How does sympatric speciation work

This page has been archived and is no longer updated. Discussion of most topics within Evolutionary Biology begins with Darwin. Darwin viewed evolution by natural selection as a very gradual mechanism of change within populations, and postulated that new species could be the product of this very same process, but over even longer periods of time. This eventual process of speciation by natural selection is illustrated by a sketch drawn by Darwin in his personal notebook nearly 20 years before the Origin of Species was published Figure 1.

Here, he proposed a model whereby lineages form from their ancestors by evolving different characters over relatively long periods of time. Darwin indicated that species could form by the evolution of one species splitting into two, or via a population diverging from its extant ancestor to the point it was a new species.

Darwin's insights into evolution were brilliant, especially in light of their being made in the absence of genetics. Indeed, ideas about heredity and the introduction of new genetic material via mutation were to come long after Darwin's founding theories of evolution.

Image via Wikimedia Commons. Some rights reserved. The integration of genetics with models of natural selection shed tremendous light on, and strengthened Darwin's views on, evolution — here was the missing mechanism that introduced new variation into populations via mutation and recombination.

Indeed, thanks to the Modern Synthesis, much of current research in Evolutionary Biology is strongly tied to genetics, and current methods for studying speciation are no exception. As discussed below, the Modern Synthesis led to advances not only in the study of evolution within populations, but also changes in the way species were defined, and in how new species were considered to form.

Thus, new species form when individuals from diverging populations no longer recognize one another as potential mates, or opportunities for mating become limited by differences in habitat use or reproductive schedules. In some cases, these pre-zygotic isolating mechanisms fail to prevent inter-breeding among individuals from separate populations. In these cases, viable hybrids may form, or the consequences of a successful mating attempt may end in failure, either due to the production of inviable zygotes or sterile, non-reproductive offspring.

These diverse pre- and post-zygotic barriers are of great importance to speciation biologists because they determine how reproductively-isolated populations are from one another, which indicates how far along the often continuous process of speciation that populations are.

For example, reproductive isolation is weak in the early stages of speciation, but changes to strong or complete in later stages of speciation Figure 2.

One or more of the many types of isolating mechanisms may play a role in the evolution of species along a continuum Figure 2. But how and why might reproductive barriers to genetic exchange evolve? Figure 2: Schematic illustration of the continuous nature of divergence during speciation, with three arbitrary points along the speciation continuum depicted.

Numerous types of differentiation can vary quantitatively, with the magnitude of differentiation representing a measure of how far speciation has proceeded. Two headed arrows represent mating between individuals. All rights reserved. A major area of debate among speciation biologists is the geographic context in which it occurs Figure 3. Ernst Mayr emphatically defended his view that speciation was most likely when populations became geographically isolated from one another, such that evolution within isolated populations would lead to enough differences among them that speciation would be an eventual outcome.

The central idea here is that when populations are geographically separated, they will diverge from one another, both in the way they look and genetically.

These changes might occur by natural selection or by random chance i. This view of speciation of geographically isolated populations — termed allopatric speciation — is still widely held among speciation biologists as playing a major role in the evolution of biodiversity e.

However, speciation might also occur in overlapping populations that are not geographically isolated i. The problem here is how do populations that are living in the same area, and exchanging genes, diverge from one another? This could occur, for example, if insects adapted to living on different plants within the same geographic region Feder et al. It will be interesting to see how many new examples emerge now that the idea of sympatric speciation is becoming less controversial.

Parapatric speciation refers to populations that are situated in geographic proximity to one another, usually with abutting but non-overlapping ranges. Here, a small proportion of each population are in actual contact with one another, and thus considered in sympatry, whereas the majority of individuals reside far enough apart that frequent encounters with one another are rare Figure 3. There are putative examples of parapatric speciation in salamanders Niemiller et al.

Reprinted from Mallet et al. The s saw a reclassification of modes of speciation away from schemes that focus solely on the geographic mode of divergence and towards a focus on the evolutionary process driving genetic divergence i.

This reclassification was motivated — at least in part — by renewed interest in the extent to which the evolutionary processes which cause adaptation within species also tend to create new species. Further, although the geographic mode of divergence has important implications for speciation via patterns of gene flow and sources of selection, speciation research has reached the point where we can directly test the role of different evolutionary process in driving speciation Butlin et al.

We outline several processes that can drive speciation. Recent years have seen renewed efforts to address these questions. For example, populations living in different ecological environments e. These same evolutionary changes can also result in the populations evolving into separate species.

For example, adaptation to different environments might cause differences between populations in the way individuals tend to look, smell, and behave. In turn, these differences might cause individuals from different populations to avoid mating with one another, or hybrids exhibit reduced fitness if mating occurs.

Thus, the populations cease exchanging genes, thereby diverging into separate species because of the adaptive changes that occurred via natural selection. More specifically, ecological speciation is defined as the process by which barriers to gene flow evolve between populations as a result of ecologically-based divergent selection between environments. This process makes some simple predictions. For example, ecologically-divergent pairs of populations should exhibit greater reproductive isolation than ecologically-similar pairs of populations of similar age Funk Figure 4 illustrates an example that supports this prediction.

