4 Requirements for Natural Selection to Occur

1. Overpopulation of offspring: this puts pressure on limited resources e.g. food… not all of them can survive

2. Variation: all species have natural variations! Increased variation leads to an increased chance the species will survive

3. Selection: the organism with traits more suited to the environment survives

4. Reproduction of Favorable Traits: eventually the “fittest” individuals survive and make up the majority of the population

• One example of natural selection taking place in the wild is the evolution of the peppered moth in England during the Industrial Revolution. Prior to the widespread use of coal, the light-colored form of the moth was more common because it blended in with the light-colored trees. However, as pollution from coal burning darkened the trees, the dark-colored form of the moth became more common because it was better camouflaged. This is an example of natural selection because the environment (in this case, pollution) selected certain traits (in this case, dark coloration) that gave the moth a survival advantage.

Genetic Isolation and Speciation

Speciation occurs when a new species is formed

1. Phyletic Evolution: population of a species progressively changes overtime to become a new species

2. Divergent Evolution: population is divided into two or more populations that are prevented from interbreeding e.g. island split like Pangea!

   • When different selective pressures act on each population, different characteristics are “favourable” and therefore those organisms survive

   • overtime the two populations become so different to each other, they can no longer interbreed to produce fertile offspring and are now two different species

Artificial Vs. Natural Selection

• Artificial Selection

  • Humans consciously select for/against traits in an organism

  • Results in less genetic variation because few individuals are chosen (and not necessarily the ones that suit the environment best… just the ones that humans like best!)

    • Cattle that produce a high yield of good quality milk

    • Plants that grow abundant sweet apples

• Issues of Natural Selection

  • In the wild, there is a large variation between plants. People select very similar traits in agricultural plants, which is an issue when a disease strikes… as there is limited variation so there is a risk all the plants will die

  • The Chihuahua is the world’s smallest dog but due to its breeding, it has many health issues like susceptibility to fractures, jawbone disorders, eye and heart conditions, teeth and gum disease. Many purebred species would not be able to survive in the wild!

• Similarities between Natural and Artificial Selection

  • Traits are inherited from parents (or parent through asexual reproduction)

  • Results cause a change in the genetic traits of a species

  • Change occurs over many generations