The Concept of a Species

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There is no simple definition for a ‘species’. The recently founded idea that species may ‘evolve’ gradually over time has changed the concept of a species. Before the idea of evolution, it was believed by almost everyone that a God created all species individually, and that they were fixed and unchanging. But later, much evidence such as the fossil record and extensive studies on the adaptations of certain creatures found by scientists, including Charles Darwin, led more and more people to believe that species have evolved very gradually over long periods of time. And because of this, the concept of a species has been modified.

Ernst Mayr, one of the founders of modern evolutionary philosophy and theories defined biological species as:

“Groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”.

This definition is also known as the biological species concept (BSC). Advocates of the BSC say that forms that are only slightly different (for example different geographical forms) may interbreed if they had the opportunity. Therefore they should be considered as the same species. However, supporters of the BSC also say that because two different forms seldom interbreed does not mean that they belong to the same species.

A good example of this is wolves and coyotes, as they can mate and have fertile young. The phylogenetic species concept (PSC), however, states that different forms of the same kind of bird (for example) should be treated as separate species as these forms have evolved separately and have unique evolutionary histories. The PSC is becoming more popular definition, as it is less restrictive than the BSC. There is no worry about whether slightly different geographic forms may interbreed, and no need to find and carry out an experimental method of finding out whether the two kinds would breed.

The argument may well be a rhetorical one because every kind of organism presents us with a different situation. It is possible that neither of the two definitions can be consistently applied throughout nature. Read about evidence of evolution

Species come about as the result of gradual changes. New traits that result from these changes are selected through a process of natural selection. As environments are continuously changing, natural selection favours different characteristics in different situations. Natural selection over long periods of time eventually produces new species. If an animal or plant does not operate as effectively as others of the same species (low fitness of survival) then it will have less chance to pass its genes through reproduction to the next generation. Therefore natural selection brings about adaptation to environment as traits that make an organism less effective than others are not passed on.

A good example of successful natural selection is that of the peppered moth, Biston betularia. During the Industrial revolution, soot and industrial wastes darkened tree trunks and killed off lichens. The grey form of the moth became rare and the melanic form (the black form) became more abundant. In 1819, the first melanic moth was seen. But by 1886 it was far more common, suggesting rapid evolutionary change. This increase in the melanic form of the moth in polluted areas is because the dark colour of the moth protected it from predation by birds and so as it was able to survive better in polluted areas, the melanic form of the peppered moth became more abundant than the grey form.

This example illustrates Darwin’s four postulates:

1. Not all young produced in a generation survive to reproduce — many more moth eggs are produced than can survive; many moths will be eaten by birds before they reproduce.

2. There is variation among individuals — some are black in colour, others are grey

3. The variation is genetic — black moths have different alleles for colour than do grey moths

4. The different forms differ in fitness — in industrial environments, the melanic moths survive to reproduce better than do grey moths since grey moths get eaten by birds.

An individual moth does not adapt — it is genetically either black or grey and cannot change. Individuals do not adapt. The population of moths adapts because those individuals with lower fitness traits do not reproduce as much (because they do not survive), so their genes are not carried on from generation to generation as much as the genes for higher fitness traits, so the traits coded for by those genes die out over time, and traits with high fitness become common.

There are three forms of natural selection, directional selection, stabilising selection and disruptive (diversifying) selection. Directional selection is where the more successful trait is favoured, leading to an increase in the population with that certain trait. The example of the peppered moth is evidence for this. Stabilising selection is where the selective pressure favours the existing successful traits and the number of individuals with the less successful traits is reduced and some even die out. This kind of selection is typical of an unchanging environment. Disruptive selection is where the two extremes of variance are both favoured, so that if there were a change in a species, the new type and the original type would both be favoured.

This leads to the divergence of the phenotype (actual appearance of the organism) and therefore leads to the emergence of two distinct phenotypes so the effect is that to split the population into two subpopulations. If gene flow (this is also known as gene migration or as migration. The loss or gain of individuals from a species can easily change gene pool frequencies. For example, if all red haired people were to leave England, then the next generation would be likely to have very few people with this trait. The English population would have evolved, as would the population to which the red haired people would have migrated) between the populations is prevented then the process may give rise to a new species. This kind of selection is uncommon but is of relative importance as it is of theoretical interest because it suggests a mechanism for species formation without geographical isolation.

Artificial selection is similar but is conscientiously carried out by humans in order to change the evolution of those certain organisms to obtain the desired traits. A good example of this is the creation of new breeds of animals through the control of their reproduction. Controversially, examples of artificial selection are seen in humans themselves as they choose their mates according to their own preferences. Darwin had relied upon domestication through artificial selection to demonstrate the selection process and ultimately prove that evolution exists in species.

Charles Robert Darwin.

Darwin’s main studies were in the similarities between groups of organisms as evidence of evolution. With persistence and extensive studies trying to find as much evidence as possible, he managed to bring into society the idea of evolution through his book ‘On the origin of species by means of natural selection’ (first publication in November 1859), which was at first rejected and thought of as absurd, causing a lot of controversy as many people refused to believe that they were related to apes. Charles’ ideas were opposed by the Church and through his letters and private notebooks, we now know that he had lost his faith in God and became atheist.

Many people before Darwin had thought that species were created by God and that they were fixed and unchanging. But the evidence that Darwin had gathered proved them wrong. He proposed the idea that modern humans were closely related to apes. This also caused uproar in the society as he at first failed to prove his theory. Many scientists at the time had adhered to the ‘catastrophe theory’. Where it was thought that the Earth had experienced a succession of creations of animal and plant life and that each creation had been destroyed by a sudden catastrophe such as a convulsion of the Earths surface. It was thought that the catastrophes were ‘localised’ and eventually repopulated by species from elsewhere on the Earth. Often, catastrophists also believed that God created new life after each global catastrophe.

When he was 22, he served as a naturalist aboard the HMS Beagle on an expedition to the southern hemisphere for five years (1831-1836). This where he had begun to develop his ideas of evolution by natural selection. In South America Darwin found fossils of extinct animals that were similar to modern species. On the Galapagos Islands in the Pacific Ocean he noticed many variations among plants and animals of the same general type as those in South America (near the Galapagos Islands). In the Finches of the Galapagos Islands, he found evidence of adaptive radiation (this is the development of a variety of forms adapted to various environments from a single ancestral group).

It is thought that around 0.5 to 1.5 million years ago the original finches in Equador were swept to the nearby islands by a strong hurricane. The finches had to adapt to the new environments in order to survive. On each different island and in different areas of each island, there were different situations to live in, although the climates were similar. There were differing numbers of predators to the finches, different kinds of predation, there were different kinds of seeds and fruit to eat. To survive, they had to adapt their beaks for the different kinds of food (larger beaks could crack nuts and smaller beaks could pick up seeds), they had to adapt their diets, they had to adapt their sizes to the predators, they had to adapt their colours for the predators and along with these there were more adaptations to the finches. These adaptations gave rise thirteen new species of finch.

Adaptive radiation is also divergent evolution. The species evolve apart from each other as they split into different species. This is the opposite to convergent evolution, where there is evolution of the same trait in two or more species that have come from different evolutionary lineages, so the two species have adapted to similar ecological and environmental conditions. The two species evolve to look similar. An example of convergent evolution is the evolution of wings in bats flies, and birds or the hydrodynamical and streamlined body of fish, dolphins and other aquatic animals. This kind of evolution can be represented by the diagram below:

The two lines that join to become one represent the two different ancestral species that evolve to have a similar trait or phenotype (so they look similar).

Adaptive radiation would be represented by this diagram:

The single line at the top of the diagram represents the original ancestral species. Then, moving towards the bottom it splits into two lines that represent the two new species that evolved from the original. The number of lines splitting off from the original can be altered to mark the actual number of species that arise from an ancestral species (e.g. for the Gal�pagos finches there would be thirteen lines coming off the single line).

These diagrams can be more advanced. They can be larger to show the relationships between many organisms and their ancestors. They are called cladograms. Phyletic trees are also diagrams that represent relationships between organisms. These are mostly 2D diagrams, but Charles Darwin altered it so that it became a 3D diagram. The 3D tree improves visualization and qualitative analysis since it concurrently provides topological (tree structure) and spatial information (based upon genetically measured distances).

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