evolution

• Definition of evolution


  Descent with modification
• Natural selection


  the sequence of events in which a certain inheritable characteristic that confers advantage to individual, increasing its likelihood of survival to reproduction, is preserved by environment
• Extended phenotypes


  manifestations of genes that occur outside of the organism that possess those genes
• Population definition in evolution

  a group of individuals of the same species that live and interbreed in a particular geographic area
• the definition of evolution

  changes in characteristics of a species over time, several generations. relies on natural selection, all species are related and gradually change over time. evolution relies on genetic variations.
• Sexual selection

  The evolution of elaborate, and apparently non-adaptive, sexual traits that would clearly not aid in the survival of their bearers, but increase the individual's reproductive success
• Charles Darwin's proposal

  • All living species were derived from common ancestors
  • The primary mechanism to explain this was natural selection: organisms better adapted to their environment would benefit from higher rates of survival than those less well equipped to do so
  • There were many examples of elaborate, and apparently non-adaptive, sexual traits that would clearly not aid in the survival of their bearers
• Sexual selection mechanisms

  • Intrasexual selection: competition between members of the same sex (usually males) for access to mates
  • Intersexual selection: members of one sex (usually females) choose members of the opposite sex
• The idea of female mate choice was received with ridicule, and was not seriously reconsidered until nearly 80 years later
• Bateman's principle

  Female reproduction is primarily limited by their access to resources to nourish and produce large gametes, whereas male reproduction is mainly limited by access to females
• In sexually reproducing species, the average reproductive success is equal for both males and females
• Reproductive variance


  A successful male can potentially sire many offspring, leading to a high reproductive variance among males. A successful female will not take away reproductive opportunities from other females, leading to a smaller variance in reproductive success.
• Stronger sexual selection

  • Results in sexually dimorphic traits that are exaggerated, or more elaborate, in the sex with highest reproductive variance
• Evidence of evolution

  • Anatomy: species may share similar physical features because the feature was present in a common ancestor (homologous structures)
  • Molecular biology: DNA and the genetic code reflect the shared ancestry of life. DNA comparisons can show how related species are
  • Biogeography: The global distribution of organisms and the unique features of island species reflect the evolution and geological change
  • Fossils: Fossils document the existence of now-extinct past species that are related to present-day species
  • Direct observation: We can directly observe small-scale evolution in organisms with short lifecycles (e.g., pesticide-resistant insects)
• homologous structure

  If two or more species share a unique physical feature, such as a complex bone structure or a body plan, they may all have inherited this feature from a common ancestor. Physical features shared due to evolutionary history (a common ancestor) are said to be homologous
• analogous structure

  not all physical features that look alike are marks of common ancestry. Instead, some physical similarities are analogous: they evolved independently in different organisms because the organisms lived in similar environments or experienced similar selective pressures. This process is called convergent evolution. (To converge means to come together, like two lines meeting at a point.)
• Hardy-Weinberg equilibrium


  A principle stating that the genetic variation in a population will remain constant from one generation to the next in the absence of disturbing factors
• Hardy-Weinberg equilibrium

  • When mating is random in a large population with no disruptive circumstances, the law predicts that both genotype and allele frequencies will remain constant because they are in equilibrium
• Factors that can disrupt the Hardy-Weinberg equilibrium


  • Mutations
  • Natural selection
  • Nonrandom mating
  • Genetic drift
  • Gene flow
• Mutations disrupt the equilibrium of allele frequencies by introducing new alleles into a population
• Natural selection and nonrandom mating disrupt the Hardy-Weinberg equilibrium because they result in changes in gene frequencies
• Genetic drift occurs when allele frequencies grow higher or lower by chance and typically takes place in small populations
• Gene flow occurs when breeding between two populations transfers new alleles into a population
• Because all of these disruptive forces commonly occur in nature, the Hardy-Weinberg equilibrium rarely applies in reality
• The Hardy-Weinberg equilibrium describes an idealized state, and genetic variations in nature can be measured as changes from this equilibrium state
• genetic variation?

  Differences in the genetic makeup of individuals within a population.
• Three primary sources of genetic variation

  • Mutations
  • Gene flow
  • Sex
• Mutations


  Changes in the information contained in genetic material (for most of life, this means a change in the sequence of DNA)
• A single mutation can have a large effect, but in many cases, evolutionary change is based on the accumulation of many mutations with small effects
• Gene flow

  Any movement of genetic material from one population to another (e.g., through migration)
• Gene flow is an important source of genetic variation
• Sex

  Introduces new gene combinations into a population
• Genetic shuffling through sex is an important source of genetic variation
• Synthetic evolutionary theory


  The fusion of Darwinian selection theory with Mendelian genetics
• The evolutionary synthesis period

  1930-1950
• Evolutionary synthesis


  The term coined by Ernst Mayr and William B Provine for the period that saw the fusion of Darwinian selection theory with Mendelian genetics
• Evolutionary synthesis


  • Emergence of the discipline of evolutionary biology
  • Creation of a new journal, Evolution
  • Creation of the first international Society for the Study of Evolution
• Modern Evolutionary Synthesis

  • All evolutionary phenomena can be explained in a way that is consistent with known genetic mechanisms and the observational evidence of naturalists
  • Evolution is gradual and is caused by small genetic changes, recombination and natural selection
  • Discontinuities among species are explained as originating gradually, through geographical separation and extinction rather than saltation
  • Selection is the main driver of change, even if fitness variations are slight
  • The object of selection is the phenotype, the externally observable traits of an organism
  • The genotype is passed through reproduction, the gene is the unit for selection
  • The genetic diversity of natural populations is a key factor in evolution
  • Natural selection destroys variation which it feeds upon
  • Barriers to gene flow are key influences in genetic drift and speciation
  • The fossil record can be explained by extrapolating micro-evolutionary observations to macro-evolutionary events
• The rate of evolutionary change is not constant, several mass extinctions in the evolutionary history of life give us evidence that massive extinctions leave niches vacant, allowing other species to achieve rapid diversification and speciation
• The role of genetic drift is equivocal