The most important concept central to evolutionary biology is the study of species or groups of organisms that are adapted to a part of the environment and it is at the level of species. As the Modern Synthesis became established, there were not one but two definitions of species, and one of the two, the biological species states that species are groups of organisms that can interbreed with one another but not with members of other species . This concept became so important that it has achieved status in many biology textbooks afterwards but as studies of how organisms reproduce, whether animals, plants, and microbes, it became clear that even that definition had its limits.
In science, such as biology for example, you can only investigate only a small part of nature but not all of nature and that is true when studying the biological species concepts which applies only to organisms that reproduce sexually where there is a male organism which produces sperm and a female organism that produces the egg and through what are called “isolating mechanisms” or a set of mechanisms where each member of a population recognize one another through mating rituals for example. That is one of the definitions applicable to the biological species concepts but this one concept applies only to sexually reproducing organisms and we know now that the rest of life that defines the biosphere are microbes, notably bacteria which do not reproduce sexually and in fact the biological species does not apply so a different definition for species needs to be applied.
In fact, from studies of other organisms such as bacteria, species definitions became more pluralistic or in other words various kinds of species definitions had to have been defined in order to study evolution for each group of organism and to do that, species concepts became more and more diversified depending on the research on those organisms that sexually reproduce such as animals or asexual organism such as bacteria.
Linnaeus , Species, and the Science of Taxonomy
For those of you who had to study biology in high school or in college, you may, no doubt, recall that the science of taxonomy or the field of biology that classifies living organisms had its modern form when Carolus Linnaes established what is called the binomial classification or that each organism, described by biology has a two part name, the species or specific epithet and the genus. Although in reality, Linnaeus was not the first scientist to propose classification whereas previously others such as Aristotle had done so but Linnaues gets the credit for establishing the science of taxonomy which stands to this day.
For starters, we will first review the basics of taxonomy by first focusing on the taxonomy of biological hierearchy what this means is that first an organism is described by the species and is then placed in the genus. The species describes the organisms while the genus includes those members of the species that share similarities such as body structure, habit, and genetic similarities. An example to illustrate is the insect or fly that is common to genetic research, the common fruit fly or Drosophila melonagaster and you may notice that when writing the binomial name, both words are italicized and one word is capitalized. That one word that is capitalized is the genus and Drosophila is the genus for which the species melanogaster belongs. Starting with the binomial nomenclature of genus and species as established by Linnaues, this says that melanogaster , which is a Greek derivation of a word ” yellow bellied” in reference to a physical trait that the common fruit fly which is a yellow colored abdomen, while Drosophila, is also Greek for “dew lover” and what this means is that fruit flies, as what the common name implies, are attracted to rotting fruit as well as any yeast that may be present on the rotting fruit which is what the fruit flies are attracted to and hence the scientific two part name accurately describes.
The genus Drosophila can have two or more species with melanogaster as and example with species such as D. pseudobscura as an example which is a slightly different species than melanogaster but both species in the genus Drosophila have some traits in common such as the presence of a pale yellow bodies and living off decaying fruit so in a genus two or more species can share the same genera but have different species names and reveal similarity.
Above the genus, there is the family and that includes all the genera where there can more than two , with many different genera sharing certain traits in commons and in the case of fruit flies, all genera of fruit flies have one thing in common and that is their presence in living on rotting fruit and nothing else. There may be some variations in genera and as long as the variation is not too great as to affect classification, then several genera are lumped together into a family.
Different families that share another trait in common is called an order and for our fruit fly example, the order is called “diptera” and this order represents what are called “true flies” and every species, genera, and family is placed within the order for sharing the characteristic of “true flies” and these are insects that have one pair of wings and are thus capable of flight but these insects have a feature that sets them apart from other insects and that is the presence of a pair of knobs called halters and these allow the flies to not only fly but to perform crazy aerial maneuveres, which is one reason that it is almost impossible to swat a house fly, another example of a dipteran. The life cycle of dipterans are characterized by the eggs that are laid in either decaying flesh or fruit, and only the larvae or maggots feed in the organic substrate and undergo metamorphosis from larvae to pupa, where the larvae is then transformed into the adult.
Several orders that share another similarity then define the class and the order dipterans share similarities with another order, the order coleopteran, the largest order which are known as the beetles and what both beetles and flies have in common is that both are insects so fruit flies and beetles are insects and that is an animal with an exoskeleton or outer covering, three pairs of legs, and the body divided into three parts, head, thorax, and abdomen. Life cycles are characterized by metamorphosis which is divided first by an fertilized egg which then releases a larvae or immature form of the insect, that is more or less identical, then pupae or transformation stage then the adult which has the form that we can recognize as either a fly or a beetle.
