IB Environmental systems and societies 3.1B Natural Selection and Speciation Notes

Natural Selection: The Driving Force of Evolution

1. Natural selection acts continuously on the genetic variation within a population, favouring traits that improve survival and reproduction.
2. Over millions of years, this process has produced the huge diversity of life across ecosystems.

How Natural Selection Works: Four Principles

1. Four linked principles drive evolution by natural selection: variation, overproduction, competition, and differential survival.

Variation

1. Individuals vary in their traits because of genetic differences and mutations.
2. This variation is the raw material for evolution, and it arises through mutation and sexual reproduction.

Overproduction

1. Most species produce more offspring than the environment can support.
2. Only a fraction survive to adulthood, so those with advantageous traits are more likely to reach reproductive age.

Competition and the Struggle for Survival

1. When resources such as food, water, and shelter are scarce, individuals compete to survive.
2. Selection pressures such as predation, disease, and climate favour some individuals over others.

Differential Survival and Reproduction

1. Individuals with advantageous traits survive and reproduce more, passing those traits to their offspring.
2. Over generations the beneficial alleles become more common, producing adaptation and eventually new species.

'Survival of the Fittest' Explained

Speciation: How New Species Form

1. Speciation occurs when populations become isolated and evolve independently, building up genetic differences that eventually prevent interbreeding.
2. Once reproductive isolation is complete, new species emerge.

Types of Reproductive Isolation

1. Isolation prevents gene flow between populations, and it takes several forms:
2. Geographical isolation
   1. Physical barriers such as mountains, rivers, or oceans separate populations.
3. Behavioural isolation
   1. Differences in mating calls or courtship prevent interbreeding.
4. Temporal isolation
   1. Populations reproduce at different times or seasons.
5. Ecological isolation
   1. Populations occupy different niches within the same area.

Adaptive Radiation: Darwin's Finches

1. When one ancestral species spreads into varied niches, it can diverge into many species, called adaptive radiation.

Example

• On the Galápagos Islands, finches evolved different beak shapes suited to different food sources.
• Birds whose beaks matched the available seeds survived and reproduced more successfully.
• Over time, distinct species emerged from the single ancestor.

Natural vs Artificial Selection

1. Natural selection is driven by environmental pressures such as predation and climate.
2. Artificial selection is driven by humans choosing which individuals breed, to get desired traits.

Artificial selection and its effect on genetic diversity are covered in detail in 3.1F.