Evolution (SL IB Biology)

• Species do not stay the same over time; the species that we see around us today have developed over millions of years
  • This process of species change is known as evolution
• Evolution can be defined as:

Changes in the heritable characteristics of organisms over generations

• Heritable characteristics are those that can be inherited by, or passed on to, the next generation
  • Changes in characteristics that are not inherited, e.g. a plant having its leaves eaten, do not lead to evolution
  • Heritable characteristics are determined by the alleles of genes that are present in an individual
  • Alleles may change as a result of random mutation, causing them to become more or less advantageous
• Heritable characteristics that are advantageous are more likely to be passed on to offspring, leading to a gradual change in a species over time
  • This is the process of natural selection 
• Changes in the heritable characteristics of organisms can also lead to the development of completely new species
• The formation of new species via the process of evolution has resulted in a great diversity of species on Earth
  • Theoretically, at the origin of life on Earth, there would have been just one single species
  • This species evolved into separate new species 
  • These species would then have divided again, each forming new species once again
  • Over millions of years, evolution has led to countless numbers of these speciation events, resulting in the millions of species now present on Earth

Evolution diagram

Evolutionary change over a long period of time has resulted in a great diversity of species

Darwinian evolution

• Charles Darwin, as a result of observations on a round-the-world expedition, and backed by years of experimentation and discussion, proposed the theory of evolution by natural selection
• Darwin’s theory is as follows:
  • Individuals in a species show a wide range of variation due to random mutations in their DNA
  • Individuals within a population must compete for survival due to selection pressures
  • Individuals with characteristics most suited to the environment have a higher chance of survival and so are more likely to reproduce
  • Advantageous alleles are passed down to offspring
  • Over many generations the advantageous alleles become more frequent in a population
• Darwinian evolution by natural selection requires that characteristics are heritable

Natural selection diagram

Natural selection acts on genetic variation in populations. Here the allele for white shells is advantageous, so becomes more frequent in the population over time.

Lamarckian evolution

• Another theory of evolution, developed at the start of the 19th century (before Darwin announced his theory), was that of French scientists Jean-Baptiste Lamarck
• Lamarck’s theory was based mainly on the idea that changes that occur in an organism during its lifetime can be inherited
  • Such changes are known as acquired characteristics
• His theory is as follows:
  • A characteristic that is used frequently by an organism becomes better and stronger, whereas a characteristic that isn't used gradually disappears
  • The beneficial characteristics that are used frequently are passed to offspring
  • For example, Lamarck suggested that:
    • Giraffes had a short-necked ancestor that would frequently stretch its neck to reach the high branches so it could feed on the leaves
    • This repeated stretching could very slowly elongate the giraffe's neck and this elongated neck would be passed to the giraffe’s offspring
    • Over time and many generations, the giraffe would evolve to have the very long neck it has today
• Lamarck’s ideas were incorrect because they lack the component of heritability; acquired characteristics are not passed on to offspring
  • The new science of epigenetics may provide an exception to this rule, but changes like these are unlikely to be major drivers of natural selection

Lamarckian evolution diagram

Lamarck proposed that characteristics acquired during an organism's lifetime could be passed on to offspring

NOS: The theory of evolution by natural selection predicts and explains a broad range of observations and is unlikely to ever be falsified

• Scientists can gather information about the world by observing events
• They formulate theories that seek to explain observed events
• The theory of natural selection explains many observations, and is widely accepted as a correct explanation of observed events; no other reasonable theories have ever been proposed, and so this theory is likely to remain as the scientific explanation for species change over time
  • It is worth noting that there are some minor aspects of Darwin's original theory that have been falsified since they were proposed:
    • 'Evolution by natural selection is always slow'
      • We know that, e.g. antibiotic resistance can evolve in bacteria very quickly
    • 'The fossil record cannot provide evidence for evolution'
      • There are multiple examples of fossils that appear to show intermediate species
  • These errors have resulted in updates to Darwin's theory, but not to its falsification
• Due to the geological time periods over which evolutionary change has occurred, it is not possible to formally prove that natural selection has given rise to the species that we see today, hence the continued use of the term 'theory'