Ecological Speciation And Its Natural Occurrances Biology Essay


The term 'ecological speciation' refers to the process whereby reproductive isolation arises within a population. (Schluter 2001) This is an evolutionary process which is the result of contrasting environments causing different selection pressures on phenotypic traits (which may be morphological, physiological or behavioral) within the population, thus leading to divergent natural selection. (Schluter 2001)

In the 'classical' model of ecological speciation, the development of such isolation begins in allopatry as a 'by-product' mechanism. Rather than being selected for directly, reproductive isolation (premating and/or postmating) arises as a consequence of genetic differentiation caused by natural selection acting upon other traits. (Schluter 2001) This process has been demonstrated in separate studies on different Drosophilla spp by Kilias et al (1980) and Dodd (1989). Kilias et al raised different lines of Drosophilla melanogaster for five years in environments which differed in terms of temperature, light and moisture.(Kilias et al; cited in schluter 2001) Dodd studied the mating preferences of Drosophilla pseudoobscura raised for one year, whose environments differed in terms of the larval medium they were raised on. (Dodd; cited in Schluter 2001) On secondary contact, both of these studies indicated some level of premating reproductive isolation between individuals raised in contrasting environments, but no reproductive isolation between control groups raised in the same environments. These studies provide evidence for the occurrence of classical ecological speciation in nature. (Schluter 2001)

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The 'classical' process of speciation is completed by reinforcement during the sympatric phase (Schluter 2001). Genetic differentiation built up during allopatry may lead to reduced fitness in hybrids/ hybrids that are maladapted during secondary contact in sympatry. During this process, postzygotic reproductive isolation is favored directly by selection as a result of a lack of viable offspring during hybridization. (Schluter 2001)

It is theoretically possible for ecological speciation to occur in pure allopatry or sympatry, in which the sympatric and allopatric phases are absent respectively. (Schluter 2001) During sympatric speciation, where the build up of differentiation that we see in the allopatric phase of the classical scenario is absent, reproductive isolation is a result of selection pressures that act directly upon the continuum of intermediates within a single population, and favor extreme phenotypes. (Schluter 2001) This may occur for example, if intermediate phenotypes are less successful at resource exploitation/competition than their counterparts at the extreme ends of the phenotypic continuum. (Schluter 2001) Reproductive isolation arises from positive assortative mating and this becomes an internal barrier to gene flow. (Bilton 2006)

Question 2- What are the main causes of reduced hybrid fitness and how does the study of hybrid fitness contribute to the case of ecological speciation?

The main causes of reduced hybrid fitness are ecological selection, genetic incompatibility and sexual incompatibility (Schluter 2001) Reduced fitness caused by ecological selection is driven by the fact that hybrid individuals are maladapted to their environment in comparison to their non hybrid counterparts. They may for example be less efficient at resource exploitation, and/or more susceptible to predation or parasitism. (Schluter 2001) A reduction in hybrid fitness as a result of genetic incompatibility refers to the intrinsic incompatibility of genes inherited from each parent, which in turn results in either hybrid inviability, where their survivorship (either in terms of surviving to full term or to adulthood) is reduced, or hybrid sterility whereby the hybrid is either sterile or more commonly exhibits a reduction in fertility (producing fewer gametes for example) (Bilton 2006²). Reduced hybrid fitness as a result of sexual incompatibility arises from mate recognition systems and assortative mating favoring non hybrid individuals. (Bilton 2006)

Genetic mechanisms of reduced hybrid fitness will be expressed in any environment, during non ecological as well as ecological speciation. (Schluter 2001) Unlike genetic and sexual incompatibility, ecological selection against hybrids is dependant upon the organism's environment and would not manifest itself in the laboratory. (Schluter 2001) Therefore laboratory experiments are valuable in demonstrating the process of ecological selection and have thus provided evidence for ecological speciation. One such example is that of F1 hybrids between limnetic and benthic species of threespine sticklebacks, which in the laboratory demonstrate a high fitness, but when studied in the field their intermediate phenotype results in a reduced ability to exploit their food resource and a consequential slower growth rate (Hatfield et al 1999; cited in Schluter 2001) Similar results of hybrids exhibiting a high viability within the laboratory but a low viability within the field have been demonstrated in several studies and provide support for ecological selection leading to ecological speciation. (Schluter 2001)

Question 3- Using a named example; explain what is meant by parallel speciation. How does this contribute evidence for ecological speciation?

