speciation the how and why of species. what is a species? a species is one or more populations of...

Speciation

The How and Why of Species

What is a Species?

A species is one or more populations of organisms with the potential to interbreed with one another but NOT with members of other such groups naturally.

“But… What about the liger?”

We’ll get there. I promise!

Determining Separate Species

Biological Species Concept Proposed by Ernst Mayr who said:

“Species are groups of interbreeding natural populations that are reproductively isolated from other such groups."

Explains why the members of a species resemble one another and differ from other species. Breeding organisms pass genes to offspring By contrast, genes are not transferred to other species,

and different species therefore look different

Has some fallacies: asexual organisms, hybrids, ring species, chronospecies

What leads to New Species?

• Diversifying or Directional Selection can lead to new species.• More on this later!

• When differences between subpopulations become large enough that gene flow between them may stop.

Reproductive Isolating MechanismsPrezygotic Barriers

Reproductive Isolating Mechanisms• Prezygotic Barriers

• Geographic Isolation• Ecological Isolation. • Temporal(Time) Isolation• Behavioral Isolation• Mechanical Isolation• Gametic Isolation

Reproductive Isolating Mechanisms

• Postzygotic Barriers• Reduced Hybrid Viability• Reduced Hybrid Fertility• Hybrid Breakdown

Modes of Speciation

Allopatric speciation of squirrels in the Grand Canyon

Allopatric Speciation

New species arise as a result of geographic isolation“Rivers change course, mountains rise,

continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations”

Allopatric means “different homelands”

Sympatric Speciation

Cichlids from Lake Victoria

Sympatric Speciation

Sympatric Speciation

Two subpopulations become reproductively isolated within the same geographic area.

First proposed by Darwin in the 1850s.

Parapatric Speciation

Images from Evolution Berkeley

Parapatric Speciation

No specific extrinsic barrier to gene flowContinuous population exists but the

population does not mate randomly Individuals are more likely to mate with their

geographic neighbors than with individuals in a different part of the population’s range

Divergence may happen because of reduced gene flow within the population and varying selection pressures across the population’s range

Competitive Exclusion

Competitive Exclusion

Also known as Gause’s LawTwo species that compete for the exact same

resources cannot stably coexist. As a result, competing related species often

evolve distinguishing characteristics in areas where they both coexist

Rate of Speciation

Rate of Speciation

• Often can take millions of years, but can occasionally occur faster.• Banana trees moth species

• Gradualism • Punctuated equilibrium

And Now, Those Hybrids…

And Now, Those Hybrids… Ligers – hybrid between lions and tigers Zebroids – hybrid between horses and zebras Cama – hybrid between a camel and a llama

(artificial insemination) Wolphin - bottlenose dolphin and a false killer

whale

Remember, most of these hybrids are sterile and cannot reproduce with each other.