Transcript mass extinctions

NATURAL SELECTION
A GENETIC CHANGE IN A POPULATION
OF ORGANISM THAT OCCURS OVER
TIME.
Charles Darwin
The Origin of Species by
Means of Natural Selection
(1859)
WHAT DID DARWIN SUGGEST?

Inherited traits favorable to survival in a given
environment would tend to be preserved and
unfavorable ones would be eliminated.
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Adaptations: Evolutionary modifications that
improves the chances of survival and
reproductive success of the population in a
given environment.
WHAT DOES NATURAL SELECTION INVOLVE ?
Those individuals with a combination of genetic
traits better suited to environmental conditions are
most likely to survive and reproduce.
Four components of Natural Selection:
1. overproduction
2. variation
3. limits on population growth , or a
struggle for existence
4. differential reproductive success
MICRO VS. MACRO EVOLUTION
Gene pool: The sum total of all genes possessed by
the individuals of the population of a species.
Microevolution: small genetic changes that occur in
a species gene pool over time.
Example: Camouflage coloration in the
peppered moth
REMEMBER THE STORY?????
First an environmental change occurred:
Soot caused a change in the background color of the
tree trunks. Then the environmental change led to a
change in selective forces: Predators were able to
find and eat the moths with the coloration that no
longer blended in with the background.
SOOOOOOO….
APPLICATIONS OF DARWIN’S OBSERVATIONS
1. There were two colors forms
(Variability in the genes)
2. Color form was genetically based
( heritability of the genes)
3. There was greater survival and reproduction by
one of the color forms
( differential reproduction)
Microevolution took place!!!!
KEYS TO REMEMBER
Environmental conditions do not create
favorable heritable characteristics…
 Instead, natural selection favors some
individuals over others by acting on inherited
genetic variations (alleles) already present in
the gene pool of a population.
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NATURAL SELECTION ACTS ON GENETIC
VARIATION
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Natural selection changes
characteristics through:
Directional selection = drives
a feature in one direction
Stabilizing selection = favors
intermediate traits
 Preserving the status quo
Disruptive selection = traits
diverge in two or more
directions
EVIDENCE OF NATURAL SELECTION IS
EVERYWHERE
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It is evident in every adaptation of every organism
Artificial Selection = the process of selection
conducted under human direction
 Producing the great variety of dog breeds and
food crops
SPECIES RICHNESS:THE NUMBER OF
SPECIES PRESENT IN A COMMUNITY
•
Tropical Rainforest and
coral reefs have
extremely high species
richness.

Isolated islands and
mountain tops have
low species richness
Creates
Geographical
Isolation
BIODIVERSITY: VARIETY OF DIFFERENT SPECIES

The number of
species in existence
at any time is equal
to the number
added through
speciation minus
the number
removed by
extinction.
WHAT DETERMINES SPECIES RICHNESS?
1. Abundance of ecological niches
Greatest at the margins of adjacent communities
ecotone – a transitional zone where two or
more communities meet
edge effect- the change in species composition
produced at ecotones
2. Inverse relationship to the geographical isolation of
a community.
3. Reduced when one or more species is dominant in
a community (competition)
4. Inversely related to the stress on a habitat
5. Geological history – (climate changes)
NICHES
Fundamental niche: the potential ecological
niche that an organism could have if there
were no competition from other species.
 Realized niche: the life style that an
organism actually pursues because of
competition from other species.