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The term parallel speciation refers to the parallel evolution of reproductive isolation mechanisms in independent populations of a former species. (Johannesson 2001) This process is important as it can occur purely as a result of natural selection (Schluter et al 2001). The result of parallel speciation is the lack of reproductive isolation between independently evolved populations which have experienced similar environments and therefore selection pressures; i.e. 'ecotypes' (Richmond 2002). Once again, threespine sticklebacks provide a clear example of the occurrence of this process within nature (Schluter 2001). Limnetic and Benthic species of the stickleback have arisen on up to four separate occasions, within different lakes (Schluter 2001). The limnetic species is small and slender, with a small mouth adapted for plankton foraging, and the benthic species is superior with a larger mouth adapted to feed on bottom dwelling invertebrates (Johannesson 2001). These phenotypic differences have been driven by divergent selection due to different environmental pressures acting upon individuals occupying different depths of the lakes (Johannesson 2001). Despite no extrinsic barrier to gene flow, the two species rarely hybridize within their natural environment and furthermore within the lab the frequency of hybrid mating is just 10-15% in 'no choice' mating trials (Schluter 2001). This is in stark comparison to the mating frequency between individuals from similar environments (Schluter 2001). It is thought that traits influencing mating compatibility e.g. body size and coloration have evolved in parallel within independent populations as a result of their comparable environmental conditions and subsequent selection pressures (Schluter 2001).

The process of parallel speciation provides compelling evidence for the ecological speciation concept as it reiterates the importance of divergent natural selection in being the principal driving force which acts upon phenotypic traits and ultimately results in reproductive isolation (Johannesson 2001, Schluter 2001). Furthermore it highlights the fact that the evolution of reproductive isolation can occur even when there is no extrinsic barrier to gene flow and that the environment plays such a crucial role in this that we find the parallel evolution of reproductive isolating mechanisms within geographically isolated locations (Johannesson 2001).

Section B

Question 1-What characteristics of the Indonesia-Philippines region might contribute to the high endemism of reef fish described by Mora et al.?

By analyzing the geographic pattern of reef fish endemism, Mora et al were able to distinguish between the Centre-of-accumulation and Centre-of-origin hypotheses, confirming the latter and identifying The Indonesia-Philippine region (IPR) as the major centre of endemism in the Indian and Pacific oceans (Mora et al 2003).

The IPR occurs in a highly interconnected area, and contains among the highest number of islands per unit geographical area (Mora et al 2003). It is likely that recent changes in sea level have caused this area to demonstrate frequent allopatric speciation events. (Mora 2003) Mora et al found a negative correlation between labrid/ pomacentrid species richness and an increasing distance from the IPR. This is most likely a result of a barrier or limitation in the dispersal of these species (Mora et al 2003).

Mora et al presume that the regions of shallow water reef habitats are poorly interconnected in longitude due to deep water gaps between the islands, but are highly interconnected in latitude as a result of continental margins. (Mora et al 2003) However, despite the theoretical ability for species dispersal across the latitudinal gradient, ecological barriers such a temperature and current systems may limit species expansion, and/or cause local extinction at higher latitudes (Mora et al 2003)

The 'coral triangle' of Indonesia, New Guinea and the Philippines contains 90 endemic fish species (Smith 2006). The East Australian current moves from tropical waters into significantly cooler waters towards the poles which are inhabitable by such species (Smith 2006). This abiotic barrier prevents the expansion of species which have originated within that geographical range and thus lead to an unusually high proportion of endemics within these regions (Smith 2006). Furthermore certain species such as anemonefish from the family pomacentridae exhibit extended periods of parental care, where the young rarely deviate from their place of birth and quickly settle on their local reef. This behavior also contributes to high levels of endemism within the region (Smith 2006).

Question 2- Why should there be fewer species of reef fish at higher latitudes as described by Mora et al.? You should consider both general and specific reasons in your answer, don't limit yourself solely to those described in the paper.

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Mora et al describe fewer species of reef fish at higher latitudes (Mora et al 2003). In this study the species richness of labrid and pomacentrid species of reef fish were correlated with their mean pelagic larval duration (PLD). This was in order to determine whether longitudinal and/ or latitudinal gradients of species richness are a direct result of species dispersal abilities (Mora et al 2003).

Species richness was found to be negatively correlated with distance from the Indonesian-Philippine region (IPR); i.e. as the distance from the IPR increases, the numbers of species present within that location decreases. Furthermore the mean PLD was positively correlated with distance from the IPR; i.e. as the distance from the IPR increases, so to does the mean PLD of the species found in that locality (Mora et al 2003) These findings suggest that species dispersal is dependant upon a high larval duration, but that only a small proportion of the species within that region exhibit this trait, resulting in poor species dispersal from the region (Mora et al 2003).

An exception to this rule was found in localities of high latitude, whereby species richness declined regardless of the mean PLD (Mora et al 2003). In this instance, therefore, there must be another constraint on species dispersal which both overrides the PLD of the organism and the fact that due to continental margins there is a high level of latitudinal continuity allowing for dispersal (Mora et al 2003). Ecological barriers occurring in a latitudinal manor such as current and temperature may be the limiting factors resulting in fewer species within these higher latitudes (Mora et al 2003, Smith 2006).

