11.3 Evidence for Evolution and Speciation

Evolution describes cumulative change in heritable population characteristics. Common descent adds the claim that present lineages arose by repeated divergence from ancestral populations. These claims do not require every lineage to change at the same rate or progress toward greater complexity. A lineage in a stable niche may show little morphological change while its genome continues to accumulate neutral substitutions.

No single fossil, sequence or breeding experiment establishes the entire history of life. Confidence comes from independent evidence converging on compatible relationships and mechanisms. Fossils order forms through geological time; biogeography connects distribution to isolation; homologous structures preserve modified ancestral plans; direct observations reveal population change; and molecular comparisons quantify inherited similarity.

Fossils include mineralized remains, impressions, tracks and other preserved traces. Sedimentary layers provide relative order, while radiometric dating uses predictable isotope decay to estimate absolute ages of suitable material or surrounding rock. Carbon-14 is useful only for relatively recent organic remains because little remains after many half-lives; older geological samples require isotopes with longer half-lives.

The fossil record is biased. Hard parts preserve more readily than soft tissues, rapid burial is uncommon, erosion destroys deposits and only a tiny fraction is discovered. A sequence of related forms supports evolutionary change, but one fossil species cannot usually be declared the direct ancestor of another. Both may instead lie on neighboring branches descending from an unsampled common ancestor.

Transitional fossils combine features associated with different groups and can test the order in which traits evolved. Their value is not that they are half-made organisms; each was a functioning organism adapted to its own environment. Gaps are expected from preservation probabilities and do not erase the chronological and anatomical patterns present in the surviving record.

Artificial selection demonstrates inherited change under controlled mate choice. Field and experimental studies can observe natural selection directly. In guppies, for example, male coloration reflects opposing pressures: conspicuous males may attract mates but also predators. Transplant or predator-manipulation studies can connect a changed pressure to subsequent phenotype-frequency change, especially when replicated and followed across generations.

DNA evidence provides an enormous record of descent. Shared unusual mutations, aligned non-coding sequences, gene order and pseudogenes can be especially informative because their agreement is unlikely to arise independently. Mitochondrial DNA is inherited mainly through the maternal line in humans, does not undergo the same recombination as nuclear chromosomes and occurs in many copies per cell, making it useful for relatively recent maternal ancestry. It traces one lineage, not the ancestry of the entire genome.

Amino-acid sequences change more slowly than their genes because synonymous substitutions do not alter protein sequence and harmful protein changes are removed by selection. Close protein similarity supports common ancestry but identical short sequences do not prove species identity. Evidence becomes stronger when many independent loci yield the same branching pattern.

Speciation is the evolution of reproductive isolation between populations. The essential first step is reduced gene flow. Without it, interbreeding continually mixes alleles and opposes divergence. Once exchange is restricted, mutation, selection and drift alter each gene pool independently. Reproductive barriers may strengthen until individuals no longer mate successfully or no longer produce fertile offspring.