3.1B Natural selection and speciation Notes

Natural selection: Driving force of evolution and biodiversity

1. It operates continuously, acting on genetic variation within populations, favoring traits that improve survival and reproduction.
2. Over millions to billions of years, this process has resulted in the diversity of life across different ecosystems.

Mechanism of Natural Selection

1. Natural selection relies on four fundamental principles: variation, overproduction, competition, and differential survival.
2. Together, these processes explain how populations evolve over time.

Example

Darwin’s Finches

• In the Galápagos Islands, finches evolved different beak shapes suited for different food sources.
• Birds with stronger beaks survived better on hard seeds, passing this trait to their offspring.
• Over time, distinct species emerged, illustrating adaptive radiation driven by natural selection.

1. Variation

1. Within every population, individuals vary in physical and behavioral traits due to genetic differences and mutations.
2. These variations form the raw material for evolution.
3. Genetic diversity arises through:
   1. Random mutations during DNA replication
   2. Sexual reproduction, which combines genetic material from two parents
   3. Gene flow, where genes move between populations through migration

2. Overproduction

1. Most species produce more offspring than the environment can support.
2. This creates competition for limited resources such as food, mates, and shelter.
3. Only a fraction of offspring survive to adulthood.
4. Those with advantageous traits are more likely to reach reproductive age.

3. Competition and Struggle for Survival

1. When resources such as food, water, or shelter are scarce, individuals compete for survival.
2. This “struggle for existence” determines which individuals are best adapted to current environmental conditions.
3. Selection pressures, such as predation, disease, and climate, favor some individuals over others.

4. Differential Survival and Reproduction

1. Individuals with advantageous traits, those that improve survival or reproduction, are more likely to survive and pass on those traits to their offspring.
2. Over generations, the frequency of these beneficial alleles increases in the population.
3. This gradual change leads to adaptation and can eventually result in new species.

Speciation: The Formation of New Species

1. Speciation occurs when populations of a species become isolated and evolve independently, accumulating genetic differences that eventually prevent interbreeding.
2. Once reproductive isolation is achieved, new species emerge.

1. Reproductive Isolation

1. Populations must become isolated to prevent gene flow.
2. Isolation can occur in several ways:
   1. Geographical isolation: physical barriers (mountains, rivers, oceans) separate populations.
   2. Behavioral isolation: differences in mating calls or courtship behaviors prevent interbreeding.
   3. Temporal isolation: populations reproduce at different times or seasons.
   4. Ecological isolation: populations occupy different niches within the same area.

2. Divergent Evolution and Adaptation

1. Once isolated, populations adapt to their specific environments.
2. Different selective pressures cause divergent evolution, where populations evolve distinct traits.
3. Over many generations, genetic differences accumulate to the point that interbreeding is no longer possible.

Artificial Selection vs. Natural Selection

1. Natural selection: Driven by environmental pressures (e.g., predation, climate).
2. Artificial selection: Controlled by humans selecting for desired traits in crops or animals.

Interconnection Between Natural Selection, Adaptation, and Speciation

1. Natural selection acts on existing variation within populations.
2. Over time, advantageous traits accumulate, creating adaptations.
3. Isolation can separate populations, allowing adaptations to diverge.
4. With sufficient divergence, speciation occurs, increasing biodiversity.