Biology · AP Biology
Natural selection
how populations change over generations
A population evolves when its genetic makeup changes from one generation to the next. Natural selection is the change that comes from some heritable traits leaving more offspring than others.
In the figure, moths rest on bark darkened by soot. Birds find the light moths easily and eat them; the dark ones are hard to see. The survivors breed, and their offspring look like them. Generation by generation, the dark form spreads: 2 of 44 moths at the start, 41 after 8 generations.
No moth changed colour. Selection acts on individuals, but it is the population that evolves.
This chapter covers how selection works and what it acts on, the other forces that change allele frequencies (drift, gene flow, mutation), the Hardy–Weinberg model of a population that is not evolving, the evidence for evolution and common ancestry, how trees of relationship are read, how new species form, and how life may have begun.
variation, inheritance, overproduction, differential success
Natural selection follows from four observations about any population.
| 1 | Individuals vary in their traits. |
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| 2 | Some of that variation is heritable: passed from parent to offspring in genes. |
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| 3 | More offspring are produced than can survive, so individuals compete for limited resources. |
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| 4 | Those whose heritable traits suit the environment better survive and reproduce more. |
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So the favoured heritable traits become more common in the next generation. The environment decides which traits are favoured, and when it changes, so does the direction of selection.
The peppered moth. In industrial England, soot darkened tree trunks and the dark form became common. When air pollution was reduced, trunks lightened and the light form returned.
A model. Let birds eat 50% of the light moths each generation and none of the dark. If a fraction f is dark, the dark fraction among the survivors is
| generation | 0 | 2 | 4 | 6 | 8 |
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| dark, expected | 0.05 | 0.17 | 0.46 | 0.77 | 0.93 |
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| dark, in the figure | 2/44 | 7/44 | 21/44 | 34/44 | 41/44 |
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Selection is not random, but the variation it sorts arises by random mutation. A moth cannot become dark because dark would help.
directional, stabilizing and disruptive selection
Selection acts on phenotypes, the traits an organism shows, and through them on the genes that produce them. Its pressure comes from the environment: predators, climate, food, disease, competitors.
In each panel, grey is the trait’s distribution before selection, blue is how well each trait value survives and reproduces, and red is the population after 20 generations.
| selection | favours | the population |
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| directional | one extreme | mean shifts toward it |
| stabilizing | the middle | variation narrows |
| disruptive | both extremes | splits toward two forms |
Directional. After a drought on the Galápagos left mainly large, hard seeds, ground finches with deeper beaks survived better, and the next generation’s average beak depth was larger.
Stabilizing. Human babies of very low or very high birth weight have historically survived less often than those of medium weight.
Disruptive. In one African finch, birds with small bills eat soft seeds and birds with large bills crack hard ones; medium bills do neither well.
Sexual selection favours traits that win mates, such as a peacock’s tail, even when they make survival harder.
humans choose who breeds
In artificial selection, people decide which individuals breed, choosing for traits they want. The mechanism is the same as natural selection; the selective agent is human.
A breeder keeps the tallest 20% of a crop as parents each generation. In generation 0 the crop averages 50 cm; every plant kept is at least 55.0 cm, and the kept plants average 58.4 cm. Their offspring average 54.2 cm: taller, but less tall than their parents, because part of the variation in height came from soil and water rather than genes.
| generation | 0 | 1 | 2 | 3 | 4 | 5 |
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| mean height (cm) | 50.0 | 54.2 | 58.4 | 62.6 | 66.8 | 71.0 |
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Selection only sorts variation that already exists and is heritable.
One wild species, many crops. Wild mustard, Brassica oleracea, was bred into:
| selected for | crop |
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| leaves | kale |
| terminal bud | cabbage |
| lateral buds | Brussels sprouts |
| flowers and stems | broccoli |
| stem | kohlrabi |
Dog breeds, all descended from wolves, are another example. Their great differences arose in a few thousand years.
drift, bottlenecks, founders and gene flow
Evolution is a change in a population’s allele frequencies. Natural selection is one cause; random chance, migration and mutation are others.
Mutation is the source of new alleles; it is the raw material of all the others.
Genetic drift is a random change in allele frequency from one generation to the next, because which individuals happen to reproduce is partly chance. It is strongest in small populations. In the figure every population starts at p = 0.5:
| N = 10 | N = 500 |
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| allele lost or fixed by generation 60 | 6 of 6 | 0 of 6 |
| p at generation 60 | 1, 0, 0, 1, 0, 0 | 0.39 to 0.59 |
Drift ignores fitness: a harmful allele can be fixed and a useful one lost. It reduces genetic variation within a population.
