Editing Sympatric speciation (section)

==Controversy==

For some time it was difficult to prove that sympatric speciation was possible, because it was impossible to observe it happening.<ref name="Fitzpatrick et al. 2008" /> It was believed by many, and championed by [[Ernst Mayr]], that the theory of evolution by [[natural selection]] could not explain how two species could emerge from one if the subspecies were able to interbreed.<ref name="Mayr 1947">{{cite journal |first1=Ernst |last1=Mayr |date=December 1947 |title=Ecological Factors in Speciation |journal=Evolution |volume=1 |issue=4 |pages=263–88 |jstor=2405327 |doi=10.2307/2405327}}</ref> Since Mayr's heyday in the 1940s and 50s, mechanisms have been proposed that explain how speciation might occur in the face of interbreeding, also known as [[gene flow]].<ref name="Kondrashov & Kondrashov 1999">{{cite journal |doi=10.1038/22514 |year=1999 |last1=Kondrashov |first1=Fyodor A. |last2=Kondrashov |first2=Alexey S. |journal=Nature |volume=400 |issue=6742 |pages=351–4 |pmid=10432111 |title=Interactions among quantitative traits in the course of sympatric speciation|bibcode=1999Natur.400..351K |s2cid=4425252 }}</ref> And even more recently concrete examples of sympatric divergence have been empirically studied.<ref name="Savolainen et al 2006">{{cite journal |doi=10.1038/nature04566 |title=Sympatric speciation in palms on an oceanic island |year=2006|author1-link=Vincent Savolainen |last1=Savolainen |first1=Vincent |last2=Anstett |first2=Marie-Charlotte |last3=Lexer |first3=Christian |last4=Hutton |first4=Ian |last5=Clarkson |first5=James J. |last6=Norup |first6=Maria V. |last7=Powell |first7=Martyn P. |last8=Springate |first8=David |last9=Salamin |first9=Nicolas |last10=Baker |first10=William J. |journal=Nature |volume=441 |issue=7090 |pages=210–3 |pmid=16467788|bibcode=2006Natur.441..210S |s2cid=867216 }}</ref><ref>{{cite journal |last1=Jaenike |first1=John |last2=Dyer |first2=Kelly A |last3=Cornish |first3=Chad |last4=Minhas |first4=Miranda S |last5=Noor |first5=Mohamed |title=Asymmetrical Reinforcement and Wolbachia Infection in Drosophila |journal=PLOS Biology |date=10 October 2006 |volume=4 |issue=10 |pages=e325 |doi=10.1371/journal.pbio.0040325 |pmid=17032063 |pmc=1592313 |doi-access=free }}</ref>  The debate now turns to how often sympatric speciation may actually occur in nature and how much of life's diversity it may be responsible for.

===History===
{{further|Modern synthesis (20th century)}}

The German evolutionary biologist Ernst Mayr argued in the 1940s that speciation cannot occur without geographic, and thus reproductive, isolation.<ref name="Mayr 1947"/> He stated that gene flow is the inevitable result of sympatry, which is known to squelch genetic differentiation between populations. Thus, a physical barrier must be present, he believed, at least temporarily, in order for a new biological species to arise.<ref name="Mallet et al 2009">{{cite journal |doi=10.1111/j.1420-9101.2009.01816.x |title=Space, sympatry and speciation |year=2009 |last1=Mallet |first1=J. |last2=Meyer |first2=A. |last3=Nosil |first3=P. |last4=Feder |first4=J. L. |journal=Journal of Evolutionary Biology |volume=22 |issue=11 |pages=2332–41 |pmid=19732264|s2cid=9722101 |url=https://kops.uni-konstanz.de/bitstream/123456789/8588/1/space_sympatry_and_speciation.pdf |doi-access=free }}</ref> This hypothesis is the source of much controversy around the possibility of sympatric speciation. Mayr's hypothesis was popular and consequently quite influential, but is now widely disputed.<ref name="Jiggins 2006">{{cite journal |doi=10.1016/j.cub.2006.03.077 |title=Sympatric Speciation: Why the Controversy? |year=2006 |last1=Jiggins |first1=Chris D. |journal=Current Biology |volume=16 |issue=9 |pages=R333–4 |pmid=16682343|s2cid=16947323 |doi-access=free |bibcode=2006CBio...16.R333J }}</ref>

The first to propose what is now the most pervasive hypothesis on how sympatric speciation may occur was [[John Maynard Smith]], in 1966. He came up with the idea of disruptive selection. He figured that if two ecological niches are occupied by a single species, diverging selection between the two niches could eventually cause [[reproductive isolation]].<ref>{{cite journal |first=J. Maynard |last=Smith |year=1966 |title=Sympatric Speciation |journal=The American Naturalist |volume=100 |issue=916 |pages=637–50 |jstor=2459301 |doi=10.1086/282457|bibcode=1966ANat..100..637S |s2cid=222329634 }}</ref> By adapting to have the highest possible fitness in the distinct niches, two species may emerge from one even if they remain in the same area, and even if they are mating randomly.<ref name="Kondrashov & Kondrashov 1999"/>

===Defining sympatry===
Investigating the possibility of sympatric speciation requires a definition thereof, especially in the 21st century, when mathematical modeling is used to investigate or to predict evolutionary phenomena.<ref name="Jiggins 2006"/> Much of the controversy concerning sympatric speciation may lie solely on an argument over what sympatric divergence actually is. The use of different definitions by researchers is a great impediment to empirical progress on the matter. The dichotomy between sympatric and allopatric speciation is no longer accepted by the scientific community. It is more useful to think of a continuum, on which there are limitless levels of geographic and reproductive overlap between species. On one extreme is allopatry, in which the overlap is zero (no gene flow), and on the other extreme is sympatry, in which the ranges overlap completely (maximal gene flow).

