Evolution and Natural Selection

Every wrong answer in this topic comes from the same misconception, which makes it unusually easy to study: the trait comes first, the need does not create it.

Darwin and the finches

Charles Darwin proposed evolution by natural selection, published in 1859, and he is the name the test wants. The observations that set him thinking came from the Galapagos Islands, where each island's finches had a beak suited to the food on that island - thick beaks for cracking seeds, thin ones for probing for insects. One ancestral finch, many islands, many beaks.

Darwin also leaned on artificial selection - selective breeding - which farmers and dog breeders had practiced for centuries. If people choosing which animals breed can turn a wolf into a dachshund, nature doing the choosing over far longer can do far more.

An adaptation is any inherited trait that improves survival and reproduction in a particular environment: camouflage that hides prey from predators, a thick coat in the cold, a cactus's water-storing stem.

The four conditions

Natural selection follows whenever four things are true, and nothing more is required.

1. Variation. Individuals in a population differ.
2. Heritability. Some of that variation is passed to offspring.
3. Competition. More offspring are produced than can survive on the available resources.
4. Differential survival and reproduction. Individuals with certain variations leave more offspring.

Given those four, the frequency of the useful variant rises over generations, and that rise is evolution.

The misconception the test is built on

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Wrong: giraffes stretched for high leaves and their necks grew longer, and they passed the longer necks on.

Right: giraffes varied in neck length. In conditions where high leaves mattered, the longer-necked ones fed better, survived better and left more offspring. The population's average neck length rose.

Nothing was acquired during a giraffe's life and nothing was passed on because it was used. That older idea is Lamarckism, and it is wrong, and the test offers it as a distractor.

The generalization worth memorizing: organisms do not adapt because they need to. Populations change because some individuals already differ.

A bacterium does not become resistant because it meets an antibiotic. Some bacteria are already resistant by chance, and the antibiotic kills the rest, leaving the resistant ones to multiply.

Fitness

Fitness is reproductive success. It has nothing to do with being strong, fast or healthy except where those help produce offspring.

A peacock's tail makes it slower and more visible to predators, and it persists because it improves mating success. That is high fitness with reduced survival, and it is the example that shows fitness is not fitness in the everyday sense.

Variation and where it comes from

Mutation is the ultimate source of new alleles. It is random with respect to what would be useful.

Sexual reproduction shuffles existing alleles into new combinations, through meiosis and fertilization, which is why siblings differ.

Neither process is aimed. Mutation is copying error; recombination is shuffling. Selection is the only part of the process that is non-random, and it acts afterward.

Types of selection

Artificial selection is the same mechanism with a human choosing who breeds, which is how every domestic dog descends from wolves. It is the strongest evidence that selection produces large change, because it has been observed doing so.

Speciation

A species is a group whose members interbreed and produce fertile offspring.

New species form when a population is divided and the parts diverge:

1. Isolation. A barrier separates two groups - a river, a mountain range, a distance too far to cross.
2. Divergence. Each group faces different conditions and different mutations, and their gene pools drift apart.
3. Reproductive isolation. Eventually the two can no longer interbreed successfully, and they are separate species.

Geographic isolation is the usual first step, and it is what a question about an island population is describing. Species formed this way are called allopatric - "other homeland." Barriers need not be physical; two populations that stop recognizing each other's courtship displays are separated by behavioral isolation.

Other ways gene pools change

Natural selection is not the only force, and two others are asked:

• Genetic drift is random change in how common an allele is, from pure chance in who happens to breed. Its effect is largest in small populations.
• Gene flow is alleles moving between populations when individuals migrate and breed. It makes populations more alike.

Natural selection is the only one that consistently improves fitness, because it is the only one that is not random with respect to the environment. Mutation is the source of new variation; the others shuffle and filter what mutation provides.

When a species cannot adapt fast enough, the result is extinction - which is the fate of the great majority of species that have ever lived.

The evidence

The test asks what kinds of evidence support common descent.

Homologous means common ancestry; analogous means common function. That pair is the distinction most often asked, and the words are easy to swap. Homologous structures point to a shared ancestor; analogous ones do not.

Convergent evolution is what produces analogous structures: unrelated organisms facing similar conditions arrive at similar solutions, like the streamlined shape of sharks and dolphins.

What you can skip

Evolution has a large vocabulary and the questions use a small corner of it. Across all 2,104 General Science questions in our bank:

• Punctuated equilibrium, the founder effect and bottlenecks: zero mentions each. Genetic drift covers the idea they are special cases of.
• Sympatric speciation and gradualism: one mention each, and Hardy-Weinberg never. Know allopatric speciation and move on.
• Mimicry never appears; camouflage appears once.

The misconception section above is where the points are. Nearly every hard question in this topic is some version of "did the organism change because it needed to?", and the answer is always no.

Where people lose points

Saying an organism developed a trait because it needed it.

Saying an individual evolves. Populations evolve, over generations.

Treating fitness as strength or health.

Saying mutations occur in response to the environment. They are random with respect to need.

Swapping homologous and analogous.

Thinking selection creates variation. It filters what mutation and recombination already produced.

Work one in under a minute

A population of insects is sprayed with a new pesticide. Most die, but the survivors multiply and within a few generations the spray is ineffective. What happened?

Not that the insects became resistant in response to the spray.

Some insects were already resistant because of existing variation. The pesticide killed the susceptible ones, and the resistant survivors produced the next generation. The frequency of the resistance allele rose.

That is directional selection, it required the variation to pre-exist, and it is the same story as antibiotic resistance in bacteria.

Where this leads

The variation selection acts on comes from the mutations and recombination in the genetics lessons, and the classification of species is what speciation produces.