Evolution, illustrated
How does a process with no designer, plan or goal produce eyes, wings and brains? The answer is an algorithm short enough to state in one line: copy with small errors, test the copies against the world, keep what works, repeat for a very long time. Biology gives us both the source code (genomes) and a partial version history (fossils), and the two agree. Darwin's revolution appears in The revolutions of our time. This post is the illustrated tour.
+--------------------+ +--------------------+ +--------------------+
| VARIATION | | INHERITANCE | | SELECTION |
| offspring differ | |traits are passed on| |some variants leave |
| (mutation, mixing |-->| (genes, copied |-->| more descendants |
| of parental genes) | | with small errors) | |in this environment |
+---------+----------+ +--------------------+ +---------+----------+
^ |
| |
+-------------------------------------------------+
TIME: repeat for thousands to billions of generationsFour ingredients
If a population has variation, if that variation is inherited, and if some variants leave more descendants than others (selection), then given enough time the population changes. Nothing else is needed: no foresight, no goal. Richard Lewontin spelled out these conditions in 1970. Darwin knew nothing of genes, and the logic worked without them.
Selection is not magic. It acts only on variation that already exists, and only in the environment where it happens. Even a modest edge compounds quickly:
generation share of the population (# dark morph, . pale morph)
0 |####....................................| 10.0%
5 |############............................| 29.2%
10 |########################................| 60.5%
15 |##################################......| 85.0%
20 |######################################..| 95.5%
30 |########################################| 99.7%
assumed: dark moths leave 1.3 times as many offspring as pale
ones. Illustrative numbers: one gene, no mutation, no drift.Notice the shape: a slow start while the variant is rare, a fast middle, a slow finish. Real populations are messier. Environments change, so yesterday's winner can become tomorrow's loser, and most traits carry trade-offs.
Darwin, Wallace and the long wait
Charles Darwin sailed on HMS Beagle (1831-36), had the core idea by 1838 after reading Malthus on population, and then held it back for twenty years while he gathered evidence. In June 1858 an essay arrived from Alfred Russel Wallace, who was collecting specimens in the Malay Archipelago and had reached the same mechanism independently. Friends arranged a joint presentation at the Linnean Society on 1 July 1858, and On the Origin of Species followed on 24 November 1859. The phrase "survival of the fittest" came from the philosopher Herbert Spencer (1864), and Darwin adopted it in later editions.
The missing piece was inheritance. Mendel's pea experiments (1866) went unnoticed until 1900. The modern synthesis of the 1930s and 1940s (Fisher, Haldane, Wright, Dobzhansky, Mayr) joined Darwin to genetics, and Watson and Crick (1953) supplied the molecule. An influential story, associated with Ernst Mayr, says Darwin replaced fixed "essences", the kind discussed in What is existence? A short tour of ontology, with "population thinking". Some historians think that story oversimplifies, but the shift of attention from the ideal type to the varying population is real.
More than selection
- Mutation creates new variants. A human child carries on the order of seventy new mutations that neither parent had.
- Genetic drift is change by chance, strongest in small populations. In Motoo Kimura's neutral theory (1968), most molecular differences between species are neutral and spread by drift.
- Gene flow moves genes between populations. It can spread a useful variant or blur differences.
- Sexual selection, which Darwin developed in 1871, favours traits that win mates, even costly ones like a peacock's tail.
Biologists still argue about how much weight each force deserves, and whether development and non-genetic inheritance need a larger role in theory. Those are debates about how evolution works, not whether it happened.
The evidence
Several independent lines point the same way.
- Fossils that fit predictions. Searching 375-million-year-old rocks on Ellesmere Island in Arctic Canada, where fish-to-land-animal transition sediments were expected, Neil Shubin's team found Tiktaalik in 2004. Fossil series also trace whales back to four-legged land mammals, and DNA places their closest living relatives among hippos.
- Shared code and dead code. A bat wing, a whale flipper and a human arm contain the same bones. In genomes, humans, other apes and monkeys carry a broken copy of GULO, a gene that makes vitamin C in most other mammals. Human chromosome 2 is a fusion of two chromosomes that stay separate in other apes, with telomere remnants in the middle. A reverse engineer knows the logic: two programs with the same odd bug share ancestry. Life is full of shared bugs.
- Molecular clocks. DNA differences accumulate roughly steadily (Zuckerkandl and Pauling, 1960s). Calibrated against fossils, they date splits, such as human and chimp lineages at about 6-7 million years ago. Rates vary with generation time, so the dates are ranges.
- Evolution you can watch. Penicillin-resistant bacteria appeared within a few years of its mass use in the 1940s, and Fleming warned of it in 1945. A 2019 estimate attributes over a million deaths a year directly to resistant bacterial infections. Richard Lenski's E. coli lines have evolved since 1988 through more than 75,000 generations, and one lineage learned to use citrate around generation 31,000. On the Galapagos island of Daphne Major, Peter and Rosemary Grant saw finch beaks deepen after the 1977 drought.
