Natural Selection

• Darwin's theory of evolution

  Explains how modern organisms have evolved over long periods of time through descent from common ancestors
• Darwin's observations on the Beagle


  • Noticed many different, yet ecologically similar, animal and plant species occupied different, yet ecologically similar, habitats around the globe
  • On the Galápagos Islands, noticed that the traits of many organisms-such as the shell shapes of tortoises-varied from island to island
  • Noticed that different, yet related, animal and plant species occupied different habitats within a local area
  • Collected fossils and noticed that some fossils of extinct species resembled living species
• Darwin's findings led him to think that species are not fixed and that they could change by some natural process
• Beliefs about Earth and life before Darwin


  • Most Europeans believed that Earth and all its life forms were only a few thousand years old and had not changed very much in that time
• Influence of other scientists on Darwin


  • Geologists James Hutton and Charles Lyell 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
• Knowing that Earth could change over time helped Darwin realize that species might change as well. Knowing that Earth was very old convinced Darwin that there had been enough time for life to evolve
• Lamarck's evolutionary hypothesis


  • All organisms have an inborn drive to become more complex and perfect
  • An organism could gain or lose traits during its lifetime by using or not using certain organs
  • Acquired characteristics could be passed on to an organism's offspring leading to evolution of the species
• Scientists now know that most of Lamarck's ideas about evolution are incorrect. However, he correctly suggested that life is not fixed and was the first to offer a natural and scientific explanation for evolution. Further, he recognized that an organism's traits are linked to its environment
• Thomas Malthus' ideas


  If the human population continued to grow unchecked, it would run out of living space and food
• Darwin realized that this was true of all organisms, not just humans
• 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 experimented with artificial selection. The results from his experiments indicated natural variation was very important because it provided the raw material for evolution
• 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
• From generation to generation, populations continue to evolve as they become better adapted, or as their environment changes
• Common descent


  All species are descended, with modification, from common ancestors. Through descent with modification, all organisms-living and extinct-are linked on a single tree of life
• Genetics and evolutionary theory


  • Traits are controlled by genes and many genes have at least two forms, or alleles
  • The combination of different alleles is an individual's genotype
  • Natural selection acts on phenotype, not genotype
• Genetic variation and evolution

  • Studied in populations. Members of a population share a common group of genes, called a gene pool
  • Allele frequency is the number of times an allele occurs in a gene pool compared with the number of times other alleles for the same gene occur
  • Evolution is any change in the allele frequency in a population
• Sources of genetic variation


  • Mutations
  • Genetic recombination during sexual reproduction
  • Lateral gene transfer
• Single-gene and polygenic traits


  • A single-gene trait is controlled by one gene
  • A polygenic trait is 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, forming a bell curve

  2. Directional selection takes place when individuals at one end of the bell curve have higher fitness

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

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


  • In small populations, alleles can become more or less common simply by chance
  • The bottleneck effect is a change in allele frequency following a dramatic reduction in the size of a population
  • The founder effect is a change in allele frequency that may occur when a few individuals from a population migrate to and colonize a new habitat
• Genetic equilibrium and evolution


  • If allele frequencies in a population do not change, the population is in genetic equilibrium. Evolution is not taking place
  • The Hardy-Weinberg Principle states that allele frequencies in a population should remain constant unless one or more factors cause those frequencies to change
  • Populations are rarely in genetic equilibrium. Most of the time, evolution is occurring
• Sexual selection


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


  • For one species to evolve into two new species, the gene pools of two populations must become separated, or reproductively isolated
  • Reproductive isolation can develop through behavioral, geographic, or temporal isolation
• Speciation in Darwin's finches may have occurred in a sequence of events that involved the founding of a new population, geographic isolation, changes in the gene pool, behavioral isolation, and ecological competition
• Binomial nomenclature


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


  • Seven different levels from smallest to largest: species, genus, family, order, class, phylum, and kingdom
  • Each ranking level is called a taxon
• 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
  • Organisms are placed into groups called clades, which are monophyletic groups that include a single common ancestor and all descendants of that ancestor
• Cladograms


  • Diagrams that show how species and higher taxa are related to each other
  • The place where the ancestral lineage splits is called a fork, or a node
  • The bottom of the diagram, or the root, represents the ancestor shared by all of the organisms on the cladogram
  • Cladistic analysis relies on specific shared traits, or characters, with derived characters being traits that arose in the most recent common ancestor of a particular lineage
• DNA in classification


  The more derived genetic characters two species share, the more recently the species shared a common ancestor and the more closely related they are