Species And Speciation

Summarising the thoughts expressed in the module Genes-Evolution-Environment a group of birds of the same species, found in a clearly defined geographical area, is called a population. Within this population genes, acting in conjunction with various environmental factors, determine which phenotypes will appear.

Those phenotypes which are adapted to the prevailing environmental conditions are supported. Non adaptive phenotypes are not supported and are eventually eliminated. If the gene pool within the population shows consistent change from generation to generation, we say that the population is evolving.

We talked about spatially separated populations and the effect of isolation leading to the evolution of new species. We need to be clear what we mean by “species”.

Species definition.

Newton (The Speciation and Biogeography of Birds) comments on the need for an objective system of decision making in defining a species. He quotes the old adage that “a species is a species when a competent taxonomist says so”.

He says that a species “looks somewhat different from all other species, mates only with others of its kind producing offspring that look like their parents and is found in particular parts of the world where it is associated with particular habitats and feeding behaviour”.

Biologically “each species forms an independent and closed genetic system, reproductively isolated from other species, occupying a distinct geographical range and a specific ecological niche”. “It also occupies a discrete geological time frame from its formation to its extinction or evolutionary change to another species”.

From speciation to extinction.

Evolution is a continuing process so “species” exist in all stages of formation. We call them “sub-species” or “races” but they may not meet all the above criteria. To me this is the most interesting part of the evolutionary process. We have seen that interaction with the environment determines which phenotype will actually appear. The more diverse the environment the greater the number of phenotypes.

The extent to which environmentally induced phenotypic change is adaptive and therefore selected is clearly important. In fact the potential phenotypes can be influenced, not just by their own environment but by previous environmental effects on its parents, which are transmitted to the offspring in various ways and at various stages of development.

Existing patterns of biodiversity have been shaped both by speciation and by extinction. Thus, when members of a population die they are replaced by the progeny of parents that were better adapted to survive and reproduce in the environment in which natural selection took place. This process creates and preserves traits that are seemingly fitted for the roles they perform.