ECOSYSTEM STABILITY
•
Ecosystems with greater species richness are
better able to supply ecosystem services –
environmental benefits, such as clean air, clean
water, and fertile soil.
•
Community stability – the ability of a community to
withstand environmental disturbances
Example:
• monocultures and pest vs. blight on a specific
trees in a forest of other species
MACROEVOLUTION
Long term large-scale
evolutionary changes
among groups of a
species. New species are
formed from ancestral
species and other species
are lost through
extinctions.
Phylogenetic tree (cladogram)
MACROEVOLUTION: HOW EVOLUTION TAKES PLACE
ABOVE THE LEVEL OF SPECIES AND OVER MUCH LONGER
PERIODS THAN MICROEVOLUTION.
Macroevolutionary patterns include:
1. Genetic persistence – the inheritance of DNA
molecules from the origin of the first cells through
all subsequent lines of descent ( basis of the
unity of life)
2. Genetic divergence – long-term changes in
lineages of species (basis of the diversity of life)
3. Genetic losses – the steady loss through
background extinction or mass extinctions
(abrupt, catastrophic loss of lineages)
GENETIC DRIFT: CHANGES
IN THE GENETIC
MUTATION: CHANGES IN COMPOSITION OF A
DNA OF A CELL
POPULATION BY CHANCE.
MORE VOCABULARY
Gene Flow:
Movement of
genes between
populations. This
can lead to
changes in the
genetic
composition of
local populations.
SPECIATION: THE PROCESS BY WHICH NEW SPECIES ARE
GENERATED
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Allopatric speciation:
species formation due to
physical separation of
populations
 The
main mode of
speciation
 Populations can be
separated by glaciers, rivers,
mountains
 Each population gets its own
set of mutations
GEOGRAPHICAL ISOLATION
When two populations of a species or two groups
of the same population become physically
separated for fairly long periods into areas with
different environmental conditions.
ANOTHER TYPE OF SPECIATION

Sympatric speciation = species form from
populations that become reproductively isolated
within the same area
 Feed
in different areas
 Mate in different seasons
 Hybridization between two species
 Mutations
REPRODUCTIVE ISOLATION:
Occurs when species are unable to
reproduce because of long term
geographical separation.
Divergent Evolution: When natural
selection operates in two geographically
isolated population and change the allele
frequencies.
SELECTIVE PRESSURES INFLUENCE ADAPTATION

Related species in
different environments
 Experience different
pressures
 Evolve different traits
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Convergent evolution =
unrelated species may
evolve similar traits
 Because they live in
similar environments
COEVOLUTION
Organisms evolve together
SPECIATION RESULTS IN DIVERSE LIFE FORMS
 How do major groups diverge over time?
 Phylogenetic trees (cladograms) = show
relationships among species, groups, genes,
etc.
 Scientists
can trace how certain traits evolved
EXTINCTION:
PERMANENTLY GONE!
Previous 5 Mass Extinctions:
Are we in the 6th
Mass Extinction???
MASS EXTINCTION VS. BACKGROUND
EXTINCTION
In mass extinctions, large numbers of species become
extinct each year for tens of thousands to millions of years.
The five great mass extinctions have occurred during the
past 500 million years and have been 20-60 million years
apart. There have also been shorter mass extinctions (loss
of 15-24% of all species) in between. A period of mass
extinction is often regarded as having a loss of 25-70% of all
species. The extinction of the dinosaurs is an example of
mass extinction.
MASS EXTINCTIONS
All mass extinctions have been followed by periods of recovery,
known as adaptive radiations. After almost every mass
extinction, numerous new species have evolved (speciation) to
fill new or vacated ecological niches in the changed
environment. The extinction of dinosaurs, for example, was
followed by an explosive rise of mammals. Fossil records
suggest about 10 million years or more are required for adaptive
radiations to rebuild biological diversity after a mass extinction.
BACKGROUND EXTINCTION – NATURAL
EXTINCTION
Background extinction refers to the extinction that occurs
naturally in the evolution process. 0.00003% of species become
extinct naturally according to fossil records. During the
ecological process, the natural extinction occurs following the
evolution of the species. If a species cannot succeed in adapting
to its surroundings it eventually becomes extinct.
FACTORS OF BACKGROUND EXTINCTION INCLUDE
1. gradual changes in temperature
 2. predator-prey relationships
 3. tectonic plate movements.
It is estimated that the average species stays
around for 4 -22million years before it becomes
extinct and that 99.9 of all the species that
have ever existed are now extinct.
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EXTINCTION IS A NATURAL PROCESS, BUT …
Humans profoundly affect rates of extinction
 Biodiversity loss affects people directly
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 Food,
fiber, medicine, ecosystem services