Jugami Biology

Biology · Lesson 3

Evolution by Natural Selection

Small differences, sorted over many generations.

Every population varies. Some of those differences are inherited, and some help their owners survive and reproduce. Over many generations, the helpful versions become more common. That simple process, natural selection, explains how living things come to fit their surroundings and how one ancestor can give rise to many species.

This lesson builds on DNA and Genetics: the variation that selection sorts begins as changes in DNA.

Ink sketch of a branching tree of descent: three finch heads with different beak shapes at the living branch tips, and one branch marked extinct.extinctfig. 1: one ancestor, many beaks

1. Darwin, Wallace and the mechanism

Charles Darwin began forming his ideas after the voyage of HMS Beagle (1831–1836), and by 1838 he had worked out natural selection. He spent the next twenty years gathering evidence. In 1858 Alfred Russel Wallace, collecting in the Malay Archipelago, sent Darwin an essay describing the same idea. Their work was presented together at the Linnean Society of London on 1 July 1858 and published as a joint paper that year. Darwin's book On the Origin of Species followed on 24 November 1859.

Natural selection follows from a few observations anyone can check:

  1. Variation. Individuals in a population differ in size, color, behavior and more.
  2. Inheritance. Some of those differences are passed from parents to offspring through genes.
  3. Overproduction. More offspring are born than can survive, so they compete for food, mates and safety.
  4. Selection. Individuals with helpful inherited traits tend to survive and reproduce more, passing those traits on.
  5. Adaptation over time. Across many generations, helpful traits become more common in the population.

Selection does not create variation; it sorts variation that already exists. New variation comes from mutation and from the shuffling of genes in sexual reproduction (see mutations and inheritance).

2. Darwin's finches

The Galápagos Islands hold more than a dozen species of finch that differ mainly in beak size and shape, each suited to different foods. Darwin collected some in 1835, though he did not label them carefully by island. Later studies showed they descend from a single ancestral species that reached the islands from South America.

Starting in 1973, Peter and Rosemary Grant measured nearly every finch on the small island of Daphne Major, year after year. In 1977 a severe drought hit. Small, soft seeds ran out, leaving mostly large, hard seeds that only birds with deep, strong beaks could crack. The medium ground finch population (Geospiza fortis) crashed, with roughly 85 percent dying. The survivors had beaks that were, on average, about 4 percent deeper than before the drought, and because beak depth is strongly inherited, their offspring had deeper beaks too.

Selection can also reverse. After the 1980s, a larger seed-eating species, the large ground finch, settled on Daphne Major. During a drought in 2004–2005 the two species competed for big seeds, and this time medium ground finches with smaller beaks did better.

A change of a few tenths of a millimeter sounds tiny, but the Grants' measurements showed natural selection working within a few years, in the wild.

3. Peppered moths

The peppered moth (Biston betularia) rests on tree bark by day, and birds hunt it by sight. Most were pale and speckled, camouflaged on lichen-covered bark. A dark form, carbonaria, was first recorded near Manchester in 1848. As factories coated trees in soot and killed the lichens, dark moths became hidden and pale ones exposed. By the 1890s the dark form made up the large majority of moths around Manchester. This change is called industrial melanism.

In the 1950s Bernard Kettlewell released marked moths in a polluted wood near Birmingham and a clean wood in Dorset, then recaptured them. More dark moths survived in the polluted wood, and more pale moths in the clean one. Critics later questioned parts of his methods, so Michael Majerus ran a larger experiment in Cambridge from 2001 to 2007, watching where moths rested naturally and how often birds ate them. Pale moths survived better in the now cleaner air, supporting bird predation as the main cause. His results were published in 2012, after his death.

After clean-air laws in the 1950s and 1960s cut soot, the dark form became rare again. In 2016 researchers found the DNA change behind it: a stretch of jumping DNA (a transposable element) inserted into a gene called cortex, estimated to have arisen around 1819.

4. Selection simulator

This model follows a large moth population. The dark allele is dominant, so moths with one or two copies are dark. Each generation, the camouflaged color survives 20 percent better than the other. It starts with dark moths rare, as in early 1800s England. Change the bark and watch what happens.

A patch of tree bark with 40 resting peppered moths. The share of dark and pale moths matches the current population.

Turn on JavaScript to run the simulator. Without it: on sooty bark, dark moths rise from rare to the large majority in roughly 30 to 60 generations; switch to clean bark and they fall back.

