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Species: the story on HearLore | HearLore
Species
Imagine a world where the boundary between one creature and another is not a solid wall, but a shifting, often invisible fog. This is the reality of the species, the fundamental unit of life that scientists have struggled to define for centuries. While the concept seems simple enough to the casual observer, a group of animals that can breed and produce fertile offspring, the truth is far more complex. Biologists have identified over 24 different ways to define a species, and no single definition works for every organism on Earth. From bacteria that swap genes like trading cards to plants that clone themselves without ever mating, the traditional rules of reproduction break down. The most recent rigorous estimate suggests there are between 8 and 8.7 million species of eukaryotes, yet only about 14 percent of these had been described by 2011. This vast gap between what exists and what we know highlights the difficulty of pinning down life's diversity. The species is not a fixed category carved in stone, but a dynamic, evolving concept that changes as our understanding of genetics and evolution deepens.
The Great Chain of Being
For over two thousand years, the history of life was viewed through a lens of fixed categories, a rigid hierarchy known as the great chain of being. Aristotle, the ancient Greek philosopher, used the terms genos and eidos to describe kinds and forms, believing that these categories were distinct and unchanging. A bird was a bird, and a fish was a fish, with no organic connection between them. This view persisted through the Renaissance and into the 18th century, where European scholarly and religious education reinforced the idea that species were created by God and remained immutable. John Ray, an English naturalist, was the first to attempt a biological definition of species in 1686, suggesting that species breed true and do not change, even though variations exist. Carl Linnaeus, the Swedish scientist who created the binomial system for naming species, classified organisms based on shared physical characteristics, establishing a taxonomic hierarchy that reflected natural relationships. However, Linnaeus did not consider that new species could emerge, maintaining a view of divinely fixed species that might alter through hybridization or acclimatization. The idea that species could change form over time was a radical departure from this thinking, one that would eventually shatter the ancient hierarchy and replace it with the dynamic model of evolution.
Darwin's Disruption
The year 1859 marked a seismic shift in how humanity understood life, when Charles Darwin published On the Origin of Species. Darwin argued that it was populations, not individuals, that evolved by natural selection from naturally occurring variation among individuals. This required a new definition of species, one that acknowledged change over time. Before Darwin, species were seen as fixed entities, but he concluded that species are what they appear to be: ideas, provisionally useful for naming groups of interacting individuals. The concept of evolution meant that species were not static but were constantly in flux, changing form over time. Jean-Baptiste Lamarck had previously proposed the transmutation of species in his 1809 Zoological Philosophy, but it was Darwin and Alfred Russel Wallace who provided a compelling account of evolution and the formation of new species in 1858. Their work showed that species could arise by natural selection, and that the history of the planet provided enough time for major changes. This understanding was greatly extended in the 20th century through genetics and population ecology, revealing that genetic variability arises from mutations and recombination, while organisms are mobile, leading to geographical isolation and genetic drift with varying selection pressures. The species was no longer a fixed point on a ladder, but a moving target in a vast, interconnected web of life.
Common questions
What is the most recent estimate for the number of eukaryotic species on Earth?
The most recent rigorous estimate suggests there are between 8 and 8.7 million species of eukaryotes. Only about 14 percent of these had been described by 2011.
When did Charles Darwin publish On the Origin of Species?
Charles Darwin published On the Origin of Species in the year 1859. This publication marked a seismic shift in how humanity understood life and evolution.
Who proposed the most widely used definition of a species based on reproductive isolation?
Ernst Mayr proposed the most widely used definition of a species in 1942. This definition relies on reproductive isolation and defines a species as the largest group of organisms that can produce fertile offspring.
How many species of blackberry exist in Britain according to the text?
The blackberry Rubus fruticosus has perhaps 400 microspecies in Britain alone. This number is complicated by hybridization, apomixis, and polyploidy.
What percentage of all species that ever lived on Earth are now extinct?
Over 99 percent of all species that ever lived on Earth are now extinct. This figure represents some five billion species that have disappeared throughout history.
The most widely used definition of a species, proposed by Ernst Mayr in 1942, relies on reproductive isolation. It defines a species as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. However, this definition breaks down in several critical situations. When organisms reproduce asexually, as in single-celled organisms such as bacteria and other prokaryotes, the concept of a reproductive species fails. DNA barcoding and phylogenetics are commonly used in these cases to distinguish between groups. The problem also arises when scientists do not know whether two morphologically similar groups of organisms are capable of interbreeding, which is the case with all extinct life-forms in paleontology, as breeding experiments are not possible. Hybridization permits substantial gene flow between species, challenging the idea of reproductive isolation. In ring species, members of adjacent populations in a widely continuous distribution range interbreed successfully, but members of more distant populations do not. Such non-breeding, though genetically connected, end populations may co-exist in the same region, closing the ring. These complexities show that the reproductive definition, while useful for many multi-celled organisms, is not a universal solution to the species problem.
