The species concepts is a framework to define and distinguish different species.
Table of Contents
Species Concepts:
Here are some of the main species concepts with explanations and examples:
Biological Species Concept
Definition: A species is a group of interbreeding individuals that are reproductively isolated from other such groups.
Example: Horses (Equus ferus caballus) and donkeys (Equus africanus asinus) can interbreed to produce mules, but mules are sterile, so horses and donkeys are considered separate species.
Morphological Species Concept
Definition: A species is defined by its morphological characteristics, or physical form and structure.
Example: The African elephant (Loxodonta africana) and the Asian elephant (Elephas maximus) can be distinguished by their size, ear shape, and trunk characteristics.
Ecological Species Concept
Definition: A species is a set of organisms adapted to a particular set of resources in the environment.
Example: Darwin’s finches on the Galápagos Islands exhibit different beak shapes adapted to specific food sources, representing different species like the ground finch (Geospiza) and the cactus finch (Geospiza scandens).
Phylogenetic Species Concept
Definition: A species is the smallest group of individuals that share a common ancestor and can be distinguished from other such groups by unique characteristics.
Example: The red panda (Ailurus fulgens) and the giant panda (Ailuropoda melanoleuca) are distinguished as separate species despite both being called “panda” because they have distinct evolutionary lineages.
Genetic Species Concept
Definition: A species is defined by genetic similarity and differences. Species boundaries are identified based on genetic markers.
Example: Cryptic species of giraffes have been identified through genetic analysis, revealing that what was once thought to be a single species, the giraffe (Giraffa camelopardalis), actually consists of multiple species with distinct genetic differences.
Evolutionary Species Concept
Definition: A species is a single lineage of ancestor-descendant populations that maintains its identity from other such lineages and has its own evolutionary tendencies and historical fate.
Example: The coelacanth (Latimeria) is considered a distinct evolutionary species because it has maintained a separate evolutionary lineage for millions of years.
Isolation Mechanism
Isolation mechanisms are biological features or behaviours that prevent species from interbreeding with each other, thereby maintaining species boundaries.
They can be broadly classified into prezygotic and postzygotic isolation mechanisms.
(I) Prezygotic Isolation Mechanisms
Prezygotic mechanisms prevent fertilization and the formation of a zygote. Here are the main types:
1. Geographical Isolation:
Geographical isolation, a form of isolation mechanism, occurs when physical barriers like mountains or rivers prevent populations of a species from interbreeding.
Example: The Kaibab and Abert’s squirrels, which were separated by the formation of the Grand Canyon, leading to the evolution of distinct species.
2. Temporal Isolation
Definition: Different species breed at different times (seasons, time of day, or years).
Example: The American toad (Bufo americanus) and the Fowler’s toad (Bufo fowleri) live in the same area but breed at different times of the year, preventing interbreeding.
3. Habitat Isolation
Definition: Species live in the same region but occupy different habitats, so they rarely encounter each other.
Example: The lion (Panthera leo) and the tiger (Panthera tigris) historically lived in overlapping ranges in India but used different habitats (open grasslands for lions and dense forests for tigers).
4. Behavioural Isolation
Definition: Species have different mating behaviors or rituals that prevent them from recognizing each other as potential mates.
Example: The courtship dances of different species of birds of paradise are unique to each species, ensuring that they mate only with their own kind.
5. Mechanical Isolation
Definition: Differences in reproductive structures prevent successful mating between species.
Example: In many insects, the shapes of the genitalia are species-specific, preventing mating between different species.
6. Gametic Isolation
Definition: Even if gametes (sperm and egg) from different species meet, they are unable to fuse to form a zygote.
Example: In sea urchins, the sperm and eggs of different species are not compatible due to differences in the proteins on the surfaces of the gametes
(II) Postzygotic Isolation Mechanisms
Postzygotic mechanisms occur after fertilization and result in the offspring being unviable or infertile. Here are the main types:
1. Hybrid Inviability: Hybrids fail to develop properly and die before reaching reproductive maturity.
Example: Crosses between certain species of frogs result in embryos that fail to develop beyond early stages.
2. Hybrid Sterility: Hybrids are sterile and cannot reproduce.
Example: A mule, the offspring of a horse (Equus ferus caballus) and a donkey (Equus africanus asinus), is sterile.
These isolation mechanisms are essential for maintaining the genetic integrity of species and facilitating the process of speciation by preventing gene flow between populations.
