Biological evolution refers to a process through which a specific trait within a population of species becomes increasingly dominant. This process results from the differentiation in the ability of a member of species that confers a fitness to withstand an environmental factor better or utilize a resource in the environment more than another member of the same species. This capability or vulnerability that relate to a trait makes an individual in the population more reproductive leading to an increase in the number of its kind in a population.
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Many factors involved in the process of evolution are external and internal to the individual in a population inhabiting a specific geographical location. These factors may be favorable or unfavorable for the survival of a group in a population that share a phenotype or underlying genotype that accounts for the disappearance or increase of the group in a population comprising of individuals with distinct traits. An understanding of how these factors interact may provide the basis for human intervention towards conserving individuals with a trait that suffer extinction due to vulnerability to forces of natural selection.
“Heterogeneity in different species determines the impact of selective forces in optimizing the fitness of individual in unique environments” – a process biology scholars call local adaptation (Fraser, Wier, Bernatchez, Hansen, & Taylor, 2011, p. 404). Members of species in local population show superior fitness in their own local environment relative to members from a divergent species population and environment. However, the ability to adapt to local characteristics is not a predictable result even where different selective pressure interplay.
Assessment of local adaptation in popular salmonid fishes has a longstanding tradition (Garcia de Leaniz, Fleming, Einum, Verspoor, Jordan, & Consuegra, 2007). These salmonid fishes group include graylings, whitefishes, charr, trout and salmon. Salmonids have complex morphology based on their biological homing and often show low dispersal between habitation areas. Their trait differentiation is usually a factor of local environmental features and most possess a heritable element. The trait differences between populations confer individual fitness differentiation between populations (Garcia de Leaniz et al., 2007).This studies coupled with past failures of transplants of salmonid populations from their indigenous habitat support the key paradigm that salmonid are adapted to their local environments.
In Atlantic salmon, the most prominent adaptive differentiation expresses as phenotypic differentiation fitness –related phenotypic trait (Garcia de Leaniz et al., 2007, p. 182). Numerous morphological, behavior characteristics and life history indicate sufficient heritable differentiation within and between salmon populations of the Atlantic. This conviction indicates differentiation in fitness and survival in freshwater and marine phases, and is, therefore, probably adaptive. In fact, many studies infer that distinct genotypes appear to be optimal in diverse environments, generating conditions that support local adaptation (Kawecki & Ebert, 2004, in Garcia de Leaniz et al., 2007).
Indigenous population of Atlantic salmon can vary significantly in morphometric and meristic characters and much of these morphological variations seem adaptive. For instance, in 1991, Claytor, Maccrimmon, and Gots studied 47 natural salmon populations in all Atlantic species in Western Europe and North America and concluded that salmon populations in high-gradient rivers had more streamlined bodies and longer heads. Breeding experiments verified that morphological differentiation was genetic, because the traits variances persisted in Atlantic salmon populations even when researchers reared them under a different habitat (Garcia et al., 2007).
A connection between the velocity of water and morphology is also obvious in other salmonids, which most signify an adaptive response to the flow of water. In fact, juvenile salmonids reared in high gradient waters diverge in morphology from those reared under low gradient waters, and the extent of phenotypic plasticity seems to be prominent.
Progressively, as juveniles start to smolt, their body structure appears to converge when preparing for a change to the prominent homogenous marine environment. Afterwards, when spawners go back to breed in freshwater, differentiation in adult body structure and secondary sexual characteristics may strengthen once more and have significant fitness implications (Garcia de Leaniz et al., 2007).
Mutation and Loss of Fitness
Consistent with the principles of Fisher (1958 cited in Garcia de Leaniz et al., 2007) of two divergent evolution forces, the fitness of individuals is enhanced in every generation through natural selection and diminished through the process of mutation and change in environment. Therefore, in a stable environment, genetic diversity can have both gains and costs as well. The gain represent enhanced future adaptive potential while cost represent reduced adaptation. Genetic alterations creating loss of adaptive capacity may arise from deleterous mutations, random genetic drift or gene introgression. Two probable scenarios can be espoused including one in which the population becomes increasingly vulnerable to change in habitat and another in which the genotype and/or phenotype move from an adaptive optimal.
The accidental and deliberate initiation of exotic salmon may cause introgression of mutated or poorly adapted genes into indegenous salmon populations, which may cause depression and maladaption from outbreeding (Garcia de Leaniz et al., 2007). Indegenious salmon populations survive and perform generally better than exotic populations. This implies that accidental break out of farm salmon or intentional stocking may reduce the survival and vitality of natural indegineous populations after they interbreed. Reccurrent introductions will generate cumulative reduction in fitness and could likely lead to an extinction pull in susceptible populations (McGinnity, et al., 2003).
However, the density of native dominant gene in the river may determine the effect of mutation. Therefore, the imported mutation may thrive alongside the indigenous individuals in sections of low carrying capacity, which may increase the production of smolts and adults at first. Hybridisation between indigenous and exotic individuals may possibly augment the overal fitness of the indigenous population for the earliest generation. Depending on the level of hybridization, fitness tend to diminish in subsequent generations, to a strength lower than it was before the initiation of the mutant gene. Conversely, in parts that have reached carrying capacity, initiation can diminish the production of wild smolt and diminish fitness in the first generation (Garcia de Leaniz et al., 2007).
