MYP Biology 5.9 Natural Selection Notes

Why Do Some Traits Become More Common Over Time?

1. Natural selection is the process by which individuals with advantageous traits are more likely to survive and reproduce.
2. These traits are passed to offspring and become more common in the population over generations.

How Does Natural Selection Work?

Natural selection follows a predictable sequence explaining how populations change over time:

1. Variation exists Individuals differ in traits such as size, color, behaviour, or physiology.
2. Some variations give advantages: certain traits improve survival or reproduction under specific environmental conditions.
3. Individuals with advantageous traits survive longer, escaping predators, finding food more easily, resisting disease, or tolerating stress.
4. As these individuals reproduce more, they pass their alleles to more offspring.
5. The trait becomes more common over generations, increasing the frequency of these advantageous alleles.

What Creates the Variation That Natural Selection Acts On?

1. Variation is generated through genetic processes described previously:
   1. Meiosis shuffles alleles
   2. Fertilization combines alleles
   3. Mutations create new alleles
   4. Polygenic traits produce continuous variation
2. Again, you must understand natural selection does not create variation; it filters existing variation.

How Do Environments Decide Which Traits Succeed?

1. The environment “chooses” traits by rewarding individuals who already have traits that match current conditions.
2. In other words, the environment determines which traits are favorable, not an organism’s intentions.
3. These environmental factors include:
   1. Temperature
   2. Predators
   3. Availability of food or water
   4. Light levels
   5. Disease
   6. Competition
   7. Human-driven pressures (e.g., antibiotics, pollution)

Why Do Certain Traits Become Common?

1. A trait becomes common when individuals carrying beneficial alleles contribute more offspring to the next generation.
2. Evolution therefore, isn't about an individual "becoming" anything.
3. It's abut populations shifting genetically over time.
4. When individuals with advantageous alleles leave more offspring, those alleles occupy a larger share of the gene pool in the next generation.
5. That measurable shift is the core signal of evolution.

How Does Natural Selection Explain Real Biological Changes?

1. Real-world cases show natural selection operating on existing variation to shift allele frequencies over generations.
2. Each example follows the same sequence: variation → environmental pressure → survival → reproduction → allele frequency change.

Antibiotic resistance in bacteria

1. Random mutations create resistant cells.
2. Antibiotics kill non-resistant bacteria.
3. Resistant bacteria survive and reproduce.
4. Resistance becomes common.

Peppered moths during industrialization

1. Dark moths were camouflaged against soot-covered trees.
2. They survived predation better.
3. Dark allele frequency increased.

Darwin’s finches

1. Droughts favored strong beaks able to crack tougher seeds.
2. Birds with larger beaks survived and reproduced.
3. Mean beak size increased.
   

What Are Adaptations?

1. There exists three main types of adaptations:
   1. Structural (body shape, claws, fur)
   2. Behavioral (migration, hunting strategies)
   3. Physiological (enzymes, venom, heat tolerance)

Can Natural Selection Be Observed in Real Time?

1. The short answer is yes, especially in species with short generation times:
   1. Bacteria evolving antibiotic resistance
   2. Insects evolving pesticide resistance
   3. Viruses evolving new variants

Why Doesn’t Natural Selection Always Produce “Perfect” Organisms?

1. Selection can only act on existing variation.
2. Environments change, trade-offs exist (e.g., brighter feathers attract mates but also predators), and mutations occur randomly.