10.8 changing population characteristics

selection pressures

these are factors that affect the organism’s chances of survival or reproductive success

steps of natural selection

1. organisms within a species show variation in their characteristics that are caused by differences within their genes. For example, they may have different alleles of a gene for a particular characteristic. New alleles can arise by mutation

2. organisms whose characteristics are best adapted to a selection pressure such as predation, competition or disease have a better chance of surviving and then reproducing. Less well-adapted organisms die or fail to reproduce. This is known as ‘survival or the fittest’

3. successful organisms pass the allele encoding the advantageous characteristic onto their offspring. Conversely, organisms that possess the non-advantageous allele are less likely to successfully pass it on

4. This process is repeated for every generation. Over time, the proportion of individuals with the advantageous adaptation increases. Therefore the frequency of the allele that codes for this particular characteristic increases in the population’s gene pool

5. over very long periods of time, many, many generations and often involving multiple genes, this process can lead to the evolution of a new species

antibiotic resistant bacteria

• bacteria reproduce very rapidly and so evolve in a relatively short time

• when bacteria replicate, their DNA can be altered and this usually results in the bacteria dying. However, a mutation provided resistance to antibiotics

• when bacteria were exposed to this antibiotic, resistant individuals survived and reproduced, passing the alleles for resistance on to their offspring

• non-resistant individuals died

• over time, the number of resistant individuals in the population increased

peppered moths as an example of evolution

• before the industrial evolution, most peppered moths were pale coloured

• this provided camouflage against the light-coloured tree bark, increasing their chance of survival

• those that were dark were easily spotted by birds and eaten

• the different colours were due to different alleles

• during the industrial evolution, many trees became darker

• the dark moths were better adapted as they were better camouflaged

• the dark peppered moths were more likely to and reproduce, passing on their alleles and increasing the frequency of dark moths in the population

sheep blowflies as a modern example of adaptation

• sheep blowflies lay their eggs in faecal matter around a sheep tail - the larvae then hatch creating a fatal condition called flystrike

• in the 1950s in Australia, the pesticide diazinon was used to kill the blowflies and prevent the condition. Within 6 years, the blowflies had developed a high level of resistance to the diazinon

• individual insects with resistance survived higher exposure to the insecticide and passed on this characteristic through their alleles allowing a resistant population to evolve

• they adapted so quickly because of pre-adaptation which is where an organisms existing trait is advantageous for a new situation

flavobacterium as an example of evolution

• scientists found a new strain of flavobacterium living in waste water from factories that produce nylon

• this strain of bacteria is able to digest nylon and it therefore beneficial to humans as they help to clear up factory waste

• the bacteria use enzymes to digest the nylon known as nylonases and these provide the bacteria with another source of nutrition