Evolutionary Processes (HL IB Environmental Systems & Societies (ESS))

Evolutionary Processes

• Biodiversity arises from evolutionary processes

  • Evolution is the cumulative change (i.e. the overall change over time) in the heritable characteristics of a population or species

  • Natural selection is the name of the mechanism that drives this evolutionary change

    • Natural selection occurs continuously and can take place over billions of years

    • The result of this process of natural selection is the biodiversity of life on Earth we see today

Natural Selection

• In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce

• This results in natural selection:

  • Individuals in a species show a range of variation caused by differences in genes (genetic diversity)

  • When organisms reproduce, they produce more offspring than the environment is able to support

  • This leads to competition for food and other resources, which results in a "struggle for survival"

  • Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce

  • Therefore, the genes resulting in these characteristics are passed on to offspring at a higher rate than those with characteristics less suited to survival

  • This means that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics

• This theory of natural selection was put forward by Charles Darwin and became known as "survival of the fittest"

Example of natural selection

• Imagine a population of rabbits shows variation in fur colour

• The rabbits have natural predators like foxes

  • This acts as a selection pressure

• Rabbits with a white coat do not camouflage as well as rabbits with brown fur

  • This means predators are more likely to see white rabbits when hunting

• As a result, rabbits with white fur are less likely to survive than rabbits with brown fur

• The rabbits with brown fur therefore have a selection advantage

  • This means they are more likely to survive to reproductive age and be able to pass on their genes to their offspring

• Over many generations, the frequency of the gene for brown fur will increase and the frequency of the gene for white fur will decrease

• Remember that organisms better suited to their environments are more likely to survive

  • However, this does not mean their survival is guaranteed

• Organisms that are less suited to an environment are still able to survive and potentially reproduce within it

  • However, their chance of survival and reproduction is lower than the individuals that are better-adapted

• Also, it is important to be aware that an environment, and the selection pressures it exerts on an organism, can change over time

  • When a change occurs then a different characteristic may become more advantageous

• Finally, remember that all organisms (not just animals) experience selection pressures as a result of the environment they are in

Speciation

• Speciation is the generation of new species through evolution

• It occurs when populations of a species become isolated and adapt to their environments in different ways

• Over time, these populations become so different that they can no longer interbreed with each other to produce fertile offspring

• When they cannot interbreed in this way, they are considered separate species