Natural Selection and Adaptation

• Natural selection:

  • a mechanism of evolution where the members of a population best suited to their environment have the best chance of surviving to reproduce and so pass on beneficial alleles
  • results in a change in the allele frequency
• Evolution:

  • a change in the allele frequencies and inherited characteristics of a population over time, which may result in the formation of a new species
  • evolution can occur due to natural selection or genetic drift
  • genetic drift is the change in the allele frequency due to chance
  • the founder effect and genetic bottlenecks are both examples of genetic drift
• Natural selection:

  • genetic diversity is the number of different alleles of genes in a population - so it is all the genetic characteristics in the genetic makeup of a species

  genetic diversity is caused by;

  • mutations
  • meiosis (crossing over and independent segregation)
  • random fusion of gametes
• Genetic diversity:

  • genetic diversity serves as a way for populations to adapt to changing environments and so is a factor in enabling natural selection to occur
  • a key feature of Darwin's theory of Natural Selection was that some individuals are more likely to reproduce than others (they all show differential reproductive success). This is due to their genotype
  • however, although it is the alleles that are passed on, it is the phenotype that are subject to selection. Organisms with advantageous genotypes and therefore phenotypes are more likely to survive the current selective pressures. They are more likely to have more offspring than others
• Natural selection in the evolution of populations:

  • organisms are subject to various selection pressures e.g. predation, competition (intraspecific and interspecific), disease, environmental factors
  • there is variation between organisms due to random mutation which can result in a new allele of a gene
  • the new allele could be advantageous in the environment
  • individuals with the beneficial allele are more likely to survive, reproduce and pass on the advantageous allele to their offspring
  • as a result, over many generations, the frequency of this beneficial allele increases in the population
• Natural selection leads to better adapted species:

  adaptations may be;

  • anatomical (e.g. fur colour in mice or beak size in finches)
  • physiological (e.g. having haemoglobin with a different affinity for oxygen or having the ability to produce venom)
  • behavioural (e.g. tool use in chimps or elaborate songs in whales)
• Types of selection:

  • natural selection usually acts against one or more of the extremes in a range of phenotypes. As a result, a certain phenotype (or phenotypes) becomes rare, and an alternative phenotype becomes common. Selection can cause the mode and/or distribution of the phenotypes to change

  types of selection include;

  • directional selection
  • stabilising selection
  • disruptive selection
• Directional selection:

  • this type of natural selection acts against one of the extremes in a range of phenotypes
  • as a result, one phenotype becomes rare and an alternative phenotype becomes more common. This results in an increase in the frequency of one phenotype relative to another
  • this often happens when there are two different alleles for a gene, and one is more beneficial than the other so become more frequent in a population
  • the mode changes as a result
  • example - bacterial antibiotic resistance shows directional selection
• Stabilising selection:

  • this type of selection acts against both the extremes in a range of phenotypes i.e. it favours the middle phenotypes and so acts to prevent change
  • the mode stays the same, but the range of phenotypes decreases
  • example - human birth weight has become stabilised around the optimum size for maximum survival
• Disruptive selection:

  • in this type of selection, individuals with both extremes of a phenotype have a selective advantage over those in the middle
  • although the least common form of selection, it is the most important in bringing about evolutionary change and thus speciation (development of a new species)
  • example - Pacific Coho salmon where large males and small males have a selective advantage over middle-sized males in passing on their alleles to the next generation by being more successful in fertilising eggs laid by females on the riverbed
• Natural selection can lead to antibiotic resistance:

  • there is a mutation in a bacterial cell in a colony
  • this could result in a bacterial cell with an allele for resistance for a particular antibiotic
  • when this antibiotic is used, the resistant bacterial cell would survive and reproduce by binary fission
  • all offspring would inherit the allele for resistance and the frequency of the allele increases
  • this process would continue until the whole bacterial population are resistant to the antibiotic
• Required practical 6 - use of aseptic techniques to investigate the effect of antimicrobial substances of microbial growth:

  • bacteria can be grown on a plate of agar in a petri dish. This lawn of bacteria would be produced using aseptic techniques
  • the effect of different antibiotics can be investigated by using paper discs soaked in different antibiotics and placing them around the agar plate, then incubating
  • the incubator maintains an optimum temperature for bacterial growth. Incubating upside down prevents condensation falling on bacterial colonies and interfering with growth
• Required practical 6 - use of aseptic techniques to investigate the effect of antimicrobial substances of microbial growth:

  • the antibiotics will diffuse outwards from each paper disc into the agar with bacteria. If the bacteria being investigated is vulnerable to an antibiotic, then a clear area will be visible around the disc. There are no bacteria present in the clear area as they have been killed by that antibiotic
  • more effective antibiotics require a lower concentration to kill bacteria and so they will produce larger clearer zones
  • if a bacterium is completely resistant to an antibiotics, then there will be no clear zone around that paper disc
  • the clear area is called the zone of inhibition
  • the lid should not be removed after incubation. The dish should be placed in a plastic bag and sterilised using an autoclave (steams at 120°C under high pressure to kill bacteria)
• Required practical 6 - use of aseptic techniques to investigate the effect of antimicrobial substances of microbial growth:

  when investigating the effect of antimicrobial substances on microbial growth, it is essential that aseptic techniques are used. Aseptic techniques ensure that;

  • the microbes being investigated don't escape and contaminate the environment / lab worker
  • the microbes don't become contaminated by other unwanted / pathogenic microbes
• Aseptic techniques:

  • washing hands thoroughly with soap and disinfectant - to remove/kill microbes
  • wearing gloves/goggles/apron - to prevent contamination
  • having a lit Bunsen burner in the room - to create a convection (upward) current in the room to draw away airborne bacteria
  • disinfecting work surfaces with disinfectant or alcohol - to kill microbes / prevent contamination
  • flaming culture bottlenecks to prevent contamination - to kill microbes / prevent contamination
• Aseptic techniques:

