Transcript Evolution - Siegel Science

Evolution
What is Evolution?

The theory that organisms gradually and
slowly change over time into new species.
 Results from natural selection acting on
genetic variation present among individuals
of a species
 Sources of variation: mutations and genetic
recombination (mutations can be good, bad,
or indifferent)
Example of Evolution:

Pesticide resistance
 In the 1960s DDT was used to kill mosquitoes (to stop the
spread of malaria). At first, the results were good, but
soon DDT use had to stop. Why?
 The pesticide killed most insects, but a few survived. These
survivors are mosquitoes with genes that somehow enable
them to resist the chemical.
 These survivors then reproduce and may pass on their
pesticide resistant genes to their offspring.
 In each generation, the number of pesticide resistant
insects increase, making the pesticide spraying less
effective.
Theories of Evolution

1766 Georges Buffon (French)-said that some
fossil forms may be
ancient versions of
living species
Theories of Evolution

Jean Baptiste Lamarck (1809-French)
 Organisms
constantly strive to improve themselves and
become more advanced
 This effort to improve causes the most used body
structures to develop,
while the unused structures
waste away (principle of use
and disuse)
 Once the structure is modified,
the modification is inherited by
the organism’s offspring
(inheritance of acquired characteristics)
Theories of Evolution continued…

Charles Darwin (1859-British)
 Voyage on H.M.S. Beagle (18311836) to the Galapagos Islands;
author of On the Origin of Species
 Purpose of voyage was to map
the South American
coastline
Darwin was the
naturalist on the ship.
H.M.S. Beagle voyage
Theories of Evolution continued…
Darwin collected several type of finches on the islands.
He noted that the birds beaks were adapted to the
type of food they ate. Darwin hypothesized that the
finches strayed from the mainland and then diversified
on the different islands.
 While on the voyage he read Principles of Geology
(Charles Lyell) which discusses the gradual changes the
earth goes through over time (ex. mountain building)
 Through his research Darwin composed his theory of
evolution (1840s). He delayed publication, however,
fearing the public outcry that would occur against his
ideas.

Darwin’s Theory of Natural Selection
1. Overproduction – Many more offspring are born
than can be supported by the environments’ food,
space, and resources. These offspring then compete
for survival.
 2. Individual variation – Individuals within a species
have different traits
 Sources of variation:
a. Mutations (rare)
b. Sexual recombination -independent assortment
and crossing over during meiosis, and fertilization

Darwin’s Theory of Natural Selection

3. Natural Selection – Those individuals with traits
best suited to the environment are the ones that
survive long enough to reproduce and pass their
traits on to their offspring. Those organisms less fit
tend to die before they are able to reproduce.
Their genes are not passed on.
 This leads to an accumulation of favored traits in
the population over generations (evolution).
Theories of Evolution continued…
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Alfred Wallace – He also
developed a theory of evolution
based on natural selection. He sent
a memoir concerning his ideas to
Darwin in 1858. This prompted
Darwin to publish his book.
1859 – Darwin publishes his book,
On the Origin of Species.
• Living species of today arose from
a succession of ancestors through
evolution
• Natural selection is the mechanism
for how life evolves.
Populations evolve, individuals do not!
Population = a group of individuals of the
same species living in the same place at
the same time
 Common MISCONCEPTION – Individual
organisms evolve during their lifetime
 Populations evolve over generations!

Natural Selection at Work
1. Directional Selection- characteristics at one end of
the phenotypic range become more common than
midrange characteristics
 Examples:
Widespread use of chemical pesticides in
agriculture
Antibiotic Resistance
 2. Stabilizing Selection- midrange characteristics are
favored (extremes are not favored)
 Example: babies that weigh far more or far less
than average at birth have a lower chance of
survival

Natural Selection at Work (continued)

3. Disruptive Selection – characteristics at both
ends of the range are favored (midrange
characteristics are selected against)
Example: black-bellied seedcracker bird found in
rainforests. These birds have either a small beak or
a large beak. Birds with small bills can eat the soft
seeds available to them, large-billed birds are
better at cracking larger, harder seeds. Birds of this
type with medium sized bills do not exist.
Types of Natural Selection
Mechanisms of Microevolution

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Microevolution = the generation to generation change in the
frequency of alleles in a population
Population Genetics – the study of the genetic make-up of
populations over time
1. Genetic Drift – occurs when populations shrink
 A. Bottleneck effect – disasters such as earthquakes,
floods, and fires may kill large numbers of individuals.
The small surviving population has a smaller sample of
alleles left to pass on to the future generations.
 Reduces the overall genetic variability in a population
because some alleles are likely to be lost from the gene
pool
 Gene pool = all the alleles (genes) in a population
Endangered Species
Cheetahs were once
widespread in Africa
and Asia. Their numbers have significantly
decreased. Only a few populations exists in
the wild. They have very low genetic
variability (as low as highly inbred lab mice!)
The cheetahs remaining are being crowded
into nature preserves thus increasing the
chance for the spread of disease. With little
genetic variability within the species, they may
have a reduced ability to adapt and survive
any environmental changes.
Mechanisms of Microevolution
 B.