Within this region, The East Australian current moves from tropical waters into significantly cooler waters towards the poles which are inhabitable by many species of the IPR (Smith 2006). This ecological barrier of current and temperature will prevent the expansion of species which have originated within the IPR and as a result will lead to an unusually high proportion of endemics found within these regions (Smith 2006). There are a wide range of additional potential ecological barriers which may (theoretically) contribute to the poor latitudinal dispersal of these species. These may include resource availability, predation and the lack of a suitable habitat for example.

Question 3- Explain how endemism arises, how it contributes to global biodiversity, and why endemics are so important.

The term 'endemic' refers to species which are exclusively native and unique to a defined place or region (Crass 2003). Endemism usually arises within regions which are isolated in some way by an isolating mechanism which may be geographical and/ or behavioral in nature (Crass 2003).

Continental drift whereby the continents we recognize today dispersed from a single land mass 'Pangea' between 80 and 200 million years ago resulted in the isolation of land masses and the subsequent evolution of highly distinct flora and fauna (WWF 2007). These areas of endemism are relatively large in comparison to oceanic islands which also often exhibit very high levels of endemism (Gaston et al 2004).

The colonization of oceanic islands by species originating elsewhere may result in a high level of endemism as a result of unusual environmental conditions whose selection pressures drive the evolution of local adaptations causing the species to be highly specialized, but consequentially restricted to the specific region in which they evolved (Gaston et al 2004). Isolation in terms of distance and/ or a physical barrier to dispersal play an important role in the independent evolution of such endemic taxa (Gaston et al 2004).

Endemic species may also be found within small ranges of a large geographical area due to the fact that they occupy a narrow ecological niche, and are surrounded by different habitats of which they are not adapted to suit (WWF 2007). These ranges are in effect an 'island' for the inhabitant and as such the forces driving the evolution of endemic species within them are similar to those found on oceanic islands (WWF 2007).

A 'Biodiversity hotspot' usually refers to an area with an unusually high proportion of endemic species. Over 50 percent of the world's plant species and 42 percent of all terrestrial vertebrate species are endemic to 34 of these biodiversity hotspots, which cover just 2.3% of the Earths surface. Endemic species therefore contribute significantly to the world's biodiversity, not only because they constitute a considerable proportion of global species numbers, but also because they are 'irreplaceable' if they disappear from just one geographical area. What's more, as a direct result of them being exclusive to a particular region, they are at an especially high risk of extinction as a result of invasive species, exploitation, habitat destruction and fragmentation. It is important therefore for these valuable species to be protected and prioritized in terms conservation management and policy.

Section C

Question 1- Neutral theories are often used to investigate ideas in ecology and evolution. What makes a theory a "neutral theory" and what are such neutral theories used for? Do not limit your answer to Hubbell's neutral theory- this is only one of the latest- but instead make sure you discuss neutral theories in general.

The fundamental principal of a neutral theory is that it assumes all individuals of a population or community, regardless of their species, are equal in terms of their probability of birth and death, immigration and emigration and (in the neutral theory of molecular evolution) acquiring a mutation that may lead to a speciation event (Norris 2003, Xin-Sheng et al 2006).

The term neutral theory may be used to refer to one of two related theories; 'the Neutral Theory of Molecular Evolution' and/ or 'the Unified Neutral Theory of Biodiversity and Biogeography' (Wikipedia 2007). The Neutral Theory of Molecular Evolution is a more established theory than the latter, boasting a 40 year history since its development by Motoo Kimura in the late 1960s (Xin-Sheng et al 2006). It attempts to tackle questions at the population level, placing importance on genetic drift as the driving force behind evolutionary change, (Xin-Sheng et al 2006) and claims that the majority of molecular differences within a genome are 'selectively neutral'(Wikipedia 2007²). The Neutral Theory of Biodiversity and Biogeography is young in comparison, published in 2001 by ecologist Stephen Hubbell, and in contrast to Kimura's theory focuses on macroecology and the patterns of community structure, placing importance on ecological drift as the principal factor shaping the structure of a community (Norris 2003, Xin-Sheng et al 2006).

The neutral theory is in effect a null hypothesis because it assumes that individuals of a population/ community are 'per capita equivalent' and that there is no difference in individual responses to ecological forces/ pressures acting upon them (Xin-Sheng et al 2006). This should lead to the development of a testable hypothesis (Xin-Sheng et al 2006). Neutral theories contrast with more traditional theories which, in macroecology, place importance on competition for resources and niche differentiation in shaping community structure and relative species abundance, (Norris 2003) and in population genetics place emphasis on ecological pressures driving the natural selection and selective evolution of species (Xin- Sheng et al 2006).

