THE SPECIES AS AN ECOLOGICAL UNIT

OBJECTIVE: To recognize the fact that a "species" is not genetically homogeneous and to consider the influence of that fact on the ecology of a species.

DEFINITIONS OF A SPECIES

1. A morphological definition: A species is a population of organisms whose members, within a considerable range of natural variations are structurally different from all other organisms.

2. A biosystematic definition: The members of a species can interbreed and produce fertile offspring, but cannot do so with members of a different species.

3. An ecological definition: A species consists of groups of morphologically and ecologically similar natural populations that may or may not be interbreeding but that are reproductively isolated from other such groups.

Three key elements to this definition:

a. morphology (similarity of appearance)

b. breeding behavior (generally are not interbreeding with other species)

c. ecologically similar (e.g. members of the same species tend to occur in the same type of habitat)

GENOTYPE AND PHENOTYPE

Genotype is the genetic constitution of an individual.

Phenotype refers to the observable properties of an organism produced by the genoptype interacting with the environment.

P = G + E + GE, where P is phenotype or a phenotypic character, G is genotype or genetic information coded in the chromosome, E is the environment, and GE is the interaction of genotype and environment. GE is usually small and includes interactions such as pre-conditioning or acclimation (e.g. the recent history of environmental conditions affects the way in which the genotype responds to current environmental conditons)

TWO MAJOR SOURCES OF PHENOTYPIC VARIATION

1. The short-term influence of the plant's environment during the life of the plant.

Phenotypic plasticity refers to the degree to which a character of a given genotype can be modifed by environmental changes. Some traits are highly "plastic" and others are relatively "non-plastic."

Heritability refers to the strength of genetic control (rather than environmental control) of a particular trait.

2. The influence of genotypic variation.

Major sources of genetic variation are:

a. mutation -- both changes in the molecular structure of genes at individual loci and chromosomal aberrations such as duplications, deletions, translocations; net result is to add to the pool of genetic variability by incrasing the number of alleles (different forms of a gene) available for recombination at each locus.

b. recombination of genes in sexual reproduction.

Gene flow plus selection results in genetic differentiation of populations.

GENECOLOGY AND THE ECOTYPE CONCEPT

G. Turreson in the 1920s defined genecology as the study of the adaptive properties of a population in relation to its environment.

An ecotype is a population distinguished by morphological and/or physiological characterisitics, interfertile with other ecotypes of the same species, but usually prevented from naturally interbreeding by ecological barriers. It is a subpopulation of a species which is genetically differentiated so that its survival in a particular habitat is enhanced.

Key aspects of the ecotype concept:

a. differences are genetically based;

b. differences may be morphological, physiological or phenological;

c. ecotypes occur in distinct habitats;

d. the genetic differences are "adaptive" (i.e. enhance survival and reproduction in that particular habitat);

e. ecotypes are potentially interfertile;

How distinct are ecotypes? Turreson stressed that they are discrete entities but often the variation is gradual along environmental gradients forming clines or ecoclines.

The much older concept of "provenance" in forestry refers to a local population of tree species distinguished by its growth and survival from other such populations.

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What is the significance of the ecotype concept in the contexts of understanding plant community dynamics or forest management?