Ch. 24 The Origin of Species - Katy Independent School ...staff.katyisd.org/sites/thsbiologyapgt/Documents... · Species – Species A Note: The Original Species A is back to its

AP Biology 2007-2008

Ch. 24

The Origin of Species

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So…what is a species?

Biological species concept

defined by Ernst Mayr

population whose members can interbreed &

produce viable, fertile offspring

reproductively compatible

Western Meadowlark Eastern Meadowlark

Distinct species: songs & behaviors are different enough to prevent interbreeding

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How and why do new species originate?

Species are created by a series of evolutionary processes

populations become isolated geographically isolated

reproductively isolated

isolated populations evolve independently

Isolation

allopatric geographic separation

sympatric still live in same area

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Obstacle to mating or to fertilization if mating occurs

PREZYGOTIC-reproduction barriers

behavioral isolation

geographic isolation habitat isolation temporal isolation

mechanical isolation gametic isolation

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Geographic isolation

Species occur in different areas

physical barrier

allopatric speciation

“other country”

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

Ammospermophilus spp

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Habitat isolation

Species occur in same region, but occupy

different habitats so rarely encounter each other

reproductively isolated

2 species of garter snake,

Thamnophis, occur in

same area, but one lives in

water & other is terrestrial

lions & tigers could hybridize,

but they live in different habitats

and have different behaviors:

lions are live in prides

tigers are solitary

This results in them not seeing

each other as suitable mates.

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Temporal isolation Species that breed during different times of day,

different seasons, or different years cannot mix gametes

reproductive isolation

sympatric speciation “same country”

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 Unique behavioral patterns & rituals isolate species

identifies members of species

attract mates of same species • courtship rituals, mating calls


Blue footed boobies mate only

after a courtship display

unique to their species

sympatric speciation?

AP Biology firefly courtship displays

courtship display of

Gray-Crowned Cranes, Kenya

courtship songs of sympatric

species of lacewings

Recognizing your

own species

AP Biology Well, 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.

Plants


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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

Animals

I can’t even imagine!

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Gametic isolation

Sperm of one species may not be able to fertilize

eggs of another species

mechanisms

biochemical barrier so sperm cannot penetrate egg

receptor recognition: lock & key between egg & sperm

chemical incompatibility

sperm cannot survive in female reproductive tract

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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Prevent hybrid offspring from

developing into a viable, fertile adult

reduced hybrid viability

reduced hybrid fertility

hybrid breakdown

zebroid

POSTZYGOTIC- reproduction barriers

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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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Mules are vigorous,

but sterile

Reduced hybrid fertility Even if hybrids are vigorous

they may be sterile

chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes

Donkeys have 62

chromosomes

(31 pairs)

Horses have 64

chromosomes

(32 pairs) Mules have 63 chromosomes!

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Hybrid breakdown

Hybrids may be fertile & viable in first

generation, but when they mate offspring

are feeble or sterile

In strains of cultivated rice,

hybrids are vigorous but

plants in next generation are

small & sterile.

On path to separate species.


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Allopatric and Sympatric Speciation

Reproductive Isolation is the key!

Allopatric speciation geographic separation

populations experience

different mutations/adaptations

over time the two genomes

diverge to the point that

reproductive isolation has

occurred.

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Sympatric Speciation Organisms still live in same area

Can occur by polyploidy speciation

(usually in plants)

Habitat differentiation – a subpopulation exploits a habitat or resource not utilized by the parent population

Sexual selection ex: fig. 24.12

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Hybrid Zones Populations start diverging genetically.

A hybrid zone occurs where interbreeding is possible and gene flow can resume.

The outcome depends on the strength of the isolating factor: either reinforcing the isolation, genetically fusing the populations or creating stable areas of hybrid populations.

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Hybrid Zones

Female hybrid Male hybrid

Grizzly Polar Bear

Enquiry Moment:

Polar bear and grizzly bear

hybridization can occur and

produce fertile offspring.

What does this say about bear

evolution and the selection

factors involved?

And, discuss the possible

outcome of the Hybrid Zone if

climate change is occurring

with continued shrinkage of

the polar ice pack before the

next Glaciation Event occurs?

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Polyploidy Species A

2n = 6

Species B

2n = 4

Normal

gamete

n = 3

Meiotic error;

chromosome number not

reduced from 2n to n

Unreduced gamete

with 4 chromosomes

Hybrid with

7 chromosomes

Unreduced gamete

with 7 chromosomes

Normal

gamete

n = 3

New species:

viable fertile hybrid

(allopolyploid) 2n = 10

Polyploidy is the

presence of

extra sets of

chromosomes

due to accidents

during cell

division

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Polyploidy

Polyploidy is much more common in

plants than in animals

An autopolyploid is an individual with

more than two chromosome sets,

derived from one species

Many important crops (oats, cotton,

potatoes, tobacco, and wheat) are

polyploids

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Niles Eldredge

Curator

American Museum of Natural History

Rate of Speciation

Current debate:

Does speciation happen gradually or

rapidly

Gradualism

Charles Darwin

Charles Lyell

Punctuated equilibrium

Stephen Jay Gould

Niles Eldredge

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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

rapid bursts of

change

long periods of little

or no change

species undergo

rapid change when

they 1st bud from

parent population Time

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Stephen Jay Gould (1941-2002)

Harvard paleontologist & evolutionary biologist

punctuated equilibrium

prolific author popularized evolutionary thought

Example: Punctuated Equilibrium- Slide 1

Original mouse Species – Species A Adapted to do best on the wetter, more tropical eastern side of the island under current conditions.

Species A lives in this region as the climatic conditions favor the environmental factors it is adapted to survive within.


Over a few tens of thousands of years, climatic changes expand the wetter, dryer regions suitable for Species A into the Central Valley and Northeast Coast. Over time, Species A increases in numbers and range to expand into these regions. Note: since the suitable environment is expanding, there are no significant selection forces to cause genetic differences to build up.




Climatic changes swing back. The northern coast becomes dryer/colder while the central valley stays warm, but dryer. Over time, most in these populations are not suited to survive the conditions and die out. If the proper mutations occur in the northern coastal area some individuals will vary with larger body size and darker fur, both adaptations to retain body heat and survive colder conditions. These individuals will have higher reproductive success. If they survive, it will create an isolated pocket of these mice. Species A continues on in numbers in its southeastern range.


Original mouse Species – Species A

Note: The Original Species A is back to its original range. The north coastal mice are surviving, but mutations, natural selection forces are creating genetic changes making them larger and darker.

The central valley mice are undergoing selection forces that favor drought survival. It may or may not produce visual changes, but you can tell when an overall drought it’s the island, as the Central Valley mice will survive, reproduce in higher numbers compared to Species A.


Original mouse Species – Species A

Note: • The Original Species A is unchanged. • The north coastal mice are undergoing genetic changes making them larger and darker. • The central valley mice are undergoing genetic changes making them more drought resistant. • They are separated physically in different habitats. • All three variants are surviving simulataneously, populations are “branching off.” One species IS NOT replacing the other.

If enough genetic changes occur that prevent successful reproduction, then they have become reproductively isolated and can be considered as different species!

AP Biology 2007-2008

Any Questions??