Species Definition

Explore the various definitions of species, from the biological species concept to the morphological and ecological species concepts. Discover how species boundaries are defined and applied in different contexts.


Species definition is a complex and debated topic in the field of biology. It is essential to have a clear understanding of what constitutes a species to study and classify different organisms accurately. In this article, we will explore the various definitions of species and examine how they are applied in different contexts.

Biological Species Concept

The most widely accepted definition of a species is the biological species concept, proposed by Ernst Mayr. According to this concept, a species is a group of organisms that can interbreed and produce fertile offspring. This concept emphasizes reproductive isolation as the key criterion for defining a species.

Morphological Species Concept

Another approach to defining species is the morphological species concept, which focuses on physical characteristics. Organisms that share similar morphological features are considered to belong to the same species. This concept is useful in cases where reproductive isolation is not a feasible criterion.

Ecological Species Concept

The ecological species concept defines a species based on its ecological niche. Organisms that occupy the same ecological niche are considered to be the same species. This concept highlights the role of an organism’s interactions with its environment in defining its species status.

Case Studies

  • Finches in the Galapagos Islands: Charles Darwin’s observation of finches with different beak shapes on the Galapagos Islands led to the development of the concept of adaptive radiation. These finches, despite their morphological differences, belong to the same species.
  • Polar bears and grizzly bears: Despite their physical similarities, polar bears and grizzly bears are considered separate species due to their reproductive isolation. This example illustrates the importance of the biological species concept in defining species boundaries.

Challenges in Species Definition

Defining species can be challenging, especially in cases where hybridization and introgression blur traditional boundaries. Additionally, the discovery of cryptic species, which are morphologically similar but genetically distinct, poses further complications in species classification.


Species definition is a complex and dynamic field that continues to evolve with advancements in genetic and ecological research. By understanding the different concepts of species definition and their applications, biologists can better classify and study the diversity of life on Earth.

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