Line graph of the percentage of dark moths in each generation 100% 50% 0% 0 50 generations % of moths that are dark

Try this: run 40 generations on sooty bark, then switch to clean bark. Then try the tiny population. Its line wobbles because of chance (genetic drift), and the dark allele can even be lost before selection gets a chance to act.

5. The evidence

Fossils

Transitional forms

Evolution predicts in-between forms in rocks of the right age. In 2004 Neil Shubin's team searched 375-million-year-old rocks on Ellesmere Island, in Arctic Canada, and found Tiktaalik: a fish with a flat head, a neck, and sturdy fins with wrist-like bones. Whale fossils show a series from land mammals such as Pakicetus (about 50 million years ago) through Ambulocetus and Rodhocetus to fully aquatic Basilosaurus, which still had tiny hind legs.

Anatomy

Homologous and vestigial structures

Homologous structures share the same bones, inherited from a common ancestor, but do different jobs (fig. 3). Vestigial structures are reduced and have lost most or all of their old function: the small pelvic bones in whales, the human tailbone, and the muscles that raise goosebumps.

Embryology

Shared beginnings

Early vertebrate embryos look alike. Fish, chicken and human embryos all have pharyngeal arches and a tail. In fish the arches become gills; in humans they become parts of the jaw, ear and throat. (Ernst Haeckel's famous 1870s drawings exaggerated the likeness, but photographs confirm the shared features.)

Biogeography

Where species live

Island species most resemble those on the nearest mainland: Galápagos finches resemble South American birds, not African ones. Australia's marsupials diversified in isolation to fill roles that placental mammals fill elsewhere.

Molecules

DNA and proteins

All life uses the same genetic code. The more closely related two species are thought to be from fossils and anatomy, the more similar their DNA. Human and chimpanzee DNA differ at only about 1.2 percent of single letters in sequences that line up, and both carry the same old viral DNA insertions at the same places.

Watching it happen

Experiments

Since 1988, Richard Lenski's lab has grown 12 populations of E. coli bacteria, over 75,000 generations. One population evolved the ability to feed on citrate, a new trait, around generation 31,500. Bacteria evolving antibiotic resistance is selection happening in hospitals today.

Forelimbs of a human, a cat, a whale and a bat drawn side by side. Each has the same set of bones, colored the same way: one upper arm bone, two forearm bones, wrist bones and five digits, in different proportions.Human armCat legWhale flipperBat wingupper arm (humerus)forearm (radius, ulna)wrist (carpals)digitsfig. 3: same bones, different jobs (not to scale)
Fig. 3. Homologous forelimbs: the same four sets of bones, colored alike, reshaped for grasping, walking, swimming and flying.

6. Other mechanisms and new species

MechanismWhat happensExample
MutationChanges in DNA create new alleles, the raw material for all other mechanisms.The cortex insertion behind dark peppered moths.
Genetic driftAllele frequencies change by chance. Strongest in small populations, including after a bottleneck or when a few founders start a new population.Northern elephant seals were hunted to perhaps a few dozen animals in the 1890s and now have very little genetic variation.
Gene flowIndividuals moving between populations carry alleles with them, making populations more alike.Pollen blown between plant populations.
Sexual selectionTraits that help win mates spread, even when they cost survival.A peacock's tail.

Speciation

A species is usually defined as a group whose members can interbreed and produce fertile offspring. New species form when populations become reproductively isolated. Most often a barrier such as a mountain range, river or stretch of sea separates them (allopatric speciation). Each population then mutates, drifts and adapts in its own way until, even if they meet again, they no longer interbreed. Speciation can also happen without a geographic barrier, for example when plants double their chromosome number.

7. Common misconceptions

Myth

"It's just a theory"

In everyday speech a theory is a hunch. In science a theory is a well-tested explanation that ties together many facts, like germ theory or the theory of gravity. Evolution is both an observed fact (populations change) and a theory that explains how.

Myth

"Individuals evolve"

A finch cannot grow a deeper beak because it needs one. Individuals survive or not; populations evolve as the mix of inherited traits shifts across generations.

Myth

"Evolution is a ladder toward humans"

Evolution is a branching tree, not a ladder. Humans did not evolve from chimpanzees; we share a common ancestor that lived roughly 6 to 7 million years ago. Every living species is equally "evolved" for its own way of life, and evolution has no goal.

