Evolution (Biology)

• Speciation occurs when populations of organisms become isolated from each other, accumulate genetic differences over multiple generations, and become incapable of interbreeding.
• Science does not like absolutes
• Scientists prefer to use the term OBSERVATIONS
• How a population evolves

  A shift in the gene pool so that a higher ratio of individuals have a particular adaptation, adaptation is a particular structure, physiology, or behavior that helps an organism survive and reproduce in a particular environment
• Populations evolve to become better suited to their habitat for increased survival and better chance of passing on characteristics to offspring, takes place over many generations
• Individuals cannot evolve as they cannot change their genes, populations are the smallest unit that can evolve
• Evolution is the cumulative change in the heritable characteristics of populations that occurs over successive generations
• Variation becoming a characteristic or trait of the population


  1. Consider a litter of puppies or kittens...they usually do not all look the same as they inherit different parts of the DNA (alleles) from their parents

  2. Some genetic information is expressed in each individual's phenotype (physical appearance and behavior) while much of it has no visible effect but remains part of the genetic make-up and continues to be passed on to next generations
• Variations

  • Structural, functional, or physiological differences between individuals
  • NOT all variations become adaptations
  • Individuals possessing a helpful variation are more likely to survive and have a higher chance of passing this variation on to their offspring
  • Variation will become more frequent
• Genetic Variation


  • In a population, results from the variety of genetic information in all individuals of the population
• Mutations

  1. Changes in the genetic material (DNA) of an organism

  2. Only source of new genetic variation when inherited

  3. Continuously happening in DNA of all living organisms

  4. Can occur spontaneously when DNA is copied or be caused by environmental agents like ultraviolet radiation

  5. Mutations that drastically change the shape or structure of DNA can be harmful to an organism but not all mutations are harmful

  6. Mutations are only passed on through gametes, not somatic cells
• Selective Advantage

  • A genetic advantage of one organism over its competitors
  • Causes organism to be favored in terms of survival and reproduction
• Insects, bacteria, and viruses are examples of organisms that have very short life spans, reproduce, and adapt quickly to changing environments
• Organisms that reproduce quickly and have short generations show the passing of traits more quickly
• Darwin's actions

  1. Wrote papers describing his collections & observations

  2. Brought back thousands of specimens and fossils

  3. Draft of his theory of species formation in 1844
• Natural Selection = a mechanism of evolution whereby traits become more or less common in a population due to effects on survival and reproduction (survival of the fittest)
• Inferences of Darwin's Theory of Evolution by Natural Selection

  1. Members of the same species compete with each other for survival

  2. Individuals with more favourable variation are more likely to survive. SURVIVAL IS NOT RANDOM

  3. Survivors contribute proportionately more offspring to succeeding generations - the favourable variation will become more common. THIS IS NATURAL SELECTION
• Darwin's Theory of Evolution by Natural Selection

  1. Individuals within a species vary in many ways

  2. Some variability can be inherited

  3. Each generation produces way more offspring than can survive and passes on variations

  4. Populations tend to remain stable in size
• In 1858, Darwin received a letter from Alfred Russel Wallace that changed everything
• Darwin was concerned about the impact of his ideas in a religious culture
• Essence of Darwin's ideas

  • Variation exists in natural populations
  • Many more offspring are born each season than can possibly survive to maturity
  • There is a struggle for existence - competition
  • Characteristics that are beneficial in the struggle for existence will tend to become more common in the population, changing the average characteristics of the population - adaptations
  • Over long periods of time, and given a steady input of new variation into a population, these processes lead to the emergence of new species
• Founder Effect


  • Founder Effect = genetic drift that results when a small number of individuals separate from their original population and establish a new population. Example: Polydactyly (extra fingers and toes, a symptom of Ellis-van Creveld syndrome) are more common in Amish communities in the US
• Disruptive Selection


  • Example 1: Mimicry = similarity of one species to another which provides protection. Similarities: appearance, behavior, sound, scent
• Evolutionary Change Without Selection

  Some changes in the genetic makeup of a population are NOT influenced by the traits of individuals
• Types of Natural Selection

  • Directional Selection
  • Stabilizing Selection
  • Disruptive Selection
  • Sexual Selection
• Sexual Selection

  • Sexual Selection = differential reproductive success caused by variation in the ability to obtain mates. Results in sexual dimorphism and mating courtship behaviors
• Genetic Drift


  • Genetic Drift = changes to allele (genetic traits) frequency as a result of chance are much more pronounced in small populations. When populations are small, chance can play a significant role in altering allele frequencies
• Directional Selection


  • Selection that favors an increase or decrease in the value of a trait from the current population
• Natural Selection


  Natural Selection = a mechanism of evolution where traits become more or less common in a population due to effects on survival and reproduction (survival of the fittest)
• Bottlenecks


  • Genetic bottleneck = a dramatic, often temporary, reduction in population size, usually resulting in significant genetic drift
• Stabilizing Selection


  • Example 1: Human babies have an average size. Too big and they can't get through the birth canal. Too small and they have low survivability
• Fossil Record shows that today’s organisms descended from ancestral species
• Paleontology


  The scientific study of fossil remains
• Fossil Record

  1. Layers of sedimentary rock contain fossils

  2. New layers cover older ones, creating a record over time

  3. Fossils within layers show that a succession of organisms have populated Earth throughout a long period of time
• Convergent Evolution


  • Fish: aquatic vertebrates, Dolphins: aquatic mammals - Similar adaptations to life in the sea, not closely related
• Artificial Selection

  1. Humans "improving" domesticated plant and animal species for thousands of years through selection

  2. Selecting offspring with desirable traits as breeding stock for succeeding generations
• Analogous features are structures similar in function but not in origin or anatomical structure
• Lines of Evidence to support Darwin’s theory

  • Fossil records
  • Artificial selection
  • Anatomical evidence
  • Genetic evidence
• Fossils are any preserved remains or traces of an organism or its activity; many fossils are of hardened body parts like bone
• Evolutionary change in horses

  • Increase in size
  • Loss of toes
  • Increase in size of molars