Ecology is a branch of biology that is primarily interested in how organisms in the wild interact with one another and with their physical environment. Historically, these interactions were studied through field observations and experimental manipulations. These provided phenotypic data, which are based on one or more aspects of an organism's morphology, physiology, biochemistry or behaviour. What we may think of as traditional ecological studies have greatly enhanced our knowledge of many different species, and have made invaluable contributions to our understanding of the processes that maintain ecosystems.
At the same time, when used on their own, phenotypic data have some limitations. We may suspect that a dwindling butterfly population, for example, is suffering from low genetic diversity, which in turn may leave it particularly susceptible to pests and pathogens. If we have only phenotypic data then we may try to infer genetic diversity from a variable morphological character such as wing pattern, the idea being that morphologically diverse populations will also be genetically diverse. We may also use what appear to be population-specific wing patterns to track the movements of individuals, which can be important because immigrants will bring in new genes and therefore could increase the genetic diversity of a population. There is, however, a potential problem with using phenotypic data to infer the genetic variation of populations and the origins of individuals: although some physical characteristics are under strict genetic control, the influence of environmental conditions means that there is usually no overall one-to-one relationship between an organism's genotype (set of genes) and its phenotype. The wing patterns of African butterflies in the genus Bicyclus, for example, will vary depending on the amount of rainfall during their larval development period; as a result, the same genotype can give rise to either a wet season form or a dry season form (Roskam and Brakefield, 1999).
The potential for a single genotype to develop into multiple alternative phenotypes under different environmental conditions is known as phenotypic plasticity. A spectacular example of phenotypic plasticity is found in the oak caterpillar Nemoria arizonaria that lives in the southwest USA and feeds on a few species of oaks in the genus Quercus. The morphology of the caterpillars varies, depending on which part of the tree it feeds on. Caterpillars that eat catkins (inflorescences) camouflage themselves by developing into catkin-mimics, whereas those feeding on leaves will develop into twig mimics. Experiments have shown that it is diet alone that triggers this developmental response (Greene, 1996). The difference in morphology between twig-mimics and catkin-mimics is so pronounced that for many years they were believed to be two different species. There
Table 1.1 Some examples of how environmental factors can influence phenotypic traits, leading to phenotypic plasticity
Growth patterns in plants
Temperature during embryonic development
Soil nutrients and water availability
Migration Age and nutritional between host quality of plants host plants
Feeding-related Food availability morphology
Eggs of the American snapping turtle Chelydra serpentina develop primarily into females at cool temperatures, primarily into males at moderate temperatures, and exclusively into females at warm temperatures (Ewert, Lang and Nelson, 2005) Southern coastal violet (Viola septemloba) allocated a greater proportion of biomass to roots and rhizomes in poor-quality environments (Moriuchi and Winn, 2005) Dandelions (Taraxacum officinale) produce larger leaves under conditions of relatively strong light intensity (Brock, Weinig and Galen, 2005) Diamond-back moths (Plutella xylostella) are most likely to migrate as adults if the juvenile stage feed on mature plants (Campos, Schoereder and Sperber, 2004). Sea-urchin larvae (Strongylocentrotus purpuratus and S. franciscanus) produce longer food-gathering arms and smaller stomachs when food is scarce (Miner, 2005) The plumage of male house finches (Carpodacus mexicanus) shows varying degrees of red, orange and yellow depending on the carotenoids in each bird's diet (Hill, Inouye and Montgomerie, 2002)
is also a behavioural component to these phenotypes, because if either is placed on a part of the tree that it does not normally frequent, the catkin-mimics will seek out catkins against which to disguise themselves, and the twig-mimics will seek out leaves or twigs. Some other examples of phenotypic plasticity are given in Table 1.1.
Phenotypic plasticity can lead to overestimates of genetic variation when these are based on morphological variation. In addition, phenotypic plasticity may obscure the movements of individuals and their genes between populations if it causes the offspring of immigrants to bear a closer resemblance to individuals in their natal population than to their parents. Complex interactions between genotype, phenotype and environment provided an important reason why biologists sought long and hard to find a reliable way to genotype wild organisms; genetic data would, at the very least, allow them to directly quantify genetic variation, and to track the movements of genes -- and therefore individuals or gametes -- between populations. The first milestone in this quest occurred around 40 years ago, when researchers discovered how to quantify individual genetic variation by identifying structural differences in proteins (Harris, 1966; Lewontin and Hubby, 1966). This discovery is considered by many to mark the birth of molecular ecology.
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