Impact of Environmental Changes on Evolution

Environmental Changes and Evolution

Variations in environmental conditions have acute significance for the structure of ecological communities. Changing global environmental conditions have fostered new lines of ecological research to predict how environmental change will occur in a community and how it can be managed in order to preserve species (Falkenberg, 2013). Changes in environmental conditions can impact both directly and indirectly on all species and are a source of unique selection pressures for adaptive evolution.  Climate change can influence evolution by changing sizes of populations and altering patterns of gene low and hybridization.  Evolutionary impacts on wild populations due to climate change are expected to be large and widespread, however ecological and genetic constraints can preclude some organisms from evolving in response to climate change (Merilä, 2016).  

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When faced with changes in environmental conditions species respond by one of three different means – they can remain adapted to conditions by changing their genetic makeup through the process of evolution; they can respond by expressing different phenotypes in response to prevailing environmental conditions or they can migrate to locations more suited to them, with conditions similar to those preceding environmental change (Merilä, 2016). 

Organisms and populations are using all of these methods to remain adapted to their environment in the face of environmental change.  Massive shifts of organisms towards the poles and away from warmer equatorial waters have been reported in the last few decades in both terrestrial and marine ecosystems.  Phenotypic plasticity has also been observed in organisms as a response to climate change.  The migration and breeding seasons of many species are being brought forward, and are of a shorter period (Merilä, 2016).

Concerns have been raised that the rates of environmental changes caused by climate change are occurring too rapidly to allow many organisms to adapt, and thus it has been reasoned that widespread extinctions and maladaptions will occur, especially in larger organisms which have longer generational times. These concerns have been compounded by the fact that climate change is also expected to fragment and destroy habitats (Merilä, 2016).  However both Cameron (2013) and Bloudoff-Indelicato (2012) have found that it takes only a relatively small number of generations for evolutionary adaptations to occur in some species.

Throughout evolutionary time, organisms and population have continually evolved, but in the last decade, it has been found that climate change can drive evolutionary changes in animal species in just a few generations, which refutes the popular assumption that evolution occurs only gradually over hundreds or thousands of years (Tom Cameron, 2013).   Researchers studying laboratory populations of soil mites have found that significant genetically-transmitted changes have occurred in just 15 generations, demonstrating that evolution and short-term ecological change are interlinked. Cameron’s study found that populations evolve rapidly in response to environmental change which can have major consequences for species preservation or harvest yields (Tom Cameron, 2013).  Genetic changes in population affect the timing of major life events such as development, reproduction, dormancy and migration.  Smaller animals with shorter lifecycles and larger populations will probably adapt, but larger animals with long lifecycles and smaller populations will probable move closer to the poles in response to warming climate conditions, or cease to exist altogether (Bradshaw, 2006).

Scientists have proposed that enhanced rates of gene flow may assist in expediting evolutionary adaptation. Some radical solutions such as hybridization and genetic introductions across large distance are being considered as ways to enhance adaptedness in organisms such as corals, where changes in environment immediately threaten survival (Merilä, 2016).  One example of this is the recent harvesting of coral spawn on the Great Barrier Reef, where warmer sea temperatures are causing coral bleaching over huge areas of the Reef.  This year’s annual spawning event is the largest seen in many years, and scientists are capturing eggs and sperm to raise coral larvae and eventually disperse them to the most damaged parts of the reef (Hartley, 2018).

Tawny Owls

   Environmental change is forcing many animals to adapt in order to survive.  The tawny owl (Strix aluco) of Finland is one such animal.  This small to medium-sized owl is a common bird of prey in Finland and primarily occupies broad-leaved forests and woodland across Europe and parts of Asia.   They are nocturnal and prey at night on birds and small rodents such as voles and mice, insects and reptiles.  They are sedentary and during their breeding season between March and June (the northern hemisphere spring) they produce a clutch of between three and five eggs. They have been known to mate for life and during the breeding season the female incubates the eggs, while the male provides her with food. After hatching, the parents raise the chicks until they are two to three months old.  They appear in two color morphs (brown and grey) which have been found to be related to a change in rates of survival (Nilsson, 2018).  Their natural predators include other birds of prey such as buzzards, hawks and eagles and larger species of owls as well as foxes, cats and dogs together with squirrels and rats which prey on the owl’s eggs.

