Pre-zygotic (before mating) vs. post-zygotic (after mating).
Pre-zygotic: different breeding seasons (either yearly or time of day), different signals: sight, sound, smell, behavior, mechanical problems—genitalia don’t fit together. Also gametic isolation: sperm don’t detect the egg or can’t fertilize it.
Post-zygotic: hybrid inviability: hybrids don’t survive to birth or adulthood; hybrid sterility: as with the mule, a sterile hybrid.
Even having hybrids that are significantly less fit (able to survive and reproduce) than purebreds is an isolating mechanism, a way to prevent the two populations from fusing into one population.
Geographical barriers: mountains, oceans, rivers. A few members of a species manage to cross by a rare chance event.
This is the mechanism by which Darwin’s finches evolved into separate species in the Galapagos islands. Only very rarely can birds cross the ocean to get to other islands.
Or, the barrier develops slowly as conditions change: the gradual formation of the Grand Canyon split a population into 2 isolated groups, that have diverged into separate species, the Kaibab and Albert squirrels.
Geographical isolation is the easiest way for species to form, but there are other possible mechanisms. “Sympatric speciation” means speciation that occurs within the same geographical location.
An example: cichlid fish in Lake Barmobi Mbo in Cameroon, Africa—an isolated volcanic lake. Nine species, all more closely related to each other (by DNA evidence) than to similar fish in other lakes. Lake has no distinct geographical zones, and the fish can easily swim anywhere in it. They feed in different locations, but all breed in the same location, close to the bottom. An example of sympatric speciation, but the mechanism is not clear.
About half of all flowering plants are polyploid: more than 2 copies of each gene.
Polyploids are the result of failure of cell division (mitosis or meiosis) to separate the chromosomes into 2 cells.
New polyploids are usually sterile, or their offspring are sterile: extra chromosomes with no homologue to pair with in meiosis leads to unbalanced (aneuploid) gametes, which will produce dead offspring.
Triploids (3 copies of each gene) are sterile—the source of seedless fruit.
In animals, sexual reproduction is essential every generation, so most polyploids fail to reproduce: need a similarly polyploid mate to produce fertile offspring.
In plants, vegetative propagation is possible for many generations without sex, and in many species one plant contains both sexes. So, reproduction with a polyploid partner is easier in plants than in animals.
When two populations of a species are separated by a geographical barrier, they diverge genetically. Sometimes the barrier is removed and the two groups come into contact with one another. The region of contact is a “hybrid zone”.
Several possibilities exist:
If the two groups have only diverged a bit, fertile offspring will result, and the two groups will merge back into a single species. Geographical differences may exist within the species: different subspecies or varieties, but all can interbreed freely.
If the two groups have diverged to the point that no fertile offspring result from their matings, sexual selection will occur to deter further matings. New pre-zygotic reproductive isolation mechanisms, especially behavioral differences, arise to reinforce the division between the two species.
This is called “parapatric speciation”: 2 species forming while in contact with each other in a restricted region.
Shrimp in Panama. Up until 3 million years ago, North and South America were separated by the ocean. As dry land rose between the continents, shrimp who once freely interbred were separated by a barrier. When put together in the lab (or by going through the Panama Canal) now, they attack each other rather than mating.
What happens after 2 species separate from each other?
In some cases, the species exists for millions of years, gradually changing in response to external conditions but always maintaining as a single distinct species.
In other cases, many new species will form from a single species in a very short time: this is “adaptive radiation”. This often happens on isolated islands, where a new species is blown in by a storm, and finds many different ecological niches to fill. Darwin’s finches are an example of this.
They are thought to have originated with a small group of finches that blew over about 1 million years ago, to islands with no dangerous predators and very few other land birds.
Also, sometimes a “key innovation” will arise, and new niches will suddenly be open to the possessors of the innovation. Or, a disaster will allow a sudden expansion of a few surviving species. Mammals grew rapidly in number of species following the extinction of the dinosaurs.
Various events can cause extinction: being outcompeted for a critical resource, having the climate change too rapidly to adapt.
“ Mass extinctions” are caused by catastrophic events. The Earth has had several mass extinction events, where the vast majority of species die out over a short period of time. This is what is seen when one moves between various geological ages. Asteroids hitting the Earth are responsible for at least some of these, but probably not all.