Classes with similarities define the phylum and the class insecta is similar to the class arachinida which includes the spiders and although insects and spiders are different species of animals that are invertebrates or those animals without an internal backbone, with the insects having six legs, antennae, and wings, whereas spiders lack wings and antennae and have eight legs, what both insects and spiders have in common is the presence of an exoskeleton, jointed legs, and body parts divided into a head and abdomen. In regards to the fact that spiders and insects have jointed legs, there is a word that describes the presence of having jointed legs in that both spiders and insects are called “arthropods” which means “jointed feet” and not just insects and spiders but other animals that are arthopods such as the class crustacean or rather crustacean namely crabs are also related to the insects and spiders in possession of jointed legs and a hard exoskeleton. Other classes of invertebrates such as the class myriapoda includes the millipedes. Thus any animal that is above the class level , however different they may be at the lowest level or genus level, that has a trait in common such as the exoskeletons and jointed legs of insects, millipedes, and crabs are placed at the level of phylum such as the phylum arthropoda.
Above the phylum, there is the kingdoms and invertebrates as diverse as arthropods, the phyla that includes the insects and spiders, along with other phyla of invertebrates such as phylum annelida and these include segmented worms, phylum Mollusca or molluscs which includes snails and slugs, and the phylum Echinodermata which includes the marine invertebrates sea stars and sea urchins and all these kinds of animal phylum define the animal kingdom and in this kingdoms, an animal is an organism that can only get ready food and has a degree of movement and is composed of various kinds of cells for specialized function such as nerve cells and muscle cells, the former for taking in sensory information while the latter, which is a true characteristic of animals, and something that is not present in other different kingdoms, the presence of which allows for locomotion as well as the possession of a degree of complexity of the nervous system, which is a good trait for distuingishing animals from other kindgoms such as plants and fungi which possesse no nervous or muscular system.
The highest level of classification above the kingdom is the domain and that is a classification that is based on something present in all kingdoms, and that is the presence of an eukaryotic cell or a cell with a nucleus where the DNA is organized into special proteins called histones which is then arranged into chromosomes which carries genes or units of hereditary information and in every chromosome of an eukaryote there are actually two kinds of genes, exons which are genes that code for protein molecules and introns which do not code for any protein. Also the nucleus is surrounded by cytoplasm with various subcellular structures called organelles which are membrane bound structures that carry out specific biochemical functions such as the mitochondria which are present in animals as well as in plants and fungi where biochemical energy is released in a step by step process into a molecule called ATP which is the energy currency of all cells. Eukaryotic cells undergo two forms of cell division, mitosis, a complex process where one cell becomes two, identical cells, and meiosis, a cell process where chromosomes are reduced half and then through a process called recombination produce a special set of cells called gametes or sex cells, which carry half the number of chromosomes and through recombinations genes are exchanged between pairs of chromosomes resulting either in sperm cells or egg cells. Meiosis is only used in organisms that carry out sexual reproduction and is responsible for genetic variation within a population. Animals, plants, and fungi belong to the domain Eukarya which is the domain of eukaryotic cells.
From this tour of biological classification, you see how it goes from specific to general, that is starting from species well up to the domain, with every organism described using the Linnaean classification which has been expanded in light of new discoveries not just in finding new animal or plant species but also findings in molecular biology, cell biology, and microbiology which broadened the Linnaean classification scheme.
When the foundation of this classification scheme was developed, although it placed species based on similar traits which went up to the kingdom level, at first you might think that Linnaues, through his system of classification would reveal an evolutionary relationship. You would be wrong for Linnaues was operating within the typological mindset where he only thought that in the beginning God created organisms that are distinct from one another and would be considered classes of the typological kind where each species is represented as a kind of unchanging essence, the basis behind what is called “typological species” and although one aspect of taxonomy as established by Linnaeus is for the purpose of identifying known species with those that have not yet be formally classified, or in other words Linnaean taxonomy as a form of identification, which is still used when trying to identify an animal or plant in the wild for purposes such as knowing which insect inhabits a specific habitat or to know which animal is likely to go extinct.
Ever since The Origin of Species up until the Modern Synthesis, when Darwin first adopted population thinking and with this radical form of thinking which became part of the science of evolutionary biology, the original Linnaean concept of species as fixed types is no longer tenable but each species is an endpoint of a process of evolution , through natural selection, where each species is adapted to its habitat because of natural selection while many other species of animals and plants have gone extinct only to be replaced with better adapted forms as the science of paleontology reveals.
