LECTURE 8:Macroevolution

What is microevolution?

– Evolution on a small scale– Change in allele frequencies from one generation to

the next– A process that leads to a change in a species

• Natural selection• Genetic Drift• Gene Flow • Mutation

• Microevolution explains how populations evolve

What is Macroevolution?

• Origin of different species – SPECIATION

• Extinction of species• Evolution of major features

What are the Macroevolution Processes?

• Divergent Evolution:– Formation of 2

descendent species from an ancestral species

– Homologous structures

• Convergent Evolution:– 2 species acquiring

same characteristics from 2 different ancestral species

– Analogous Structures

What is Speciation?

• Origin of new species

How would we identify if a species is “new”?

How can we see Speciation in the Fossil Record?

What are the types of speciation?

• Coevolution:– A species evolves due another species evolving– Predator/prey relationships

• Adaptive Radiation– Appearance of numerous species over a short period

of time– Filling open niches

– When would this happen?

What is adaptive radiation?

• Explosion of new species from common ancestor that results in diverse species adapted to different environments

ADAPTIVE RADIATION

What is the tempo of speciation?

• Punctuated Equilibrium– Long periods of little

change – Occurs after fast/rapid

changes • Adaptive radiation

PUNCTUATED EQUILIBRIUM

ADAPTIVE RADIATION

PUNCTUATED EQUILIBRIUM

Where was there an extinction?

What is the Biological Species Concept?

– Biological• Group of individuals capable of interbreeding

• Is this an accurate definition of a species?

• CASE 1• A mule is the offspring of a female horse and a

male donkey. In contrast, the hinney is the offspring of a male horse and female donkey. The mule is easier to breed and larger in size than the hinney. For these reasons, the mule became an important domesticated animal. Horses have 64 chromosomes, donkeys have 62. The mule has 63 and cannot evenly divide, that is why the animal is sterile.

• CASE 2• A liger is a cross between a female tiger and a

male lion. In contrast, the tigon is a cross between a male tiger and a female lion. These two species do not breed in nature because their habitats are so different. Lions live in open grasslands while tigers live in forests. In captivity, it is possible to produce ligers and tigons. Male ligors are sterile, but female ligers are fertile and may reproduce with either tigers or lions.

• CASE 4• E.coli is a bacterium normally found in the

intestines. It is harmless and may actually be beneficial to the human digestive system. There is a pathogenic strain of E.coli that produces a toxin that can kill its human host. The two strains look very similar under a microscope. Comparison of their genomes reveals that the pathogenic strain lacks 528 genes found in the normal strain and has 1,387 genes not found in the normal strain.

What are other species concepts?

• 5 Types:– Morphological

• Group of individuals sharing similar characteristics

– Recognition• Behavior/chemical recognition between individuals

– Genetic• Range of variation in DNA- similar in individuals

– Cladistic• Species defined as a branch in a cladogram

– Biological• Group of individuals capable of interbreeding

Why don’t similar species interbreed?

• Reproductive Isolation– Biological barriers preventing members of 2

species from producing fertile, viable offspring hybrids

• Geographic Isolation

What is Geographic Isolation?

• Geographic Isolation:– Separation of a species due to obstruction

• Allopatric Speciation

What is Reproductive Isolation?• Sympatric Speciation• Prezygotic (before fertilization)

– Timing• Diff breeding seasons

– Behavior• Mating rituals

– Habitats• Diff living areas (niches)

– Mechanical• Diff Reproductive Structures

– Gametic• Sperm cannot survive in female

reproductive tract

• Postzygotic (After fertilization) – Reduced Hybrid Viability

• Abort embryonic development– Reduced Hybrid Fertility

• Cannot breed

What are the Modes of Speciation?

• Two general modes of speciation are distinguished by the way gene flow among populations is initially interrupted

• Allopatric speciation– geographic separation of populations restricts gene

flow

• Sympatric speciation– speciation occurs in geographically overlapping

populations when biological factors, such as chromosomal changes and nonrandom mating, reduce gene flow

What is Allopatric Speciation?

• Geological processes can fragment a population into two or more isolated populations– Mountain ranges, glaciers, land bridges, or splintering

of lakes may divide one population into isolated groups.

• Some individuals may colonize a new, geographically remote area and become isolated from the parent population– For example, mainland organisms that colonized the

Galápagos Islands were isolated from mainland populations.

What is Allopatric Speciation?

• Once geographic separation is established, the separated gene pools may begin to diverge through a number of mechanisms– Mutations – Sexual selection favors different traits in the

two populations– Different selective pressures in differing

environments act on the two populations.– Genetic drift alters allele frequencies.

• HOW DO WE CONFIRM THAT ALLOPATRIC SPECIATION HAS OCCURRED?

– EXPERIMENT!!!!!

What is Sympatric Speciation?

• New species arise within the range of the parent populations– Reproductive barriers must evolve between

sympatric populations• In plants, sympatric speciation can result from

accidents during cell division that result in extra sets of chromosomes, a mutant condition known as polyploidy

• In animals, it may result from gene-based shifts in habitat or mate preference.

IN SUMMARY:• In allopatric speciation, a new species forms while geographically

isolated from its parent population.– As the isolated population accumulates genetic differences due to

natural selection and genetic drift, reproductive isolation from the ancestral species may arise as a by-product of the genetic change.

– Such reproductive barriers prevent breeding with the parent even if the populations reestablish contact.

• Sympatric speciation requires the emergence of some reproductive barrier that isolates a subset of the population without geographic separation from the parent population.– In plants, the most common mechanism is hybridization between

species or errors in cell division that lead to polyploid individuals.– In animals, sympatric speciation may occur when a subset of the

population is reproductively isolated by a switch in food source or by sexual selection in a polymorphic population.

Microevolution or Macroevolution

• Change in allele frequencies• Creation of new species• Giraffe necks getting longer• A species of birds going extinct• A species of beetle have evolved harder shells,

so a species of bird has evolved beaks that can crush the beetles shells

• -What is this called?• There is variation within a snake species, one

variation survives the other does not survive