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!
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?
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 Mechanisms Prezygotic 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
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”
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 flow • Continuous 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 • Also known as Gause’s Law • Two 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 • Often can take millions of years, but can occasionally occur faster. • Banana trees moth species • Gradualism • Punctuated equilibrium
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.