With an understanding of evolution as first hypothesized by Darwin but later confirmed through the Modern Synthesis, the taxonomic scheme as outlined by Linnaeus but seen through evolutionary biology does reveal evolutionary relationships between species, as well as genera whereas in pre Darwinian biology, similarities, as used for classification would have been seen as just a coincidence through typology but with population thinking where because of genetic variations which is a real phenomenons, these kinds of heritable differences are enough to cause species to adapt and evolve into different species and taxonomy. according to Darwin can reveal which species are evolving and which remain constant so a historical dimension is added to taxonomy, which would make taxonomy something of a historical science where each species, genus, and family are the endpoints of an evolutionary history.
Darwin may have reasoned that species are not fixed but have a long, history that is as old as the earth itself and that each species is different from one another because of adaptation which is the result of natural selection, a testable concept of species as a evolutionary unit was not really defined by Darwin himself and it is interesting that even Darwin was rather vague on the term “species” and the species he was likely referring to was the definition of species as defined by Linnaues, but a testable definition of species that is different from the Linnaen version only became apparent after the Modern Synthesis where two species concepts became part of evolutionary biology one of which I will focuse is the one you probably have heard which is the biological species concept, which was originally formulated by Ernst Mayr as well as the ecological species concept.
Before reviewing these two concepts, first we must shift focus from Linnaeus to Darwin, starting with a basic definition of natural selection, how it is the main mechanism of adaptation, and how Darwin challenged the belief of species as fixed types.
Darwin and the Problem of Species
The Linnaen scheme of taxonomy may be useful for certain problems in biology such as conservation purposes or for identifying which species of plant or animal but the scheme itself reveals nothing about the history of how the species is adapted to its habitat and it was Darwin (1859) who realized that taxonomy beginning at the genus up to the family for example does reveal that various species share a common ancestor. Whereas according to Linnaesus, similarities can only be considered coincidence, relations between species reveals evolutionary descent so taxonomy had to accommodate the fact that evolution by natural selection is what makes species different from the ancestors and so taxonomy and biology in general now had a historical dimension where all of life is related through one another through the vast expanse of time.
If you have read my blogs on the history of evolutionary thought, I outline the step by step process on how Darwin arrived at the theory of natural selection, which as outlined by Mayr (1988) is really a collection of theories that are related to one another as to form the whole of natural selection, but I will not do so for this blog, rather I will summarize in a single paragraph or two on how natural selection and evolution are inevitable.
From Aristotle well up to the Middle Ages and from the Renaissance up to the time of Linnaeus, western thinking, which included a belief in a supernatural God that created the world as well as all the living inhabitants, it was believed that , as far as the western world was concerned, that every living creature, small or large, was more or less the same as its ancestors. Although fossils, which are the remains of extinct life forms, went unnoticed, it was thought that there were just modern forms of todays life forms that were destroyed by Noah’s flood.
However as far as paleontology, it wasn’t until the 1790’s, when the French biologist Georges Cuvier, studied fossils and by comparing each fossil to the living species, he concluded that fossils represent life forms that are completely different than the ones living today and in addition, as sedimentary rock layers were being studied, it was found that in each layer, there are fossils that tend to be different with the more complex ones at the top while going towards the bottoms, the forms of the fossil become more and more simple and also completely different from similar living forms observed today.
Despite these findings, it was still assumed that even if there were extinctions, as was popularized also by Cuvier, then at each layer, the presence of fossils that were different , it was still assumed that either these species were created by God or else when trying to take a natural explanation, instead of a supernatural explanation, species suddenly appeared by a process called spontaneous generation, which later was proven to be false. Although some thinkers thought that even before Darwin, life forms evolve from one another, but exactly how was not known, except for another Frenchman Lamarck who postulated an evolutionary mechanism, now later proven to be incorrect, that through inheritance of acquired characteristics, every life form becomes more and more complex because of some inner drive to be perfect.
From Linnaeus up to Lamarck, all were in the typological mindset where each life form, according to typological thinking, is an unchanging essence or “type”, and Linnaeus assumed that every species in his classification were types and also with Lamarck, his evolutionary theory is really an example of a type changing to a different type. When Charles Darwin introduced his theory of evolution, it was not only challenging to many beliefs that many people held dear such as the belief that humans were special and were set apart from the rest of the animal world, but really the most radical thinking, and this was one of the reasons for Darwin’s genius, is his breakaway from the typological mindset to a population thinking mindset, that few biologists of his era took seriously but was later accepted during the Modern Synthesis.