8. How do we know?

  1. 1809 Jean-Baptiste Lamarck proposes that species change over time, though his mechanism (inheriting traits acquired during life) turned out to be wrong.
  2. 1831–1836 Darwin sails on HMS Beagle, visiting the Galápagos in 1835.
  3. 1858 Darwin and Wallace's joint paper is read at the Linnean Society.
  4. 1859 On the Origin of Species is published.
  5. 1866 Gregor Mendel publishes his pea-plant experiments on inheritance, largely ignored until 1900.
  6. 1930s–1940s The modern synthesis joins Darwin's selection with Mendel's genetics, through the work of Fisher, Haldane, Wright, Dobzhansky, Mayr and others.
  7. 1953 Watson and Crick publish the structure of DNA, revealing the molecule that varies and is inherited. In the same year Kettlewell runs his first moth experiments.
  8. 1973 Peter and Rosemary Grant begin their study of finches on Daphne Major.
  9. 1977 A drought on Daphne Major drives measurable selection for deeper beaks.
  10. 1988 Richard Lenski begins his long-term E. coli evolution experiment.
  11. 2004 Tiktaalik is discovered on Ellesmere Island (described in 2006).
  12. 2012 Majerus's peppered moth predation results are published.
  13. 2016 The DNA change behind dark peppered moths is identified.

Quiz: Field Station Check

Talk about it

  1. Antibiotics kill most bacteria in an infection. Use the five steps of natural selection to explain why stopping a course of antibiotics early can lead to resistance.
  2. If the peppered moth's dark allele had never arisen by mutation, could soot have made the moths turn dark? Why or why not?
  3. The Grants saw beak depth go up in 1977 and down in 2004–2005. Does that mean evolution "went backwards"? What does it tell you about selection?
  4. Why might genetic drift matter more for an endangered species than for a common one?
  5. Someone says, "If humans came from monkeys, why are there still monkeys?" How would you answer using the idea of a branching tree?

Sources

  1. Darwin, C. and Wallace, A. R. (1858). On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection. Journal of the Proceedings of the Linnean Society of London, Zoology 3: 45–62. doi:10.1111/j.1096-3642.1858.tb02500.x
  2. Darwin, C. (1859). On the Origin of Species by Means of Natural Selection. London: John Murray. Full text: Darwin Online
  3. Boag, P. T. and Grant, P. R. (1981). Intense natural selection in a population of Darwin's finches (Geospizinae) in the Galápagos. Science 214: 82–85. doi:10.1126/science.214.4516.82
  4. Grant, P. R. and Grant, B. R. (2006). Evolution of character displacement in Darwin's finches. Science 313: 224–226. doi:10.1126/science.1128374
  5. Kettlewell, H. B. D. (1955). Selection experiments on industrial melanism in the Lepidoptera. Heredity 9: 323–342. doi:10.1038/hdy.1955.36
  6. Cook, L. M., Grant, B. S., Saccheri, I. J. and Mallet, J. (2012). Selective bird predation on the peppered moth: the last experiment of Michael Majerus. Biology Letters 8: 609–612. doi:10.1098/rsbl.2011.1136
  7. Van't Hof, A. E. et al. (2016). The industrial melanism mutation in British peppered moths is a transposable element. Nature 534: 102–105. doi:10.1038/nature17951
  8. Daeschler, E. B., Shubin, N. H. and Jenkins, F. A. (2006). A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440: 757–763. doi:10.1038/nature04639
  9. Gingerich, P. D. et al. (2001). Origin of whales from early artiodactyls: hands and feet of Eocene Protocetidae from Pakistan. Science 293: 2239–2242. doi:10.1126/science.1063902
  10. Thewissen, J. G. M. et al. (2001). Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls. Nature 413: 277–281. doi:10.1038/35095005
  11. Chimpanzee Sequencing and Analysis Consortium (2005). Initial sequence of the chimpanzee genome and comparison with the human genome. Nature 437: 69–87. doi:10.1038/nature04072
  12. Blount, Z. D., Borland, C. Z. and Lenski, R. E. (2008). Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli. PNAS 105: 7899–7906. doi:10.1073/pnas.0803151105
  13. National Academy of Sciences and Institute of Medicine (2008). Science, Evolution, and Creationism. Washington, DC: National Academies Press. nap.nationalacademies.org/catalog/11876
  14. University of California Museum of Paleontology. Understanding Evolution. evolution.berkeley.edu