Intentional, initiation of mutant in the mentioned circumstances are especially destructive because behavioral displacement of native fish by mutants with consequential deprived marine survival of the mutants cause a general decline in adult returns (McGinnity et al., 2003). Mutants entering a river lead to hybrids instead of pure mutants or indigenous because of differential spawning characteristics of males and females. Such mutants may displace native fish and diminish the overall fitness of the population. Therefore, the declined fitness in the mutants of exotic wild fish imply that the deliberate initiation of these fishes are as damaging as the mutants of farm escapes. In fact, such initiation may be more harmful because more numbers may be involved in yearly than in periodical introduction; characteristics of farm escapes.
Contemporary Microevolution in Threespine Stickleback
Recognizing the target of natural selection is crucial though problematic. One major reason for the problem is that evolution scientist have not identified the genetic architecture of the adaptive charactesristics. Natural selection targets phenotypes inspite of their genetic basis, although the underlying genes determines the evolutionary consequence of selection (Dalziel et al. 2009 in Raeymaekers, 2011, p. 2465). Incorpoating genetic information parallel to phenotypic information makes it possible to evaluate the impact on adaptive differentiation by selection promoting a unique allele oneer over another thereby connecting genetic differentiation, phenotypic differentiation, and fitnesss.
Evolution studies in respect to this organism has focused on the dramatic differentiation in the count of amourplates in its lateral side of threespine stickleback. This differnetiation is continuous although typically divided into three plate morph classes including the partially plated morph, low-plated morph, and the completely-plated morph. The variations relate to the habitats of organism. Higher counts of lateral plates characterize the populations in coastal and marine habitats, whereas remarkably reduced numbers of the object chracterizes the populations in freshwater (Raeymaekers, 2011, p. 2465).
Beginning with Heuts in 1947, (cited in Raeymaekers, 2011, p. 2465), numerous reserachers have attempted to isolate the selective agents responsible for evolution pro low-platedness, which have happened independently in many occasions of marine stickleback colonizing freshwater. The current discovery of the connection of plate number to the Ectodysplasin gene (Colosimoet al. 2005 cited in Raeymaekers, 2011, p. 2465) drove many researches to investigate this connection indepth at the gene level.
Measuring evolution rate, nevertheless, can take place in real time (Hendry & Kinnison, 1999).This condition is true with respect to plate numbers on threespine stickleback individuals, because certain studies have proven that when completely-palted population of this organism inhabit freshwater, they evolve low plate counts through some decades. The precise time series ever was that of a marine stickleback, which inhabitted Loberg Lake in Alaska (Bell, Aguirre, & Buck, 2004) in which the ratio of the completely-plated morph declined to 11 percent from 96 percent.
Le Rouzicet, Ostbye, & Klepaker studied the relationship between the Eda genotype and the plate phenotype using Nygaards Park samples (2011). Their progressive step was to apply a modelling approach to relate an event of phenotypic evolution propelled by selection on plate structure with an event of phenotypic evolution that selection on Eda genotype drives. This comparison is siginificant, because even though lateral plates are certainly a selection target, say by predators, Eda may exert pleiotropic influence on other characteristics that are under selection as well.
For instance evolution experts have suggested that Eda may also determine growth rate of the this species, which confers fitness property (Barrett, Rogers, & Schulter, 2009). The genotype-selection paradigm seem to match the plate morph time series than the morph-selection one. Although this may appear impossible since all selection must operate via the phenotype, the genotype-selection paradigm could be more sensitive to pleiotropic influence (Raeymaekers, 2011). This situation palces less limitations on the various ways selection may influence phenotypes.
In addition, the scholars appraised models using constant or frequency-dependent selection following freshwater habitation. Frequency-dependent selection mean that the weigth of selection relies on the frequency of genotypes or morphs throughout. In this light, the model proved that individuals with minimal plates count have a weighty fitness advantage provided their frequency remains down, as is the case during the start of freshwater invasion.
Spontaneous forces that acts on individuals in a population drives evolution of these organism. Individuals are equal subject to such forces, but the extent of their impact depends on the fitness of these individuals. The fitness of individuals towards the effect of environmental factors relate to their phenotypes (traits), which inturn is a factor of their genotype. Individuals in a population express differential traits and, thus, have varied level of fitness to a specific environmental factors. This situation determines the capacity of such individuals to colonize a habitat. Individuals with favourable traits dominate the environment than those with unfavourable traits who eventual become extinct from that environment.
Natural selection is the overiding factor that propells evolution. Because this factor is ongoing, the process of evolution still continuous even today although at a microlevel. In addition, the dynamic of the forces of nature continuously exposes individual in a population to new conditions that are not caterred for by their phenotype. Nevertheless, individual variation confers varied level of fitness to the new force such that frequency of different traits vary in a population. Because this traits are heritable, a genotype of an organism detrmines its fitness to an environemntal force and subsequent survival success.
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