Natural selection is one of the best-supported explanations in biology, and one of the most misunderstood. In Living Earth you are asked to explain, using evidence, how evolution happens and how natural selection leads to adaptation. The explanation rests on four factors. If all four are present, a population changes over generations. If any one is missing, it does not.
The four factors
- Variation. Individuals in a population differ in their traits, and many of those differences come from differences in their genes. Mutations and the shuffling of genes in sexual reproduction keep producing new variation.
- Inheritance. Some of the traits are heritable, so parents pass them to offspring. A trait that is not inherited, such as a scar, cannot be selected for.
- Overproduction and competition. Populations can grow faster than their food, water, shelter or mates allow. More offspring are born than can survive and reproduce, so individuals compete for limited resources.
- Differential survival and reproduction. Individuals with a heritable trait that suits their environment tend to survive and have more offspring. Over generations, that trait becomes more common in the population.
Notice that the environment does the selecting. It does not create the variation. It only filters what is already there.
Worked example 1: a beetle population
Suppose a population has 200 beetles, 100 green and 100 brown, living on brown soil. Color is heritable. Birds find green beetles more easily. In one season the birds eat 60 green beetles and 20 brown beetles. The numbers are invented to show the logic.
- Survivors: 100 − 60 = 40 green, and 100 − 20 = 80 brown. That is 120 beetles in total.
- Share of brown among survivors: 80 ÷ 120 = 0.667, or about 67%. It started at 50%.
Now each survivor has two offspring that match its color. That gives 80 green and 160 brown young, so 240 in total. Brown beetles are 160 ÷ 240 = 67% of the next generation. No beetle changed color. The population changed because brown beetles left more offspring than green ones. Repeat this for many generations and brown becomes more common still.
Worked example 2: how antibiotic resistance spreads
Bacteria show all four factors very quickly. Imagine a flask with 1,000,000 bacteria. By chance, 10 of them carry a mutation that makes them resistant to an antibiotic. That is 10 ÷ 1,000,000 = 0.001% of the population.
- Variation: a few cells are resistant, and most are not.
- Selection: the antibiotic is added. Suppose it kills every non-resistant cell. Only the 10 resistant cells survive.
- Inheritance: when a resistant cell divides, both daughter cells are resistant.
- Reproduction: if the survivors double 20 times, there are 10 × 2²⁰ = 10 × 1,048,576 = 10,485,760 bacteria, all resistant.
The antibiotic did not cause the mutation. The resistant cells were already there. The antibiotic simply removed their competition, and so resistance went from vanishingly rare to universal in the population. Each unneeded use of an antibiotic gives resistant cells this kind of advantage.
What natural selection is not
- It is not a choice or a goal. Organisms do not change because they need to.
- It acts on individuals, but it is populations that evolve.
- It works only on variation that already exists and is heritable.
- It is not the only cause of change. Mutation, gene flow between populations and chance events can also change how common a trait is.
Common mistakes
- Saying that individuals evolve. An individual has the traits it was born with. A population changes from one generation to the next.
- Saying that organisms adapt because they need to. Traits that help survival become common because their carriers leave more offspring, not because anyone wanted them.
- Counting acquired traits. A bodybuilder's muscles or a lost tail from an accident are not passed on through genes.
- Reading fitness as strength. In biology, fitness means success in surviving and reproducing in a particular environment.
- Forgetting the environment. A trait that helps in one place can be neutral or harmful in another.
Practice Living Earth questions with an explanation for every answer, or read the guide to California Living Earth practice.
Source: the California Next Generation Science Standards (CA NGSS), as set out in the California Science Framework, HS-LS4-2, HS-LS4-4.