Modes of Speciation
Speciation is the evolutionary process by which populations evolve to become distinct species. The modes of speciation are generally classified based on the geographic and reproductive contexts in which they occur. Here are the primary types of speciation, along with their definitions and examples:
1. Allopatric Speciation
Speciation that occurs when biological populations of the same species become geographically isolated from each other, preventing gene flow.
Examples: Darwin’s Finches: Different species of finches on the Galápagos Islands evolved from a common ancestor due to geographic isolation on different islands.
2. Sympatric Speciation
Speciation that occurs without geographic isolation. This can happen through mechanisms like polyploidy, habitat differentiation, and sexual selection within the same geographic area.
Examples: Apple Maggot Fly (Rhagoletis pomonella): In North America, this fly originally laid eggs on hawthorns but began to lay eggs on introduced apples. Over time, flies that preferred apples became reproductively isolated from those that preferred hawthorns.
3. Peripatric Speciation
A form of allopatric speciation that occurs when a small peripheral population becomes isolated from the main population. The small population size can lead to rapid genetic changes.
Examples: London Underground Mosquito (Culex pipiens molestus): A form of the mosquito Culex pipiens, thought to have become isolated in the London Underground, leading to genetic divergence and speciation.
4. Parapatric Speciation
Speciation that occurs when populations are adjacent to each other and have a narrow zone of contact. Gene flow is possible but limited, often due to differing environmental conditions or behaviors.
Examples: Grass Species (Anthoxanthum odoratum): This grass grows on mine waste contaminated with heavy metals and on uncontaminated soils nearby. Populations adapted to heavy metals and those in normal soils are diverging due to selection pressures.
Adaptive Radiation
Definition: Adaptive radiation is an evolutionary process in which a single ancestral species rapidly diversifies into a multitude of new species, each adapted to exploit different ecological niches. This phenomenon typically occurs when organisms colonize a new environment with diverse habitats or when a major environmental change creates new opportunities.
Significance of Adaptive Radiation
- Biodiversity Increase: Adaptive radiation leads to a rapid increase in species diversity, contributing significantly to the biodiversity of ecosystems.
- Ecological Niches: It enables organisms to occupy a variety of ecological niches, leading to specialization and the exploitation of different resources.
- Evolutionary Innovation: Adaptive radiation often drives the evolution of new adaptations and innovations, promoting overall biological complexity.
- Understanding Speciation: It provides insights into the mechanisms of speciation and evolutionary processes, helping scientists understand how new species evolve.
Reasons for Adaptive Radiation
- Ecological Opportunity: When new habitats or resources become available, organisms can diversify to exploit these new opportunities.
- Evolutionary Innovation: The development of a new trait or set of traits that allows organisms to exploit resources in new ways.
- Release from Competition: The extinction of competitors can free up resources and habitats, allowing surviving species to diversify.
- Adaptive Zones: The availability of various adaptive zones or ecological niches in an environment can drive diversification.
Examples of Adaptive Radiation
Darwin’s Finches:
Location: Galápagos Islands
Example: A single ancestral finch species diversified into multiple species with different beak shapes and sizes, each adapted to different food sources, such as seeds, insects, and nectar.
Galapagos Finches
Darwin’s finches are a classic example of adaptive radiation. These finches evolved from a common ancestor into multiple species with varying beak shapes and sizes, each suited to different food sources.
Initial Colonization
A small group of ancestral finches likely colonized the Galapagos Islands, finding an environment with diverse, unoccupied niches and little competition.
Ecological Opportunities
The islands provided various habitats and food sources, such as seeds, insects, and nectar, which created ecological opportunities for the finches to exploit.
Diversification and Speciation
Over time, the ancestral finches underwent rapid diversification through natural selection, leading to the evolution of different species, each specialized for a particular niche.
Beak Adaptations
- The primary adaptation involved beak size and shape:
- Large, strong beaks: Adapted for cracking large seeds.
- Medium-sized beaks: Suited for eating smaller seeds.
- Long, slender beaks: Perfect for probing cactus flowers for nectar.
- Sharp, narrow beaks: Ideal for catching insects.
In summary, adaptive radiation as exemplified by the Galapagos finches shows how an ancestral species can give rise to multiple new species through the process of adapting to different ecological roles, driven by natural selection and environmental opportunities.