Population thinking became the foundation of modern evolutionary theory, from 1859 up until the present and starting at the population, each individual is not exactly alike in terms of phenotypes, and this is a consequence of reproduction and also starting from the problem of population growth from Malthus, human populations grow faster exponentially, far faster than the food supply and Darwin applied this to all of life, not just humans, but not every offspring produced in abundance will survive until reproduction so in each generation , no two are alike and that makes difference between survival or death. From variations of phenotypes of each individuals, those with phenotypes that allow survival or adaptation will have a better chance of surviving or more roughly, those that are fit have a better chance than their less fit relatives who will likely die and the ability to adapt will be passed on to each individual. At the individual level, it is the environment that determines who will live to reproduction or not, and this is what Darwin meant by natural selection for nature selects those that can adapt and filter out the ones that cannot.
Inevitably above the population, the population will slowly become different than in the previous generation, if the environment changes and the result, because of natural selection, populations will evolve into new forms and that is evolution which is the result of natural selection which depends on a few rules. 1. Populations grow but in each life cycle, few offspring survive while the rest die. 2. The ones that survive are the result of variations and variations are present in populations and are heritable, and 3. Those individuals with variations that favor survival will pass on these phenotypes to the next generation. This form of “differential survival” as Mayr (2001) preferred to call it, is really just natural selection.
As natural selection is assumed to be the primary form of adaptation and that natural selection is assumed to act on the individual as was implied in Darwin’s reasoning, as elucidated by Gould (2002), then an ancestral population may either evolve into many species, some will evolve into two or three species, or another lineage could go extinct. Darwin (1859) was the first to illustrate in what he called “the tree of life”, where two or more species either evolved into many species or subspecies while another lineage either evolve into two lineages or none at all. This form of speciation or process where species evolve into different species, as outlined by Darwin (1859) was different in that not every species evolve into complex forms but some will and some will stay the same.
This is ultimately dependent on population thinking with natural selection which is a inevitable consequence of population thinking wherease other evolutionary theories which are based on typology where later doomed to fail with Darwinism and later Neo-Darwinism being victorious because of population thinking and although Darwin illustrated the consequence of speciation with the only diagram from his famous book, there was one problem that Darwin never made clear and that is his definition of species.
Darwin never gave a rigorous definition of species in his work but talks about species in a rather vague way and that seems rather unfortunate given that the real problem was not just about definition of species but rather the process itself or in other words speciation. He may illustrated how species evolve from his then hypothesis of natural selection but he does not give a clear definition since at the time that Darwin presented his revolutionary idea, species were rather defined in terms of taxonomy in the Linnaen sense and originally Linnaeus assumed that species were the same in the past as they are today but with Darwin, this is not likely the case for in the distant past the ancestors of all animals and plants were completely different than they are today because of evolution by natural selection which Darwin stressed was a slow and gradual process.
As natural selection became recognized through the Modern Synthesis as the “force” that shapes adaptation, it became apparent that a scientific definition of species had to formulated and two species concepts which became part of biology and one of the two concepts became popularized mainly through the work of Ernst Mayr which is called the “biological species concept” and from this concept, starting with a population, a population is defined as a group of individuals that are adapted to one habitat and can interbreed with one another in the same population, and from this definition of populations, the biological species concept is stated as individuals through what are called isolating mechanisms which are a set of phenotypes such as mating rituals to protein surfaces on gametes which sets one population or species apart from one another. This concept was based on sexual reproduction and although Darwin placed emphasis on reproduction as the source of variation, he was ignorant of the fine details of cellular reproduction, mainly meiosis and wasn’t until genetics was expanded on the cellular level together with field observations of various species of animals and plants that the biological species concepts matured into a form that is part of the definition of species as found in biology textbooks.
The biological species concepts has it advantages in that it is able to separate one group of organisms, based on certain characterstics of sexual reproduction and as far as speciation goes, speciation would involve the evolution of somewhat different isolating mechanism resulting in two species or more, this concept is applicable only to species that reproduce sexually but it does not work for species that reproduce asexually, so species concepts had to broaden to include criteria other than isolating mechanism but still it is worth exploring the biological species concept in detail from its original inception up to its drawbacks of where it could not be applied.
The Biological Species Concept: Advantages and Disadvantages.
It is a fact that in the plant and animal kingdom, there is sexual reproduction where a set of cells, the gametes, have half the number of chromosomes in comparison to the total number of chromosomes which are different for each species and through meiosis, gametes are produced with half the number and through recombination, each gamete has a different arrangement of chromosomes, so that no two are alike in terms of composition of genes as the arrangements are different from one gamete to another. Gametes fuse together to form the zygote which has both sets of chromosomes from the mother and father and that results in the fully developed organism characteristics of each species.
