Evidence of Evolution

Separate three questions that evidence can answer

Has a population or lineage changed? Do organisms share ancestry? What process caused a particular change? These questions are related, but they are not interchangeable. A change in allele frequency documents evolution at a locus; it does not by itself identify selection rather than drift. A homologous structure supports shared ancestry, but it does not measure the strength of past selection.

The strength of evolutionary explanation comes from connecting evidence that makes different measurements. A proposed history must fit chronological order as well as anatomical and molecular relationships. Revising one fossil's placement or one branch of a tree does not erase the broader evidence for descent with modification.

Fossils record organisms, not a complete family album

Fossils include preserved body material, impressions, and traces such as tracks or burrows. A sequence of fossil-bearing layers can show appearances, changes in trait combinations, and disappearances. Fossilization and discovery are uneven: hard parts and organisms in depositional environments are more likely to be represented than many soft-bodied or upland organisms.

A transitional fossil combines traits informative about an evolutionary transition. It need not be the direct ancestor of a living species, and it is not an unfinished organism. It lived as an organism in its own environment and may belong to a side branch close to the lineage of interest. Evolution branches; fossils need not form a single ladder leading toward modern humans.

An absent fossil in one layer is therefore weaker evidence than “the organism certainly did not exist.” Preservation, exposure of the right rocks, and sampling must also be considered.

A radioactive clock must suit the sample

Radiometric methods use predictable isotope decay. In a simple closed-system model, each half-life leaves half the original parent isotope. If \(N_0\) is the starting amount, \(N(t)\) the amount remaining after time \(t\), and \(T_{1/2}\) the half-life:

For an idealized piece of once-living material with one-eighth of its original carbon-14 remaining, \(1/8=(1/2)^3\). Three half-lives have elapsed. Using 5,730 years gives \(t=3(5730)=17{,}190\) years. This is an illustrative decay calculation, not a calibrated calendar age: actual radiocarbon work accounts for changing initial carbon-14 levels, contamination, and other sample-specific effects.

Carbon-14 is useful for suitable carbon-containing remains within roughly the last 50,000 years, not for million-year-old fossils. Older volcanic minerals can be dated using other isotope systems. In the ash example, choose a method appropriate to the minerals and age range rather than assuming every fossil is dated with carbon-14.

Shared structure is more informative than shared purpose

A homology is a similarity inherited from common ancestry. Compare the forelimb skeletons of a human and a whale: the corresponding bones retain a related arrangement even though the limbs perform different tasks. Homology concerns the history of the structure, not whether the organisms use it in the same way.

Analogy describes independently evolved similarity in function or form. Bird and bat wings provide a useful level-of-comparison check: their underlying forelimb bones are homologous, while their specialized flight surfaces evolved independently. “Both fly” is not enough to infer that they inherited flight from their most recent common ancestor.

Similar environmental demands can favor similar solutions through convergent evolution. To distinguish convergence from shared ancestry, examine detailed organization, development, and molecular evidence rather than only an external outline.

A vestigial structure is reduced or modified relative to an ancestral role. It need not be useless; another function can remain. Likewise, shared developmental features can be informative without implying that embryos are identical or that an embryo passes through the adult forms of its ancestors.

Compare molecular records at corresponding positions

DNA and protein comparisons are useful when the regions being compared have a justified correspondence. Align homologous regions and account for insertions, deletions, or gene duplication. Otherwise, a difference count may compare unrelated positions or different gene copies.

Here are invented 12-base DNA segments from the same hypothetical locus in three sampled taxa. All are written in the same 5′ to 3′ orientation. Spaces separate groups of four for readability, not codons. They are not sequences from real species and have no assigned protein function.

A and B differ only at position 9. A and C differ at positions 5, 9, and 11; B and C differ at positions 5 and 11. The pairwise counts are therefore 1, 3, and 2, respectively. A–B sequence identity is:

A–C identity is \(9/12=75\%\), and B–C identity is \(10/12\approx83.3\%\). In this fragment A and B are most similar. With comparable evolutionary rates and an appropriate locus, that pattern is consistent with a closer relationship—but twelve sites alone do not establish the species tree.

Multiple changes at one site can hide past substitutions. Selection can conserve some regions, and different genes can have different histories. Use longer alignments, several independently informative loci, and suitable models before drawing a strong phylogenetic conclusion. Do not translate “one mismatch” directly into “one million years” without a justified clock calibration.

Percent identity is a measured similarity, not a “percent homology.” Homology is an inference about shared origin. Conserved biochemical systems, DNA sequences, and amino acid sequences offer related evidence, but a protein alignment and the DNA encoding that same protein are not completely independent datasets.

Geography adds another prediction

Biogeography asks how organisms and fossils are distributed across space. Shared ancestry combined with colonization, barriers, and isolation predicts distributions shaped by history, not only by current climate. Island lineages can diversify after arrival from a source region, while similar habitats far apart can contain distantly related organisms with convergent adaptations.

In a hypothetical island study, plants on two islands share several unusual molecular variants with plants on a nearby continent, but not with look-alike plants in a distant, similar climate. That pattern supports a historical connection to the nearby source more directly than appearance alone. It does not determine the date or direction of dispersal without additional evidence.

Build a conclusion that uses evidence without overstating it

Agreement is strongest when the methods have different sources of error. A geological age, an anatomical comparison, and several molecular datasets can constrain the same history from different directions. If evidence conflicts, check sample identity, dating context, alignment, and alternative evolutionary histories; disagreement is a reason to refine the explanation, not to hide the observation.

Practice making a bounded inference