Transcript File - Siegel Science

Theories Of Evolution
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)
Lamarck’s Theory of Evolution

More examples
 Traits
Acquired During Ones Lifetime Would
Be Passed To Offspring
Clipped ears and tails of dogs could be passed to offspring!
Lamarck’s Mistakes

Was he correct??

NO!

Traits are passed down from one generation to the
next by genes, not by an individual’s life
experiences or activities

Lamarck did NOT know how traits were
inherited (Traits are passed through genes)
Genes Are NOT Changed By Activities In
Life
Change Through Mutation Occurs Before
An Organism Is Born


Theories of Evolution continued…

Charles Darwin (Born Feb. 12,1809-British)
 Voyage on H.M.S. Beagle (18311836) voyage around the world;
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
Voyage of the Beagle
During His Travels, Darwin Made
Numerous Observations
Collected Evidence That Led Him
To Propose a
Revolutionary Hypothesis
about The Way Life Changes Over
Time
Darwin’s Belief’s
Survival of the fittest
OR
Natural selection
 Food
and resources are limited
 So
organisms have to fight to get
them (lions fight for food, etc)
• Too many organisms, they will
fight to survive
• not all offspring will survive
What happened to the
Giraffe’s?
Survival of the fittest
or
Natural selection
 Natural
selection said the giraffes with
short necks had less food to eat
 Why?

the food resources changed to leaves only
on the upper branches
 What


happened?
short necks could not reach upper branches
and did not survive
Long neck giraffes survived because they
were able to reach the food
Evolution
is the slow,
gradual change in
a population of
organisms
over time…
a looooooooong time
Darwin’s Theory
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
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).
Where do we
see this
change?
The unequal ability of individuals to
survive and reproduce
leads to a gradual change in a population,
with favorable characteristics accumulating
over generations
“natural selection”
New species evolve
Let’s look at a classic example of
natural selection
Peppered moths!
Theories of Evolution continued…




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
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.
Mechanisms of Microevolution
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

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
Macroevolution



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
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