The owl usually presents in colors of brown or pale grey, the cold icy winters of Finland traditionally favoring the grey owl.  The grey owl uses its coloring to avoid predators by blending into the snowy landscape.   However, over the last 50 years, researchers have noticed a shift in the balance of owl colors.  The grey owls are in decline and the brown owls are thriving.  This is due to winters becoming milder and the brown owls becoming more suited to hiding in the bare-branched brown forest.  The more brown owls that survive, the more brown owl genes that are passed to the next generation.

A study by Karrel et al. (2011) examined the links between climate change and phenotype selection of plumage color in the tawny owl.  Plumage color in the tawny owl is dependent upon the amount of melanin deposited in their plumage and is a highly heritable phenotypic trait, unaffected by age or sex (Patrik Karell, 2011).  Karell’s study (2011) found that there were two distinct coloration phenotypes, brown and grey.  Earlier studies (Brommer, 2005) have shown that the brown tawny owl had a reduced rate of survival when compared with the grey tawny owl, and this resulted in lower overall reproductive success for the brown color.

Karell et al. (2011) have observed that their study provides the first empirical evidence of selection of a heritable trait, driven by climate change.  They found that climatic conditions in early winter were a powerful driving force in plumage color selection, and also found that higher proportions of brown tawny owls were being seen in the population.  They believe that, within the population of Finnish tawny owls, phenotypic plasticity or random genetic drift are not sufficient explanations to contribute the micro evolutionary changes which have occurred in the population over the last two decades (Patrik Karell, 2011).  They found that warmer winter temperatures contributed towards selection of brown color, although they found that survival for both colors was approximately equal during winter.  They discovered that brown coloration had no reproductive advantages over grey color, and as a result determined that the increase in the brown color of the owl population was driven by other elements related to environmental change including reduction in winter temperatures, amount of snow cover and reduction of food supply (Patrik Karell, 2011). 

Pink Salmon

Scientists believe that climate- related changes will have major impact on the Pink Salmon by affecting changes to the physical environment such as changes in precipitation, temperature, groundwater discharge and decreased period of ice in lakes.  Not only will these events affect the timing of migration, but will also impact on the spawning, hatching, growth, survival and distribution of salmon species (Beamish, 2009).Specific factors which are responsible for recent declines are not well understood, however they are believed to be related to human activities. It is thought that increases in global temperature will increase water temperatures and decrease river flows during spawning migration.  This will have the effect of reducing egg deposition and increasing mortality.  Warmer waters will also affect the stages of egg incubation, causing fry to emerge prematurely and increasing fry mortality.  A warmer climate will also increase the frequency and severity of winter floods, reducing the survival rates of eggs and fry (Beamish, 2009).

The life cycle of the pink salmon commences in freshwater, where the female’s eggs are fertilized and remain in the gravel of the river through the winter, where they develop into embryos.  The eggs hatch in the spring when tiny fish emerge with the yolk sac attached.  These tiny fish stay close to the spawning ground for a few months until the egg sac is consumed, and then emerge from the gravel as fry to head directly to sea (National Park Service, 2018). Pink salmon spend 18 months at sea and then migrate homeward. On reaching freshwater, the female deposits eggs in several places on the journey upstream, which are fertilized by the male.  The fish die soon after spawning, supplying the river with nutrients (National Park Service, 2018).

Salmon fry move into lakes after hatching, where they usually remain for approximately one year.  Increased lake water temperatures resulting from climate change may have the effect of inhibiting the growth of salmon to sizes necessary to survive in the open ocean.  Increased water temperatures may also have the effect of reducing food supply.  Changes in water temperature are also thought to decrease the salmon’s energy stores which are necessary for successful migration and growth.  These decreases may also increase pre-spawning mortality during the grueling upstream phase of migration (Beamish, 2009).

