Evolutionary trend

• Evolution


  A process of gradual and orderly change from one condition to another
• Evolution is a scientific theory proposed by Charles Darwin
• Evolution occurs at the population level and is made possible by genetic variations
• Evolutionary trend


  A steady change in a given adaptive direction
• Evolutionary biology


  • A subfield of biology concerned with the study of the evolutionary processes that produced the diversity of life on Earth
  • Evolutionary biologists study the descent of species, and the origin of new species
• Evidences that species evolve

  • Comparative anatomy
  • Biogeography
  • Fossils
• Earth and the other planets of the solar system formed about 4.6 billion years ago
• Conditions on early Earth made the origin of life possible, as evidenced by fossils of microorganisms that are about 3.5 billion years old
• There are approximately two million species of animals and plants living today
• Progressive evolution


  Simpler forms of organisms move towards becoming more complex structural and physiological organizations
• Retrogressive evolution


  Structurally more complex organic forms produce simpler individuals
• Examples of retrogressive evolution in plants

  • Derivation of fungi from algal ancestors as a result of loss of chlorophyll
  • Development of structurally simplified types of flowers from more elaborate and more complex flowers
  • Change from autotrophic nature to parasitic mode of nutrition in parasitic Cuscuta (Dodders) due to loss of chlorophyll
• Evolution proceeds either on progressive or on retrogressive lines, and both these processes are proceeding simultaneously in nature
• Microevolution


  Changes in allele frequencies in a population over generations, occurring at the genetic level
• Causes of changes in allele frequency

  • Natural selection
  • Genetic drift
  • Gene flow
• Only natural selection can cause adaptive radiation
• Macroevolution


  Larger changes, such as when a new species is formed, including the idea that all life forms are connected and can be traced back to one common ancestor
• Macroevolution is cumulative microevolution
• The sweeping changes in life on Earth revealed by fossils illustrates macroevolution
• The process of evolution


  1. Genes mutate

  2. Individuals are selected

  3. Populations evolve
• Individuals do not evolve, but populations evolve
• Evolution


  Can be viewed as a pattern or as a process
• One way to assess whether natural selection or other factors are causing evolution at a particular locus is to determine what the genetic makeup of a population would be if it were not evolving at that locus, and then compare with the real scenario
• Species


  A group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring
• Types of speciation


  • Allopatric
  • Sympatric
• Allopatric speciation


  The most common form of speciation, occurring when a population is split into two (or more) geographically isolated subdivisions that organisms cannot bridge
• Sympatric speciation


  Occurs when two subpopulations become reproductively isolated without first becoming geographically isolated
• Example of sympatric speciation


  • Insects that live on a single host plant, where a group of insects switched host plants and did not breed with other members of their species still living on their former host plant
• Example of observed speciation

  • In the plant genus Tragopogon, two new species (T. mirus and T. miscellus) have evolved within the past 50-60 years, formed when one diploid species fertilized a different diploid species and produced a tetraploid offspring
• Factors that promote sympatric speciation

  • Polyploidy
  • Habitat differentiation
  • Sexual selection
• Extinction


  The ultimate fate of all species, due to numerous reasons such as competitive exclusion, habitat loss, or the development of unbeatable defenses by the organisms they exploit
• Geologic eons


  • Archean
  • Proterozoic
  • Phanerozoic
• Eras within the Phanerozoic eon

  • Paleozoic
  • Mesozoic
  • Cenozoic
• Evolutionary trends in animals

  • Trends in symmetry
  • Trends in digestive tracts
  • Trends in body cavities
  • Trends in cephalization
  • Trends in segmentation
  • Trends in skeletal modification
  • Trend in nervous system
• Evolutionary trends in symmetry

  • Asymmetry
  • Radial symmetry
  • Bilateral symmetry
• Evolutionary trends in digestive system

  • Incomplete
  • Complete
• Evolutionary trends in body cavities

  • Acoelomate (no coelom)
  • Pseudocoelomate (false body cavity)
  • Coelomate (true coelom)
• Advantages of coelom and pseudocoelom

  • Reproductive and digestive organs can evolve more complex shapes and functions
  • Gut tube and other organs are cushioned and better protected
  • Can act as hydroskeleton providing support and rigidity
  • Activities of the suspended gut can take place undisturbed
• Cephalization


  The evolutionary trend whereby nervous tissue becomes concentrated toward one end of an organism, producing a head region with sensory organs and a sophisticated brain
• Evolutionary trends in body plan

  • Protostomes (mouth forms first)
  • Deuterostomes (anus forms first)