Question 2- Hubbell's Unified Neutral Theory of Biodiversity and Biogeography is closely related to two other theories: (a) the Neutral Theory of Molecular Evolution and (b) the Equilibrium Theory of Island Biogeography. Briefly explain each one of these later two theories, making it explicit why each one is a neutral theory. Try to explain each theory in a way that highlights the similar philosophy and approach that underlie them both.

The Neutral Theory of Molecular Evolution was developed by Motoo Kimura in the late 1960s (Wikipedia 2007²) The theory suggests, in contrast to more traditional views, that the majority of molecular differences and variations we see within a genome are 'selectively neutral'; neither subject to, nor the consequence of natural selection (Wikepedia 2007²). Whilst it "Does not deny the role of natural selection in determining the course of adaptive evolution" (Kimura 1986; cited in Wikipedia 2007²) the theory places emphasis on genetic drift as being the principal driving force in population genetics and evolutionary change (Xin- Sheng et al 2006). In the infinite allele model, a continuous influx of mutations causes 'old' alleles to be replaced with new alleles over time (Xin- Sheng 2006). These mutations will alter the allele frequencies within a population as a result of genetic drift, and will have a greater effect on species with smaller abundances (Xin- Sheng 2006). The theory should lead to the development of a testable null hypothesis whereby the individuals of a population exhibit no difference in their responses to ecological selective pressures (Xin- Sheng). This is in contrast to perhaps more traditional theories in which population genetics and allele frequencies within a population are principally driven by natural selection as a result of selective pressures.

In MacArthur and Wilson's (1967) 'Equilibrium Theory of Island Biogeography', the equilibrium number of species inhabiting an island (be it an oceanic island or an 'island' of any habitat which is surrounded by a contrasting habitat) is determined by three main principles; the number of species inhabiting the island is dependant upon the balance between immigration and emigration rates. This balance is dynamic and is largely determined by the continual extinction of species and their replacement (via immigration) by another, and finally, such rates are dependant upon the island size and its isolation (Begon et al 2006) The theory makes the assumption that species immigration/ emigration rates are solely determined by the physical properties of the island, and that the species themselves are equivalent in their abilities to colonize and maintain their population (Lomolino 2000). The theory, therefore, is species neutral; ignoring inter-specific interactions, and as a consequence is unable to tackle the reasons behind the patterns of community structure that we find in such ecosystems (Lomolino 2000). As Lomolino points out, 'some of the most interesting patterns in biogeography concern not just how many, but which species inhabit islands'(Lomlino 2000).

Question 3- Hubbell's model does not take into account interactions between species, event though it has been demonstrated beyond doubt that such interactions do take place. Despite this, Hubbell's theory is still considered to be useful. Explain four ways in which the theory might be valid even though species are not the same and use resources and respond to disturbance differently. Do not repeat the same point more than once in a different form of words!

Despite the fact that Hubbell's model does not take into account species interactions such as competition and niche differentiation, some of its fundamental principals may still be valid. The problem may be that ecologists view deterministic processes as completely exclusive from the random processes of neutral theory, when in fact the two theories may complement each other and lead to the understanding of the wider picture of community structure. It is without doubt that species exhibit different roles, tolerances and requirements within the environment, but it is also apparent that alongside species interactions and niche differentiation, random processes such as ecological drift account for the lack of community structure predictability (Norris 2003). It may simply be that Hubbell places too much importance on these processes as being the principal driving force in shaping communities, when in fact they may be important but none the less additional factors (Norris 2003).

The neutralist response may also be valid in situations where there is limited biological diversity, and in fact it is in these very cases that the theory is designed to apply (Norris 2003). A strong advocate of the neutral approach, Graham Bell of McGill university, explains that "Neutral models refer only to ecologically similar species, and not, emphatically not, to trophically complex communities" (Norris 2003). It is understandable then, why the neutral theory does not make ecological sense when applied to complex communities whose structure will be principally driven by species interactions, but will be valid in communities where inter-specific interactions such as competition do not play a major roll, and random processes dominate (Norris 2003).

The theory may also be useful in practical situations when researching and analyzing community structure. The theory may be used on data in order to establish what aspects of community structure are attributable to chance processes such as ecological drift, and thus leave ecologists with a starting point to attribute the remaining variability to biological factors (Norris 2003).

In addition to this, despite the fact that species are different, the theory may be wholly applicable and valid, if as Hubbell claims, the demographic parameters across which species vary do not affect an individuals chance of occupying a vacancy within the community (Norris 2003). Under this assumption, variation, competition and niche differentiation are all possible, but do not directly determine community composition (Norris 2003). At an individual level, the equal chance of occupying a vacancy within the community simply leads to no predictable 'winner' or 'loser' (Norris 2003).