genetic diversity and adaptions

• genetic diversity is the number of different alleles of genes in a population
• genetic variation is the small differences in DNA base sequences between individual organisms within a species population

  genetic variation transferred from one generation to the next and result in genetic diversity
• mutation means new alleles contributed to genetic diverstiy or size of gene pool

  gene pool is the different alleles of genes of all individuals in a population at one time

  • The new allele may be advantageous, disadvantageous or have no apparent effect on phenotype (due to the fact that the genetic code is degenerate
  • New alleles are not always seen in the individual that they first occur in
  • They can remain hidden (not expressed) within a population for several generations before they contribute to phenotypic variation
• There needs to be some level of genetic diversity within a population for natural selection to occur
• Differences in the alleles possessed by individuals within a population result in differences in phenotypes
• Environmental factors

  Affect the chance of survival of an organism; they, therefore, act as a selection pressure
• Selection pressures increase the chance of individuals with a specific phenotype surviving and reproducing over others
• Fitness


  The ability of an organism to survive and pass on its alleles to offspring
• Organisms with higher fitness possess adaptations that make them better suited to their environment
• Population with a large gene pool or high genetic diversity


  • Has a strong ability to adapt to change
• Population with a small gene pool or very low genetic diversity

  • Much less able to adapt to changes in the environment and so can become vulnerable to extinction
• natural selection

  • Genetic variation exists within populations due to the presence of different alleles
  • There is differential reproductive success between the organisms with different alleles of the same gene
  • Under certain environmental conditions, individuals with certain alleles will have an increased chance of survival and reproduction
  • New alleles can arise in populations through random mutation
  • Natural selection can cause the frequency of alleles in a population to change over time
• Principles of Natural Selection

  • Random mutation can produce new alleles of a gene
  • Many mutations are harmful or neutral but, under certain environmental conditions, the new alleles may benefit their possessor, leading to an increased chance of survival and increased reproductive success
  • The advantageous allele is passed onto the next generation
  • As a result, over several generations, the new allele will increase in frequency in the population
• natural selection

  Darwin's theory to explain evolution, process by which organisms better adapt themselves to environment survive and reproduce and pass on their advantageous alleles to offspring

  • Natural selection causes a change in allele frequencies over time
  • Selection pressures (caused by the environment an organism is in) increase the likelihood that certain individuals with specific alleles survive to reproductive age, enabling them to pass on their alleles to their offspring
• other factors or processes that can affect allele frequencies in a population:

  • The founder effect
  • Genetic drift
  • The bottleneck effect
• Gene pool


  The complete range of DNA sequences (alleles) that exist in all the individuals of a population or species
• The Founder effect

  • The Founder effect occurs when only a small number of individuals from a large parent population start a new population
  • As the new population is made up of only a few individuals from the original population only some of the total alleles from the parent population will be present
  • not all of the gene pool is present in the smaller population
  • Which alleles end up in the new founding population is completely up to chance
  • As a result, the changes in allele frequencies may occur in a different direction for the new small population vs the larger parent population
• Genetic drift

  • When a population is significantly small, chance can affect which alleles get passed onto the next generation
  • Over time some alleles can be lost or favoured purely by chance
  • When there is a gradual change in allele frequencies in a small population due to chance and not natural selection then genetic drift is occurring
• Bottleneck effect

  • It occurs when a previously large population suffers a dramatic fall in numbers
  • A major environmental event can massively reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost
  • The surviving individuals end up breeding and reproducing with close relatives
• processes that cause allele change

  • natural selection: selection pressures producing gradual change in allele frequencies over several generations
  • founder effect: changes in allele frequencies occurring in different direction for new isolated small population in comparison to large parent population due to chance
  • genetic drift: gradual change in allele frequencies in small population due to chance and not natural selection
  • bottleneck effect: reduction in gene pool of population due to dramatic decrease in population size
• Environmental factors that affect the chance of survival of an organism are selection pressures. These selection pressures can have different effects on the allele frequencies of a population through natural selection

  • There are different types of selection:
  • Stabilising
  • Directional
• Stabilising selection

  • Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations
  • This means things stay as they are unless there is a change in the environment
  • A classic example of stabilising selection can be seen in human birth weights
  • Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights
• Directional selection

  • Directional selection is natural selection that produces a gradual change in allele frequencies over several generations
  • This usually happens when there is a change in environment/selection pressures or a new allele has appeared in the population that is advantageous
• example of directional pressures

  • For example, antibiotic-resistant bacteria strains are becoming more common due to the overuse of antibiotics
  • The presence of antibiotics is a selection pressure
  • Mutations are occurring in bacteria populations randomly
  • A mutation arises that confers antibiotic resistance - it is a beneficial allele
  • Bacteria with this mutation are more likely to survive and reproduce
  • Most bacteria without the resistance mutation die
  • Over generations, this leads to an increase in the frequency of beneficial allele that produces antibiotic resistance
• • Environmental factors that affect the chance of survival of an organism are selection pressures
  • For example, there could be high competition for food between lions if there is not plentiful prey available; this environmental factor ‘selects’ for faster, more powerful lions that are better hunters
• • Certain alleles within a species population can produce features that make an organism better suited to its environment - adaptations
  • When new alleles of genes result from mutation there is the potential for relatively rapid change in a species if their environment changes
• Natural selection will select for favourable alleles that produce adaptations
• Natural selection will select against unfavourable alleles
• • This means that over time natural selection will cause favourable allele frequencies to increase and unfavourable allele frequencies to decrease, making the species better adapted to their environment
• Adaptations enable organisms to survive in the conditions in which they normally live
• Types of Adaptations

  • Anatomical
  • Physiological
  • Behavioural
• Anatomical adaptations

  Structural/physical feature
• Anatomical adaptation

  • The white fur of a polar bear provides camouflage in the snow so it has less chance of being detected by prey
• Physiological adaptations

  Biological processes within the organism
• Physiological adaptation

  • Mosquitos produce chemicals that stop the animal's blood clotting when they bite, so that they can feed more easily
• Behavioural adaptations


  The way an organism behaves
• Behavioural adaptation

  • Cold-blooded reptiles bask in the sun to absorb heat
• • Evolution is the change in adaptive features of a population over time as a result of natural selection
• • If the environment changes or a chance mutation produces a new allele, selection pressures may favour individuals with different characteristics or with the new allele
  • Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
  • This means whole populations of organisms become better suited to their environment
• • If two populations of one species are isolated from each other and become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species
  • The formation of new species (speciation) from pre-existing species over time, is a result of accumulated genetic differences
  • Evolution drives speciation and so is responsible for the large number of species that exist on earth