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• Sound travels through air at 340 m/s at sea level at 20°C. The speed of sound varies according to the temperature and material through which it travels. At higher temperatures, particles have more kinetic energy so they can compress more easily. Therefore, sound travels faster at higher temperatures. The more closely packed the particles in a substance, the faster the sound wave travels. The speed of sound is affected by the closeness of the particles in a material and how far they can move. For example, the particles in water are much closer together than in air so they collide more easily. Thus, they transfer energy more quickly. This means a sound wave can travel faster through water. The particles in a solid are packed even closer together. Therefore, at a constant temperature, sound waves travel the fastest through solids and the slowest through gases. If you are a drummer, you have probably been told more than once to ‘keep the noise down!’ But is there somewhere you could play your drum kit as hard and as loud as possible with absolutely no sound being heard? The answer is ‘yes’, but it is not a place you can get to easily. A famous sci-fi movie was advertised with the tagline ‘In space, no one can hear you scream.’ The moviemakers were right. In outer space, you can play your drum kit without anyone hearing a sound – but you wouldn’t hear it either. You could even see an explosion without hearing a thing. This is because sound needs something to travel through; it needs a substance (or medium) that contains particles that can be compressed to create the sound waves. The medium could be a solid, a liquid or a gas. When a wave reaches a boundary between two media, it might return into its original medium. The returning of a wave into its original medium is called reflection of wave. In all wars since World War I, reflected waves have been used to detect enemy submarines under water. In a similar way to radar (which sends out radio waves), sonar sends out sound waves and records how long the sound takes to reflect or echo back after striking an object. The longer the sound takes to return, the further away the object is. An exact location can be calculated by knowing how fast sound travels in water. This information, along with the time taken for the sound to return, allows the exact location of a submarine to be determined. Sonar is widely used today and can help map the ocean floor, check the depth of water and locate schools of fish.
• Evolution is the permanent change in the frequency of alleles in a population due to natural selection. Mutations introduce new alleles into the gene pool. Selection pressures cause these new phenotypes to survive or die out. If an organism displays traits that make it suited to its environment, then it is able to mate and produce offspring. The offspring will have the same survival characteristics (and alleles) as their parent. This gradually changes the frequency of alleles in the gene pool. Individuals of the same population generally have the same number and types of genes but different alleles (variations of the genes). For example, all humans will have the gene for eye colour, but the alleles they have for this gene may be blue, brown or even hazel. New alleles arise because of small changes in the DNA sequence. Some mutations cause variations in the physical appearance (phenotype) of the individual. For example, it was a single mutation about 6000–10,000 years ago that resulted in one of our ancestors having blue eyes. All the different types of genes in the entire population can be thought of as a gene pool – a pool of genetic information. The gene pool includes all the alleles for all the genes in the population. New alleles arise through changes (or mutations) in the DNA that makes up the genes. A mutation may give an individual an advantage, making them better able to survive than other individuals within the population. This means they have a greater chance of mating and passing their genetic advantage on to their offspring. Populations are always evolving. The frequency of an allele is how common that allele is within a population. The allele frequency is affected by environmental conditions. If the environmental conditions are favourable, then more of that allele is passed on to the next generation. When a variation within a species is favoured by the environmental conditions, it is referred to as an adaptation. Variations provide ‘options’ for the species when environmental conditions change. Although individual organisms may die, some members of the population with the favourable phenotype will survive and continue the species gene pool. Under normal conditions, genes in a given population are exchanged through breeding. This means the genes will be passed from one generation to the next as different families or groups in the population choose partners and mate. This is called gene flow. However, the gene flow is interrupted if the population becomes divided into two groups so that the groups experience isolation from each other. Isolation is not always physical. Temporal isolation occurs when different populations reproduce at different times. Behavioural isolation occurs when there are differences in behaviour in a species, such as mating rituals. Mechanical isolation occurs when two potential mates are physically incompatible. For example, flowers of certain plants vary in size and shape; this excludes certain pollinators from being able to access them. Over time, different selection pressures occur in the two groups. Given enough time for evolution to occur, the two populations may become so different that they are incapable of interbreeding to produce fertile offspring should they ever come together again. They become reproductively isolated and therefore are different species (speciation). The two species have then diverged. Allopatric speciation occurs when species divide as a result of geographic isolation, for example, a mountain range might form or a river might widen. Continental drift also results in geographic isolation. These changes lead to different environmental selective pressures. An example of this is thought to have occurred among rock wallabies in Australia. There are approximately 20 different species of rock wallaby in Australia, which are separated by long distances. Common structures that are found in different species often have a similar pattern but different function. These structures are known as homologous structures. Analogous structures are structures in organisms that perform the same function but are structurally different (suggesting no recent common ancestor). For example, a dolphin (mammal) and a shark (fish) have the same environmental selection pressures. This is an example of convergent evolution. The wings of birds and butterflies are also analogous structures. Sympatric speciation is where new species arise within an existing species that share the same geographical location. Sympatric speciation is more common in plants than animals and may be the result of the failure of chromosome separation during meiosis or the cross between two species. The new species that results from this cannot breed with the parent species, though plants may be able to reproduce asexually.

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