The Genetic Revolution
The 20th and 21st centuries brought a revolution in how species are defined, shifting the focus from morphology to genetics. Biologists and taxonomists have made many attempts to define species, beginning from morphology and moving towards genetics. In microbiology, genes can move freely even between distantly related bacteria, possibly extending to the whole bacterial domain. As a rule of thumb, microbiologists had assumed that members of Bacteria or Archaea with 16S ribosomal RNA gene sequences more similar than 97 percent to each other need to be checked by DNA, DNA hybridization to decide if they belong to the same species. This concept was narrowed in 2006 to a similarity of 98.7 percent. Using a single easy-to-use locus to distinguish taxa is called DNA barcoding, and one of the barcodes for eukaryotes is a region of mitochondrial DNA within the gene for cytochrome c oxidase. A database, Barcode of Life Data System, contains DNA barcode sequences from over 190,000 species. However, scientists such as Rob DeSalle have expressed concern that classical taxonomy and DNA barcoding, which they consider a misnomer, need to be reconciled, as they delimit species differently. Genetic introgression mediated by endosymbionts and other vectors can further make barcodes ineffective in the identification of species. The surefire way to capture all gene flow among populations is to compare their entire genomes, using the average nucleotide identity method to quantify genetic distance between entire genomes.
The Chaos of Hybridization
Nature often defies the neat categories scientists try to impose, and nowhere is this more evident than in the phenomenon of hybridization. Natural hybridization presents a challenge to the concept of a reproductively isolated species, as fertile hybrids permit gene flow between two populations. For example, the carrion crow Corvus corone and the hooded crow Corvus cornix appear and are classified as separate species, yet they can hybridize where their geographical ranges overlap. In the case of the northern spotted owl, which is protected, hybridization with the unprotected California spotted owl and the barred owl has led to legal debates and conflicts between lawmakers, land owners, and conservationists. The resulting single species has been termed a compilospecies, a term that highlights the fluidity of species boundaries. Horizontal gene transfer between organisms of different species, either through hybridization, antigenic shift, or reassortment, is sometimes an important source of genetic variation. Viruses can transfer genes between species, and bacteria can exchange plasmids with bacteria of other species, including some apparently distantly related ones in different phylogenetic domains. This makes analysis of their relationships difficult and weakens the concept of a bacterial species. The existence of microspecies, groups of organisms with very little genetic variability, further complicates the picture. For instance, the dandelion Taraxacum officinale and the blackberry Rubus fruticosus are aggregates with many microspecies, perhaps 400 in the case of the blackberry and over 200 in the dandelion in Britain alone, complicated by hybridization, apomixis, and polyploidy.
The Extinction Crisis
A species is extinct when the last individual of that species dies, but it may be functionally extinct well before that moment. It is estimated that over 99 percent of all species that ever lived on Earth, some five billion species, are now extinct. Some of these were in mass extinctions such as those at the ends of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods. Mass extinctions had a variety of causes including volcanic activity, climate change, and changes in oceanic and atmospheric chemistry, and they in turn had major effects on Earth's ecology, atmosphere, land surface, and waters. Another form of extinction is through the assimilation of one species by another through hybridization. The practical implications of species definition are profound, as biologists and conservationists need to categorize and identify organisms in the course of their work. Difficulty assigning organisms reliably to a species constitutes a threat to the validity of research results, for example making measurements of how abundant a species is in an ecosystem moot. Surveys using a phylogenetic species concept reported 48 percent more species and accordingly smaller populations and ranges than those using nonphylogenetic concepts, which was termed taxonomic inflation. This could cause a false appearance of change to the number of endangered species and consequent political and practical difficulties. Conservation laws in many countries make special provisions to prevent species from going extinct, but the fluidity of species boundaries often creates conflicts between legal frameworks and biological reality.
The Future of Naming
The naming of species remains a hypothesis about the evolutionary relationships and distinguishability of that group of organisms. As further information comes to hand, the hypothesis may be corroborated or refuted. Sometimes, especially in the past when communication was more difficult, taxonomists working in isolation have given two distinct names to individual organisms later identified as the same species. When two species names are discovered to apply to the same species, the older species name is given priority and usually retained, and the newer name considered as a junior synonym, a process called synonymy. Dividing a taxon into multiple, often new, taxa is called splitting. Taxonomists are often referred to as lumpers or splitters by their colleagues, depending on their personal approach to recognizing differences or commonalities between organisms. The nomenclatural codes that guide the naming of species, including the ICZN for animals and the ICN for plants, do not make rules for defining the boundaries of the species. Research can change the boundaries, also known as circumscription, based on new evidence. Species may then need to be distinguished by the boundary definitions used, and in such cases the names may be qualified with sensu stricto to denote usage in the exact meaning given by an author, while the antonym sensu lato denotes a wider usage. The future of species naming lies in reconciling these historical practices with modern genetic data, creating a system that reflects the dynamic nature of life itself.