Evolution

• Mechanisms of Evolutionary Change

  • Artificial Selection
  • Natural Selection
  • Genetic Drift
  • Mutations
  • Gene Flow
• Evolution

  The theory of development from earlier forms; the process by which all species develop from earlier forms of life; the process by which new species are formed from pre-existing ones over very long periods of time
• Population genetics studies the different "forces" that might lead to changes in the distribution and frequencies of alleles, in other words, to evolution
• Allele frequencies

  Measure the genetic diversity of a species population or the richness of its gene pool
• Allele frequency is a measure of the relative frequency of an allele on a genetic locus in a population
• In a population, allele frequencies are a reflection of genetic diversity; changes in allele frequencies over time can indicate genetic drift or new mutations
• Forces of evolution that can change allele frequencies

  • Natural Selection
  • Genetic Drift
  • Mutations
  • Gene Flow
• Mutations


  The ultimate source of new alleles in a gene pool
• The Hardy-Weinberg equation is a mathematical equation used to calculate the genetic variation of a population at equilibrium
• The Hardy-Weinberg equilibrium states that the amount of genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors
• Artificial Selection

  Can be done through selective breeding; individuals with desirable traits are bred to increase the chances of having offspring with the same desirable traits
• Artificial selection is the process where people select which organisms get to reproduce, causing major changes in the features of plants and animals
• Evolution through artificial selection

  • Common vegetables cultivated from forms of wild mustard; variety of dog breeds due to targeted selection of particular traits
• Natural Selection


  Leads to an evolutionary change when some individuals with certain traits in a population have a higher survival and reproductive rate than others and pass on these inheritable genetic features to their offspring
• Natural selection is the process through which species adapt to their environments and is the engine that drives evolution
• Natural selection operates by differential reproductive success (fitness) of individuals
• Natural selection

  1. Process through which populations of living organisms adapt and change

  2. Can only take place if there is variation or differences among individuals in a population
• Individuals with adaptive traits are more likely to survive and reproduce
• Individuals pass adaptive traits on to their offspring
• Advantageous traits become more common in the population over time through natural selection
• Natural selection

  Can lead to speciation, where one species gives rise to a new and distinctly different species
• Species may be defined as a group of organisms which share a large number of common characteristics and can interbreed to produce fertile offspring
• If a group of organisms becomes sufficiently different over time, they may no longer interbreed to produce fertile offspring, becoming two distinct species
• Genetic Drift

  1. Random fluctuations in the frequency of appearance of a gene, usually in a small population

  2. May cause gene variants to disappear completely, reducing genetic variability

  3. Not driven by environmental or adaptive pressures like natural selection
• Genetic drift is a stochastic process that influences or changes allele frequency within a population due to sampling error from generation to generation
• Some alleles may be completely lost within a generation due to genetic drift, even if they are beneficial traits
• Population bottleneck and founder effect are examples of random drift that can have significant effects in small populations
• Bottleneck effect occurs when there is a sudden sharp decline in a population's size, leading to a reduction in genetic diversity
• Founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals cleaved from a larger population
• Founder effect results in the new population lacking the genetic diversity of the previous one, leading to certain genetic diseases becoming more prevalent
• Natural selection vs. Genetic Drift

  Genetic drift is a random process, while natural selection is non-random and adaptive
• In bacteria, natural selection has led to an increase in the frequency of alleles that confer antibiotic resistance, increasing survival rates
• Biological evolution


  Any heritable change in a population of organisms over time, involving populations, not individuals
• Evolution occurs when there is a change in gene frequency within a population over time
• Genetic differences in evolution are heritable and can be passed on to the next generation
• Evolution is descent with modification from a common ancestor
• Speciation


  Creation of new and distinct biological species by branching off from the ancestral population
• Geographically isolated populations

  No longer interbreed, leading to differences building up between the populations
• Homologous structures provide evidence for common ancestry, while analogous structures show that similar selective pressures can produce similar adaptations
• Similarities and differences among biological molecules can be used to determine species' relatedness