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- The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.
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Biological species concept. The biological species concept defines a species as members of populations that actually or potentially interbreed in nature, not according to similarity of appearance. Although appearance is helpful in identifying species, it does not define species.
By far the most well-known species concept is the biological species concept, which was proposed by evolutionary biologist Ernst Mayr. The biological species concept states that a species is a group of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.
- Overview
- Key points
- Introduction
- The biological species concept
- What keeps species distinct?
- Prezygotic barriers
- Postzygotic barriers
- How do new species arise?
- Allopatric speciation
- Case study: squirrels and the Grand Canyon
What defines a species. How new species can arise from existing species.
•According to the biological species concept, organisms belong to the same species if they can interbreed to produce viable, fertile offspring.
•Species are separated from one another by prezygotic and postzygotic barriers, which prevent mating or the production of viable, fertile offspring.
•Speciation is the process by which new species form. It occurs when groups in a species become reproductively isolated and diverge.
•In allopatric speciation, groups from an ancestral population evolve into separate species due to a period of geographical separation.
•In sympatric speciation, groups from the same ancestral population evolve into separate species without any geographical separation.
•According to the biological species concept, organisms belong to the same species if they can interbreed to produce viable, fertile offspring.
•Species are separated from one another by prezygotic and postzygotic barriers, which prevent mating or the production of viable, fertile offspring.
•Speciation is the process by which new species form. It occurs when groups in a species become reproductively isolated and diverge.
•In allopatric speciation, groups from an ancestral population evolve into separate species due to a period of geographical separation.
On some level, the idea of a species is pretty intuitive. You don't need to be a zoologist to classify organisms like humans, giant pandas, or sunflowers into groups based on their appearance. This method works well when the species in question look very different from one another. You probably wouldn’t mistake a panda for a sunflower—unless you really needed your glasses!
But when we get right down to it, what really make a species a species? Organisms that look alike often belong to the same species, but this isn’t always the case. I for one can't tell the African fish eagle and the bald eagle apart from the photos below. But they are, in fact, different species.
On the flip side, organisms that belong to the same species can look very different from one another. For instance, dogs come in all shapes and sizes—from tiny Chihuahuas to massive Great Danes—but they all belong to the same species: Canis familiaris, the domestic dog.
If appearance doesn’t reliably define a species, then what does? For most eukaryotes—such as animals, plants, and fungi—scientists tend to define a species based on reproductive compatibility. That is, organisms are usually considered to be members of the same species if they can successfully reproduce with one another.
According to the most widely used species definition, the biological species concept, a species is a group of organisms that can potentially interbreed, or mate, with one another to produce viable, fertile offspring.
In this definition, members of the same species must have the potential to interbreed. However, that doesn't mean they have to be part of the same interbreeding group in real life. For instance, a dog living in Australia and a dog living in Africa are unlikely to meet but could have puppies if they did.
In order to be considered to be a single species in the biological species concept, a group of organisms must produce healthy, fertile offspring when they interbreed. In some case, organisms of different species can mate and produce healthy offspring, but the offspring are infertile, can’t reproduce.
For example, when a female horse and a male donkey mate, they produce hybrid offspring called mules. Although a mule, pictured below, is perfectly healthy and can live to a ripe old age, it is infertile and cannot have its own offspring. Because of this, we consider horses and donkeys separate species.
The biological species concept connects the idea of a species to the process of evolution. Because members of a species can interbreed, the species as a whole has a common gene pool, a collection of gene variants.
On the other hand, genes are not exchanged between different species. Even if organisms of different species combine their DNA to make offspring, the offspring will be sterile, unable to pass on their genes. Because of this restricted gene flow, each species evolves as a group distinct from other species.
The biological species concept defines organisms as being, or not being, of the same species based on whether they can interbreed to make fertile offspring. But why is it that different species can't successfully interbreed? This question may seem silly for very different species (like a plant and an animal), but for others like the horse and the donkey above, it's much less obvious.