  • using flamed/sterile loops when transferring cultures - to kill microbes / prevent contamination
  • sterilising or disposing of all used equipment - to kill microbes / dispose of any contaminated equipment
  • only removing petri dish lids when necessary / only lift the lid of the petri dish slightly - to prevent entry of any other microbes / prevent contamination
  • not allowing the growth of microorganisms at body temperature - do not want to encourage the growth of any other harmful bacteria
  • do not tape around the entire dish - this would prevent oxygen entering and so promotes the growth of more harmful anaerobic bacteria
• Factors that reduce genetic diversity:

  • the founder effect
  • genetic bottlenecks
  • inbreeding
• The founder effect:

  • it sometimes happen that a few organisms from a population become isolated from the rest of the population
  • for example, when a small number of individuals emigrate from the parent population and colonise a new region such as an island. These individuals will carry with them only a small fraction of the alleles present in the parent population. This leads to a reduction in genetic diversity / a smaller gene pool as there is a reduction in the variety of alleles
• Genetic bottlenecks:

  • a population is said to go through a genetic bottleneck when it is temporarily reduced to a very small number when most individuals are killed. This could be due to a chance event such as disease, volcanic eruption, flooding or interference by humans e.g. hunting
  • the few survivors will contain only a small proportion of the alleles present in the original population. This may result in a dramatic reduction in genetic diversity / a smaller gene pool
• Inbreeding:

  • if organisms are only able to breed with a limited number of individuals, then this can reduce genetic diversity further and sometimes amplify genetic disorders
  • for example, many pedigree dog varieties have high rates of genetic disorders due to inbreeding. This includes hereditary deafness in Dalmatians and digestive problems in Yorkshire terriers
• Speciation:

  • selection changes gene pools, causing a species to change greatly over time
  • the greatest change occurs though when existing species gives rise to two or more new species i.e. at the branching points in the evolutionary tree
  • this process is called speciation
  • it can arise through disruptive selection
  • speciation is a process whereby one gene pool gives rise to more than one gene pool

  this occurs because of reproductive separation of two populations

  • not all organisms within a population will be able to interbreed, so there is no gene flow
  • therefore, when there are new mutations and changes in frequency of alleles in each population, these are not passed to the other population
  • over time their gene pools become so different they are no longer classified as the same species
• Speciation:

  there are two types of speciation:

  • allopatric speciation
  • sympatric speciation
• Allopatric speciation:

  this type of speciation arises due to geographical isolation of a part of a population from a main population e.g. a mountain range, river or separation of land masses;

  • the original population becomes geographically isolated so there is no gene flow between the 2 populations (reproductive isolation)
  • random mutations occur in each separate population leading to variation
  • each population experiences different selection / environmental pressures
  • different alleles will be advantageous in the different populations and will be selected for
  • those with advantageous alleles are more likely to survive, reproduce and pass on the advantageous allele to their offspring
  • allele frequencies change in the different populations
  • eventually, the gene pools of the 2 populations is so different that they can no longer interbreed to give fertile offspring - they are 2 separate species
• Sympatric speciation:

  this type of speciation arises when there is no geographical isolation of populations (the organisms are in the same area), but a small number of individuals within the population become reproductively isolated from the others;

  • random mutations cause reproductive isolation of some members of the population - no gene flow
  • different alleles will be selected for and passed on
  • allele frequencies change
  • results in disruptive selection
  • eventually, the gene pools are so different that they can no longer interbreed to give fertile offspring - they are 2 separate species
• Sympatric speciation:

  in sympatric speciation, the reproductive isolation of some individuals in a population can happen due to a variety of reasons. Some members of the population acquire mutations that cause;

  • reproduction / mating / 'coming into season' at different times of the year to the rest of the population
  • anatomical differences for some individuals that may physically prevent mating occurring
  • different courtship patterns / behaviours
  • gamete incompatibility - e.g. a change in gamete receptor protein may prevent it being recognised and fusing, with another gamete without the complementary mutation
  • polyploidy - a change in the number of chromosomes. Gametes with differences in the number of chromosomes cannot successfully fuse
• Sympatric speciation:

  • sympatric speciation is very rare in organisms that use sexual reproduction, as it requires for complementary random mutations to occur in at least one male and one female within a population at the same time
  • organisms that use asexual reproduction e.g. plants, do not have the same problem, therefore conditions such as polyploidy are more common
• Genetic drift:

  • evolution (a change in allele frequency of a population over time) occurs as a result of natural selection and genetic drift
  • genetic drift is the idea that allele frequencies can change simply due to chance
  • some individuals fail to survive or reproduce simply due due to bad luck, not because they are poorly adapted
  • this is only prevalent in small populations as chance has a larger influence than in larger populations
• Genetic drift:

  • individuals within a population show variation in their phenotypes
  • by chance, the allele for one genotype is passed on to more offspring than others
  • the number of individuals with this allele increases
  • if by chance the same allele is passed on more often again and again, it can lead to evolution as the allele becomes more common in the population

  genetic drift can lead to differences in allele frequency between two isolated populations. If enough differences in allele frequency accumulate over time, this could eventually lead to speciation
• Natural selection and genetic drift:

  • natural selection and genetic drift usually work alongside each other to drive evolution, but one can drive evolution more depending on the population size
  • evolution by genetic drift usually has a bigger effect in smaller populations
  • in larger populations, chance factors tend to even out across the whole population, so evolution by natural selection will have the biggest impact

  the founder effect and genetic bottleneck are both example of genetic drift