Introduction

In this section, we will explore the concept of species and how they evolve over time. Species are defined as a group of organisms that are capable of breeding with one another and produce offspring.

Learning

Natural Selection, on its own, does not explain how related species evolved to become distinct and separate.

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 mating’s 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.

According to this definition, members of a species are ‘reproductively isolated’ from members of other species ensuring separation of their gene pools. Because there is no introduction of new alleles from members of other species, this allows each species to evolve on its own.

Species physical appearance (phenotype) can be misleading and suggest an ability or inability to mate. In other words, looks can be deceiving. For example, interbreeding between different types of domestic dogs is possible, even though they may visibly differ substantially in size, build, or coat type.

In other cases, individuals may appear similar even though they are not members of the same species. An example would be Bald Eagles and African fish eagles. Both are birds and eagles but are different species and cannot breed/produce offspring. The African fish eagle is pictured on the left and the bald eagle is pictured on the right.

Isolating “Forces”

There are several ‘forces’ that maintain reproductive isolation of species. These include:

Behavioral Isolation

Different species display different courtship behaviors or rituals. Birds often exhibit complex, intricate courtship behaviors such as dances and calls that members of the opposite sex find ‘attractive’. If these behaviors differ significantly enough between two closely related species, interbreeding will not occur since the behaviors would not be found to be ‘attractive’.

The male (left) has a smaller pupil, lighter colored feet, and is smaller in size than the female. This image was taken near Punta Suarez on Española Island, Galápagos Islands, Ecuador.

Geographic Isolation

Physical separation of populations will prohibit interbreeding between members of different species. A great example of this is seen in remote island locations where species will often appear quite differently from ancestors on the mainland or on neighboring islands.

Temporal Isolation

Different closely related species occupying the same locale may maintain isolation by adhering to different breeding seasons. A good example of this is found amongst frogs where some species will mate and reproduce during the early to mid-Spring months, while members of other species may the mate and reproduce during the late-Spring and Summer months.

Additional factors are also at play in maintaining isolation of the gene pools amongst species. In the event interbreeding between species occurs, there are other incompatibilities which may prohibit the mixing of the gene pools between species, including:

Other Isolating Mechanisms

What might keep two subpopulations from interbreeding when reunited geographically? There are several mechanisms.

• Gamete Incompatibility – sperm from one species is incapable of fertilizing eggs of another species.
• Hybrid Non-Viability – Hybrid offspring do not survive to birth.
• Hybrid Infertility – Hybrid offspring are sterile and cannot themselves reproduce (this is true in the case of a cross between a horse and a donkey; while they can produce offspring – called ‘mules’ – these offspring are non-fertile and cannot reproduce to pass on their genes).

Summary

• A species is a group of organisms that is capable of interbreeding with one another.
• Interbreeding is prohibited by:
  • Behavioral Isolation
  • Geographic Isolation
  • Temporal Isolation
• Post-breeding isolation mechanisms can also maintain genetic distinction between species, including:
  • Gamete Incompatibility
  • Hybrid Non-viability
  • Hybrid Infertility

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Sources:

“Species and Speciation.” http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.53 Licensed CC BY-NC-SA 4.0

“Formation of a New Species.” By OpenStax Biology 2e. Retrieved from https://openstax.org/books/biology-2e/pages/18-2-formation-of-new-species Licensed under: CC-BY: Attribution

Blue-footed Booby Comparison.jpg, Digital Image of Blue Footed Booby. Jan 13, 2008. GNU Free Documentation License, Version 1.2

Habitat Isolation, Digital Image of Grasshoppers, By OpenStax Biology 2e, Creative Commons Attribution-NonCommercial–ShareAlike 4.0 International License

Temporal Isolation, Digital Images of Frogs/ Credit a: modification of work by Mark R. Jennings, USFWS; Credit b: modification of work by Alessandro Catenazzi, By OpenStax Biology 2e, Creative Commons Attribution-NonCommercial–ShareAlike 4.0 International License.

Speciation. (2020, December 31). Retrieved May 26, 2021, from https://bio.libretexts.org/@go/page/5918