Speciation

• • • • • • • • Speciation: splitting of one species into 2 different species.

What is a species? Based on ability to reproduce.

“Biological species concept”: a species is a group of organisms that interbreed under natural conditions and that are reproductively isolated from each other. – Reproductively isolated: don’t produce fertile hybrids.

– Natural conditions: artificial breeding doesn’t count. For example, artificial insemination, keeping 2 species locked up together.

In contrast, the older “morphological species concept”: members of the same species look similar to each other. Many examples of organisms that look similar but can’t produce fertile offspring.

Problems with biological species concept: Doesn’t work with fossils or extinct species. Doesn’t work with asexual species , such as most bacteria. How to deal with what is “natural”.

Reproductive Isolation

• How do populations become reproductively isolated?

• Result of gene mutations and altered allele frequencies due to selection and genetic drift.

• To maintain as a single species, there must be gene flow between populations: matings between members of separated populations that allow mixing of alleles.

• In the absence of gene flow, mutations in different populations will be independent, and allele frequencies will change independently of each other. • In most cases, migration is the key to gene flow. • Once gene flow stops: genetic divergence occurs. The two populations gradually become genetically different.

• Speciation sometimes occurs very quickly, other times more slowly. In most cases it is not an instantaneous event.

Genetic Divergence

Reproductive Isolation Mechanisms

• • • 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.

Allopatric Speciation • The simplest and most common mechanism of speciation is allopatric speciation: 2 groups of one species are isolated geographically, and diverge into separate species.

More Allopatric Speciation

• • • 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.

Sympatric Speciation

• • 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.

Sympatric Speciation by Polyploidy

• • • • • • 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.

Hybrid Zones

• • • 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.

Parapatric Example

•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.

Patterns of Speciation

• • • • • 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.

Extinction

• • •

Extinction

can happen: none left of the species.

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.