Macroevolution Macroevolution & Speciation& Speciation

Accel Bio Accel Bio 2014 2014

What is a species?What is a species?• Species means “kind” or “type”Older way of looking at this question:

• Morphological Species ConceptMorphological Species Concept– Classified organisms into separate species based on

________________ differences.– Used by ____________ (Swedish doctor & naturalist, c.1735) but not

adequate today.structural / physical

Carl Linnaeus

What is a species?What is a species?With the help of population genetics, in 1940 Ernst Mayr came up with the:

• Biological Species ConceptBiological Species Concept– A species consists of groups of populations

whose members are capable of ____________ in nature to produce ______ offspring AND who do not interbreed with members of different species.

(this last part is called ____________________)

interbreedingfertile

reproductive isolation

Defining a speciesDefining a speciesBiological Species Concept issues:Biological Species Concept issues:

– Still this definition is not perfect: what about asexually reproducing organisms like ________________ ?!• These are classified by structural and biochemical

differences

bacteria or fungi

Budding Yeast

Asexual reproduction in E.coli bacteria

Defining a speciesDefining a speciesBiological Species Concept issues:Biological Species Concept issues:

– Additionally, some different species that do not interbreed in nature can be made to do so in an artificial environment (ranch, circus, zoo, aquarium, or laboratory).Ex) Zebroids, interspecies hybrids:a cross between a horse and a zebra

Zebroid, Mt. Kenya Game Ranch and

Animal Orphanage, Kenya

How do you create a new species?How do you create a new species?

Usually through geographic isolation (separation) of some members of a population. The mechanisms of genetic drift, mutation, & natural selection all (separately) act upon the newly founded population and the original population. Over time, these now separate populations evolve independently.

If the two populations are brought back together and are capable of successfully reproducing in nature, we say they are still the same species. If they are NOT capable of this, then we declare them different species.

Species boundaries are maintained Species boundaries are maintained through geographic and/or through geographic and/or

reproductive isolationreproductive isolation

• What separates two species whose space / range overlaps? Reproductive barriers usually keep species boundaries intact.

• Reproductive barriers can be put into two categories: – Prezygotic barriersPrezygotic barriers: prevent fertilization– Postzygotic barriersPostzygotic barriers: prevent hybrid from

developing into viable, fertile adult

Prezygotic Barriers to Species Prezygotic Barriers to Species InterbreedingInterbreeding

• Ecological / Habitat isolationEcological / Habitat isolation– Two species occupy different

microenvironments (in same area) and thus do not interbreed

Ex) Garter snakes: water v. land

• Temporal isolationTemporal isolation– Reproduce at different times (of day,

year, season, etc.)Ex1) flowers: open/pollinated at different time of dayEx2) frogs: mate at different time of season (temp dependent)

• Behavioral isolationBehavioral isolation– Different courtship rituals / mating

behaviorEx1) blue-footed boobies “dance”Ex2) frog “calls” --> different “songs” among different species

More Prezygotic Barriers to Species More Prezygotic Barriers to Species InterbreedingInterbreeding

• Mechanical isolationMechanical isolation– Differences in anatomy / physical structure do not permit

interbreedingEx) flowers: adapted for specific (insect) pollinators

– Larger flowers only pollinated by larger bees, smaller bees cannot reach

• Gametic isolationGametic isolation– Egg & sperm of two species biochemically incompatible

(different recognition proteins on surface usually)– This especially important to externally fertilizing aquatic organisms

or organisms with wind-borne gametesEx1) flowers: will not accept/recognize “foreign” pollenEx2) fish that fertilize externally, in open water

Postzygotic Barriers to Species Postzygotic Barriers to Species InterbreedingInterbreeding

• Hybrid inviability (reduced hybrid viability)Hybrid inviability (reduced hybrid viability)– Aborted development, usually at early embryonic stage

Ex1) bullfrog eggs & leopard frog spermEx2) different species of irises

• Hybrid breakdownHybrid breakdown– F1 hybrid can successfully reproduce with other F1 hybrids or a Parental

individual but F2 hybrid unable to reproduce.Ex1) sunflower speciesEx2) cotton species

• Hybrid sterility (reduced hybrid fertilityHybrid sterility (reduced hybrid fertility– Hybrid offspring survive (& may be strong)

but are sterile (often due to abnormal gametes of hybrid)

Ex) female horse (2N = 64) and male donkey (2N = 62) results in sterile mule offspring (2N = 63).

