Speciation prezygotic and postzygotic

Warm up question

• What is a zygote?

Quick Write @ end of class

• Normal watermelon plants are diploid (2n = 22), but breeders have produced tetraploid (4n = 44) watermelons. If tetraploid plants are hybridized with their diploid relatives they produce triploid (3n = 33) seeds. These offspring can produce triploid seedless watermelons and can be further propagated by cuttings. Are the diploid and tetraploid watermelon plants different species? Explain.

CH 22: The Origin of CH 22: The Origin of SpeciesSpecies

Overarching Question:Overarching Question: How does How does speciationspeciation (the (the development of a new species) fit development of a new species) fit into evolutionary theory?into evolutionary theory?

Biological Species Concept

Decide whether the organisms in each of the following examples belong to the same species:

Example A Example B Example CHarris’ antelope squirrel vs. white-tailed antelope squirrel

Various breeds of dogs Donkey, horse, mule

The biological species concept =population whose members have the potential to naturally produce viable offspring

What are some things that What are some things that might keep organisms from might keep organisms from

breeding successfully??breeding successfully??Modes of reproductive isolation (blocking gene flow between populations)Achieved through mechanisms that can be classified into two main categories: 1. Prezygotic barriers- impede mating before fertilization can even

take place 2. Postzygotic barrier- prevent a hybrid zygote from developing into

a viable, fertile adult.

The Grand Canyon separating land animals-pre/postzygotic?

Pre/postzygotic?

Prezygotic BarriersPrezygotic Barriers

Habitat/geographic isolation: - live in different placese.g. Two snakes of the same genus live in the same area, but one is aquatic & the other is terrestrial-they just don’t have many encounters because they spend their time in different places

Temporal isolation – timing is different these skunks live in the same area, but they are reproductively active during different seasons

Do a little dance….Behavioral isolation – mating rituals different e.g. certain courtship rituals are performed before mating takes place

Prezygotic BarriersPrezygotic Barriers

Prezygotic BarriersPrezygotic BarriersMechanical Isolation – parts don’t fit or come

together e.g. physical differences inhibit cross-fertilization2 flowers of the same genus have different colors and therefore attract different pollinators

Gametic Isolation: biochemical mechanisms may prevent gametes from fusing, or the reproductive tract in the female may be an unfavorable

environment for sperm

Postzygotic BarriersPostzygotic Barriers• Reduced hybrid viability- hybridsdo not complete development

Hybrid breakdown- F1s are viable,

but later generations are not

Reduced hybrid fertility- the hybrid offspring is sterile

Quick Think

1. Define REPRODUCTIVE ISOLATION

2. List the 5 pre-zygotic and 3 post-zygotic barriers to reproduction

Quick Think

• 2 species of bird in a forest are not known to interbreed. One species feeds and mates in the treetops and the other on the ground. But in captivity, the 2 species can interbreed and produce viable, fertile offspring. What type of reproductive barrier most likely keeps these species separate? Explain your reasoning.

Limitations of the Biological Species Concept

• You can’t define a species by reproductive isolation if the organism reproduces asexually, as is the case in many bacteria

• What if you’re studying 2 extinct organisms??? How are you going to be able to tell whether or not they can mate & produce viable offspring??

Are these the same species?

Are these the same species?

Other Definitions of SpeciesOther Definitions of Species• Morphological species concept~ a species is defined by body shape, size, & structure Pros: can be applied to fossils & asexual organisms Cons: the criteria are subjective

• Paleontological species concept~ Classifies fossils based solely on morphological differences. Pros/cons exist, but this is our only option with extinct species…unless you can get

a DNA sample• Ecological species concept~ Distinguishes between species based on

their ecological niche-its biological role in the community Pros: can be used for sexual & asexual organisms bacteria can be classified

based on their food source Cons: two organisms may have a distinct niche, but still be able to interbreed

successfully • Phylogenetic species concept~ a species is a set of organisms that share

a unique genetic history. Compares physical characteristics as well as molecular sequences. Pros: can distinguish between species that are morphologically similar. Often

times all you need is a DNA sample Cons: hard to say how genetically different something has to be before it can be

considered a different species

The Bottom LineThe Bottom Line• Species is somewhat of a vague term in that there is no

single system for classifying one group of organisms as separate from another…although there is an effort to find one “uniform species concept”

• Researchers are often forced to use a different definition for “species” based on the circumstances & info they have

Although the biological species concept is still heavily used, the advent of DNA technology reinvented the phylogenetic species concept and cladistics. In fact, genetic similarity/dissimilarity is leading to massive reclassifications. Example= in plants, the classification as a dicot is being phased out

Modes of SpeciationModes of Speciation

• Allopatric Speciation VS Sympatric SpeciationAllopatric Speciation VS Sympatric Speciation

Allopatricpart of the population is geographically isolated from the rest of the population & forms a new species since it is reproductively isolated

Sympatric speciation occurs while the organisms are still cohabiting.

