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Chapter 24.
The Origin of Species
AP Biology
“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.”
5/24/2017
— Darwin
What is a species?
 Biological species concept



AP Biology
defined by Ernst Mayr
population whose members can interbreed &
produce viable, fertile offspring
reproductively compatible
5/24/2017
Biological species concept
Eastern Meadowlark
AP Biology
Western Meadowlark
Similar body & colorations, but are distinct
biological species because their songs &
other behaviors are different enough to
prevent interbreeding
5/24/2017
Diversity & Taxonomy
 The Tree of Life
species are the
smallest unit at
the ends of
branches
 basic unit for
organizing &
categorizing
living things
 smallest unit by
which we
measure diversity

AP Biology
5/24/2017
How and why do new species originate?
 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
AP Biology
5/24/2017
Pre-zygotic barriers
 Impede mating or hinder fertilization if
mating occurs
habitat isolation
 temporal isolation
 behavioral isolation
 mechanical isolation
 gametic isolation

AP Biology
male Frigate bird
displaying to attract
females
5/24/2017
Ecological 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
AP Biology
lions & tigers could
hybridize, but they
live in different
habitats:
 lions in grasslands
 tigers in forest
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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
AP Biology
5/24/2017
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
AP species
Biology
5/24/2017
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.
AP Biology
5/24/2017
Mechanical isolation
 For many insects, male &
female sex organs of
closely related species do
not fit together, preventing
sperm transfer

AP Biology
lack of “fit” between sexual organs:
hard to imagine for us, but a big issue for insects with
different shaped genitals!
Damsel fly penises
5/24/2017
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
 receptor recognition: lock & key between egg & sperm
Sea urchins release sperm
& eggs into surrounding
waters where they fuse &
form zygotes. Gametes of
different species—red &
purple —are unable to fuse.
AP Biology
5/24/2017
Postzygotic barriers
 prevent hybrid zygote from developing
into a viable, fertile adult
reduced hybrid viability
 reduced hybrid fertility
 hybrid breakdown

AP Biology
zebroid
5/24/2017
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.
AP Biology
5/24/2017
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
Mule are vigorous,
but sterile
Horses have 64
chromosomes
(32
pairs)
AP Biology
Donkeys have 62
chromosomes
Mules have 63 chromosomes! (31 pairs)5/24/2017
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.
AP Biology
5/24/2017
Speciation
 Species are created by a series of
evolutionary processes

populations become isolated
 reproductively isolated
 geographically isolated

isolated populations evolve
independently
 Isolation

allopatric
 physical separation

AP Biology
sympatric
 still live in same area
5/24/2017
Allopatric speciation
 Allopatric = “other country”

geographic separation
 migration
 physical barrier
AP Biology
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
antelope5/24/2017
squirrel
Sympatric speciation
 Sympatric = “same country”

some type of isolation even
though populations live in same
area

what causes this isolation?
 behavioral differences
 non-random mating
 physiological differences
 chromosomal changes
 polyploidy
 mostly in plants: oats, cotton,
potatoes, tobacco, wheat
AP Biology
5/24/2017
Adaptive radiation
 Evolution of many diversely adapted
species when introduced to various
new environmental challenges &
opportunities
Drosophila
Geospiza
AP Biology
5/24/2017
Review
 Speciation is a process

populations become isolated
 geographic isolation
 different environmental conditions

food, predators, disease, habitat
 different selection pressures
 genetic drift
 reproductive isolation
 different selection pressures


AP Biology
sexual selection
isolated populations evolve
independently
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Gradualism
 Gradual divergence
over long spans of
time

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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

Time
AP Biology
5/24/2017
Stephen Jay Gould (1941-2002)
 Harvard paleontologist &
evolutionary biologist
punctuated equilibrium
 prolific author

 popularized evolutionary thought
AP Biology
5/24/2017
AP Biology
5/24/2017
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
tree with many
divergent trends.
It does not represent
the peak of
perfection. There
is compromise &
random chance
involved as well
Remember that for
humans as well!
AP Biology
5/24/2017
Some interesting evolutionary
trends….
AP Biology
5/24/2017
Convergent evolution
 Flight evolved 3 separate times

AP Biology
evolving similar solutions to similar “problems”
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Parallel Evolution
 Fill similar niches; have similar adaptations,
but are not closely related
Niche
Placental Mammals
Burrower
Mole
Marsupial mole
Anteater
Numbat
Mouse
Marsupial mouse
Anteater
Nocturnal
insectivore
Australian Marsupials
Climber
Spotted cuscus
Lemur
Glider
Stalking
predator
AP Biology
Chasing
predator
Flying
squirrel
Ocelot
Wolf
Sugar glider
Tasmanian cat
Tasmanian “wolf”5/24/2017
Mimicry
 convergent evolution based on similar
(protective) appearance
Monarch male
poisonous
AP Biology
Viceroy male
edible
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Coevolution
 Predator-prey relationships
 Parasite-host relationships
 Flowers & pollinators
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5/24/2017
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 bestowed
posthumously.
www.DarwinAwards.com
AP Biology
5/24/2017
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