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2004-2005AP Biology

Chapter 24.

The Origin of Species

“Both in space and time, we seem

to be brought somewhat near to

that great fact—that mystery of

mysteries—the first appearance of

new 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 distinct

biological species because their songs &

other behaviors are different enough to

prevent 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 may

encounter each other rarely

2 species of garter

snake, 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, or

different years cannot mix gametes

Eastern spotted

skunk (L) & western

spotted skunk (R)

overlap in range but

eastern mates in late

winter & western

mates in late summer

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Behavioral isolation

! Courtship rituals that attract mates &

other unique behaviors to a species

are effective reproductive barriers

Blue footed boobies

mate only after a

courtship display

unique to their species

The most comedic species of the Galapagos Islands is the Blue

Footed Booby, what a ridiculous outfit and expression! Their name is

in fact taken from the Spanish 'bobo' which means clown.

The Blue Footed Boobies above display part of their humorous

courtship ritual whereby they raise their feet one at a time and then

swivel their heads away from the prospective mate looking to the sky.

Other interesting Booby features are the highly evolved airbag

systems in their skulls which allow them to dive bomb into the sea for

fish from great height, and the egg and hatchling nesting boundaries

they make which are rings of Boobie poop. They aren't the only Booby

on the island — there are also Masked and Red Footed Boobies

about.

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Gray-Crowned Cranes

engaged in courtship

display, Kenya

What can you say?

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Mechanical isolation

! Morphological differences can prevent

successful mating

Even in closely related

species of plants, the

flowers often have distinct

appearances that attract

different pollinators.

These 2 species of monkey

flower differ greatly in

shape & color, therefore

cross-pollination does not

happen.

The most comedic species of the Galapagos Islands is the Blue

Footed Booby, what a ridiculous outfit and expression! Their name is

in fact taken from the Spanish 'bobo' which means clown.

The Blue Footed Boobies above display part of their humorous

courtship ritual whereby they raise their feet one at a time and then

swivel their heads away from the prospective mate looking to the sky.

Other interesting Booby features are the highly evolved airbag

systems in their skulls which allow them to dive bomb into the sea for

fish from great height, and the egg and hatchling nesting boundaries

they make which are rings of Boobie poop. They aren't the only Booby

on the island — there are also Masked and Red Footed Boobies

about.

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Mechanical isolation

! For many insects, male &

female sex organs of

closely related species do

not fit together, preventing

sperm transfer

" lack of “fit” between sexual organs:

hard to imagine for us, but a big issue for

insects with different shaped genitals!

Damsel fly penises

The selection is intense because it directly

affects offspring production -- it is affecting sex

itself

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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 surrounding

waters where they fuse &

form zygotes. Gametes of

different 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’s

development

Species of salamander

genus, Ensatina, may

interbreed, but most

hybrids do not complete

development & those

that 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 numberor structure & meiosis in hybrids may fail toproduce normal gametes

Donkeys have 62

chromosomes

Horses have 64

chromosomes

Mule have 63 chromosomes!

What’s wrong with having 63 chromosomes?

Odd number! Cannot pair up in meiosis.

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Habitat breakdown

! Hybrids may be fertile & viable in first

generation, but when they mate

offspring are feeble or sterile

Strains of cultivated rice has

accumulated recessive

alleles. Hybrids are vigorous

but plants in next generation

are 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 evolve

independently

! Isolation

" allopatric

" sympatric

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Allopatric speciation

! Allopatric = “other country”

" geographic separation

! migration

! physical barrier

Harris’s antelope

squirrel inhabits

the canyon’s

south rim (L).

Just a few miles

away on the north

rim (R) lives the

closely related

white–tailed

antelope squirrel

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Sympatric speciation

! Sympatric = “same country”

" isolation even though

members of population

remain 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 various

new environmental challenges &

opportunities

Drosophila

Geospiza

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Adaptive radiation in plants

Silversword alliance

all descended

from a common

ancestor 5mya

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Adaptive radiation

! Many ecological

niches open

! Evolution of many

diversely-adapted

species from a

common ancestor

to 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 to

environmental change

" Gradualism

! Charles Darwin

! Charles Lyell

" Punctuated equilibrium

! Stephen Jay Gould

! Niles Eldredge

Niles Eldredge

Curator

American Museum of Natural History

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Gradualism

! Gradual divergence

over long spans of

time

" assume that big

changes occur as

the accumulation

of many small ones

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Punctuated Equilibrium

! Rate of speciation is

not constant

" species undergo

most change when

they 1st bud from

parent population

" as separate species,

remain static for

long periods of time

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Stephen Jay Gould (1941-2002)

! Harvard paleontologist &evolutionary biologist

" punctuated equilibrium

" prolific author! popularized evolutionary thought

Stephen Jay Gould (September 10, 1941 – May 20, 2002) w as a New York-

born American paleontologist, evolutionary biologist, and historian of science.

He w as the most influential and w idely read w riter of popular science of his

generation. He served as a member of the faculty at Harvard University

beginning in 1967. Tow ard the end of his life he served as the Alexander

Agassiz Professor of Zoology at that university. He helped Niles Eldredge

develop Eldredge's theory of punctuated equilibrium 1972, w herein

evolutionary change occurs relatively rapidly in comparatively brief periods of

environmental stress, separated by longer periods of evolutionary stability.

According to Gould, this overthrew a key tenet of neo-Darw inism; according

to most evolutionary biologists, the theory w as an important insight but merely

modif ied neo-Darw inism in a w ay fully compatible w ith w hat had been know n

before. Gould became w idely know n through his popular science essays in

Natural History magazine, collections of essays like The Panda's Thumb and

The Flamingo's Smile, and extended studies like Wonderful Life and

others.Gould w as an emphatic advocate of evolution and w rote prolif ically on

the subject, trying to communicate his understanding of contemporary

evolutionary theory to a w ide audience. A recurring theme in his w ritings is

the history and development of evolutionary (and pre-evolutionary) thinking.

His early research involved the study of the fossil record of snails (detailed in

one of his essays). He w as also an enthusiastic baseball fan and made

frequent references to the sport (including an entire essay) and a very w ide

range of other topics.

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Evolution is not goal-oriented

An evolutionary trend does not mean that evolution is

goal oriented. The modern horse is the only surviving

twig of an evolutionary

bush with many

divergent 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 therefore

exhibit similar

adaptations

" but are not

closely related

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Coevolution

! Predator-prey relationships

! Parasite-host relationships

! Flowers & pollinators

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Darwin Awards

Named in honor of Charles Darwin, the

father of evolution, the Darwin Awards

commemorate those who improve our

gene pool by removing themselves from it.

The Darwin Awards salute the improvement of

the human genome by honoring those who

accidentally kill themselves in really stupid

ways.

Of necessity, this honor is generally

bestowed posthumously.

www.DarwinAwards.com