The unifying idea of biology
The geneticist Theodosius Dobzhansky wrote in 1973 that "nothing in biology makes sense except in the light of evolution." Evolution explains the diversity of life, the similarities between species, and why organisms fit their environments.
Darwin’s argument
Darwin’s theory of natural selection rests on a few observations and inferences:
- Organisms produce more offspring than can survive.
- Individuals vary, and much of that variation is inherited.
- Resources are limited, so there is competition.
- Individuals with traits better suited to the environment are more likely to survive and reproduce.
- Over generations, those favourable traits become more common.
In biology, fitness doesn’t mean strength. It means reproductive success: how many genes an organism passes on.
Types of selection
| Type | What it favours | Example |
|---|---|---|
| Directional | One extreme | Longer beaks in finches during drought |
| Stabilizing | The average | Human birth weight (very low and very high both riskier) |
| Disruptive | Both extremes | Seedcracker finches with very small or very large beaks |
| Sexual selection | Traits that attract mates | Peacock tail feathers |
Other mechanisms of evolution
Natural selection isn’t the only force.
- Mutation: the original source of new alleles.
- Gene flow: alleles move between populations through migration.
- Genetic drift: random changes in allele frequency, strongest in small populations.
- Bottleneck effect: a disaster sharply reduces population size (northern elephant seals were hunted down to perhaps 20–30 individuals in the 1890s; today’s population has very low genetic diversity).
- Founder effect: a small group starts a new population (some genetic conditions are more common in isolated communities).
Evidence for evolution
Fossil record
Fossils show life changing over time and include transitional forms:
- Tiktaalik (about 375 million years old): a fish with limb-like fins and a neck
- Archaeopteryx: feathered with teeth and a clawed tail
- The whale series, from land-dwelling Pakicetus to Ambulocetus to modern whales
Comparative anatomy
| Structure type | Meaning | Example |
|---|---|---|
| Homologous structures | Same basic structure, different functions; common ancestry | Human arm, cat leg, whale flipper, bat wing all have the same bones |
| Analogous structures | Same function, different structure; no common ancestry for that trait | Bird wings and insect wings (convergent evolution) |
| Vestigial structure | Reduced, little function | Human tailbone, whale pelvis, flightless bird wings |
Molecular biology
- All life uses DNA and nearly the same genetic code.
- The more closely related two species are, the more similar their DNA and proteins.
- Cytochrome c, a protein in respiration, differs by 0 amino acids between humans and chimps but by about a dozen between humans and dogs.
Biogeography
Species on islands resemble species on the nearest mainland, not species in similar habitats far away. Marsupials dominate Australia because the continent was isolated for millions of years.
Direct observation
- Antibiotic-resistant bacteria, such as MRSA
- Pesticide-resistant insects
- The Lenski E. coli experiment, running since 1988, has tracked more than 75 000 generations
Speciation
Speciation is the formation of a new species.
- Allopatric speciation: a geographic barrier separates populations (Kaibab and Abert’s squirrels on either side of the Grand Canyon).
- Sympatric speciation: new species form in the same area, often through changes in chromosome number in plants or different mating preferences.
Reproductive isolation can be prezygotic (different mating seasons, behaviours or habitats) or postzygotic (hybrids that are sterile, like mules).
Common mistakes
- "Survival of the fittest" means the strongest survives. It means the best reproducer.
- Evolution has a goal. It doesn’t; it responds to current conditions.
- A theory is just a guess. The theory of evolution is supported by evidence from many independent fields.
Try it yourself
- Explain, step by step, how a population of bacteria becomes resistant to an antibiotic.
- Decide whether each pair is homologous or analogous: (a) bat wing and butterfly wing; (b) human hand and dolphin flipper.
- Why does genetic drift affect small populations more than large ones?
Quick recap
- Natural selection favours inherited traits that improve reproductive success.
- Selection can be directional, stabilizing, disruptive or sexual.
- Mutation, gene flow and genetic drift also change populations.
- Evidence comes from fossils, anatomy, DNA, biogeography and observation.
- Speciation occurs when populations become reproductively isolated.