Variation and Evolution Notes

Variation

  • Variation means members of a species show different characteristics.
  • Species: A group of organisms capable of interbreeding to produce fertile offspring.
  • Variation can be acquired or inherited.
  • Inherited variations are caused by sexual reproduction and mutations.

Sexual Reproduction

  • Sexual reproduction is responsible for most variations in offspring.
    • Independent assortment of chromosomes into gametes.
    • Crossing over during meiosis: produces chromosomes that are a combination of both sets of genes

Mutations

  • A mutation is a change in the amount or structure of DNA.
  • Mutations can arise randomly on a chromosome.
  • If a gene is altered, the change in its sequence of bases may prevent the correct protein from being produced.
    • This altered gene is known as a recessive allele.
  • Many mutations result in no change because the dominant allele on the second chromosome can still produce the original protein.

Causes of Mutations

  • Mutations may arise naturally when DNA does not produce exact copies or doesn’t repair properly (spontaneous mutations).
  • Mutagens are agents that cause mutations; they increase the spontaneous rate of mutation.

Categories of Mutagens

  • Ionizing radiation: e.g., X-rays and ultraviolet (UV) radiation.
  • Chemicals: e.g., tobacco smoke and pesticides.

Types of Mutations

  • Gene mutations: Changes in a single gene caused by changes in a single pair of bases.
    • The altered gene is called the allele.
    • Examples: cystic fibrosis, albinism, and hemophilia.
  • Chromosome mutations: Large changes in the structure or number of one or more chromosomes.
    • Humans normally have 46 chromosomes in each somatic cell; gaining a chromosome results in 47.
    • Example: Down syndrome.

Evolution

  • Evolution is the way in which genetic changes produce different types of organisms over long periods of time.
  • Up until the early 1800s, people believed that species were fixed and not changing.
  • Since the start of the 19th century, a number of theories of evolution have been suggested, the most widely accepted theory was based on the work of Charles Darwin and Alfred Wallace.
  • Darwin and Wallace called this theory “The Theory of Natural Selection”.

The Theory of Natural Selection

  • Based on three observations and two conclusions:
    • Observation 1: Organisms overbreed.
    • Observation 2: Population numbers tend to remain static.
    • Conclusion 1: Not all organisms in a population can survive, so there is a struggle for existence.
    • Observation 3: Inherited variations arise in a population.
    • Conclusion 2: Nature selects those organisms most suited (or best adapted) to their environment.

Natural Selection

  • Natural selection is the process by which organisms with genetically controlled characteristics that allow them to be well adapted to their environments will survive and reproduce to pass on their genes to following generations.

Evidence for Evolution

  • One of the best sources of evidence for evolution is paleontology (the study of fossils).
  • A fossil is the remains of something that lived a long time ago.
  • Examples of fossils include entire organisms, shells, bones, teeth, seeds, pollen grains, leafprints, and footprints.

Fossil Evidence for Evolution

  • Fossils provide evidence for evolution in the following ways:
    • Fossils can be aged by reference to the depth at which they are found or by measuring the amount of radioactive decay.
    • Fossils discovered to date show changes when compared to modern organisms.
    • The more modern fossils show increased complexity.
    • Fossil evidence can be linked to environmental change (organisms had new environments to which they had to adapt).