The varying definitions of sympatric speciation fall generally into two categories: definitions based on biogeography, or on population genetics. As a strictly geographical concept, sympatric speciation is defined as one species diverging into two while the ranges of both nascent species overlap entirely – this definition is not specific enough about the original population to be useful in modeling.<ref name="Fitzpatrick et al. 2008"/>

Definitions based on population genetics are not necessarily spatial or geographical in nature, and can sometimes be more restrictive. These definitions deal with the demographics of a population, including allele frequencies, selection, population size, the probability of gene flow based on sex ratio, life cycles, etc. The main discrepancy between the two types of definitions tends to be the necessity for "panmixia". Population genetics definitions of sympatry require that mating be dispersed randomly – or that it be equally likely for an individual to mate with either subspecies, in one area as another, or on a new host as a nascent one: this is also known as panmixia.<ref name="Fitzpatrick et al. 2008"/> Population genetics definitions, also known as non-spatial definitions, thus require the real possibility for random mating, and do not always agree with spatial definitions on what is and what is not sympatry.

For example, micro-allopatry, also known as macro-sympatry, is a condition where there are two populations whose ranges overlap completely, but contact between the species is prevented because they occupy completely different ecological niches (such as diurnal vs. nocturnal). This can often be caused by host-specific parasitism, which causes dispersal to look like a mosaic across the landscape. Micro-allopatry is included as sympatry according to spatial definitions, but, as it does not satisfy panmixia, it is not considered sympatry according to population genetics definitions.<ref name="Fitzpatrick et al. 2008"/>

Mallet et al. (2002) claims that the new non-spatial definition is lacking in an ability to settle the debate about whether sympatric speciation regularly occurs in nature. They suggest using a spatial definition, but one that includes the role of dispersal, also known as cruising range, so as to represent more accurately the possibility for gene flow. They assert that this definition should be useful in modeling. They also state that under this definition, sympatric speciation seems plausible.<ref name="Mallet et al 2009"/>

===Current state of the controversy===
Evolutionary theory as well as mathematical models have predicted some plausible mechanisms for the divergence of species without a physical barrier.<ref name="Kondrashov & Kondrashov 1999"/> In addition there have now been several studies that have identified speciation that has occurred, or is occurring with gene flow (see section above: evidence). Molecular studies have been able to show that, in some cases where there is no chance for allopatry, species continue to diverge. One such example is a pair of species of isolated desert palms. Two distinct, but closely related species exist on the same island, but they occupy two distinct soil types found on the island, each with a drastically different pH balance.<ref name="Savolainen et al 2006"/> Because they are palms they send pollen through the air they could freely interbreed, except that speciation has already occurred, so that they do not produce viable hybrids. This is hard evidence for the fact that, in at least some cases, fully sympatric species really do experience diverging selection due to competition, in this case for a spot in the soil.

This, and the other few concrete examples that have been found, are just that; they're few, so they tell us little about how often sympatry actually results in speciation in a more typical context. The burden now lies on providing evidence for sympatric divergence occurring in non-isolated habitats. It is not known how much of the earth's diversity it could be responsible for. Some still say that panmixia should slow divergence, and thus sympatric speciation should be possible but rare (1). Meanwhile, others claim that much of the earth's diversity could be due to speciation without geographic isolation.<ref name="Nosil 2008">{{cite journal |doi=10.1111/j.1365-294X.2008.03715.x |title=Speciation with gene flow could be common |year=2008 |last1=Nosil |first1=Patrik |journal=Molecular Ecology |volume=17 |issue=9 |pages=2103–6 |pmid=18410295|s2cid=164288751 |doi-access=free |bibcode=2008MolEc..17.2103N }}</ref> The difficulty in supporting a sympatric speciation hypothesis has always been that an allopatric scenario could always be invented, and those can be hard to rule out – but with modern molecular genetic techniques can be used to support the theory.<ref name="Nosil 2008"/>

In 2015 [[Cichlid]] fish from a tiny volcanic crater lake in Africa were observed in the act of sympatric speciation using DNA sequencing methods. A study found a complex combination of ecological separation and [[mate choice]] preference had allowed two [[Ecomorphology|ecomorphs]] to genetically separate even in the presence of some genetic exchange.<ref>{{cite news |url=http://phys.org/news/2015-12-darwin-puddle-species-emerge-geographic.html |title='Darwin's puddle' shows how new species can emerge without geographic separation |date=18 December 2015 |work=Phyorg.com }}</ref><ref>{{cite journal |title=Genomic islands of speciation separate cichlid ecomorphs in an East African crater lake |last1=Malinsky |first1=M. |display-authors=etal |date=2015 |journal=Science |doi=10.1126/science.aac9927 |pmid= 26680190|volume=350 |issue=6267 |pages=1493–1498 |pmc=4700518|bibcode=2015Sci...350.1493M }}</ref>