Based on studies at the cellular level on how genes are arranged on chromosomes as was made apparent from the science of genetics along with the fact that members of a population can only interbreed with one another, it became apparent that reproduction not only allows offspring to be alike from their parents, but sexual reproduction appears to be the key for defining species as members where each one can recognize one another for mating but not with members of other species. If individuals can only mate with other individuals in the same populations, then questions were asked such as: How do individuals recognize their own kind during mating? What is it about sexual reproduction that sets one population apart from another sexually reproducing populations? How, during speciation, does an ancestral population evolve into new lineages and how does sexual reproduction play a role?
During the Modern Synthesis where genetics and natural selection were combined together, one of the problems on defining species along with understanding speciation was partially solved through the forming of the biological species concepts and this concept depends on what are called “isolating mechanisms” also called reproductive barriers, and as these two different terms imply, these are barriers that a population posseses which sets them apart from other populations and are part of each individual.
These isolating mechanisms consists of two main groups: one are the pre-mating isolating mechanisms and these are the barriers that prevent mating from two individuals of two different populations while the second, the post-mating mechanisms take effect after there is mating between individuals from two different populations. Evolutionary theory , when attempting to explain how species evolve from one another must also take into account the evolution of these kinds of isolating mechanisms for it is the evolution of different isolating mechanisms that separates species from one another if the biological species concept is to have any degree of validity.
For the pre-mating isolating mechanisms these act to prevent species of the same genera as well up to the family level from interbreeding and there are a variety of pre-mating isolating mechanisms such as pollinator preference where different species of insects pollinate only one species of flowers. Other examples that are more general than just pollinator preference include ecological habitats where various species are prevent from interbreeding because of inhabiting various kinds of niches. An example would be a species of lizard that can feed and live in tree canopies whereas another related species only lives on the trunk as well as the ground. These two species if they occupy the same tree will not interbreed because of the different habitats and selection would have favored the survival of these two different species of lizards because of varying habitats. Temporal isolation involves breeding in different seasons such as various species of coral in marine habitat that spawn at different time in order to decrease the chances of species interbreeding.
Despite the evolution of various kinds of pre-mating isolating mechanisms in keeping species separate, sometimes there will be a chance that individuals from different taxa such as from two different species will mate, and aside from isolating mechanisms that are either ecological or temporal, there are the postmating mechanisms that are present in the reproductive biology of each species and one such isolating barrier is mechanical isolation and that is in animals, the shape of the genitalia are different for each species while in plants the surface structure of the pollen or the plant equivalent of male sperm, is different for each flowering plant species. This is like a key fitting into a specific lock and it would make sense for natural selection to favor certain biological structures for copulation in order to prevent sperm transfer, as in the case for animals from members of different species. Another form of post-mating isolating mechanisms also includes gamete incompatibility where the pattern of proteins on the surface of gametes such as sperm and egg are different depending on the species.
With pre and post mating isolating mechanisms, these can greatly reduce the chances of species intercrossing in the wild, but sometimes even with these kinds of isolating mechanisms, rarely will a hybrid or organism that is born from two individuals of two different species will result. Another set of post mating isolating mechanisms includes what is called hybrid inviability or when the hybrid dies before reaching sexual maturity. There is also hybrid sterility where the hybrid is unable to reproduce. An example of the mule, which is the cross between two different species of equids, the horse and donkey comes to mind.
It is not enough to define what a species or rather what each species concept is, it also important to describe the process by how species evolves and how natural selection favors speciation. The forming of the biological species concept was an attempt to do so and it was proposed by German American biologist Ernst Mayr in 1942, based on his research of observing various species of birds in the wilds of New Guinea and previously Russian American biologists Theodosius Dobzhansky, a student of Thomas Morgan, observed the fact of isolating mechanisms in species of fruit fly so Mayr along with Dobzhansky generalized isolating mechanisms as a consequence of natural selection favoring heritable difference between potential species diverging from an ancestral species, as part of the definition of a biological species.
Speciation, according to Mayr, occurs when in a population that is adapted to a given environment, a small change in the external conditions of the environment will favor any heritable changes and through sexual reproduction which is the source of genetic variation, any phenotype that allows survival in the specie’s new conditions will be favor but for speciation to occur, there must be a corresponding evolution of the pre and post mating mechanisms and these must not only be heritable but also be different in keeping the species apart while adapting to the new environments.
Because the science of genetics matured in the first part of the 20th century, this field of biology merged with evolution and one of the findings in genetics that helped reinforced the biological species concept is what is called gene flow and that since there is a degree of movement whether in animals with their natural ability to move from one place to another and even plants with their spreading of spores, seeds, and pollens, organisms carry their genes and their alleles through out space and such movement of genes is what is called gene flow and gene flow acts to prevent populations from diverging into different species. If a natural barrier acts to prevent moving populations, the amount of gene flow is reduced and with a reduction in gene flow, the chances of variations including variations in the isolating barriers present in the species will also diverge resulting in the possible creation of two or more new species.
