Chapter 18: Evolution & The Origin of Species

• Evolution by natural selection describes a mechanism for how species change over time
• Wallace and Darwin both observed similar patterns in other organisms and independently developed the same explanation for how and why changes could take place
• Natural selection (Darwin)/“Survival of the Fittest”: the more prolific reproduction of individuals with favorable traits that survive environmental change because of those traits, leading to evolutionary change
• Darwin argued that natural selection was an inevitable outcome of three principles that operate in nature:

  • Most characteristics of organisms are inherited
  • More offspring are produced than are able to survive, leading to competition for limited resources
  • Offspring vary among each other in regard to their characteristics and those variations are inherited
• Studies of evolution by natural selection are difficult to conduct, requiring analyses of several generations to document changes in evolutionary change
• • Natural selection can only take place if there is variation, or differences, among individuals in a population.
• • The evolution of species has resulted in enormous variation in form and function.
• • We call two species that evolve in diverse directions from a common point divergent evolution.
• Convergent evolution: similar traits evolve independently in species that do not share a recent common ancestry
• Allopatric speciation involves geographic separation of populations from a parent species and subsequent evolution
• Sympatric speciation involves speciation occurring within a parent species remaining in one location
• Speciation: A long time apart from the original group (population) the two groups that are divided then form a different species
• Population divides into different locations through dispersal, where individuals move away from each other
• Vicariance occurs when the environment causes division among populations
• Adaptive Radiation results in the evolution of more than one new species, often observed more frequently in islands
• Polyploidy involves having extra chromosome sets, with auto-polyploidy having two or more complete sets of chromosomes from the same species
• Allopolyploid individuals are formed when individuals of two different species reproduce to create viable offspring
• Aneuploidy is characterized by missing chromosomes
• Polyploidy increases the chances of survival and is more common in plants
• Gene flow helps prevent interbreeding among populations
• Pre-zygotic Barriers are mechanisms that block reproduction from occurring at the fertilized cell stage
• Post-zygotic Barriers result in offspring that are infertile or die before reaching maturity
• Temporal Isolation is based on differences in timing
• Habitat Isolation occurs due to differences in habitat
• Behavioral Isolation is caused by differences in behaviors
• Gametic Isolation happens when gametes are not compatible
• Mechanical barriers prevent fertilization by acting as physical obstacles
• Hybrid inviability refers to offspring that are unable to form, are not born, or do not survive after birth
• Hybrid: offspring of two different species
• Reinforcement: less fit than the parent = continue as different species
• Fusion: Reproductive barriers weaken until the two species become one - as fit as the parents
• Stability: Fit hybrids continue to be produced but also the two individual species continue
• Gradual Speciation: evolution occurs every so often, a little at a time
• Punctuated Equilibrium: stays the same for a long period of time, then another change occurs
• The primary factor of speciation rate is the environmental conditions
• Natural Selection acts on individual organisms
• Evolution acts on populations