2.2 Evolution

• Variation in traits arises as a result of mutation . Mutation is the original source of new  sequences  of  DNA  . These new sequences can be novel  alleles . Most mutations are harmful  or neutral , but in rare cases they may be beneficial to the fitness of an individual. 
• Populations produce more offspring than the environment can support, resulting in  selection pressures
• Individuals with variations that are better  suited to their environment tend to survive longer and produce more  offspring , breeding to pass on those alleles that conferred an advantage to the next generation.
• Selection results in the NON-RANDOM  increase in the frequency of advantageous alleles and the NON-RANDOM  decrease in the frequency of deleterious  alleles. 
• Sexual selection is the NON-RANDOM  process involving the selection of alleles that increase the individual’s chances of mating and producing  offspring . It may lead to sexual  dimorphism which is where two sexes of the same species exhibit different characteristics e.g. Lions, peacocks etc
• Male-male rivalry: Large size or weaponry increases access to females through conflict.  E.g. male deer use antlers to compete for females
• Female choice: Involves females assessing the fitness of males. this may result in males attempting to "attract" females through elaborate displays.
• Genetic drift is more important in small populations, as alleles are more likely to be lost from the gene  pool. 
• Bottleneck effect: Population bottlenecks occur when a population size is reduced for at least one generation. The reduced population can have lower genetic diversity.
• Founder effect: Founder effects occur through the isolation of a few members of a population from a larger population. The gene pool of the new population is not representative of that in the original gene pool. 
• A gene  pool is altered by genetic drift because certain alleles may be under-represented or over-represented  and allele  frequencies  change.
• The HW principle can be used to determine whether a change in allele  frequency is occurring in a population over time. Changes suggest evolution is occurring.
• p2 + 2pq + q2 = 1​

  p+q=1​

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  p = frequency of dominant allele ​

  q = frequency of recessive allele ​

  p2 = frequency of homozygous dominant genotype ​

  2pq = frequency of heterozygous genotype ​

  q2 = frequency of homozygous recessive genotype 
• Fitness is an indication of an individual’s ability to be successful at surviving and reproducing
• The ratio between the number of individuals of a particular genotype after selection, to those before selection.​

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  frequency of a particular genotype after selection​

  frequency of a particular genotype before selection 
• the ratio of the number of surviving offspring per individual of a particular genotype to the number of surviving offspring per individual of the most successful genotype ​

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  number of surviving offspring per individual of a particular genotype ​

  number of surviving offspring per individual of the most successful genotype 
• Co-evolution is the process by which two or more species evolve in response to selection pressures imposed by each other
• Symbiosis is the Co-evolved intimate relationships between members of two different species. 
• The impacts of these relationships can be positive (+), negative(-) or neutral (0) for the individuals involved.​

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  Mutualism, commensalism, and parasitism are types of symbiotic interactions 
• Mutualism:

  Both organisms are interdependent on each other for resources or other services. As both organisms gain from the relationship, the interaction is (+/+). ​

  Example: Clown fish and anemones
• Commensalism:

  only one of the organisms benefits (+/0). ​

  Example: Cattle egrets follow cattle herds, eating the insects disturbed by the moving cattle, the cattle gain no benefit.
• Parasitism:

  the parasite benefits in terms of energy or nutrients and the host is harmed as the result of the loss of these resources (+/-). ​

  Example: Mosquitos and humans.
• The Red Queen hypothesis states that, in a co-evolutionary  relationship , change in the traits of one species can act as a selection pressure on the other species. This means that species in these relationships must adapt to avoid extinction .