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Chapter 24.

The Origin of Species

“Both in space and time, we seemto be brought somewhat near tothat great fact—that mystery ofmysteries—the first appearance ofnew beings on this Earth.”

— Darwin

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Essential Questions How and why do new species

originate?

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What is a species? Biological species concept

defined by Ernst Mayr population whose members can interbreed &

produce viable, fertile offspring reproductively compatible

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Biological species concept

Eastern Meadowlark Western Meadowlark

Similar body & colorations, but are distinctbiological species because their songs &other behaviors are different enough toprevent interbreeding

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Diversity & Taxonomy The Tree of Life

species are thesmallest unit atthe ends ofbranches

basic unit fororganizing &categorizingliving things

smallest unit bywhich wemeasure diversity

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Reproductive isolation Species concept hinges on reproductive

isolation biological barriers that impede members

from producing viable offspring before vs. after fertilization

pre-zygotic barriers (before the zygote) post-zygotic barriers (after the zygote)

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Prezygotic barriers Impede mating or hinder fertilization if

mating occurs habitat isolation temporal isolation behavioral isolation mechanical isolation gametic isolation

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Habitat isolation Two species may occupy different

habitats within same area so mayencounter each other rarely

2 species of gartersnake, Thamnophis,occur in same area,but one lives in water& other is terrestrial

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Temporal isolation Species that breed during different

times of day, different seasons, ordifferent years cannot mix gametes

Eastern spottedskunk (L) & westernspotted skunk (R)overlap in range buteastern mates in latewinter & westernmates in late summer

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Behavioral isolation Courtship rituals that attract mates &

other unique behaviors to a speciesare effective reproductive barriers

Blue footed boobiesmate only after acourtship displayunique to their species

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Gray-Crowned Cranesengaged in courtship

display, Kenya

What can you say?

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Mechanical isolation Morphological differences can prevent

successful mating

Even in closely relatedspecies of plants, theflowers often have distinctappearances that attractdifferent pollinators.These 2 species of monkeyflower differ greatly inshape & color, thereforecross-pollination does nothappen.

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Mechanical isolation For many insects, male &

female sex organs ofclosely related species donot fit together, preventingsperm transfer lack of “fit” between sexual organs:

hard to imagine for us, but a big issue forinsects with different shaped genitals!

Damsel fly penises

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Gametic isolation Sperm of 1 species may not be able to

fertilize eggs of another species variety of mechanisms

chemical incompatibility sperm cannot survive in female reproductive tract

biochemical barrier so sperm cannot penetrate egg

Sea urchins release sperm& eggs into surroundingwaters where they fuse &form zygotes. Gametes ofdifferent species—red &purple —are unable to fuse.

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Postzygotic barriers prevent hybrid zygote from developing

into a viable, fertile adult reduced hybrid viability reduced hybrid fertility hybrid breakdown

zebroid

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Reduced hybrid viability Genes of different parent species may

interact & impair the hybrid’sdevelopment

Species of salamandergenus, Ensatina, mayinterbreed, but mosthybrids do not completedevelopment & thosethat do are frail.

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Mule are vigorous,but sterile

Reduced hybrid fertility Even if hybrids are vigorous they may be

sterile chromosomes of parents may difier in number

or structure & meiosis in hybrids may fail toproduce normal gametes

Donkeys have 62chromosomes

Horses have 64chromosomes

Mule have 63 chromosomes!

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Habitat breakdown Hybrids may be fertile & viable in first

generation, but when they mateoffspring are feeble or sterile

Strains of cultivated rice hasaccumulated recessivealleles. Hybrids are vigorousbut plants in next generationare small & sterile.On path to separate species.

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Speciation Species are created by a series of

evolutionary processes populations become isolated

reproductively isolated geographically isolated

isolated populations evolveindependently

Isolation allopatric sympatric

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Allopatric speciation Allopatric = “other country”

geographic separation migration physical barrier

Harris’s antelopesquirrel inhabitsthe canyon’ssouth rim (L).Just a few milesaway on the northrim (R) lives theclosely relatedwhite–tailedantelope squirrel

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Sympatric speciation Sympatric = “same country”

isolation even thoughmembers of populationremain in contact

what causes this isolation? chromosomal changes

polyploidy mostly in plants oats, cotton, potatoes,

tobacco, wheat non-random mating

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Adaptive radiation Evolution of many diversely adapted

species when introduced to variousnew environmental challenges &opportunities

Drosophila

Geospiza

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Adaptive radiation in plants

Silversword allianceall descendedfrom a commonancestor 5mya

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Adaptive radiation Many ecological

niches open Evolution of many

diversely-adaptedspecies from acommon ancestorto fill niches Darwin’s finches mammals

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Review Speciation is a process

populations become isolated geographic isolation

different environmentalconditions: food, predators, disease,habitat

different selection pressures genetic drift

reproductive isolation different selection pressures:

sexual selection

isolated populations evolveindependently

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Current debate Does speciation happen gradually or

rapidly perhaps in response toenvironmental change Gradualism

Charles Darwin Charles Lyell

Punctuated equilibrium Stephen Jay Gould Niles Eldredge

Niles EldredgeCurator

American Museum of Natural History

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Gradualism Gradual divergence

over long spans oftime assume that big

changes occur asthe accumulationof many small ones

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Punctuated Equilibrium Rate of speciation is

not constant species undergo

most change whenthey 1st bud fromparent population

as separate species,remain static forlong periods of time

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Stephen Jay Gould (1941-2002) Harvard paleontologist &

evolutionary biologist punctuated equilibrium prolific author

popularized evolutionary thought

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Evolution is not goal-orientedAn evolutionary trend does not mean that evolution isgoal oriented. The modern horse is the only survivingtwig of an evolutionarybush with manydivergent trends.

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Convergent evolution Flight evolved 3 separate times

evolving similar solutions to similar “problems”

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Parallel Evolution parallel paths filling similar

niches thereforeexhibit similaradaptations but are not

closely related

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Coevolution Predator-prey relationships Parasite-host relationships Flowers & pollinators

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Darwin AwardsNamed in honor of Charles Darwin, thefather of evolution, the Darwin Awardscommemorate those who improve ourgene pool by removing themselves from it.The Darwin Awards salute the improvement ofthe human genome by honoring those whoaccidentally kill themselves in really stupidways.Of necessity, this honor is generallybestowed posthumously.

www.DarwinAwards.com