, in their turn, may be rhythmic (like daily and seasonal cycles), directional (like the accumulation of a pollutant in a lake) or erratic (like fires, hailstorms and typhoons).
Pairs of species
The existence of one type of organism in an area immediately diversifies it for others. Over its lifetime, an organism may increase the diversity of its environment by contributing dung, urine, dead leaves and ultimately its dead body.
During its life, its body may serve as a place in which other species find homes. Indeed, some of the most strongly developed matches between organisms and their environment are those in which one species has developed a dependence upon another. This is the case in many relationships between consumers and their foods. Whole syndromes of form, behavior and metabolism constrain the animal within its narrow food niche, and deny it access to what might otherwise appear suitable alternative foods. The association of nitrogen-fixing bacteria with the roots of leguminous plants, and the often extremely precise relationships between insects pollinators and their flowers, are two good examples.

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