7 Natural Selection

Survival matters because it can create an opportunity to reproduce, but reproduction is the step that transmits a favorable trait. Repeated differences in reproductive contribution allow a trait to become more common over generations.

Environments change, so a phenotype that increases fitness under one set of biotic or abiotic conditions may have no advantage—or may decrease fitness—under another. Variation therefore gives a population multiple possible responses to changing pressures.

Consider enzyme variants that function differently at different temperatures. If one variant maintains a needed cellular reaction more effectively under the current temperature, organisms carrying it may perform and reproduce better. If temperature changes, the relative advantage can also change.

A population containing different molecular variants is more likely to include individuals whose cellular machinery functions well under a range of environments, increasing the population's capacity to survive and reproduce when conditions differ.

For example, if humans repeatedly breed only plants with a heritable early-flowering phenotype, early flowering can become more common in later generations while alternative flowering-time variants become less represented.

Artificial selection therefore changes diversity within a population by shifting the relative representation of variants. When breeding is restricted to a narrow subset, variation not chosen by humans may decline.

In drift, bottlenecks, and founder events, which alleles become common can depend on who survives or establishes the population by chance. The outcome can therefore differ from what natural selection alone would predict.

Two isolated small populations can experience different chance frequency shifts, so drift can increase their genetic difference. If migration resumes, transferred alleles make their gene pools more similar, which can prevent divergence into separate species.

Mutation, selection, genetic drift, and gene flow can all contribute to frequency change. The defining evidence is the population-level change across time, regardless of which mechanism produced it.

Worked example for a hypothetical equilibrium population: if p = 0.70, then q = 1 − 0.70 = 0.30. Expected genotype frequencies are p² = (0.70)² = 0.49, 2pq = 2(0.70)(0.30) = 0.42, and q² = (0.30)² = 0.09. Check: 0.49 + 0.42 + 0.09 = 1.00. These are unitless proportions, equivalent to 49%, 42%, and 9%.

A geographical pattern alone may have several explanations. When it agrees with fossil chronology, structural homology, molecular similarity, and quantitative analysis, the combined evidence supports a more coherent history of descent and change.

Fossil sequences document extinct as well as living forms, while homologies and molecular sequences connect those forms through shared features. Agreement among chronology, morphology, and molecules is stronger than relying on appearance alone.

These examples occur at different time scales, but each records the same defining feature: inherited population characteristics are not fixed. Past fossil transitions and present genomic or resistance changes are parts of one continuing evolutionary process.

If lineages A and B meet at one recent node, while lineage C joins their branch at an earlier node, A and B are more closely related to each other than either is to C. Moving or rotating the tips around a node does not change that ancestry.

Branching hypotheses can be constructed from morphology in living or fossil species and from DNA or protein sequence similarities. The branches can illustrate speciation events that produced distinct lineages.

Two populations may look different yet remain one species if they still exchange genes through viable, fertile offspring. Conversely, mating that produces no surviving offspring or only sterile offspring does not maintain gene flow between the populations.

For example, access to several distinct habitats can expose populations to different selective pressures, promoting divergent adaptation and rapid adaptive radiation. By contrast, similar environments can favor similar solutions in separate lineages, producing convergence.

During a disease outbreak, a genetically diverse population is more likely to include some less-susceptible individuals. If those individuals survive and reproduce, their alleles become better represented in later generations, helping the population recover.

Taken together, these dates place the plausible origin of life after conditions became suitable at about 3.9 bya and no later than the earliest known fossil evidence at 3.5 bya. The fossil date is evidence that life already existed, not necessarily the exact moment it began.

Experiments that model proposed early-Earth conditions can test whether organic molecules form spontaneously under those conditions. Results can support, weaken, or revise a model as assumptions about the early atmosphere change.