Broadly speaking, different species are unable to interbreed and produce healthy, fertile offspring due to barriers called mechanisms of reproductive isolation.
Prezygotic barriers prevent members of different species from mating to produce a zygote, a single-celled embryo. Some example scenarios are below:
•Two species might prefer different habitats and thus be unlikely to encounter one another. This is called habitat isolation.
[May I see an example?]
•Two species might reproduce at different times of the day or year and thus be unlikely to meet up when seeking mates. This is called temporal isolation.
[May I see an example?]
•Two species might have different courtship behaviors or mate preferences and thus find each other "unattractive". This is known as behavioral isolation.
Postzygotic barriers keep hybrid zygotes—one-celled embryos with parents of two different species—from developing into healthy, fertile adults. Postzygotic barriers are often related to the hybrid embryo's mixed set of chromosomes, which may not match up correctly or carry a complete set of information.
In some cases, the chromosomal mismatch is lethal to the embryo or results in an individual that can survive but is unhealthy. In other cases, a hybrid can survive to adulthood in good health but is infertile because it can't split its mismatched chromosomes evenly into eggs and sperm. For example, this type of mismatch explains why mules are sterile, unable to reproduce4 .
New species arise through a process called speciation. In speciation, an ancestral species splits into two or more descendant species that are genetically different from one another and can no longer interbreed.
Darwin envisioned speciation as a branching event. In fact, he considered it so important that he depicted it in the only illustration of his famous book, On the Origin of Species, below left. A modern representation of Darwin's idea is shown in the evolutionary tree of elephants and their relatives, below right, which reconstructs speciation events during the evolution of this group.
For speciation to occur, two new populations must be formed from one original population, and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists often divide the ways that speciation can occur into two broad categories:
•Allopatric speciation—allo meaning other and patric meaning homeland—involves geographic separation of populations from a parent species and subsequent evolution.
•Sympatric speciation—sym meaning same and patric meaning homeland—involves speciation occurring within a parent species remaining in one location.
Let's take a closer look at these forms of speciation and how they work.
In allopatric speciation, organisms of an ancestral species evolve into two or more descendant species after a period of physical separation caused by a geographic barrier, such as a mountain range, rockslide, or river.
Sometimes barriers, such as a lava flow, split populations by changing the landscape. Other times, populations become separated after some members cross a pre-existing barrier. For example, members of a mainland population may become isolated on an island if they float over on a piece of debris.
Once the groups are reproductively isolated, they may undergo genetic divergence. That is, they may gradually become more and more different in their genetic makeup and heritable features over many generations. Genetic divergence happens because of natural selection, which may favor different traits in each environment, and other evolutionary forces like genetic drift.
As they diverge, the groups may evolve traits that act as prezygotic and/or postzygotic barriers to reproduction. For instance, if one group evolves large body size and the other evolves small body size, the organisms may not be physically able to mate—a prezygotic barrier—if the populations are reunited.
The Grand Canyon was gradually carved out by the Colorado River over millions of years. Before it formed, only one species of squirrel inhabited the area. As the canyon got deeper over time, it became increasingly difficult for squirrels to travel between the north and south sides.
Eventually, the canyon became too deep for the squirrels to cross and a subgroup of squirrels became isolated on each side. Because the squirrels on the north and south sides were reproductively isolated from one another due to the deep canyon barrier, they eventually diverged into different species5 .
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 Points.
The biological species concept relies on behavioral data and emphasizes reproductive isolation between groups. The lineage species concept relies on genetic data and emphasizes distinct evolutionary trajectories between groups, which result in distinct lineages (branches on a phylogenetic tree).
May 3, 2017 · 4.1. Biological species concept. In nineteen century the first who produced the most quoted definition of what he called “biological species” was the zoologist Mayr (1942) who defined species as: “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups”. Thus groups of ...
May 10, 2024 · species, in biology, classification comprising related organisms that share common characteristics and are capable of interbreeding. This biological species concept is widely used in biology and related fields of study. There are more than 20 other different species concepts, however.
