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.
