Speciation and the history of life
How does one species become two? And how has life changed across billions of years?
What a species is
The biological species concept defines a species as a group whose members can interbreed in nature and produce fertile offspring. Horses and donkeys can mate, but their offspring, mules, are sterile, so they are separate species. The concept does not work for asexual organisms or fossils, so other definitions are also used.
Reproductive isolation
Species stay separate through barriers to interbreeding: different habitats, breeding seasons, courtship behaviours, or incompatible body parts (before mating), or hybrids that die or are sterile (after mating).
How speciation happens
In allopatric speciation, a population is split by a geographic barrier, such as a river or mountain range, and the separated groups evolve independently until they can no longer interbreed. In sympatric speciation, new species arise in the same area, for example through chromosome doubling in plants.
Genetic drift and gene flow
Besides selection, allele frequencies change by genetic drift, random chance, which matters most in small populations. A bottleneck, when a population crashes, or the founder effect, when a few individuals start a new population, can shift frequencies sharply. Gene flow, migration between populations, makes them more alike.
Patterns of evolution
Adaptive radiation is rapid diversification into many forms, like the Galapagos finches or mammals after the dinosaurs died out. Convergent evolution produces similar traits in unrelated groups facing similar conditions, such as the streamlined shape of sharks and dolphins. Coevolution occurs when species evolve in response to each other, such as flowers and their pollinators.
The history of life
The first life appeared over 3.5 billion years ago as single cells. Oxygen from photosynthetic bacteria built up around 2.4 billion years ago. Complex multicellular animals diversified in the Cambrian explosion about 540 million years ago. Plants and animals then moved onto land, and humans appeared only about 300,000 years ago.
Mass extinctions
Five mass extinctions have wiped out large fractions of species. The largest, about 252 million years ago, eliminated around 90% of marine species. An asteroid impact 66 million years ago ended the non-bird dinosaurs, opening opportunities for mammals. Many scientists argue human activity is now driving a sixth.
Phylogenetic trees
A phylogenetic tree shows hypothesised relationships among species, with branch points representing common ancestors. Groups that include an ancestor and all its descendants are called clades. Birds sit within the dinosaur clade, so birds are, in evolutionary terms, living dinosaurs.
Key takeaways
A species is a group that can interbreed and produce fertile offspring. Reproductive barriers keep species apart; geographic separation drives allopatric speciation. Drift and gene flow also change populations. Adaptive radiation, convergence, and coevolution are common patterns. Life began over 3.5 billion years ago and has been reshaped by mass extinctions.
Flashcard
Interbreed and produce fertile offspring
Check your understanding
A river splits a population of mice, and over many generations the two groups can no longer interbreed. This is: