Transcript Microevolution

Chapter 16
From the 1600s on, geologists began
mapping layers of rocks at sites where
there were deep cuts in the earth.
 They found similar layers around the
world and decided that sand and
sediments had deposited at different
times to form the layers.
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Within certain layers were distinctive
fossils.
 Some deeper layers had simple marine
organisms, where, other layers had
organisms similar to those found at the
time.
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They decided that maybe all organisms
had not left one central location, but
had originated in more than one place.
 If the body structure was different in
different areas, then perhaps they had
evolved, or modified over time.
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Georges Cuvier was one of the leaders in
the naturalist field at this time.
 He founded the belief of catastrophism
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At the time of creation, the world was
populated with all species, but some great
catastrophy had destroyed some of the
species, and the survivors had repopulated
the world.
Other catastrophies through time had wiped
out other species, thus leading to the species
in existence.
Environmental pressure and internal
needs bring about a permanent change
in body form and function.
 This gradual improvement should lead
to perfection in time.
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For example the giraffe: short neck giraffes
needed to reach leaves higher in trees, so
they stretched their neck to reach them.
This stretched neck was passed on to
offspring, who in turn, stretched their own
necks and passed on that neck to their
offspring.
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A clergyman and economist, documented
human survival and the influences of famine,
disease, and war.
Humans would run out of food, living space,
and other resources because they reproduce
too much.
When this happens, they compete for the
remaining resources
Lyell traveled through Europe to find more
evidence that gradual changes, the same we
can see happening today, had produced the
features of the Earth's surface.
 He found evidence for many rises and falls of
sea level, and of giant volcanoes built on top of
far older rocks.
 Processes such as earthquakes and eruptions,
which had been witnessed by humans, were
enough to produce mountain ranges.
 Valleys were the work of the slow grinding force
of wind and water.
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Lyell's version of geology came to be known
as uniformitarianism, that the processes that
alter the Earth are uniform through time.
Lyell could never have grasped the
mechanism — plate tectonics — that makes
this kind of geological change happen.
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Lyell had an equally profound effect on
our understanding of life's history.
He influenced Darwin so that Darwin
envisioned evolution as a sort of
biological uniformitarianism.
Evolution took place from one generation
to the next before our very eyes, he
argued, but it worked too slowly for us to
perceive.
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In 1831, he boards the HMS Beagle as the
ship’s naturalist.
It sailed to South America to map the
coastline and crossed the Atlantic.
Darwin collected much information during
the stops at the islands in between.
When he returned in 1836, Darwin began
discussing his ideas with other scientists.
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Darwin wondered if he could apply Malthus’
idea to other animals.
He assumed the environment would restrict
the number of individuals, and then what
would decide who would live, and who would
die.
He then realized that one individual’s traits
could determine its survival.
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He realized that the Galapagos Island finches
were representative of this idea.
Thirteen species of finch inhabited the
islands.
Each species had a variation in traits that
made it suitable to the environment in which
it lived.
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In 1859, he published On the Origin of
Species.
He discussed how artificial selection could be
used to quickly mold species, and how natural
selection could do the same thing in a longer
time.
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Population is a group of the same species
occupying a given area.
Certain features characterize a species:
morphology (a unique body plan), physiology
(the cells operate in the same way), behavior
(the organisms respond in the same basic
way to stimulus), and they reproduce with
members of their own species.
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There are other small variations within the
population, but they are basically the same.
Variations (polymorphism) include
differences in color and size.
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Within a population is a gene pool – all of the
genes within the population.
Each gene within the pool exists in two or
more slightly different forms (alleles).
Individuals inherit different combinations of
alleles leading to different phenotypes.
Which alleles are passed on depends on five
things:
a. Gene mutations – produce new alleles
b. Crossing over at meiosis I – produces new
allele combinations
c. Independent assortment at meiosis I –
randomly puts mixes of parental alleles into
egg or sperm
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d. fertilization – combines two parents
e. change in chromosome number or structure
– can lead to loss, duplication or relocation of
genes
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All five criteria are rarely in play at one time.
Mutations are infrequent, but they are
inevitable.
These changes lead to microevolution –
small-scale changes in allele frequency as
brought about by mutation, genetic drift,
gene flow, and natural selection.
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Darwin explained natural selection after
associating inheritance of certain features
and the environment.
He made four observations to explain his
ideas:
1. All populations can increase in number
over generations
2. No population can increase indefinitely
– it will run out of resources
Inference: Individuals will end up
competing for resources
3. All individuals have the same genes,
which specify the assortment of traits
4. Most genes occur in alleles which give
rise to different phenotypes
Inference: Some phenotypes are better
for resource competitions and therefore
help them to survive and reproduce.
 These alleles increase and the genetic
change leads to fitness - an increase in
adaptation to the environment
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Conclusion: Natural selection is the
difference in survival and reproduction
among individuals that differ in
heritable traits.
 Adaptation is one outcome of the
microevolutionary process.
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Allele frequency shifts in a consistent
direction.
 This shift occurs in response to changes
in the environment.
 A trait at one end of the range becomes
dominant over the midrange traits.
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Favoring of the intermediate form of a
trait.
 This mode counteracts gene flow,
mutations and genetic drift to keep the
common phenotype.
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Forms at both extremes are favored
over the intermediate form.
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Individuals of a species may move about.
Alleles from those individuals are lost when
they leave the population.
Physical flow of genes (gene flow) tries to
counteract the loss and genetic differences.
It helps keep populations genetically similar.
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Random changes in allele frequency
brought about by chance alone.
Only one kind of allele remains in the
population and all individuals are
homozygous for it.
 It happens in all populations eventually,
but quickly in small populations.
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Severe reduction in population size
brought about by a catastrophy such as
disease, habitat loss, hunting, or natural
disaster.
Nonrandom mating among closely
related individuals.
 Human populations discourage such
things.
 In animal populations, it has lead to rare
inheritable diseases that can cause
painful death in the animals.
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