Founder effect = A few individuals colonize a habitat.
The new populations gene pool would be different from
the parent population it came from.
 Ex. During a hurricane a few birds from a population of
birds are blown off course and settle on an island.
These birds survive and reproduce eventually resulting in
a different population of birds from the original.
2. Gene Flow – a population may gain or lose alleles when
individuals move into or out of populations.
 It tends to reduce differences between populations
 Ex. The armies of Alexander the Great brought the
genes for green eyes from Greece all the way to India!
Mechanisms of Microevolution

3. Mutations – changes in the DNA
 Mutations are very rare, however, the
cumulative effect over many generations can
be significant. Mutations are the original
source of genetic variation.
 Microevolution does not necessarily lead to
the evolution of new species. Natural
selection is needed for new species to evolve.
Macroevolution: The Origin of New Species
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Macroevolution – the major changes in the history of life
 Origin of new species
 Origin of new anatomy (wings, feathers, bigger brains)
 Explosive diversification (Diversification = evolution of a
variety of species)
 Mass extinctions – clears the way for new adaptations
Speciation = when one or more species branch from a
parent species, which may continue to exist. Creates
biological diversity by increasing the number of species.
Species = a population whose members have the potential
to interbreed with one another to produce fertile offspring
How does speciation occur?
In order for a new species to evolve, some of the
species must be kept separate from the other
members of the species.
 Now the separated population can follow its own
evolutionary course.

Divergent evolution – when isolated
populations evolve independently
Reproductive Barriers
 Examples
of reproductive barriers between species:
 Temporal isolation – time based (Ex. Western spotted
skunks breed in the fall, eastern spotted skunks breed in
late winter)
 Habitat isolation – they may live in the same region but
not the same habitat (Ex. N. American garter snake: one
species lives on land, a closely related species lives in
the water)
 Behavioral isolation – traits that enable individuals to
recognize potential mates (odor, color, etc…)
 Geographic barriers – mountains may merge, land
bridges form, large lake becomes several smaller lakes;
can also occur if members of a species colonize a new
and remote area.
How does speciation occur?

Adaptive radiation – Overtime, many new species may come
from one ancestral species.
 Multiple branching
of a family tree occurs.
 A new environment
may have many new
niches.
If organisms become
isolated from one
another,
new species can evolve
To occupy the niches.
How does speciation occur?
Convergent evolution –
species with very different
ancestors may evolve to have
similar characteristics because they
evolved to occupy similar niches
(they do not have a relatively recent common
ancestry, though). Ex. Penguins
and porpoise – have similar body
structure for fast swimming

How Fast are Species Created?

Two models:
 Gradualism – Species diverge from a common ancestor gradually
and slowly
 Punctuated equilibrium – A species undergoes most of its
modifications early in its existence (punctuated). Little change
occurs after that (equilibrium).
 Paleontologist have observed in the fossil record that most species
appear in new forms relatively suddenly (in geologic terms), then
change little during their time on earth, they then disappear from
the fossil record.
 Ex. A certain species survives 5 million years on earth. Most of its
modifications occur during the first 50,000 years or so and then
little change occurs during the rest of its existence. (Gradual
changes during the first 50,000 years, but in geologic time
50,000 years is short/abrupt)
Evidence for Evolution

Fossils-reveal changes in the characteristics
of organisms
Reveals the appearance of organisms in a
historical sequence
Oldest know fossils are prokaryotes (3.5
bya)
Paleontologist = scientist who studies fossils
Evidence For Evolution

Comparative anatomyshows that organisms
have common ancestors
 Homologous structurescharacteristics that are
similar because they
were inherited from a
common ancestor
Evidence for Evolution

Comparative Anatomy
 Analogous structures-characteristics that are similar in
purpose, because the organisms share a common role
in the environment, but are not inherited from a
common ancestor
(example: wings of
bat and wings of
insects)
Homology vs Analogous
Evidence for Evolution
 Vestigial
structuresstructures that are inherited
but unused. These structures
were fully developed and
functional in some ancestor
(example: snakes-leg and
pelvic bones; horsereduced toe; humansappendix)
Evidence for Evolution continued…
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Comparative embryologycharacteristics among different
species are displayed only in
embryonic development and are
lost in adults
 example: All vertebrates have
gill pouches on the sides of the
throat during embryonic
development (fishes, frogs,
snakes, birds, apes, humans,
etc…)
Evidence for Evolution continued…
Comparative biochemistry-universal DNA!!
 The four bases that make up DNA are the same in all species. The
more similar the bases, the more related the organisms are. (The
DNA of humans is 98% similar to that of Bonobos.) The greater
number of sequences that match, the more recent the common
ancestor.
 The codon chart applies to all living things… the triplet bases code for
the same amino acids in all living things
(the process of protein synthesis is
almost identical in all eukaryotes)

More Evidence for Evolution
 All
forms of life use RNA
 All living things depend on water
 All life is based on five elements – carbon,
hydrogen, nitrogen, oxygen, and phosphorus.
These elements, along with small amounts of
others, make up the organic compounds
common to all cells: lipids, carbohydrates,
proteins, and nucleic acids