Each environment an organism inhabits holds a different set of constraints for processes related to the survival, growth, and reproductive ability of a species.
Different patterns of temperature, precipitation, seasonality, depth, salinity, pH, and dissolved oxygen among terrestrial and aquatic ecosystems are reflected in the different characteristics that an organism exhibits. These characteristics include the physiology, morphology, behaviour, and lifetime pattern of development (known as life history) of an organism. Any of these characteristics which can be transferred from one generation to the next and serves to increase the ability for the organism to survive under a given environmental condition is known as an adaptation. Not all characteristics can be considered adaptations as the characteristic which enable an organism to succeed in one environment may prove to be detrimental in another. As the environment is constantly shifting, this leads to a game of cat and mouse, with organisms adapting to try and keep up with the rapidly changing environment. The patchwork of environmental conditions present on the earth lead to gradual changes in characteristics associated with these environmental gradients and is known as a cline. Amongst this patchwork will be distinct environmental conditions, such as mountaintops and grasslands, and populations which are highly adapted to these local environments are known as ecotypes.
To fully understand the process of adaptation, we need to look into the mechanisms through which it occurs. All of life’s processes are controlled by proteins – the characteristics, the make-up, and even the behaviour of the organism is linked to the functioning of these proteins. The body can choose which proteins to make though a process known as gene expression. Genes are a type of code which contains the instructions for the body to make proteins and are arranged on small bodies known as chromosomes. Chromosomes are wound up strands of DNA and contain all the information that make up the similarities and differences between every living organism on earth. The exact position of a gene on a chromosome is known as gene’s locus. Your parents each gave you one set of chromosomes, and so you have two genes at each locus known as homologous genes. If the two copies are the same, we call them homozygous. If they are different, we call them heterozygous. If a set of genes are heterozygous, then they may lead to an intermediate characteristic. If one gene beats the other and displays only that characteristic, we call it a dominant gene, and the loser, the recessive gene. The different possibilities of a gene at a given locus is known as an allele. All the genes in an individual’s body is known as it’s genotype.
The characteristics these genes control are known as an organism’s phenotype. Phenotypes are broken down into two main categories: Those that can be counted in discrete groups known as qualitative traits (like flower color), and those that occur continuously known as quantitative traits, like height. Most traits are continuous because they have more than one gene locus affecting them and they are affected by the environment which is generally continuous in nature (temperature, rainfall, sunlight level, etc).
If an organism is able to move and there is a suitable environment nearby, it will. Otherwise, it will change it’s phenotype through a different expression of it’s genes. This ability for an organism to change it’s phenotype under different environments is known as phenotypic plasticity. Like adaptations, phenotypic plasticity occurs at the level of the individual. It can be divided into two groups: Developmental plasticity (irreversible changes that occur during growth and development) and acclimation (reversible changes to a given environmental condition, such as changing your average body temperature when it is cold outside).
Natural selection is defined as the differential success of an individual within a population to survive and reproduce resulting from their interaction with their environment, and this process acts directly on an individual’s phenotype. The fitness of an individual is known as the contribution it makes to future generations that are able to reproduce themselves. If an organism’s traits are a good match for the environment, then their fitness is high as they are able to produce more successful offspring, and they are selected for. If their traits are not a good match for the environment, then they will leave less successful offspring, and they are selected against. There are 3 main types of selection: Directional selection (where one extreme trait is selected for while intermediate traits and the other extreme trait are selected against), stabilizing selection (where both extreme traits are selected against and the intermediate trait fares better), and disruptive selection (where both extremes are selected for and the intermediate trait is selected against).
The ultimate sources of genetic variation that natural selection acts upon are known as mutations. Mutations are heritable changes in a gene or a chromosome and can be either beneficial or detrimental depending on the environment. Changes in the amount, or frequency, of alleles in a given population due to random chance is known as genetic drift. Genetic drift is like sampling error in that each generation of a species only contains a subset of the genes of the most successful parents. Patterns of genetic variation in a population can be changed by the movement of individual organisms in and out of the population, known as migration. When natural selection results in changes of properties of populations over generations, we call this process evolution. Under conditions of random mating, and in the absence of natural selection, mutation, genetic drift, and migration, the frequency of alleles and genotypes in a population will remain constant between generations and evolutionary change will not occur.