Transcript Evolution

Evolution
What is Evolution?
• Evolution involves inheritable changes in
organisms through time
– Fundamental to biology and paleontology
• Paleontology is the study of life history as
revealed by fossils
• Explains the development of life
First, Let’s Define “Life”
• The definition must be “built” on
descriptions that fit all living things.
– Living things are both complex and organized
– Living things grow and reproduce
– Living things respond to stimuli
– Living things acquire and use materials and
energy
– Living things have (use) DNA to store
information
Misconceptions of evolution
• Evolution proceeds strictly by chance
• Nothing less than fully developed
structures, such as eyes, are of any use
• There are no transitional fossils
• so-called missing links connecting
ancestors and descendants
• humans evolved from monkeys so
monkeys should no longer exist
Historical Perspective
• Evolution is usually attributed solely to Charles
Darwin, but actually considered long before he
was born.
– By some ancient Greeks and by philosophers and
theologians during the Middle Ages
• Nevertheless, the prevailing belief in the 1700s
was that Genesis explained the origin of life.
Contrary views were heresy!
Historical Perspective
• During the 18th century, naturalists were discovering
evidence that could not be reconciled with literal reading
of Scripture
Scientists gradually accepted a number of ideas:
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The principle of uniformitarianism,
Earth’s great age
Many types of plants and animals had become extinct
change from one species to another occurred
Species existed on Earth that were no longer living (extinct)
• What was lacking, though, was a theoretical framework to
explain evolution
Evolution
• A theory explains a series of observations
and often unifies related facts through
supportive evidence.
• Evolution is based on the observed
accumulated generation to generation
changes within a defined group.
• Evolution accounts for both
– Life’s unity: similarities among life forms
– Life’s diversity: differences among life forms
Lamark
• Jean-Baptiste de Lamarck
– (1744-1829) is best remembered for his theory of
Inheritance of Acquired Characteristics.
• According to this theory
– new traits arise in organisms because of their needs
– Once acquired new traits are somehow passed on to
their descendants
• Lamarck’s theory seemed logical at the time and was
widely accepted
Lamark’s Theory
Darwin
• In 1859, Charles Robert
Darwin (1809-1882) published
On the Origin of Species
– In it he detailed his ideas
on evolution formulated
20 years earlier and
proposed a mechanism
for evolution
Fig. 7-1, p. 115
Insect eaters
Berry
eater
Seed
eaters
Insect eaters
Cactus
eaters
What he noticed
• Plant and animal breeders practice artificial selection
– by selecting desirable traits and then breeding plants
and animals with those traits
• Observing artificial selection gave Darwin the idea that
a process of selection among variant types in nature
could also bring about change
• Thomas Malthus’s essay on population suggested that
competition for resources and high infant mortality
limited population size
Natural Selection (Key Points)
• Organisms in all populations posses heritable variations.
– size, speed, agility, visual acuity, digestive enzymes, color,
and so forth
• Some variations are more favorable than others
– some have a competitive edge in acquiring resources
and/or avoiding predators
• Not all young survive to reproductive maturity
– However, Those with favorable variations are more likely
to survive and pass on their favorable variations
Back to the Giraffes…
why would giraffes “develop” longer necks
over time?
because
ancestors with
longer necks
had an
advantage and
would be more
likely to live to
reproduce and
pass on their
genetic
material.
Survival of the Fittest
• In colloquial usage, natural selection is
sometimes expressed as “survival of the
fittest”
• This is misleading because natural
selection is not simply a matter of survival
but involves differential rates of survival
and reproduction
What does “Survival of the Fittest”
actually mean over time??
• Misconception:
• among animals only the biggest, strongest, and fastest
are likely to survive
– These characteristics might provide an advantage but
natural selection may favor
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the smallest if resources are limited
the most easily concealed
those that adapt most readily to a new food source
those having the ability to detoxify some substance
and so on...
So, is “natural selection” the
mechanism for evolution?
Observers knew that life on earth has
changed with time.
Some explanations were plausible, such
as the extinction or inability for an
organism to survive.
Other observations were not as easy to
explain, and more scientific investigation
and application was needed to support the
hypotheses.
Limits on Natural Selection
• Natural selection works on existing variation in a
population
– It could not account for the origin of variations
• Critics reasoned that should a variant trait arise,
it would blend with other traits and would be lost
• The answer to these criticisms existed even then
in the work of Gregor Mendel, but remained
obscure until 1900
Gregor Mendel
• During the 1860s, Gregor Mendel, performed a series of
controlled experiments with true-breeding strains of
garden peas
– strains that when self-fertilized always display the same
trait, such as flower color
• Traits are controlled by a pair of factors now called
genes
• Genes occur in alternate forms, called alleles
– One allele may be dominant over another
– Offspring receive one allele of each pair from each parent
Mendels Work
• The parental generation
consisted of true-breeding
strains
RR = red flowers
rr = white flowers
• Cross-fertilization yielded a
second generation
all with the Rr combination
of alleles, in which the R
(red) is dominant over r
(white)
Mendel’s Work
• The second generation, when self-fertilized
produced a third generation with a ratio of three
red-flowered plants to one white-flowered plant
Why is this important?
• The factors (genes) controlling traits do not
blend during inheritance
• Traits not expressed in each generation may
not be lost
• Therefore, some variation in populations results
from alternate expressions of genes (alleles)
– Variation can be maintained!
