Transcript Transport in Plants

UNIT 12
Chapter 23: The Evolution of Populations
Chapter 24: The Origin of Species
Chapter 26: Early Earth & the Origin of Life
Chapter 27: Prokaryotes & the Origin of
Metabolic Diversity
Microevolution

Microevolution is the generation-togeneration change in a population’s allele
frequency

4 factors drive microevolution
Genetic drift
 Natural selection
 Gene flow
 Mutations
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Genetic drift
Changes in gene frequency occur from one
generation to the next with small populations
 Sources of genetic drift: founder effect and
bottleneck effect
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Natural selection
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Some individuals will leave more offspring than
others
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Gene flow
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Some phenotypes make certain individuals more likely
to reproduce
Genetic exchange due to migration tends to
reduce differences between populations
Mutations

Random changes to the DNA of an organism

Original source of new alleles in populations
Characters

Characters are expressions of an individual’s
genotype
Characters can be quantitative or discrete
 Polymorphisms exist when two or more discrete
phenotypes are present

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Heterozygote advantage explains that in some
cases heterozygotes have a greater
survivorship than do homozygotes

Example: sickle-cell anemia
Sexual Selection
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Sexual selection usually leads to sexual
dimorphism
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Males are usually the larger and showier sex
Intrasexual selection involves members of the
same sex competing for mates
Intersexual selection involves one sex
choosing a member of the opposite sex to
mate with
Evolution Has No Goal

There are at least four reasons that evolution
has no goal and cannot produce “perfection”
Evolution is limited by ancestry
 Adaptations are often compromises
 Not all evolution is adaptive
 Selection can only edit existing variations
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END
Species
• A group of potentially or actually
reproducing organisms producing viable
offspring
• Prezygotic and postzygotic barriers isolate
gene pools
Prezygotic &
Postzygotic
Barriers
• Adaptive radiation is
a form of allopatric
speciation
• Many species from a
common ancestor
• Due to occupation
of previously
uninhabited areas
• Debate over the tempo (rate) of speciation
created two theories: gradualism and
punctuated equilibrium
END
A Brief History of Earth
• Earth formed ~4.5 bya (billion years ago)
• Crust cooled and hardened ~4.1bya
• Chemical origins of life appeared ~3.8bya
• Life appeared ~3.5bya (3.8bya?)
• Prokaryotes dominated life on Earth from
3.5 – 2.0bya
• Oxygen began accumulating in the atmosphere
~2.7bya
• Cyanobacteria split H2O to produce O2
• Most prokaryotes were anaerobic and therefore
went extinct
• Eukaryotic life existed as far back as 2.1bya
and multicellularity is as old as 1.2by
• The “Cambrian Explosion” (540mya)
represents a huge increase in plant, fungal
and animal diversity
• Humans diverged from other primates less
than 5mya
The Origin of Life
• Scientists in the 1920s, Oparin and Haldane
speculated that organic compounds came from
simple inorganic molecules
• Not possible now due to high O2 concentrations
• In 1953, Stanley Miller and Harold Urey set out
to test the Oparin-Haldane hypothesis
RNA as Genetic Material
• Many researchers believe RNA preceded DNA
as genetic material
• RNA is much simpler and can base pair with
other RNA nucleotides to make copies
• In a pre-biotic world, RNA was probably
capable of replication
Protobionts
• Living cells may have been preceded by
protobionts, combinations of abiotically
produced molecules
• Some of the simplest protobionts are liposomes, a
simple “bubble” created by lipids
• RNA could have become packaged in liposomes
• Hereditary material and primitive “cells” could then
evolve as units
Biogenesis
• Spontaneous generation was a long-supported
theory
• Living things arose from the non-living
• Biogenesis is the understanding today
• Life comes from preexisting living things
END
They’re (almost) Everywhere!
 Prokaryotes were the earliest organisms on
Earth and evolved alone for at least 1.5by
 Prokaryotes still dominate the biosphere
today
Collective biomass outweighs all eukaryotes
combined by 10X
 More prokaryotes in the mouth of a human
than the total number of people who have ever
lived

 We hear most about the minority of prokaryote
species that cause serious illness
During the 14th century, a bacterial disease
known as bubonic plague, spread across Europe
and killed one of every three people
 Other types of diseases caused by bacteria
include tuberculosis, cholera, many sexually
transmissible diseases and certain types of food
poisoning
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 More bacteria are benign or beneficial
 Bacteria in our intestines produce important
vitamins
 Recycle elements from the dead back into nature
 Prokaryotes often live in close association
among themselves and with eukaryotes in
symbiotic relationships

Mitochondria and chloroplasts evolved from
prokaryotes
Two Main Branches
 Molecular evidence over the last two
decades has lead to the conclusion that
there are two major branches of
prokaryote evolution

Bacteria and the archaea
Prokaryotic Structure
 Most prokaryotes are unicellular
 Common
shapes among
prokaryotes are
spheres (cocci),
rods (bacilli),
and helices
 Most are much smaller than eukaryotic cells
 Cell wall: maintains the shape of the cell,
affords physical protection, prevents the cell
from bursting
 Bacterial cell walls contain peptidoglycan
Modified chains of sugars linked by short
polypeptides
 Walls of archaea lack peptidoglycan

 Many antibiotics, inhibit the synthesis of a
functional wall
 Many prokaryotes secrete another sticky
protective layer, the capsule, outside the cell
wall
Adhere cells to a surface
 May increase resistance to host defenses and/or
antibiotics
 Glue together the cells of those prokaryotes

 Another way for prokaryotes to adhere to one
another or to the substratum is by surface
appendages called pili

Sex pilus (pili)
Prokaryotes Grow and Adapt Rapidly
 Prokaryotes reproduce only asexually via
binary fission
 Growth refers to an increase in numbers, but
NOT size
 Some bacteria form
resistant cells, endospores
Cell surrounds chromosome
with a durable wall
 Outer cell may disintegrate,
dehydrates, does not
metabolize

 Endospores are resistant to all sorts of trauma
 Can survive lack of nutrients and water, extreme
heat or cold, and most poisons
 May be dormant for centuries or more
• When environment becomes more hospitable,
endospore absorbs water and resumes growth
Metabolic Methods
 Bacteria are
diverse in their
methods in
obtaining
energy
Oxygen and Bacteria
 Oxygen accumulating in the atmosphere was
positive for some, negative for others
Obligate aerobes: O2 required for cellular
respiration
 Facultative anaerobes: O2 is used if present,
otherwise fermentation is sufficient
 Obligate anaerobes: O2 is toxic

The Evolution of Photosynthesis
 Evidence suggests that the first life forms were
chemoheterotrophs
Depleted supply of organic molecules
 Natural selection favored organisms that could
harness the sun’s power

• Cyanobacteria are the only autotrophs that release O2
(oxygenic photosynthesis)
• Some bacteria split other compounds such as H2S using
the sun’s energy
Impact on Humans
 Disease-causing bacteria have a major impact
on humans

Some are opportunistic, only causing illness if host
is weakened
 Louis Pasteur and Joseph Lister began to
connect bacteria to disease in the late 1800s
 Robert Koch was the first to connect a specific
bacterium to a disease

Koch’s methods were used to establish Koch’s
postulates
1. Researcher must find the same pathogen in all
diseased individuals
2. Isolate the pathogen and grow it in pure culture
3. Induce disease in individuals by transferring the
pathogen from pure culture
4. Isolate the pathogen from the experimental
individuals after disease develops
From Prokaryote to Eukaryote
END