Genes, Evolution and the Environment.

My aim here is to explain some of the terms which geneticists and ornithologists use in talking about the evolution and development of organisms in general and birds in particular. In the following section words and phrases in bold highlight the key points which I consider important to achieving an understanding of the subject.

It’s all in the genes.

In seeking a better understanding of variations in the characteristics which are seen in various bird species I realised that I needed to know more about genotypes and phenotypes.

Genes and alleles

Birds are multi-cellular organisms. All the things which control the form of the organism and how it functions are located in cells. Some cells contain Genes which are located on Chromosomes and are transmitted from one generation to another when reproduction takes place.

A gene is the basic unit of inheritance for a given characteristic. Genes can exist in alternative forms known as Alleles. In almost all animal cells, two alleles for each gene are inherited, one from each parent.

Paired alleles that are the same are called homozygous and those that are different are called heterozygous. In heterozygous pairings, one allele is usually dominant, and the other recessive.

Genotypes and Phenotypes.

The Genotype defines the genetic composition of the organism in respect of various alternative forms eg. two dominant, one dominant and one recessive or two recessive – which are possible. Geneticists can work out the options using Mendel’s Laws.

Most characteristics are determined by the interaction of several genes in the form of a gene-complex. This can give rise to a number of distinct options (dominant versus recessive permutations) which are called Phenotypes. Which phenotype actually sees the light of day is dependent on the nature of the environment in which development occurs.

A simple example from the plant world illustrates this point. A plant grows in two different habitats in Sweden. The genome is the same but the habitat which the seeds land in, determine the phenotype that grows:-

• One habitat is rocky, sea-side cliffs, where the plants are bushy with broad leaves.
• The other is among sand dunes where the plants grow prostrate with narrow leaves.

Talking specifically about birds, replication of genes from generation to generation perpetuates the appearance, body form and characteristics of the parents. Options in respect of the extent and appearance of certain characteristics may exist. Which option actually appears is dependent on interaction with the habitat.


Casting around for a useful example I was reminded of the Yellow-eyed Junco (right) which I had photographed in Turkey Creek, Arizona. I remembered looking at images of various sub-species of Juncos in Sibley’s field guide to birds of Western North America. An internet search revealed a great number of articles about phenotypes in relation to Juncos.

Juncos belong to a group of New World Sparrows which are listed in family Emberizidae. They are seed eating, ground foragers found in woodland and forested areas often at quite high altitudes (>3000 m) in North and Middle America. Authorities differ as to how many versions exist varying between 3 and 8 or perhaps even 12.

They are all very similar in size and body form differing only in morphology and colour as evidenced by some of the common names used:-

• Dark-eyed, Yellow-eyed, Red-backed, Grey-headed, White-winged, Pink sided, Slate coloured.
• David Sibley’s field guide to birds of Western, North America has illustrations of the Yellow-eyed species plus six sub-species of the Dark-eyed Junco.

Environmental factors are responsible for the appearance of individual phenotypes in various areas, at various levels of elevation and seem to be related to a combination of extent of vegetation, types of seed available.

The current IOC listing recognizes the Volcano, Dark-eyed, Guadalupe and Yellow-eyed Juncos as specific species so these phenotypes have presumably differentiated sufficiently to be regarded as true species.


Genes define the characteristics to be inherited and the extent to which certain characteristics may potentially exhibit small variations. The environment influences which option will actually appear.

Genetic variation, evolution and natural selection.

In the previous discussion about small variations within species, the genotype remained unchanged. This implied that the replication process worked perfectly but what if it doesn’t:-

Replication of genes.

The chromosomes and their attached genes are made from DNA (Deoxyribonucleic acid). The DNA molecules store information in the form of genetic codes needed for the synthesis of proteins to make more cells. The double-stranded DNA molecule is able to unzip and make identical copies of the molecule to replicate itself.

The replication process is not infallible and some gene copies have random errors called mutations. This can cause changes to existing characteristics or the development of new characteristics which will be inherited.

Most minor gene mutations are recessive rather than dominant so they pass unnoticed but some are not minor and have a profound effect on the phenotype. Gene mutations increase the size of the gene pool leading to an increase of variation within the population. This can lead to sub-species or entirely new species developing.


Evolution can be defined as the development of differentiated organisms from pre-existing, less differentiated organisms over a period of time. This would seem to imply that evolution is an ordered and sequential process and yet we have said (above) that “some gene copies have random errors” so how can we explain that.

Natural selection.

Although genetic variations are random, how they manifest themselves is not a matter of chance. According to Darwin's theory of evolution, only the organisms best adapted to their environment survive and transmit their genetic characteristics in increasing numbers to succeeding generations while the others tend to be eliminated. This has been called “natural selection” or “survival of the fittest”.

The fitness of an organism is manifested through its phenotype which, as we have seen above, depends on interactions with the environment.


Random mutations during cell replication cause variations in characteristics to develop, potentially leading to the evolution of a new species. The support of a favourable environment will enable the new species to survive and prosper.

Evolution and the environment.

We have learned that changed characteristics are brought about by random variations, occurring over a very long time scale. Natural selection requires that only the organisms with characteristics which are best suited to their environment will survive and transmit their genetic characteristics in increasing numbers to succeeding generations.

Population separation.

Organisms belonging to a given species do not usually exist as a single population. They exist as several small, spatially separated, interbreeding populations each with its own gene pool. This may lead to a situation where the environmental conditions in population A are not the same as those in population B. Species in population B may gradually change in character as they adapt to the different conditions.

A random variation to a species in population B may find the environment so supportive that the species become so genetically different that they will no longer mate with species in population A. We then find that a new species has evolved.

Geographic separation.

Populations which are widely distributed or have occupied different geographical habitats for a long period of time can show significant phenotypic differences. This leads to the formation of geographical races and sub-species.


In some cases the geographic separation can be relatively insignificant and the time span can be quite short. The Carrion Crow (left) is found in England and Southern Scotland. The Hooded Crow (right) is found in Northern Scotland. Hybrids are found in a narrow band across Central Scotland and they serve as an effective barrier to gene flow across the two main populations.



In addition to geographic separation features such as physical boundaries (bodies of water, mountainous ranges, differences in altitude etc) have a profound influence on the formation of new species. When a species occupies various areas where boundaries mean that each population is isolated from the others then each may adapt to different habitats and even food types.

If the adaptation is such that the various populations do not interbreed a new species has formed. Isolation of species is the key to this process and island groups are a classic example of rapid adaptation to different niches within very similar habitats.


Separation of populations of a single species can lead to them being subject to different environmental conditions and survival pressures.

If this develops into complete isolation, such that the populations will not breed if brought together, then the single species becomes two separate species.

The larger the gene pool the greater the number of phenotypes possible.

This increases the chance of diversification in terms of habitats which can be successfully occupied.1