There are many forms of speciation, and with biological species these include the biological changes such as evolving the isolating barriers as well as ecological conditions, both of which play a role in speciation and one example of speciation is allopatric speciation where a population that has adapted in one geographical area will speciate if there are changes in the environment a river slowly forms in a plain separating a once continuous population of animals forcing half of the population to adapt to these new circumstances as an example and through environment changes, two species will adapt and become different in terms of their isolating barriers that individuals from the two species will rarely interbreed.
The biological species concept has been pretty useful and it still is but it has some drawbacks and originally in what begin with Darwin, with his vague concept of species where he only had sexually reproducing organisms in mind until Mayr’s proposal of the concept which depends on sexual reproduction. Later as the field of microbiology became more advance and many species of bacteria were characterized, they reproduce mainly through binary fission where one cell becomes two and it is generally the case that the majority of bacteria reproduce asexually while the biological species concept is based on the observed fact of sexual reproduction so the biological species concept is not applicable in the case of bacteria.
In addition as far as bacteria are concerned, they even have a form of “sexual reproduction” called conjugation where one bacterial cell donates some of its DNA to another cell and it does this by giving to the recipient cell what is called a plasmid, which is a circular ring of DNA to another bacterial cell. This is not sex in the sense of meiosis where meiosis is only found in eukaryotes and there is nothing like that in bacteria which are prokaryotes and there is no meiosis. Sexual reproduction is dependent on the process of meiosis and here is where the validity of the biological species concepts breakdowns.
The biological species concept is still useful in the field of zoology and botany but it has limited use in the subfield of microbiology, bacteriology which studies bacteria where there is nothing like sexual reproduction so in keeping with species as part of evolutionary biology which is central all fields of biology and because of the validity of the foundation of Darwinism, species concepts began to broaden, it had to include to take into account in addition to sexually reproducing organisms , there are also asexually reproducing organisms like bacteria.
Since all species of life forms are the descendants of past life forms, and all are related towards a common ancestor, another species concepts was then considered the phylogenetic species concepts which we will look at next since after all of life is related by common descent.
The Phylogenetic Species Concept
Darwin, through his work from his book The Origin of Species, added another dimension to biology which is time, in arguing that all of species that are extant today were the descendants of past life forms, each adapted to current conditions which were different than the environments of what their ancestors were adapted to. Since then, biology is an historical science and through natural selection, adaptation together with inheritance is what drives speciation, whether allopatric or sympatric or speciation that occurs in populations while the former depends on environment.
Since species are the end result of a gradual process of evolution, then it would stand to reason that there should be another species concept that takes into account the fact of descent between past and present lineages and in one chapter of Darwin’s work, which is on natural selection, Darwin illustrates how species diverges using a tree of life diagram , and what was unique is that , because of population thinking which was a strong foundation for Darwinism, Darwin rules out the medieval concept of the Chain of Being where “lower” life forms are at the bottom and going on top are what we consider the “higher” life forms from birds to mammals to humans but humans are also below supernatural beings bellows angels up to God. In alternatives to Darwinisms, the chain of being takes the form of a kind of cosmic escalator where lower life forms evolve to higher high forms with humans, supposedely representing the highermost apex of the ladder and this was the alternative to Darwinism until finally as the Modern Synthesis became established, a ladder of progress was unsuitable since there was no evidence whatsoever of an inner drive to perfection and starting with Darwin, most species do diverge inhabiting various niches, even becoming complex while some lineages produce few or no descendant species.
The phylogenetic species concept which simply states that each species is the descendant of past species as well as having the potential to produce future species only became a scientific fact when the science of molecular biology made it possible to compare sequences of protein and nucleic acids revealing which specie are closely or distantly related and while taking into consideration natural selection as well as other evolutionary processes that can affect speciation, and through molecular sequence determination techniques, species are either related more or less to other species and since each species can be described on the molecular as well as on the morphological level, the phylogenetic species concept matured and had advantages as well as disadvantages when compared to other species concepts such as biological species.
Just how is it possible for the molecules of life to be used to determine how each species of organism is related to one another and how are molecules used to determine the evolutionary history of species?