Allopatric SpeciationAllopatric Speciation• Example of geographic barriers:

Mountain ranges, rivers, land bridges (the Isthmus of Panama), a big lake becoming a bunch of smaller lakes, the Grand Canyon...

Allopatric SpeciationAllopatric Speciation• An example of Allopatric Speciation in the

Grand Canyon: Antelope Squirrels. The separated gene pools diverge via mutations, natural selection, sexual selection, genetic drift, etc.

Allopatric Speciation: Allopatric Speciation: A Non-ExampleA Non-Example

• These Florida Keys natives are a non-example. Why is this a non-example???

Sympatric SpeciationSympatric Speciation• “Barriers” in sympatric speciation include

chromosomal changes, habitat differentiation, & sexual selection.– Polyploidy:

Tetraploids can only produce viable offspring with themselves (self-pollination) or other tetraploids= reproductive isolation

Sympatric SpeciationSympatric Speciation• “Barriers” in sympatric speciation include chromosomal changes, habitat differentiation, & sexual

selection.– Habitat Differentiation:

In soapberry bugs, beak length dictates which fruit on a plant the insects eat.Even though the 2 specieslive on the same plant, they don’t run into each other often since they have different food preferences = decreased gene flow.

Sympatric SpeciationSympatric Speciation• “Barriers” in sympatric speciation include chromosomal changes,

habitat differentiation, & sexual selection.– Sexual Selection: Cichlids in Lake Victoria. These 2

similar species vary in color. Females of the 2 species preferentially mate with males who have

the same coloration that they do.

Quick Think

• How are sympatric and allopatric speciation similar and how are they different?

Quick Think

• Explain why allopatric speciation would be less likely to occur on an island close to the mainland than on a more isolated island of the same size.

The Tempo of Speciation: How Fast Does All This Happen?

• A couple of suggestions: Gradualism VS Punctuated Equilibrium

Gradualism: species P.E.: new speciesdiverge slowly over emerge suddenlytime then stabilize for a while before another

divergenceWhat type of speciation do you think we might see with the finches on the Galapagos? How is speciation of this type related to the environment?

Adaptive RadiationAdaptive Radiation• Diversely adapted species stemming from a common ancestor

– Often occurs when a founder population settles in a new environment, or when extinction opens up an ecological niche.

– These speciesof silver sword share a common ancestor, but have diverged after colonizing different Hawaiian islands

Macroevolutionary Changes Can Accumulate Through Many Speciation Events

• Evolutionary Novelty: in most cases, complex structures evolved in increments from simpler prototypes in other species

•Exaption: An existing structure that is co-opted after having previously served a different function. Ex: feathers may have been favored by sexual selection before being used to assist in flight

Evolution of the Genes That Control Development Evolution of the Genes That Control Development

• Heterochrony-evolutionary change in the rate/timing of developmental events; this can result in a change in the overall appearance of a species. A chimp’s

jawbone elongates at a faster rate relative to the rest of itsskull, thus giving it its characteristic shape. Note that early in development, the human &chimp skull look similar.

Evolution of the Genes That Control DevelopmentEvolution of the Genes That Control Development

• Paedomorphosis: Sexual maturity is accelerated– Ex: most salamander species undergo metamorphosis

before sexual maturity. A relatively small genetic change accelerates sexual development to the point where this salamander is an adult

even though it still has larval features such as gills

Changes in Spatial PatternChanges in Spatial Pattern• Recall: Homeotic Genes determine where limbs

grow on the general body plans of organisms– HOX genes are specific homeotic genes in animal

embryos– Here, turning on additionalHOX genes allows the budsto give rise to digits in chickens Determine the

fate of whole groups of cells

A “Rough Sketch” of the Evolution A “Rough Sketch” of the Evolution of Vertebrates from Invertebratesof Vertebrates from Invertebrates

• Alterations are associated with HOX genes as well

Most inverts only have one cluster of Hox genes

A duplication mutation could have led to the evolution of the first verts.

A duplication mutation or a nondisjunction mutation may have given rise to the 4 sets of Hox genes seen in verts today

Remember, Evolution is NOT Goal Remember, Evolution is NOT Goal OrientedOriented

• Species are not trying to become better…

Check for Understanding

1. Two species of birds in a forest are not known to interbreed. One species feeds and mates in the treetops and the other on the ground. But in captivity, the two species can interbreed and produce viable, fertile offspring. What type of reproductive barrier most likely keeps these species separate?

Check for Understanding

• Normal watermelon plants are diploid (2n = 22), but breeders have produced tetraploid (4n = 44) watermelons. If tetraploid plants are hybridized with their diploid relatives they produce triploid (3n = 33) seeds. These offspring can produce triploid seedless watermelons and can be further propagated by cuttings. Are the diploid and tetraploid watermelon plants different species? Explain.

Quick Write

• How can the Darwinian concept of descent with modification explain the evolution of such complex structures as the vertebrate eye or heart?