Evolution, Natural Selection, Snails, and Mice

Evolution is the process in which characteristics of populations change over time, and natural selection is a mechanism by which evolution can occur. Natural selection occurs in a population in which overproduction leads to competition between offspring which exhibit variation in their heritable characteristics. Those who are the most fit to survive and reproduce pass their characteristics down to the next generation, while those which are less fit die out before they can reproduce due to the limited carrying capacity of the ecosystem to which they belong. The characteristics which best promote the survival of the organisms possessing them, giving them an advantage over those who don’t possess those characteristics, get passed down through generations and increase in frequency.

This is the case with Cepaea nemoralis, terrestrial snails which exhibit variable shell color (pink, yellow, or brown) and banding (un-banded or banded). With regards to shell color, brown (C^B) is dominant over pink (C^P), which is dominant over yellow (C^Y), and with regards to banding, band absence (B) is dominant over band presence (b). The relative simplicity with which these characteristics allowed scientists to observe the effect of evolution on Cepaea nemoralis genetics, for example, in dealing with shell color and thermal properties.

Differently colored Cepaea nemoralis shells showing genetic variation.

Brown and pink shells, the darker colors, are more efficient in absorbing solar radiation than yellow shells. Therefore, in areas where temperatures are low would need to absorb as much of the heat around them as possible, snails with dark-colored shells would be more likely to survive long enough to reproduce and pass down their adaptive traits, whereas yellow-shelled snails would be more likely to freeze to death first and are selected against. On the other hand, if these pink- and brown-shelled snails were living warm conditions, they would absorb too much heat and possibly die of heat shock, while yellow-shelled snails would be the best suited and live to reproduce. Consistent with this logic, a study conducted on snail populations across Europe showed that, as climates got warmer to the south, the proportion of yellow shells to pink and brown shells gradually increased.

The same concept of survival of the fittest was shown with respect to shell banding. The song thrush, a predator of Cepaea nemoralis, visually locates its prey. Therefore, snails better camouflaged in their environments would be less likely to be preyed on. In open grasslands, yellow-banded shells are the best camouflaged and most inconspicuous, so more yellow-banded-shelled snails survive and reproduce than do other colored snails. In woodlands, on the other hand, pink- and brown-un-banded shells are less conspicuous, and therefore snails with these characteristics are more likely to survive to pass down these genes. This may also possibly be explained by microclimatic selection (grasslands are warmer than woodlands), and with global warming and declining song thrush populations, the cause of this variation may become clearer.

TOP: Light-colored deer mouse on dark soil
BOTTOM: Dark-colored deer mouse on pale sand

Another case in which the effects of natural selection are apparent is that of the deer mice (Peromyscus maniculatus) in Nebraska. Originally dark-colored and successfully blended in with the dark soil on which they lived, deer mice evolved in terms of coat-color over the past 8,000 years due to the deposition of light-colored sand by glaciers onto their habitat 10,000 years ago and spoiled their camouflage. The deer mice living on Sand Hills, Nebraska, today have very pale, sand-colored coats compared to deer mice living in other areas where glacial deposition hadn’t taken place and soils were dark. In this case, the gene which determined fur-color was called Agouti and the allele for blonde-fur is dominant over that for brown fur. The reason why the coat colors of deer mice are so important for their survival is similar to the suggested reason for the prevalence of yellow-banded snails in grasslands, that is, visual selection--the likelihood of organisms with more camouflaged characteristics to avoid detection by their predators and therefore survive to adulthood and reproduce. Owls, hawks, and the other predators of deer mice locate their prey by sight. Therefore, dark-colored deer mice inhabiting the Sand Hills where the pale quartz grains making up the dunes made their coat color conspicuous are more likely to be eaten before they can reproduce. On the other hand, light-colored deer mice are less conspicuous and therefore can survive long enough to reproduce and pass down their genes. Conversely, dark-colored deer mice are better than light-colored deer mice to the surrounding areas which have darker soils, and therefore are more prevalent in those areas.

As evidenced from the above examples, ecology, evolution, and genetics are inextricably linked. Evolution, the process by which the characteristics of populations change over time as they adapt to their environments, can occur through natural selection. Natural selection, as mentioned earlier, involves the passing on of advantageous traits which enhance the chances of survival for organisms which possess them. The only physical traits which can be passed on are those that are genetic--acquired traits like physical fitness due to constant exercise are not heritable and cannot be passed onto offspring. And as evolution occurs in populations and not individuals, genetic variation and diversity in a population is important. This is in turn related to ecology, the relationship and interactions between organisms and the environments they inhabit, including biotic and abiotic factors, in that species evolve in response to the environments they live in. When a change in the environment a population inhabits takes place, the diversity in the genetic traits of the organisms making up the population ensures that each organism has different chances for survival. Those which have traits which are more suited to the environment after the change are more likely to survive long enough to reproduce and pass on those genes, while those which aren’t as well suited to the new environment tend to die off first. The favoring of certain genes in a population due to differing survival rates results in changes in the characteristics--evolution! For example, animals living in grasslands do best if they have mechanisms to cope with the heat and have inconspicuous characteristics to enhance their camouflage. Therefore, the animals possessing genes which best suit these environments are most likely to survive and reproduce, passing on these heritable genetic traits onto their offspring, and resulting in shifts in the allelic frequency of said traits in the population, and ultimately, evolution of the species. This also highlights the importance of biodiversity--without biodiversity, if a change in the environment takes place, then no organisms would be better suited to the new environment, and species might go extinct! Survival of the fittest and thus evolution can’t take place if none of the organisms in the population are fit enough to compete with other species!

SOURCES:

Bradt, Steve. "Mice living in Sand Hills quickly evolved lighter coloration." Harvard Gazette. Harvard College, 27 Aug 2009. Web. 6 Oct 2012. <http://news.harvard.edu/gazette/story/2009/08/mice-living-in-sand-hills-quickly-evolved-lighter-coloration/>.

"Survival of the blondest: Mice change their coat colour over 8,000 years to fool predators." Daily Mail Online. The Daily Mail, 28 Aug 2009. Web. 6 Oct 2012. <http://www.dailymail.co.uk/sciencetech/article-1209491/Survival-blondest-Mice-change-coat-colour-8-000-years-fool-predators.html>.

IMAGES:

http://idw-online.de/pages/de/newsimage?id=84635&size=screen

http://news.harvard.edu/gazette/story/2009/08/mice-living-in-sand-hills-quickly-evolved-lighter-coloration/