Species Concepts

Species Concept

• Cuvier defined the species as an assemblage descended either from each other or from common parents, as well as those that looked alike in 1829.

Different Species Concepts

• Biological species concept is based on interbreeding and reproductive isolation.
• Ecological species concept focuses on the species roles in an ecosystem.
• Phylogenetic species concept is based on evolutionary ancestry and genetic relationships.
• Monophyletic species concept considers a species as a group of organisms sharing a common ancestor.
• Evolutionary species concept is based on evolutionary history and future potential.
• Hennigan and Internodal concepts considers species in relation to evolutionary branches (cladistics).
• Taxonomic species concepts utilize traditional classification practices based on morphology and other observable traits.

Typological Species Concept

• One of the oldest ways of classifying species includes defining species by a set of unchanging characteristics.
• Plato and Aristotle believed organisms belonged to specific natural kinds that do not change over time.
• Linnaeus classified species based on morphological traits.
• Charles Darwin's Theory of Evolution introduced the idea that species changed over time through natural selection.
• Regan's (1926) definition of a species is a community, or related communities, with sufficiently definite morphological characters to entitle it to a specific name.

Criticism of Lineage Based Species

• Define species based on evolutionary history and splitting events.
• This approach acknowledges the continuity of evolutionary change.

How To Recognize Species

• Lineages between splitting events (Hennigian species) may not be observable.
• Lack of unique apomorphies.
• Continuous splitting creates new species.

Species as Individuals or Classes

• A class is a collection of things that share common properties with no specific beginning or end.
• Individuals are unique entities with a specific beginning and end.
• Ghiselin and Hull argue species are individuals, and species names reference unique historical objects.

Lineage Species Concepts

• Species are defined not as groups, but as lineages through time.
• Hennig and Simpson created the Hennigian Lineage Concept and Evolutionary Species Concept (ESC).

Hennigian Lineage Concept

• It is also known as the "cladistics species concept".
• A species is viewed as a lineage existing between two speciation events, or between speciation and extinction, emphasizing the splitting of lineages (cladogenesis).

Evolutionary Species Concept

• Species is defined as a lineage of ancestral-descendant populations
• Species maintains identity over time and space and evolves independently with unique evolutionary tendencies.

Examples of Typological or Taxonomic Species Concept

• Species are classified using the taxonomic species concept, based on their distinct morphological traits.
• Canis lupus (Gray Wolf) is a large carnivorous mammal, found in North America, Europe, and Asia, living in packs.
• Canis latrans (Coyote) is a smaller, adaptable canid found mainly in North and Central America, known for opportunistic feeding.
• Felis catus (Domestic Cat) is a domesticated feline with various coat colors and patterns, found worldwide.
• Felis lybica (African Wildcat) is a wild species native to Africa and the Middle East and is the ancestor of domestic cats; they have a uniform coat and are elusive.

Biological Species Concept

• Mayr (1942) popularized the "biological species concept" (BSC).
• A species consists of populations that replace one another geographically or ecologically and interbreed.
• "Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups."
• BSC remains a widely taught approach to delimiting species.
• It is not applicable to asexual organisms, fossils, allopatric populations, or taxa without sufficient information.
• John Ray (1686) was among the first to define species based on their ability to reproduce and maintain traits across generations.
• Dobzhansky (1937) emphasized that speciation occurs when gene flow is restricted by barriers to interbreeding, leading to genetic isolation.
• Paterson (1980, 1985) argued species are defined by their ability to recognize and select mates.

Limitations of the Biological Species Concept

• Not applicable to certain groups of organisms and situations because its definition is based on interbreeding and reproductive isolation, so if interbreeding cannot be tested, then the BSC cannot be used.
• Asexual organisms: the BSC is based on sexual reproduction, which asexual organisms do not do, so reproductive isolation does not apply.
• Fossils: fossilized remains do not provide evidence of mating behaviors or compatibility.
• Allopatric Populations: since the populations live in separate areas and do not interact, it cannot be tested whether they would interbreed if they were to meet.
• Organisms with Unknown Mating Habits: If there is a lack of data it is not possible to confirm whether they are interbreeding or are reproductively isolated, making the BSC difficult to apply.

