Natural Selection, Evolution and Classification

• Darwin's observations during his five-year trip on the Beagle

  • He noticed that many different, yet ecologically similar, animal and plant species occupied different, yet ecologically similar, habitats around the globe
  • On the Galápagos Islands, Darwin noticed that the traits of many organisms-such as the shell shapes of tortoises-varied from island to island. He noticed that different, yet related, animal and plant species occupied different habitats within a local area
  • Darwin collected fossils, the preserved remains of ancient organisms. He noticed that some fossils of extinct species resembled living species
• Geologists James Hutton and Charles Lyell


  • They argued that Earth is many millions of years old
  • They also argued that the processes changing Earth today, like volcanism and erosion, are the same ones that changed Earth in the past
• Lamarck's evolutionary hypothesis


  Lamarck hypothesized that all organisms have an inborn drive to become more complex and perfect. According to Lamarck, an organism could gain or lose traits during its lifetime by using or not using certain organs. Lamarck also hypothesized that acquired characteristics could be passed on to an organism's offspring leading to evolution of the species
• Thomas Malthus


  Thought that if the human population continued to grow unchecked, it would run out of living space and food
• Artificial selection


  Plant and animal breeders in Darwin's time used a process now known as artificial selection to improve their crops and livestock. In artificial selection, nature provides the variations, and humans select those they find desirable
• Darwin's theory of evolution by natural selection

  1. More offspring are produced than can survive to reproduce. There is competition for limited resources, or a struggle for existence

  2. Individuals exhibit variation in their traits and some of these differences can be passed on to their offspring

  3. Inherited traits that increase an organism's ability to survive and reproduce are called adaptations

  4. Differences among adaptations affect an individual's fitness-the ability to survive and reproduce in a specific environment

  5. Only the fittest organisms live to reproduce and pass on their adaptive traits to offspring. This is known as the survival of the fittest

  6. From generation to generation, populations continue to evolve as they become better adapted, or as their environment changes
• Gene pool

  Members of a population share a common group of genes, called a gene pool.
• Sources of genetic variation

  • Mutations
  • Genetic recombination during sexual reproduction
  • Lateral gene transfer
• Single-gene trait


  Controlled by one gene
• Polygenic trait


  Controlled by two or more genes, and each gene often has two or more alleles
• How natural selection works on single-gene traits


  Can lead to changes in allele frequencies and changes in phenotype frequencies
• How natural selection works on polygenic traits

  1. Populations often exhibit a range of phenotypes for a trait. When graphed, this range usually forms a bell curve, with fewer individuals exhibiting the extreme phenotypes than those with the average

  2. Directional selection takes place when individuals at one end of the bell curve have higher fitness than those near the middle or at the other end of the curve

  3. Stabilizing selection takes place when individuals near the middle of the curve have higher fitness than individuals at either end

  4. Disruptive selection takes place when individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle
• Genetic drift


  In small populations, alleles can become more or less common simply by chance. This kind of change in allele frequency is called genetic drift
• Bottleneck effect


  A change in allele frequency following a dramatic reduction in the size of a population
• Founder effect


  A change in allele frequency that may occur when a few individuals from a population migrate to and colonize a new habitat
• Hardy-Weinberg Principle


  States that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change. These factors include: non-random mating, small population size, immigration or emigration, mutations, and natural selection
• Sexual selection


  The process in which an individual chooses its mate based on heritable traits (such as size or strength)
• Speciation


  For one species to evolve into two new species, the gene pools of two populations must become separated, or reproductively isolated. Reproductive isolation occurs when members of two populations do not interbreed and produce fertile offspring. Reproductive isolation can develop through behavioral, geographic, or temporal isolation
• Speciation in Galápagos finches


  A few finches may have flown from mainland South America to one of the islands. There, they survived and reproduced. Some birds may have crossed to a second island, and the two populations became geographically isolated. Seed sizes on the second island could have favored birds with larger beaks, so the population on the second island evolved into a population with larger beaks. Eventually, these large-beaked birds became reproductively isolated and evolved into a new species
• Binomial nomenclature


  Each species is assigned a two-part scientific name: the first part refers to the genus, or a group of similar species, and the second part is unique to each species
• Linnaean classification system levels

  • Species
  • Genus
  • Family
  • Order
  • Class
  • Phylum
  • Kingdom
• Evolutionary classification


  Places organisms into higher taxa whose members are more closely related to one another than they are to members of any other group. The larger the taxon, the further back in time all of its members shared a common ancestor
• Clade


  A group of species that includes a single common ancestor and all descendants of that ancestor. A clade must be a monophyletic group
• Cladogram


  A diagram that shows how species and higher taxa are related to each other. It shows how evolutionary lines, or lineages, branched off from common ancestors. The place where the ancestral lineage splits is called a fork, or a node. Nodes represent the point where new lineages last shared a common ancestor. The bottom of the diagram, or the root, represents the ancestor shared by all of the organisms on the cladogram
• In general, the more derived genetic characters two species share, the more recently the species shared a common ancestor and the more closely related they are