Natural selection

The **differences** that exist between **individuals of the same species**.

**Heritable** variation — differences caused by **alleles** that can be passed to offspring.

**Mutation**, **meiosis** (crossing over + independent assortment) and **random fertilisation**.

A **random change in the DNA base sequence** — the only source of brand-new alleles.

**Mutation** makes new alleles. **Meiosis** and **random fertilisation** shuffle existing alleles into new combinations.

It produces **new combinations** of **existing alleles** (via meiosis and random fertilisation) — not new alleles.

Only mutations in **gametes / gamete-forming cells** are **passed to offspring**; somatic (body-cell) mutations are not.

No — most are **neutral or harmful**; only **occasionally** is one beneficial in a given environment.

It is the **only** process that creates **genuinely new alleles**; everything else just re-combines existing ones.

**Continuous** = a smooth range (e.g. height), often many genes + environment. **Discontinuous** = distinct categories (e.g. blood group), usually one/few genes.

The process where individuals with **advantageous heritable variations survive and reproduce more**, so the advantageous **allele becomes more common** over generations.

A **change in allele frequency** in the **population** over generations — the helpful allele becomes more common and the population becomes better adapted.

**No** — an individual keeps the alleles it was born with. The **population** changes over generations, not the individual.

Best able to **survive AND reproduce** — the individual that leaves the **most offspring**. Not necessarily the strongest or fastest.

Only **allele-based** variation can be **passed to offspring**, so only it can change in frequency over generations.

An environmental factor that affects which variants survive — e.g. a **predator, disease, climate, or food shortage**.

Variation → overproduction/struggle → selection pressure → differential survival → differential reproduction → **allele frequency rises** over generations.

More offspring are produced than can survive, creating a **struggle to survive** — so survival is unequal and selection can act.

Any environmental factor that affects an organism's chance of **surviving and reproducing**, so it decides which traits are favoured.

A **non-living** factor — e.g. **temperature, drought, salinity, light, pH**.

A pressure from **other organisms** — e.g. **predators, parasites/disease, competition** for food or mates.

Selection for traits that raise **mating success** (getting a mate) rather than survival.

**Mate choice (intersexual)** — one sex chooses showy partners; and **mate competition (intrasexual)** — members of one sex fight for mates.

Showy or costly mating traits — **bright plumage, antlers, horns, large size, courtship displays**.

The **gain in mating success outweighs the survival cost**, so males with it father more offspring.

Weigh a point **for** and a point **against**, then reach a **balanced judgement** — not a simple yes/no.

**Biotic** — it involves other living organisms (rivals and potential mates).

**Before.** Variation (often from a **random mutation**) already exists; the pressure only **selects** which variants survive — it never **creates** the trait.

A **random mutation** makes a resistant variant; the antibiotic **kills the susceptible** bacteria; the **resistant survive and reproduce** (incl. via **plasmids**); so the **resistance allele becomes common**.

The herbicide is a **selection pressure** — it kills non-resistant weeds, so the rare **resistant** variant survives and reproduces, and its frequency **rises as spraying continues**.

When the **heterozygote** has **higher fitness** than either homozygote, so **both alleles are kept** in the population (balancing selection).

**Carriers** (heterozygotes) are **more resistant to malaria** and avoid severe anaemia, so they are the **fittest** and reproduce most — keeping the sickle allele at **moderate frequency**.

Natural selection in **real time**: **predation** favours dull, camouflaged males while **sexual selection** favours bright males, so colouration **shifts in a few generations**.

**Variation** → a **selection pressure** removes some variants → favoured variants **survive and reproduce** → the helpful **allele becomes more common** over generations.