How does natural selection drive evolution?

Definition

Natural selection is a process in biology where organisms with traits better suited to their environment tend to survive and reproduce more than those without such traits. Over generations, these advantageous traits become more common in the population, driving evolution-the change in the genetic makeup of a population over time.

Worked Example

Suppose a population of beetles has two color variations: green and brown. Birds can spot green beetles more easily on brown soil, so green beetles are eaten more often.

Let:

• $N_g$ = number of green beetles
• $N_b$ = number of brown beetles
• $s$ = survival rate of green beetles ($s = 0.5$)
• $t$ = survival rate of brown beetles ($t = 0.9$)

Assume initial population:

• $N_g = 100$
• $N_b = 100$

After one generation:

• Surviving green beetles: $N_g' = N_g \times s = 100 \times 0.5 = 50$
• Surviving brown beetles: $N_b' = N_b \times t = 100 \times 0.9 = 90$

Total surviving beetles: $50 + 90 = 140$

Proportion of brown beetles after selection:
$$ P_b = \frac{N_b'}{N_g' + N_b'} = \frac{90}{140} = 0.643 $$

Proportion of green beetles after selection:
$$ P_g = \frac{N_g'}{N_g' + N_b'} = \frac{50}{140} = 0.357 $$

Brown beetles now make up a larger proportion of the population. Over many generations, the brown trait will likely dominate.

Takeaways

• Natural selection favors traits that increase survival and reproduction.
• Advantageous traits become more common in a population over generations.
• This process is a key mechanism driving evolution.