National 5 Biology Course Notes Part 4 : Adaptation, natural selection Unit 3 : Life on Earth Part 4 : Adaptation, natural selection and the evolution of species

A mutation is a change to an organism’s genetic material. Mutations A mutation is a change to an organism’s genetic material. Mutations are random. They are the only way in which new alleles can arise in a population. Mutations are usually harmful but some mutations can result in genes that give characteristics that give an individual a survival advantage. Mutations are rare but the frequency with which they happen can be increased by mutagenic agents- these include radiation, e.g. X-rays and UV radiation, and some chemicals, e.g. mustard gas. Adaptations An adaptation is an inherited characteristic that makes an organism more able to survive in it’s environment. e.g. adaptations of camel to survival in the desert Adaptations of a cactus to survival in the desert Can store large amounts of water in the gut Fat in hump releases water when broken down Copes with 30% water loss (most animals die at 10%) Can drink up to 30% of its body weight when it finds water (200L in 3 minutes) Low levels of sweat Can tolerate body temperature changes more than 3 times level that would kill us Low volume of urine Leaves are spines to reduce water loss. Stores water. Thick waxy cuticle reduces water loss Extensive roots to absorb as much water as possible

Natural selection Most organisms produce more offspring than the environment can sustain. Since there is variation within a species, some individuals have characteristics that make them better able to survive than others, e.g. taller individuals might see predators sooner, faster individuals may be more able to outrun predators, individuals with a certain colour may be better camouflaged. The best adapted individuals have a selective advantage and are more likely to survive, breed and pass on the genes that gave them the best characteristics to the next generation. This process where the best adapted survive at the expense of the others (survival of the fittest) is called natural selection.

Examples of natural selection The peppered moth There are two forms of the moth: a light coloured speckled form a dark form These differences are caused by two different alleles of a gene Peppered moths are active at night and during the day rest on surfaces such as tree trunks. The moths are eaten by birds. In unpolluted areas, the tree trunks are covered by light coloured lichens that camouflage the light forms – in these areas, the light forms have a selective advantage so less are eaten and more survive and pass on the light gene. Light moth Dark moth On lichen covered tree trunk Light moth Dark moth On soot covered tree trunk In polluted areas, the lichens are killed and tree trunks are stained with soot – in these areas dark moths are camouflaged and given a selective advantage so more dark moths survive and pass on the dark gene.

Examples of natural selection 2. Antibiotic resistance When an antibiotic is applied to a population of bacteria, most are killed but some will have genes that make them resistant to the antibiotic. The resistant bacteria survive antibiotic treatment and pass on the resistance genes to offspring. Since bacteria reproduce quickly and asexually (needing only one parent and producing offspring genetically identical to the parent), antibiotic resistance spreads quickly. Overuse of antibiotics has led to many bacteria becoming resistant.+

Speciation Speciation is the formation of one or more new species from an already existing species. The first step in speciation is a population being isolated from the rest of the species. The isolated population gradually becomes totally different from the rest of the species because: Different mutations occur in the two groups so their gene pools become more and more different Since they live in different environments, their selection pressures are not the same so the genes that give survival advantage and are passed on are different for the two groups. Eventually the two groups become so different they can no longer interbreed to produce fertile offspring – the isolated group has now become a new species. An example of speciation – Galapagos island finches These finches arrived from mainland South America and were isolated from the main population by the sea. A number of new species developed from the ancestral seed eating finches to fill niches occupied by other bird species on the mainland. Their beaks became adapted to eat different food sources as shown on the diagram.