Examples of Biological Species

• Examples include humans ( Homo sapiens), dogs (Canis lupus familiaris) and wolves (Canis lupus), horses (Equus ferus caballus) and donkeys (Equus africanus asinus), lions (Panthera leo) and tigers (Panthera tigris), mallard ducks (Anas platyrhynchos) and American black ducks (Anas rubripes).

Criticisms of the Biological Species Concept

• Criticisms focus on the emphasis on sexual reproduction.
• Agamotaxa (asexual creatures) reproduce independently without genetic exchange.
• A majority of taxa are asexual and thus fall outside the score of the BSC.
• Historical life on Earth lacked sexual reproduction, then raising the question of whether species existed then.
• Operational difficulties emerge where creatures are not sympatric and allopatric species present difficulties in determining interbreeding.
• Allochronic species also cannot be tested for reproductive compatibility.
• Morphology, behavior, and ecology become proxies for determining species boundaries.
• Hybridization between largely allopatric taxa raises the issue of the resulting organisms: one species or two?
• There is no fixed criterion, making decisions subjective.
• BSC emphasizes reproductive isolation as the primary mechanism of speciation.
• Rosen (1979) demonstrated that reproductive compatibility could be plesiomorphic (ancestral), leading to paraphyly.
• Species may be more closely related to members of different species than to their own.

Phylogenetic Species Concepts

• Starting in the late 1970s with Donn Rosen, cladistic ideas were applied to the species problem, which led to definitions that differed in emphasis on monophyly and tokogenetic relationships between parents and offspring.
• Reproductive compatibility holds no special place, whereas ancestral relationships are emphasized.

Examples of Phylogenetic Species Concept

• African Elephant Split: Loxodonta africana (Savanna Elephant) is larger and inhabits savannas, while Loxodonta cyclotis (Forest Elephant) is smaller with straighter tusks and lives in dense forests.
• Canis lupus (Gray Wolf) is widespread and variable across North America, Europe, and Asia, and Canis rufus (Red Wolf) is a smaller wolf in the southeastern United States.
• Ammospiza maritima (Seaside Sparrow) is a salt marsh sparrow with multiple subspecies along the U.S. Atlantic and Gulf Coasts. While Ammospiza nigrescens (Dusky Seaside Sparrow) was a darker sparrow, historically found only in the marshes of Florida but went extinct in 1987.

Autapomorphic/Monophyletic Species Concept

• Donn Eric Rosen (1979) defined a species as the smallest diagnosable monophyletic group.
• The smallest inclusive group of organisms can be identified by at least one autapomorphy (a unique derived trait that distinguishes it from all other groups).
• Example: modern snakes (suborder Serpentes).

Criticisms of Autapomorphic/Monophyletic Species Concept

• There is applicability below species level in the reticulate nature of bisexual taxa.
• Monophyly may not apply neatly to populations within a species because of the web like genetic exchange seen in reproducing taxa.
• There is also a conflict between stem or ancestral species.
• An ancestral or stem species cannot have apomorphies that are not shared by its descendant.
• If a clade is recognized as a species, its ancestor must fit that definition, but without the unique traits, apomorphies, they cannot be identified in the same way.

Monoism and Pluralism

• Monoism suggests there is one true species concept that applies to all organisms.
• Pluralism accepts that different species concepts are useful.

Pattern and Process

• Pattern classifies species based on shared ancestry (monophyly) without considering how they evolve.
• Process defines species based on how they evolve, like interbreeding and gene flow.
• No single definition can fully balance both pattern and process.
• Some propose using monophyly for pattern classification and reserving "species" for evolutionary processes.

Species Nominalism

• It originated from William of Ockham.
• Species are not real, natural categories, but names assigned by humans.
• Nature produces individuals, and species are man-made.
• Universal property is possessed by different organisms, like photosynthesis in plants.
• Particular refers to instances of an organism, like a dog.
• Darwin: for species to be real, they must be non-arbitrary.
• Arbitrary traits include reproductive isolation and genetic distinctness.
• Vrana and Wheeler (1992) state the species level is arbitrary and not fundamentally different from higher taxa like genera, families, and orders.

Example of Species Nominalism

• Differences in size, leg length, and skull width between Gray Wolf (Canis lupus) and Eastern Wolf (Canis lycaon).

How Does Species Concept Matter?

• It provides a framework for studying and describing the diversity of life.
• It defines species accurately and ensures genetic diversity and ecosystems are preserved.
• It provides insights on how populations evolve and diverge.