The pink salmon’s survival is dependent upon a high-quality stream habitat where logs, boulders and shade protect them from predators and access to side channels prevent them from being flushed downstream during floods (National Park Service, 2018).  However, rising stream temperatures, attributed to climate change, are causing the Alaskan Pink Salmon population to migrate two weeks earlier than it did in the 1970s. 

 After observing samples of 17 generations of salmon over a period from 1970 to 2011, researchers found that evolutionary change had occurred in response to warming temperatures (Bloudoff-Indelicato, 2012).  These finding support those of Cameron’s study mentioned earlier and provides evidence that animals are able to adapt to climate change in fewer generations than was previously thought.

 However, since 1989, when the second highest water temperatures since the 1970s were recorded, scientists found that substantial genetic change was occurring in the offspring.  Genetic change comes at a cost in terms of biocomplexity (Bloudoff-Indelicato, 2012).  When an organism develops an advantageous genetic attribute, another is weakened.  In the case of the pink salmon, large shifts in the ratios of late-migrating to early-migrating salmon have meant a reduction of genetic diversity, which could possibly be detrimental to salmon populations if stream temperature trends were to reverse (Bloudoff-Indelicato, 2012). Cameron’s study (2013) found that a genetic evolution occurred in soil mites whereby the mites produced more eggs at the expense of individual rates of growth.  The study found that these were indeed an evolutionary change and not just a short-term ecological response.

 The results of these studies have important implication in areas such as conservation, management of fisheries and pest and disease control, because they demonstrate that evolution can occur in short periods of time and that man-made decisions can cause major changes to the environment and life histories of populations (Tom Cameron, 2013). These studies have also showed that changes in the biology of a species can occur at the same time as an ecological response and that even small evolutionary changes in a species can have an enormous impact in population growth rates and yields (Tom Cameron, 2013).

Conclusion

Climate change is creating new environmental Global change drivers create new environmental settings and increasing selective pressures which are affecting species in a variety of ways.  One way in which species adapt is by phenotypic plasticity whereby species adapt to changing environments by selecting the attributes most beneficial to their survival in a changed environment. Through the maintenance of genetic variation, plasticity plays an important role in a species’ short-term response to climate change as well as maintaining genetic variation in the long-term (Silvia Matesanz, 2010).  Some studies have suggested that plasticity may shield populations from evolutionary change, while others suggest that it might foster evolution (Silvia Matesanz, 2010).  Due to the rapid rate of current environmental change, some disagreement exists as to whether observed responses to climate change are a result of phenotypic plasticity or evolutionary changes. Traditionally held beliefs that evolution can only occur over centuries has been challenged by two studies cited earlier in which evolutionary change has been seen to occur in two species in less than twenty generations. Although more research is needed, the impacts of climate change on selection pressures are expected to be extensive and acute.  Evidence of genetic changes in a species or population is difficult to assess and although evolutionary impacts are difficult to predict, it appears likely that climate-driven environmental changes will occur too rapidly to allow larger species with longer lifecycles and smaller populations to adapt, with extinctions and shifts in ranges being the most likely scenario.  Further research into evolutionary responses to climate change is necessary as is the establishment of management practices to enhance evolutionary resilience.

References

Bloudoff-Indelicato, M. (2012, July 13). Salmon Evolve to Cope with Climate Change. Scientific American. Retrieved from https://www.scientificamerican.com/article/salmon-evolve-to-cope-with-climate-change/

National Park Service. (2018, August 24). The Salmon Life Cycle. Retrieved from Olympic National Park, Washington: https://www.nps.gov/olym/learn/nature/the-salmon-life-cycle.htm

Tom Cameron, D. O. (2013). Eco-evolutionary dynamics in response to selection on life-history. Ecology Letters. doi: DOI: 10.1111/ele.12107