Other predictions are that traits involved in divergent adaptation will also cause reproductive isolation, and that levels of gene flow in nature will decrease as ecological differences between populations increase.

Figure 4 Ecological speciation in host-plant associated populations of Timema cristinae walking-stick insects individual populations feed on either the Ceanothus spinosus host plant or on Adenostoma fasciculatum.

Pairs of populations feeding on the same host plant species, but in different geographic localities, are ecologically similar and assumed to not be subject to divergent selection. In contrast, pairs of populations feeding on different host plant species are ecologically divergent and subject to divergent selection. This pattern is independent from neutral genetic divergence, a proxy for time since divergence. A current debate is whether sexual selection can lead to speciation in the absence of ecological divergence van Doorn et al.

Indeed, compelling examples that implicate an important role of sexual selection leading to new species sometimes also involve the evolution of different signals used in mate-selection among populations in different ecological contexts, such as light environment Seehausen et al. Here, signals used in mate-selection become adapted to new ecological environments where the transmission of these traits is more perceptible or audible in a new habitat.

Another mechanism of speciation that involves chance events is speciation by polyploidization. Polyploidy, or the presence of three or more complete sets of chromosomes, has been documented in a wide variety of taxa.

Because polyploidy can lead to hybrid infertility, it is viewed as a mechanism that can rapidly lead to the formation of new species, potentially without selection for the divergence of other characters. Recent advances in genomics now allow such studies to be taken to the genome-wide level, where biologists can examine hundreds of thousands of gene regions, rather than just a handful. A genomic island is any gene region, be it a single nucleotide or an entire chromosome, which exhibits significantly greater differentiation than expected under neutrality i.

The metaphor thus draws parallels between genetic differentiation observed along a chromosome and the topography of oceanic islands and the contiguous sea floor through which they are connected. Following this metaphor, sea level represents the threshold above which observed differentiation is significantly greater than expected by neutral evolution alone.

Thus, an island is composed of both directly selected and tightly linked loci. Major remaining questions concern the size, number and distribution i. Clear answers to these questions will likely require experimental studies that measure selection at the genomic level to directly quantify how selection acts on the genome.

Nevertheless, the integration of geographic, ecological, and new genomic approaches is likely to yield new insight into speciation over the coming decades. See text for details. Divergent natural selection : Selection that acts in contrasting directions between two populations, usually with reference to ecological differences between their environments e. Ecological speciation : A speciation process in which divergent natural selection drives the evolution of reproductive incompatibility i.

Mutation-order speciation : A speciation process in which different and incompatible mutations alleles fix in separate populations that are experiencing similar selective regimes. Dobzhansky-Muller Incompatibility : Hybrid dysfunction arising from negative interactions epistasis between alleles at two or more loci: an allelic substitution at a locus causes no reduction in fitness on its own genetic background, but leads to reduced fitness when placed on the alternative background.

Genomic Island : A region of the genome where differentiation between populations is stronger than expected in the absence of divergent selection stronger than occurs via purely neutral processes such as genetic drift alone.

When populations become separated, gene flow between them ceases. Over time, the populations may become genetically different in response to the natural selection imposed by their different environments. If the populations are relatively small, they may experience a founder effect: the populations may have contained different allelic frequencies when they were separated. Selection and genetic drift will act differently on these two different genetic backgrounds, creating genetic differences between the two new species.

Parapatric speciation is extremely rare. It occurs when populations are separated not by a geographical barrier, such as a body of water, but by an extreme change in habitat. While populations in these areas may interbreed, they often develop distinct characteristics and lifestyles. Reproductive isolation in these cases is not geographic but rather temporal or behavioral.

For example, plants that live on boundaries between very distinct climates may flower at different times in response to their different environments, making them unable to interbreed. SparkTeach Teacher's Handbook. Dunn, Margery G.

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Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Reproduction is the production of offspring.

There are two main forms: sexual and asexual reproduction. In sexual reproduction, an organism combines the genetic information from each of its parents and is genetically unique. In asexual reproduction, one parent copies itself to form a genetically identical offspring. Sea turtles are an example of an animal that reproduces sexually, a volvox green algae is an example of an organism that reproduces asexually, and a brittle star can reproduce in either way.

Help your students understand the sexual and asexual reproduction with these classroom resources. The theory of natural selection was explored by 19th-century naturalist Charles Darwin. Natural selection explains how genetic traits of a species may change over time.

This may lead to speciation, the formation of a distinct new species. Select from these resources to teach your classroom about this subfield of evolutionary biology. But the first formal genetic study was undertaken by a monk named Gregor Mendel in the middle of the 19th Century.

Mendel bred peas and noticed he could cross-pollinate them in certain ways to get green or yellow seeds. Today, the field of genetics is breaking new ground searching for new ways to treat disease or develop crops more resistant to insects or drought. Empower your students to learn about genetics with this collection of resources. A keystone species helps define an entire ecosystem. Without its keystone species, the ecosystem would be dramatically different or cease to exist altogether.

A species is often defined as a group of organisms that can reproduce naturally with one another and create fertile offspring. However, the classification of a species can be difficult—even riddled with controversy. Join our community of educators and receive the latest information on National Geographic's resources for you and your students.

Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. There are four major variants of speciation: allopatric, peripatric, parapatric, and sympatric. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.