– Why is variation important to survival of a
species?
Modern Genetics
Complex, double-stranded helical molecules of
deoxyribonucleic acid (DNA) called chromosomes
are found in cells of all organisms
except bacteria which have ribonucleic
acid (RNA)
Specific segments of DNA are the basic units of
heredity (genes)
The number of chromosomes varies from one
species to another
fruit flies 8; humans 46; horses 64
Modern Thinking
• During the 1930s and 1940s,
– paleontologists, population biologists, geneticists, and
others developed ideas that merged to form a
modern synthesis or neo-Darwinian view of
evolution
• They incorporated chromosome theory of
inheritance into evolutionary thinking
– They saw changes in genes (mutations) as only one
source of variation
Most Importantly
• They completely rejected Lamarck’s idea of
inheritance of acquired characteristics
• They reaffirmed the importance of natural
selection
• But since then, some scientists have challenged
the emphasis in modern synthesis that evolution
is gradual
Remember…
• Evolution by natural selection works on variation in
populations
– most of which is accounted for by the reshuffling of alleles
from generation to generation during sexual reproduction
• The potential for variation is enormous with thousands of
genes each with several alleles, and with offspring
receiving 1/2 of their genes from each parent
• New variations arise by mutations
– change in the chromosomes or genes
Mutations
• Mutations result in a change in hereditary
information
• ONLY mutations that take place in sex cells are
inheritable,
– Can be chromosomal mutations (affecting a large
segment of a chromosome)
– or point mutations (individual changes in particular
genes)
• Mutations are random with respect to “fitness”
– they may be beneficial, neutral, or harmful to survival!
Mutations
• If a species is well adapted to its environment,
most mutations would not be particularly useful
and perhaps could be harmful
• But what was once a harmful mutation can
become a useful one if the environment changes
The Species
• Species is a biological term for a
population of similar individuals that in
nature interbreed and produce fertile
offspring
– Species are reproductively isolated from one
another
• Goats and sheep do not interbreed in nature, so
they are separate species
• When artifically bred in captivity, offspring are most
often sterile.
Recipe for a species
• Speciation is the process by which a new
species arises from an ancestral species
• It involves change in the genetic makeup of a
population,
– which also may bring about changes
– in form and structure
• During allopatric speciation,
– species arise when a small part of a population
becomes isolated from its parent population
Insect
eaters
Berr
y
eater
Seed Cactus
eater eaters
s
Insect
eaters
Variations among “Darwin’s finches” were
naturally selected from among the
existing variations within the gene pool
and mutations that may have occurred.
What would cause the selection of the
observed variations?
Allopatric Speciation
• Reduction of the area occupied by a
species may leave a small isolated
population
– Two peripheral isolates evolved into new
species (i.e. Darwin’s finches)
Hmmm…but how long does it
take for changes to appear?
• Although widespread
agreement exists on
allopatric speciation
scientists disagree on how
rapidly a new species might
evolve
• Phyletic gradualism- the
gradual accumulation of
minor changes which
eventually bring about new
species
Punctuated Equilibrium
holds that little or no
change takes place
in a species during
most of its existence
then evolution occurs
rapidly
Misconceptions
• One antievolution argument is “If humans
evolved from monkeys, “why are there still
monkeys?”
• This involves two misconceptions
– No scientist has ever claimed that humans
evolved from monkeys
– Even if they had, that would not preclude
the possibility of monkeys still existing
Styles of Evolution
• Divergent evolution occurs when an ancestral species
giving rise to diverse descendants adapts to various
aspects of the environment
– Divergent evolution leads to descendants that differ
markedly from their ancestors
• Convergent evolution involves the development of
similar characteristics in distantly related organisms
• Parallel evolution involves the development of similar
characteristics in closely related organisms
Divergent Evolution
Convergent Evolution
Parallel Evolution
Evolutionary Novelties
• All land-dwelling vertebrate animals posses
bone and paired limbs so these characteristics
are primitive and of little use in establishing
relationships among land vertebrates
• However, hair and mammary glands are derived
characteristics.
– Only one subclade, the mammals, has them
It wouldn’t be Geology without
Death and Destruction…..
99% of all species that ever
• Perhaps as many as
existed are now extinct
• Organisms do not always evolve toward some kind of
higher order of perfection or greater complexity
• Vertebrates are more complex but not necessarily
superior in some survival sense than bacteria
– after all, bacteria have persisted for at least 3.5 billion
years
• Natural selection yields organisms adapted to a
specific set of circumstances at a particular time
Extinction
• The continual extinction of species is referred to as
background extinction
• It is clearly different from mass extinction during
which accelerated extinction rates sharply reduce
Earth’s biotic diversity
• Extinction is a continual occurrence
– …so is the evolution of new species that usually quickly
exploits the opportunities another species’ extinction
creates
– Mammals began a remarkable diversification when they
began occupying niches the extinction of dinosaurs and
their relatives left vacant
Extinction
• The mass extinction of dinosaurs and other animals at
the end of Mesozoic Era is well known…but not the
greatest loss of biologic diversity!
• The greatest mass extinction occurred at the end
of the Paleozoic Era – end of Permian
– More than 90% of all species died out
– We will discuss these extinctions and their possible
causes throughout the rest of the term