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Biology LibreTexts

18.2A: The Biological Species Concept

  • Page ID
    13420
  • A species is defined as a group of individuals that, in nature, are able to mate and produce viable, fertile offspring.

    LEARNING OBJECTIVES

    Explain the biological species concept

    KEY TAKEAWAYS

    Key Points

    • Members of the same species are similar both in their external appearance and their internal physiology; the closer the relationship between two organisms, the more similar they will be in these features.
    • Some species can look very dissimilar, such as two very different breeds of dogs, but can still mate and produce viable offspring, which signifies that they belong to the same species.
    • Some species may look very similar externally, but can be dissimilar enough in their genetic makeup that they cannot produce viable offspring and are, therefore, different species.
    • Mutations can occur in any cell of the body, but if a change does not occur in a sperm or egg cell, it cannot be passed on to the organism’s offspring.

    Key Terms

    • species: a group of organsms that, in nature, are capable of mating and producing viable, fertile offspring
    • hybrid: offspring resulting from cross-breeding different entities, e.g. two different species or two purebred parent strains
    • gene pool: the complete set of unique alleles that would be found by inspecting the genetic material of every living member of a species or population

    Species and the Ability to Reproduce

    A species is a group of individual organisms that interbreed and produce fertile, viable offspring. According to this definition, one species is distinguished from another when, in nature, it is not possible for matings between individuals from each species to produce fertile offspring.

    Members of the same species share both external and internal characteristics which develop from their DNA. The closer relationship two organisms share, the more DNA they have in common, just like people and their families. People’s DNA is likely to be more like their father or mother’s DNA than their cousin’s or grandparent’s DNA. Organisms of the same species have the highest level of DNA alignment and, therefore, share characteristics and behaviors that lead to successful reproduction.

    Species’ appearance can be misleading in suggesting an ability or inability to mate. For example, even though domestic dogs (Canis lupus familiaris) display phenotypic differences, such as size, build, and coat, most dogs can interbreed and produce viable puppies that can mature and sexually reproduce.

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    Interbreeding in Dogs: Dogs of different breeds still have the ability to reproduce. The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as (c) the cockapoo.

    In other cases, individuals may appear similar although they are not members of the same species. For example, even though bald eagles (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus vocifer) are both birds and eagles, each belongs to a separate species group. If humans were to artificially intervene and fertilize the egg of a bald eagle with the sperm of an African fish eagle and a chick did hatch, that offspring, called a hybrid (a cross between two species), would probably be infertile: unable to successfully reproduce after it reached maturity. Different species may have different genes that are active in development; therefore, it may not be possible to develop a viable offspring with two different sets of directions. Thus, even though hybridization may take place, the two species still remain separate.

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    Species Similarity & Reproduction: Species that appear similar may not be able to reproduce. The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two birds are members of different species.

    Populations of species share a gene pool: a collection of all the variants of genes in the species. Again, the basis to any changes in a group or population of organisms must be genetic for this is the only way to share and pass on traits. When variations occur within a species, they can only be passed to the next generation along two main pathways: asexual reproduction or sexual reproduction. The change will be passed on asexually simply if the reproducing cell possesses the changed trait. For the changed trait to be passed on by sexual reproduction, a gamete, such as a sperm or egg cell, must possess the changed trait. In other words, sexually-reproducing organisms can experience several genetic changes in their body cells, but if these changes do not occur in a sperm or egg cell, the changed trait will never reach the next generation. Only heritable traits can evolve. Therefore, reproduction plays a paramount role for genetic change to take root in a population or species. In short, organisms must be able to reproduce with each other to pass new traits to offspring.