All of life on earth shares a common molecule, and no matter how different each one is from one another, this molecule is universal throughout all of life and allows heritable traits to be passed from parents to offspring, and this molecule, no doubt, is the well known DNA molecule. A DNA molecule is composed of nucleotides or building blocks and each nucleotide has three parts, a sugar called deoxyribose, a phosphate and four nitrogen containing structures called bases. Along the length of a DNA molecule, there are strings of nucleotides or polynucleotides wound around together into the well known double helix, and along the outer strands of the double helix, there is a uniform pattern of deoxyribose and phosphate but inside the double helix, there are variable patterns of the four bases and the four bases are called , adenine, thymine, cytosine, and guanine which are abbreviated as A,T, C, and G respectively.
Four of the bases in DNA come in pairs and only one base pairs with a different base. Thus, A only pairs with T and C only pairs with G. If in one strand of a DNA molecule, call it A, there is the strand with the sequence of bases
then on the other strand, call it B, and since adenine always pairs with thymine and cytosine pairs with guanine, the corresponding strand will be
Two polynucleotides and with base pairs is what gives DNA its extraordinary ability to code for proteins and to replicate from one generation to another and although the pattern of bases may remain the same, once in a while, a G may be substituted where previously there was a T or three bases may go missing after replication, resulting in a DNA molecule or gene that is different than the original one and such a difference is called mutation. Mutations can result in a protein that can either confer fitness and if it does, then the organism carrying the beneficial protein will be favored by selection while an altered gene sequence may also result in a protein that is damaged and so will be harmful.
Mutations along with DNA’s ability to replicate provide the source of variation for natural selection to work on. Since each species has a genome or complete set of genes that specify which phenotypes allows it to adapt, knowing the genome of each species has allowed biologists to reconstruct the history of a species by basically comparing a sequence of bases of a gene to that from another species and to see if there are similarities, implying a common ancestry as well as differences indicating how different each species is from one another.
Here is an example to make this clear: Suppose there are two species call them species A and species B and both are species that inhabit somewhat different habitats. In Darwinian theory, these are species that can adapt to these habitats because natural selection favored any phenotype that allowed them to survive and are descended from an ancestral species.
How would comparing two genes from species A and B reveal differences as well as similarities and how would that also define a different kind of species concepts?
Suppose we had two species, call them species A and species B. If we had both the gene sequences of species A, first the gene sequence would be
and with species B
If you look closely at the sequences, will notice the similarities as well as the differences. Both sequences consists of 15 bases and 12 of these bases do match up. The similarities would indicate that both species likely had a common ancestor in the past and through natural selection as well other evolutionary forces, begin to diverge some time in the past, but because there are two species that are now separate there is also some differences in the bases sequences at position 4 where in species 2 there is a T instead of G and at position 10 in species 2, there is a G instead of C. This kind of mutation may have had a beneficial effect for species 2 if species 2 evolved in a environment that was different from that of species 1, hence the differences in base sequences.
In principle, by comparing gene sequences of each species to another, a history of speciation for whatever organisms whether animal, plant, fungi, and microbe can be reconstructed to show degrees of relationships with two or more species showing gene sequences that are related indicating close relationships while those with highly dissimilar sequences indicating distant relationships. It is then possible to reconstruct the history of any species by reconstructing a phylogenetic tree and indeed through the technique of molecular biology, phylogenetic trees have been constructed for all life that is present now thus validating a principle that was discovered by Darwin (1859) that all of life is the descendant of a common ancestor and that the diversity of life observed now represents a twig on the branch of the tree of life and indeed all of life forms, through the use of molecular sequencing, are descendants of a common ancestors.
Aside from comparing gene sequences, the evolutionary history of any species has the possession of traits that the ancestors did not have previously and these traits when constructing an evolutionary history of a species are known as derived traits. An example is mammals which possese fur and produce milk for the young. If an evolutionary history is constructed for mammals, then the presence of fur and milk are the derived characterstics and if we could the ancestors of mammals and go back in time, there would then be distant ancestors of mammals which are chordates and these are groups of animals that possese a vertebrate along with attached limbs alongside the vertebrate which would be amphibians or chordates that can live on land in water all the way to fish which can only live in water and have fins.
This is then the gist of the phylogenetic species concept in that each species is descendant of a past species and is applicable to both sexually and asexually reproduction. The phylogenetic species concept can also take into account the evolution of isolating mechanisms of sexually reproducing organisms in determining the species history of plants and animals so the biological species concept is subsumed into the phylogenetic species concept while of course it can take into account the genetic mechanisms of speciation in the asexually reproducing organisms or in other words the phylogenetic species concept is applicable to all kinds of organisms while the biological species concepts only applies for a certain group of organisms that can only reproduce sexually.