Two cases of drift. A bottleneck: a disaster or overhunting cuts a population to a few survivors, whose alleles may not represent the original population. A founder effect: a few individuals colonize a new place, and the new population carries only their alleles.
Gene flow is the movement of alleles between populations by migrating individuals or gametes. It makes populations more alike.
a population that is not evolving
For a gene with two alleles, let p be the frequency of A and q the frequency of a. Then
If gametes combine at random, an AA offspring needs A from both parents, probability p²; aa, q²; Aa, pq two ways, 2pq. That is the square in the figure. The allele frequencies come out unchanged, generation after generation, if five conditions hold:
| a large population | no drift |
| no migration | no gene flow |
| no mutation | no new alleles |
| random mating | no mate choice |
| no natural selection | equal fitness |
Real populations rarely meet all five. The equation is a null model: a population that departs from it is evolving.
Example. White flowers are recessive. In a population at equilibrium, 16 of 100 plants are white:
So about 48 of 100 plants are purple but carry the white allele, and 36 are AA. Always start from q²: the recessive phenotype is the only genotype seen directly.
A test. A count finds 45 AA, 30 Aa, 25 aa. Then p = (2·45 + 30)/200 = 0.60, and equilibrium predicts 36, 48, 16. Too few heterozygotes: something, such as nonrandom mating, is acting. A chi-square test says whether a gap is bigger than chance.
fossils, anatomy, molecules and geography
Evolution is supported by several independent kinds of evidence that agree with one another.
Homologous structures share an origin but may differ in function. In the figure, the forelimbs of a human, cat, whale and bat are built from the same bones in the same order: one upper-arm bone, two forearm bones, wrist bones, five digits. They are similar because they were inherited from a common ancestor, not because the jobs are similar.
Analogous structures do similar jobs but evolved independently: the wings of birds and of insects. That is convergent evolution, and it is not evidence of close relationship.
Vestigial structures are reduced remnants of structures that were functional in ancestors, such as the small hip bones of whales.
Fossils. Deeper rock layers are generally older, and their fossils show change over time. Radiometric dating gives absolute ages. Potassium-40 has a half-life of 1.25 billion years; a rock with 25% of its potassium-40 left has been through 2 half-lives, so is about 2.5 billion years old.
Molecular evidence. The more similar two species’ DNA and protein sequences, the more recently they shared an ancestor. Human and chimpanzee cytochrome c, a protein of the electron transport chain, are identical in amino-acid sequence.
Biogeography. Island species most resemble those of the nearest mainland, as on the Galápagos.
what every living cell shares
Features shared by all living things are best explained by descent from a common ancestor.
All three domains (Bacteria, Archaea, Eukarya) share:
| DNA and RNA | carry genetic information |
| the genetic code | the same codons specify the same amino acids, with few exceptions |
| ribosomes | translate mRNA into protein |
| core metabolic pathways | glycolysis, for example |
| a plasma membrane | a phospholipid bilayer |
That is why a human gene placed in a bacterium can be translated into the human protein.
All eukaryotes also share features bacteria lack, evidence that they descend from one eukaryotic ancestor:
| membrane-bound organelles | nucleus, mitochondria |
| linear chromosomes | bacteria have one circular chromosome |
| genes that contain introns | removed from the RNA before translation |
In the figure the red pulses run from one ancestor down to each cell: the ribosomes and the code they read were inherited, not invented three times.
resistance, evolving as we watch
All species continue to evolve. It can be seen in a few years where generations are short and selection is strong.
Antibiotic resistance. A culture holds 2 resistant cells among 110. Each treatment kills 90% of the susceptible cells and none of the resistant; the survivors divide and refill the dish.
| round | 0 | 1 | 2 | 3 | 4 |
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| resistant, expected | 2% | 17% | 67% | 95% | 100% |
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| resistant, in the figure | 2 | 19 | 85 | 108 | 110 |
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The antibiotic does not cause resistance. The resistance alleles arose earlier by random mutation; the drug selects them by killing everything else.
The same happens with insects and pesticides, weeds and herbicides, tumour cells and chemotherapy, and HIV and antiviral drugs. Influenza changes so quickly that the vaccine is remade each year. Pathogens that evolve to infect new hosts cause emergent diseases.