- Our own past. Ancient DNA shows that modern humans and Neanderthals interbred. People whose ancestors lived outside Africa carry roughly one to two percent Neanderthal DNA, a result by Svante Pääbo's group that earned the 2022 Nobel Prize.
Peppered moths deserve a caveat. The dark form was rare before 1848, then dominated near industrial cities, then declined after clean-air laws. Bernard Kettlewell's 1950s experiments were criticised for design flaws, such as moths placed on tree trunks by day, and textbooks later told the story too neatly. But a six-year repeat by Michael Majerus, published in 2012, still supports bird predation as the main driver, and in 2016 the dark form was traced to a mobile genetic element inserted in a single gene.
Deep time
The fourth ingredient is the hardest to feel. Squeeze the age of the Earth into one day and the picture below appears. Dates are approximate and revised often, but their order is robust.
clock* years ago event
/| |\
|| 23:59:59.9 5,000 years ago writing ||
||============================================================||
|| 23:59:54 300,000 years ago Homo sapiens ||
||============================================================||
|| 23:57:56 6.5 million human and chimp lines split ||
||============================================================||
|| 23:39 66 million non-bird dinosaurs end ||
||============================================================||
|| 22:01 375 million Tiktaalik, a fish with limbs ||
||============================================================||
|| 20:50 600 million first animals ||
||============================================================||
|| 14:29 1.8 billion complex (eukaryotic) cells ||
||============================================================||
|| 11:19 2.4 billion oxygen builds up in the air ||
||============================================================||
|| 05:30 3.5 billion oldest accepted fossils ||
||============================================================||
|| 00:00 4.54 billion Earth forms ||
|| ||
* Earth's 4.54 billion years squeezed into one day; top = nowThe tree of life
In 1837 Darwin sketched a branching tree in a notebook and wrote "I think" above it. The Origin's only illustration is a branching diagram. Carl Woese's ribosomal RNA work (1977) showed that Archaea are a third major group beside Bacteria and Eukarya. Many recent analyses nest the eukaryotes inside the Archaea, which would make two domains. Microbes also swap genes sideways, so the deepest roots look more like a web than a tree. Mitochondria descend from engulfed bacteria (Lynn Margulis, 1967). At the root of the sample below sits LUCA, the last universal common ancestor, a population that lived somewhere around 3.5 to 4 billion years ago; estimates vary.
older (branching order only) tips: all alive today
LUCA -+------------------------------------- Bacteria
+--+---------------------------------- Archaea
+--+------------------------------- Plants and algae
+--+---------------------------- Fungi
+--+------------------------- Insects, snails, worms
+--+---------------------- Ray-finned fish
+--+------------------- Amphibians
+--+---------------- Reptiles and birds
+--+------------- Dogs, whales, bats
+--+---------- Old World monkeys
+-+-------- Orangutan
+-+------ Gorilla
+-+---- Chimps and bonobos
+---- HumansSome surprises sit in the branching order. Fungi are closer kin to animals than plants are. Birds are dinosaurs. Whales are even-toed ungulates, with hippos as their nearest living relatives. The drawing follows our own lineage as a spine, which tempts you to read it as a ladder toward us. All tips are alive today, and none is further along than the others. Redraw it with a mushroom as the spine and it looks just as sensible.
Misconceptions
+--------------------------+-------------------------------------------+
| MYTH | WHAT THE EVIDENCE SAYS |
+--------------------------+-------------------------------------------+
| Survival of the | Fitness means leaving more surviving |
| strongest | offspring in a given environment. Winners |
| | can be small, shy, cooperative, or lucky. |
+--------------------------+-------------------------------------------+
| We descended from chimps | Humans and chimps share an ancestor from |
| | some 6-7 million years ago. Neither is |
| | the ancestor of the other. |
+--------------------------+-------------------------------------------+
| Evolution climbs a | It is a branching bush with no goal. By |
| ladder to humans | count, microbes still dominate life. |
+--------------------------+-------------------------------------------+
| It is just a theory | In science a theory is a broad, tested |
| | explanation, like germ theory or the |
| | theory of gravity. |
+--------------------------+-------------------------------------------+
| Individuals evolve | An individual develops; a population |
| | evolves over generations as gene |
| | frequencies shift. |
+--------------------------+-------------------------------------------+
| It is all random chance | Mutation is random with respect to need. |
| | Selection is not random. Together they |
| | are not blind luck. |
+--------------------------+-------------------------------------------+Two limits are worth stating. Evolution explains how life diversified, not how it began; that is a separate research question. And it is a theory about biology, not a guide to values. Many scientists who are religious accept evolution, and how it bears on questions of meaning is a conversation biology does not settle.
Closing
Reading a genome is reverse engineering with a version history attached. The result has no author, yet it shows the marks of legacy code. A giraffe's laryngeal nerve runs down the neck, around a blood vessel and back up, a detour of several metres where centimetres would do, because evolution edits what exists and cannot restart from scratch.