Are there any disadvantages to the phylogenetic species concepts? The most obvious advantage is that this concept is much more broader than the biological species concepts which does not depend on the presence of isolating mechanism which really narrows a definition of species that depends only on sexual reproduction. Only genetic differences, along with evolutionary history of species acquiring new traits are needed but there is one disadvantage in that if one studies a population of living organisms and if the phylogenetics species concepts is used, then by considering genetic differences that could potentially result in new future species or in other words if we could find the genetic sequences that is common to this population under investigation, there may be genetic differences that is so great that this one population that may be considered at first a single species may be in fact have so much genetic variation that it could consists of several species so that from the phylogenetic species concept, which at first may appear to be a single species may actually consists of several and this one drawback is that there can be so much splitting of species as to be inconvienent for identification purposes and by identification I mean knowing which species not just for scientific research but for more practical ends such as finding a species for conservation. Would a population of organisms be a distinct species worth conserving if it is found to be rare only to find that it is a really a subspecies if one uses phylogenetics for species identification? Hence the drawback with this concept.
Also notice that in both concepts discussed so far, the biological depends on reproduction while the phylogenetic depends on history. Both concepts says nothing about the environment the species is adapted too and with that, we know turn our attention to another definition that takes into account the ecology which is aptly called the ecological species concept.
The Ecological Species Concept
In addition to biological processes such as reproduction which is crucial to providing genetic differences for natural selection to favor in order to create enough differences for two species to be distinct from one another, the environment can also play a critical role in speciation.
Basically, the ecological species concept states that each species is adapted to a given part of the environment which is called the niche and each species has a set of adaptations that allow survival in its niche and both ecological and evolutionary processes defined the species from another group of species.
More simply, the environment plays a role in keeping species distinct form one another such as a species of birds that have a wider range of territory but are separate in terms of reproduction because of different habitats where one species of birds in a single genus can live in habitats that are cold while another species can only tolerate warm temperatures. Ecology such as habitats can play a role as well as reproduction which can combine together in causing separation between two species and this is known as allopatric speciation where the environment plays a role in speciation. A classic example is the widening of rivers breaking gene flow between two groups of a reproducing population where gene flow acts to keep populations from diverging but with reduced gene flow because of the widening river, the populations will split in two and natural selection will favor differences in these two populations allowing adaptation to each separate habitat caused by the widening river. Suppose that prior to a river formation, these are a population of wild mice that were adapted to a plain and next to the plain is a forest but none of them were adapted to living in a forest. Then the river separates plain from forest and one group of mice is separated from the mice already adapted to the plain and are then forced to take residence in the forest. Those mice with adaptations to survive in the forest will be favored by natural selection and as long as the wide river prevents gene flow between the two groups, both mice will end up evolving into two distinct species and they will be so distinct that both would be considered separate species in addition to reproductive isolation but by the kinds of habitats they inhabit.
The ecological species concepts takes into account both ecology and reproduction but even if species do inhabit either the same habitat or different habitats there is no guarantee that two species that occupy different species may be separate or a hybrid between two species.
An example is the problem of classifying a species of wolves that inhabit North America. One particular is a species of wolf called Canis lycaon which at first was recognized as a species in its own right until the technique of DNA sequencing revealed that it is not a separate species distinct from the well known gray wolves and coyotes but a hybrid between gray wolf and coyotes.
What this suggests is that species with habits that overlap will likely not result in distinct species but a possible combination of two different species. Since Canis lycaon has some coyote DNA which would not be the case if there were clear cut boundaries and it was found that coyotes have recently expanded into the ranges where Canis lycaon inhabit and so hybridization has occurred so the ecological species concepts is legitimate but up to a point. Like the biological species and phylogenetic species, the ecological species has it own share of problems when it comes to delimiting species.
Although there are many other species concepts which for the sake of brevity, I will not define but it does remind you that species concepts although useful are not always adequate in that previously I stated that scientists such as biologists have to study a small part of nature. Species concepts are a fine example of studying one aspect of nature. Each concept both have their advantages and disadvantages and in my mind they are all legitimate but in different ways. The biological species has it merits in the field of zoology and botany or the study of animals and plants respectively but both depend on the process sexual reproduction which is not applicable when trying to identify species of microbes such as bacteria where sexual reproduction is non existent. Likewise to understand microbial evolution you would need to take into account the environment each species of bacteria is adapted to such as temperature as well examining the genomes of each microbe and by knowing the difference in genetics and metabolism you would then proceed to use the phylogenetic species concepts as well as the ecological species concept for the former takes into account history and the latter the environment where the bacteria are adapted to and when also trying to find the geographical range of frogs in the eastern part of the US, the ecological species concept will be adequate. Of course no single concept can cover all of what is observed in nature so no concept is like the ” one size that fits all” but two or more concepts would be needed to recognize a species and its place in nature by considering each legitimate species concept that is appropriate at the task at hand.
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