– Different diploid number prevents proper matching up of homologous chromosomes in meiosis

Speciation Speciation patternspatterns

and and mechanismsmechanisms

Speciation and Adaptive RadiationSpeciation and Adaptive Radiation• Adaptive radiation refers to an evolutionary pattern in

which one species gives rise to many (also referred to as divergent evolution)

• This has happened several times in the history of the evolution of species… Evidence? Homologous structures (similar structure, different function)

Adaptive radiation most often follows mass extinctions or when a species moves into a new, unoccupied area. In both these cases, there is usually an abundance of available ecological niches, that can be filled by the new “daughter” species.

A Model of A Model of Adaptive Radiation Adaptive Radiation in an Island Chainin an Island Chain

Speciation in honeycreepersSpeciation in honeycreepersThe islands of Hawaii were colonized by a single species of finch-like birds. Eventually populations were isolated from each other on separate islands. The diagrams show how they evolved into the various modern honeycreeper species with beak shapes which adapt them for different feeding methods.

Allopatric SpeciationAllopatric Speciation

The term Allopatric Speciation is used to describe a speciation event in which the new species results due to geographic isolation and subsequent adaptation of the emigrants to a new environment. When the two species are brought together again, they do not interbreed.

Allopatric speciation of Grand Canyon sqirrelsAllopatric speciation of Grand Canyon sqirrels

Sympatric SpeciationThe term Sympatric Speciation is used to describe a speciation event in which reproductive isolation arises within the boundaries/range of the parent species (without geographic isolation). This is most common among plants, and usually results due to an increase in a plant’s chromosome number. The resulting plant is thus considered to be polyploid.

Below: Sympatric speciation by autopolyploidy (within a single species).

Allopolyploid Sympatric Speciation

One mechanism for allopolyploid (from two different species) speciation in plants:

Patterns of Evolution: Gradualism & Punctuated Equilibrium

How long does speciation take? In one survey of 84 groups of plants and animals (for which we have evidence), the answer was from 4,000 to 40 million years. The average time between speciation events was 6.5 million years. That is a LONG time!

Gradualism Punctuated Equilibrium

Image SourcesImage Sources• Zebroid: www.ceotraveler.com/adventure/kenya.shtml

• Budding yeast cells: http://www.sirinet.net/~jgjohnso/fungi.html

• Bacteria: http://library.thinkquest.org/CR0212089/meso.htm

• Snail speciation & Honeycreeper Beaks: http://www.bbc.co.uk/scotland/education/bitesize/higher/biology/genetics_adaptation/natural_selection2_rev.shtml

• Frog Mating Behavior: http://w3.dwm.ks.edu.tw/bio/activelearner/19/ch19c1.html

• Horse/Mule/Donkey: http://w3.dwm.ks.edu.tw/bio/activelearner/19/ch19c3.html

• Adaptive Radiation: http://www.cod.edu/people/faculty/fancher/AdaptiveRadiation.htm• Boobie Mating Dance: http://www.nwf.org/internationalwildlife/1998/boobies.html

• Linnaeus portrait & Cinchona watercolor: http://www.linnean.org/• Ernst Mayr: http://www.pbs.org/wgbh/evolution/library/06/2/l_062_01.html

• Honeycreepers #2: http://www.micro.utexas.edu/courses/levin/bio304/evolution/speciation.html

• Picture: