BIS 2B: Speciation 2

Polyphyletic clade

A taxonomic group that does not contain the most recent common ancestor of its members.

Monoplyletic clade

A taxonomic group that includes ALL of the taxa/species descended from a specific common ancestor. [Good!]

Allopatric speciation: Peripatric/founder effect

A part of a localized colony diverges from its main group within the same geographic location as its original group. They experience genetic drift because there is a reduction in genetic variance.

Paraphyletic clade

A taxonomic group that excludes some of the descendants from a specific common ancestor. [Bad!]

What question poses about sympatric speciation?

Did the species evolve reproductive isolation in allopatry and then subsequently merge their ranges and become sympatric OR did they evolve in sympatry?

What is the sympatric scenario in sympatric speciation?

One species enters, so they give rise to another specie (blue). Following that, the blue species gives rise to another species. It is the process through which new species evolve from a single ancestral species while inhabiting the same geographic location.

Sympatric speciation

Sympatric populations have completely overlapping ranges, so there is no obvious geographic barrier.

What is the allopatric scenario in sympatric speciation?

Two different species come into one single area where they both diverge into new species. This would create 2 pairs of sister lineages.

What is an example of sympatric speciation that was discussed in lecture? How did we come to that conclusion?

The cichlids in Barombi Mbo are monophyletic. Based on the phylogenetic tree, the most recent common ancestor made way for 12 different species of cichlids while the other branch off of that one common ancestor stayed as one. This meant that one species of cichlids paved the way for 12 different species to evolve. That one species of cichlids basically gave rise to all those cichlids that currently exist in the lake. They diverged in sympatry! However, its more complicated than that because there was gene flow from the two rivers into the lake. It did alter how the species evolved.

What is an example of vicariance discussed in the lecture?

The snapping shrimp. The rise of the Isthmus of Panama shut off gene flow between the Carribbean Sea and Pacific Ocean. (Vicariant Event) This physical barrier separated previously connected populations of snapping shrimp, so there was no longer any gene flow.

What is an example of peripatric/founder effect that we discussed in lecture?

There are at least 106 species of the Hawaiian fruit flies. Each island in Hawaii formed at different times, so dispersal by a few individuals explains why there are different species on each island. However, kipuka formations isolated fruit fly populations on islands causing the populations to further diverge. summary: Founder effect/genetic drift + diversifying selection (slightly different habitats on kipukas) are important for the evolution of new species within an island.

What were the consequences for the snapping shrimp because of this vicariant event?

There are now 7 sister species. One of the pairs lives in the Pacific Ocean while the other lives in the Caribbean Sea. The result is that they hate to mate with each other, so there is no gene flow.

What happens when 2 formerly allopatric subpopulations come into contact?

There are three scenarios: 1. They may not have genetically diverged enough to be reproductively incompatible, so they hybridize and eventually merge back into one single population. 2. They may have diverged so much that they have strong pre or post reproductive genetic barriers to gene flow. This results in 2 species where there was once 1. 3. They may hybridize, but hybrids have a lower fitness than either parental populations. Selection should act to reduce matings (pre zygotic barriers) between parental populations. This process is called reinforcement.

What is an example of parapatric speciation that was discussed in lecture?

There is a environmental divide in an area because of lead run offs from mining. One side has greater concentration of lead in the land than the other side. This made two forms of pasture grass: lead tolerant/late flowering and lead intolerant/early flowering grasses. Hybrids perform poorly on both sides, so there is no gene flow.

Allopatric speciation: Vicariance

There is a physical barrier that subdivides a species range into two or more populations that no longer freely exchange genes. Events that divide a species range are called vicariant events. Once vicariance has occurred, the separated populations can independently accumulate genetic difference through (1) diversifying selection and/or (2) genetic drift. [very important] litmus test of speciation: when populations come into secondary contact, are they reproductively isolated? (can they exchange genes?!?)

How could these two populations have diverged from each other in the first place?

There must be some pre existing EXTRINSIC barriers to gene flow, so that the populations can diverge through selection and/or drift. Extrinsic barrier that limits gene flow between members in pop. -> lineage divergence (selection and or drift) -> evolve Intrinsic (pre and post zygotic) genetic barrier to further exchange.

Parapatric speciation

These are populations that have adjacent, but non overlapping borders. (para = near) They are isolated because of dramatic environmental discontinuities in each environment that the species live in. One population is better adapted to the other environment, so migrants from one side to the other will be selected against.

What is evidence that 'reinforcement' actually reduces the relative fitness of hybrids?

This can be explained with fruit flies. For a given level of genetic divergence, sympatric sister species have much higher levels of pre zygotic isolation than allopatric pairs of sister species. Reinforcement selection increase reproductive isolation and reduce gene flow between recently diverged species that have come into secondary contact.

What is reinforcement?

This occurs when hybrid offspring has lower fitness than either parental populations, so they are selected against. This selection favors avoiding between population matings. This results in strengthening of pre zygotic reproductive isolation.

What are the two types of extrinsic barriers that focus on allopatric speciation?

Vicariance and peripatric/founder effect. They both isolate populations.

What is the difference between vicariant and founder effect/peripatric speciation?

Vicariant speciation is caused by disruptive/diversifying natural selection. They are blocked by some kind of physical barrier, so when that population is allopatric, most of the evolution stems from reproductive isolation. When they come back together, they are genetically isolated. Founder effect/peripatric speciation on the other hand may be driven by genetic drift.