Adaptation

Adaptation describes the long-term effects of individual variation in a population of individuals. No two organisms are exactly alike and despite their close relation, the members of a species differ from one another quite considerably; superimposed on the common traits, individual variations are characteristic within each species.

These variations can be heritable or environmental. Heritable variations arise through changes, or mutations, in the DNA, leading to an altered phenotype, which can be passed on to offspring. In contrast, environmental variations arise through the influence of external factors on the growth, development and survival of an individual.

The combination of these variations leads to the adaptation of the individual to the environment in which they live. For example, the ability to drink cow’s milk in adult life arose as a result of mutation, leading to prolonged expression of the enzyme lactase (Ch 14). Individuals with this mutation could then gain nutrition, as adults, from milk. In areas where alternative sources of food were available, such as sub-Saharan Africa, this would give little benefit so the incidence of lactose tolerance in these areas is low. However, in northern Europe the climate was poorer and agricultural yields unreliable. Thus food was often scarce and there was a selective advantage to being able to use animal milk as a food source later into childhood and even adulthood. Thus, even in modern humans of northern European descent, this lactose tolerance is maintained.

Response to environment

All cells exchange material and energy with their environment. In a unicellular organism, such as a bacterium, this environment can change rapidly and is strongly affected by other organisms and nonbiological factors (e.g. alterations in the concentration of compounds, pH etc.). These factors also affect multicellular organisms. The individual cells of such an organism interact with the cells and environment surrounding them, exchanging nutrients and other compounds. For example, oxygen carried by haemoglobin in red blood cells from the lungs to the tissues is donated to the oxygen-binding protein myoglobin in muscle cells; CO₂ is taken from the muscle cells by the red blood cells back to the lungs. The multicellular organism as a whole also interacts with its environment; we as humans take food, water and oxygen from our environment and, through our actions, affect and change our environment.

Reproduction

Reproduction is an essential process for species survival and occurs through the transmission of the genetic material of an organism from one generation to the next. This genetic material can then provide the blueprint for that next generation. Reproduction can occur in two ways.

Asexual reproduction occurs when there is only one parent and therefore their progeny are genetically identical. This occurs in many prokaryotes and in some eukaryotes as well. As might be expected this process does not generate genetic diversity, something to be avoided in the growth and repair processes in an organism such as a human being. However, it does allow for reproduction when potential mates are scarce.

Sexual reproduction involves two different parents, both contributing their genetic material to the offspring. This mixing of the genes leads to a greater genetic diversity in the next generation and can be a route by which advantageous variation can arise.

Growth and development

Living organisms, particularly multicellular organisms, undergo both growth and development during their life cycle. Growth occurs when the cells of an organism increase in size or number leading to an enlargement of all or part of that organism. This is easy to visualise when looking at human beings as babies become children and then adults. It is perhaps harder to imagine how this occurs in unicellular organisms such as bacteria. However, it must be remembered that these organisms rarely exist alone; they are most often found as part of a colony that may contain a mixture of different species. Mould growing on old bread contains thousands of individuals but started out as just a few, which reproduced and increased in number to grow into the patch of mould visible to the naked eye.

Multicellular organisms also undergo changes over time in the process known as development. This includes not only growth but also differentiation and morphogenesis. Differentiation occurs when changes in the structure and function of cells alter them from generalised, non-specific cells to specialised cells with particular roles. These cells then go on to form particular tissues and organs in the process of morphogenesis. Both their genetic make-up and their environment determine the growth, development and differentiation of organisms.

Organisation and function

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