The fossil record and genomes record the same continuing change over longer times.
reading and building evolutionary trees
A phylogenetic tree or cladogram is a hypothesis about how groups are related by descent.
Each node is the most recent common ancestor of everything beyond it. Two groups are more closely related the more recent their shared node. Rotating branches about a node changes nothing.
A clade is an ancestor and all its descendants. Clades are defined by shared derived characters, traits that arose in their common ancestor. Here each red bar marks where a trait arose:
| lancelet | lamprey | tuna | salamander | turtle | leopard |
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| vertebral column | – | + | + | + | + | + |
| hinged jaws | – | – | + | + | + | + |
| four limbs | – | – | – | + | + | + |
| amniotic egg | – | – | – | – | + | + |
| hair | – | – | – | – | – | + |
The outgroup, the lancelet, shares none of these traits; it shows which state is ancestral. The simplest tree, with the fewest changes, is preferred (parsimony).
Trees are built from morphology and, more reliably, from DNA and protein sequences. A cladogram shows only the order of branching; a phylogenetic tree may scale branch lengths to time or to the amount of change. A molecular clock uses a steady rate of mutation to estimate when lineages split.
A tree is not a ladder: the leopard is not more evolved than the tuna. All living groups have evolved for equally long.
one species becomes two
By the biological species concept, a species is a group whose members can interbreed in nature and produce viable, fertile offspring, and cannot do so with other groups.
A new species forms when populations become reproductively isolated. Isolating barriers:
| prezygotic | habitat, timing, courtship behaviour, mechanical fit, gametes that do not fuse |
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| postzygotic | hybrids that do not survive; sterile hybrids (a mule, from a horse and a donkey); hybrids whose offspring fail |
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Allopatric speciation, in the figure: a geographic barrier divides a population. With no gene flow, the halves diverge by mutation, drift and selection. If they no longer interbreed when the barrier goes, they are two species.
Sympatric speciation happens without a barrier. In plants, an error in meiosis can produce polyploid offspring that cannot form fertile hybrids with the parent species, a new species in one generation.
Tempo. In gradualism, species change slowly and steadily; in punctuated equilibrium, long stasis is broken by rapid change. Adaptive radiation is the rapid rise of many species from one ancestor as new habitats or resources open, as with the Galápagos finches.
diversity is a population’s insurance
A population’s genetic variation determines how well it can survive an environmental change. Selection can only favour alleles that are already present.
In the figure a blight crosses two fields. In the clone field every plant has the same genotype, and none survives. In the varied field, 12 of 30 plants carry resistance and remain to reseed it.
This happened in Ireland in the 1840s. The potatoes grown were propagated as clones, and the potato blight killed almost all of them, causing famine.
Fitness is reproductive success: the number of offspring an individual leaves that survive to reproduce, relative to others. It is not strength or lifespan. Relative fitness divides each genotype’s success by the best:
| genotype | AA | Aa | aa |
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| offspring | 50 | 40 | 20 |
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| relative fitness | 1.00 | 0.80 | 0.40 |
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Fitness depends on the environment. Where malaria is common, people heterozygous for the sickle-cell allele resist malaria and have the highest fitness; where it is absent, the same allele brings only cost. Such heterozygote advantage keeps both alleles in the population.
from simple molecules to cells
Earth formed about 4.6 billion years ago. Its surface was too hostile for life until about 3.9 billion years ago, and the earliest fossil evidence of life is about 3.5 billion years old.
Organic molecules without life. In 1953, Stanley Miller and Harold Urey boiled water into a flask of methane (CH4), ammonia (NH3) and hydrogen (H2), passed sparks through it as lightning, and cooled the products. Within a week, amino acids had formed. Experiments like this show that organic monomers can arise from inorganic molecules, given energy.
Organic molecules may also have formed near deep-sea hydrothermal vents, or arrived on meteorites.
A model of the steps:
| 1 | abiotic synthesis of monomers: amino acids, nucleotides |
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| 2 | monomers join into polymers, for example on hot rock or clay |
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| 3 | polymers are enclosed in membranes: protocells with an internal chemistry |
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| 4 | self-replicating molecules make inheritance possible |
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The RNA world. RNA can both store information and catalyse reactions (ribozymes), so it may have been the first genetic material, before DNA and proteins took over those roles.
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