Genes, inheritance and selection Study Guide

The phenotype of an organism refers to its observable characteristics, such as its physical appearance, biochemical properties, and behaviours. These characteristics are not solely determined by an organism's genetic makeup, known as its genome, but are also significantly influenced by the environment in which the organism lives. The genome contains all the genetic information encoded in DNA, organised into genes. Each gene carries instructions for building specific proteins, which then perform various functions that contribute to the organism's traits. For example, genes determine eye colour or whether an organism will develop a certain genetic condition.

However, the environment acts as a modulator of how these genes are expressed. Environmental factors like diet, climate, exposure to toxins, light, temperature, and even social interactions can switch genes on or off, or alter the level at which they are expressed. This interaction means that two organisms with identical genomes (e.g., identical twins) can still develop different phenotypes if they are raised in different environments. A classic example is height: while genes largely determine a person's potential height, a poor diet during childhood can prevent them from reaching that potential. Similarly, the colour of Hydrangea flowers is influenced by soil pH, despite the plant's genetic predisposition for flower colour. Therefore, the development of an organism's phenotype is a complex interplay between its inherited genetic information and the various environmental factors it encounters throughout its life.

Evolution is the process by which different kinds of living organisms are thought to have developed and diversified from earlier forms during the history of the Earth. The idea of evolution is supported by a wide range of scientific evidence. One major piece of evidence comes from <b>fossils</b>. Fossils are the preserved remains or traces of organisms that lived in the past. By examining fossils found in different layers of rock, scientists can observe how organisms have changed over millions of years. For instance, the fossil record shows simpler life forms in older rock layers and more complex forms in newer layers, suggesting a gradual development over time. Furthermore, the fossil record contains 'transitional fossils' which show features of both an ancestral group and a descendant group, providing links between different species.

Another key area of evidence is <b>comparative anatomy</b>. This involves comparing the body structures of different species. Homologous structures, such as the pentadactyl limb (a limb with five digits) found in mammals, birds, reptiles, and amphibians, provide strong evidence for a common ancestor. Although the limbs are adapted for different functions (e.g., flying, swimming, walking), their underlying bone structure is remarkably similar, indicating they evolved from a common design. Conversely, analogous structures, which serve a similar function but have different origins (e.g., the wings of birds and insects), show convergent evolution, where unrelated species adapt to similar environments.

<b>DNA and molecular biology</b> offer powerful evidence at a microscopic level. Comparing the DNA sequences and protein structures of different species reveals their evolutionary relationships. The more similar the DNA or protein sequences between two species, the more recently they shared a common ancestor. For example, humans and chimpanzees share a very high percentage of their DNA, supporting the idea of a relatively recent common ancestor. This molecular evidence often corroborates findings from fossil and anatomical studies.

Finally, <b>resistance to antibiotics in bacteria and resistance to pesticides in insects</b> provide observable, real-time examples of evolution in action. When antibiotics are used, susceptible bacteria are killed, leaving behind those with mutations that confer resistance. These resistant bacteria then reproduce, leading to a population dominated by resistant individuals. This process, known as natural selection, demonstrates how populations can evolve rapidly in response to environmental pressures. Similarly, widespread use of pesticides selects for resistant insects. These contemporary examples clearly illustrate the mechanisms of evolution proposed by scientists.