Transcript Evolution 3

What is a species?
It can seem obvious when two individuals are of different
species, e.g. the lion and the tiger. What about the Bengal
tiger and the Siberian tiger – are these different species?
If two populations are geographically
isolated, it can be difficult to tell if
they are capable of interbreeding to
produce fertile offspring. This is one
aspect of the species problem.
Similarly, the capacity to interbreed
cannot be tested in animals that are
extinct, such as the dinosaurs. To
overcome this problem, other
definitions of a species are needed.
Defining a species
The biological species concept is the most common
definition of a species. It defines a species as a set of
individuals who can reproduce to produce fertile offspring.
As well as the problem of geographical separation, another
disadvantage of this definition is that it only applies to
organisms that reproduce sexually.
These happy face spiders look different, but
since they can interbreed, they are considered
the same species: Theridion grallator.
Speciation
A lineage-splitting event that produces two or
more separate species.
Imagine that you are looking at a tip of the tree of
life that constitutes a species of fruit fly.
• Move down the phylogeny to where
your fruit fly twig is connected to the rest
of the tree.
That branching point, and every other branching
point on the tree, is a speciation event.
At that point genetic changes resulted in two
separate fruit fly lineages, where previously there
had just been one lineage.
How do new species arise?
Here is one scenario that
exemplifies how speciation can
happen:
The scene: a population of wild fruit flies minding
its own business on several bunches of rotting
bananas, cheerfully laying their eggs in the
mushy fruit...
Disaster strikes: A hurricane washes the
bananas and the immature fruit flies they contain
out to sea.
The banana bunch eventually washes up on an
island off the coast of the mainland.
The fruit flies mature and emerge from their slimy
nursery onto the lonely island.
At this point, speciation has not occurred — any
fruit flies that got back to the mainland could mate
and produce healthy offspring with the mainland
flies.
The populations diverge: Ecological conditions are
slightly different on the island, and the island
population evolves under different selective pressures
and experiences different random events than the
mainland population does. Morphology, food
preferences, and courtship displays change over the
course of many generations of natural selection.
So we meet again: When another storm reintroduces
the island flies to the mainland, they will not readily mate
with the mainland flies since they've evolved different
courtship behaviors. The few that do mate with the
mainland flies, produce inviable eggs because of other
genetic differences between the two populations. The
lineage has split now that genes cannot flow between the
populations.
Differing selection pressures on the two islands can
complete the differentiation of the new species.
EVIDENCE FOR EVOLUTION
1.
2.
3.
4.
Fossils Evidence
Anatomy
Embryology
Biochemistry
EVIDENCE FOR EVOLUTION
1. Fossils
Fossils tell a story…
Looking back in time
The rocks at the top of the Grand Canyon, Arizona, are
250 million years old. Those at the bottom are 2 billion
years old.
Why did the
rocks form in
layers?
Why do so
many people
want a closer
look at them?
The Evidence for Evolution
• Fossils
– Found in
sedimentary
rocks
– Fossils can be
rocks with
imprints of dead
matter.
– Can be amber;
ice; tar.
What are fossils?
The rocks are formed from sediments and many contain
fossils.
Fossils are the preserved
remains, impressions or
traces of animals, plants
and other organisms that
lived millions of years ago.
Some fossils contain the mineralized remains of the organism.
In other cases, the remains have been completely dissolved
and what is left is an impression, which may later fill with
minerals to produce a cast of the organism.
more recent sediment collects
older sediment
becomes rock
fish skeleton fossilised
land raised above
water level
recent rock
older rock
fossilised skeleton exposed
earth movements
fracture rock
When rock strata become exposed, it can be assumed that,
in most cases, the lowest layers are the oldest.
This means that the fossils of organisms preserved in the lowest
layers represent animals and plants that lived many millions
of years ago
rock strata of
increasing age
This is a fossil of a fish which lived 40 million years ago
A rare event
Fossilization is a rare event. Different types of fossil form
under different conditions and environments.
mud
amber
ice and frozen soil
Fossilized remains only form in the
absence of microbes, which need
food, oxygen, water and warmth.
What type of fossil forms whether
there are microbes present or not?
ash/mud
What does the fossil record show?
The history of life on Earth as shown by fossils is called the
fossil record.
Although there are gaps in
the fossil record, it helps to
tell the evolutionary story of
past and present-day
organisms. It can show
how the changes in an
organism were linked to
changes in its habitat.
The fossil record can also show how different species
evolved from common ancestors.
living fish
sediment from river
fish skeleton partly buried by sediment
The Evidence for Evolution
Why is the fossil record
incomplete?
– Only hard parts
become fossilized.
– Not all organisms die
in the right conditions
for fossilization
– Not all fossils have
been discovered
Whale Evolution
The Fossil
Record
• The fossil record may show slow gradual
changes and transitional forms
• It may also show jumps in evolution, either
due to an explosion in life forms or an
incomplete fossil record
We found the fossil
— no joke!
Land Mammal
?
?
?
Complete series
of transitional
fossils
Ocean Mammal
?
Evolution from sea to land
2006 fossil discovery of early tetrapod
4 limbs
Missing link from sea to land animals
Evolution of birds
Today’s organisms
descended from ancestral
species
Fossil of Archaeopteryx
• lived about 150 mya
• links reptiles & birds
claws
wing-like
forelimbs
teeth
thin
ribs
long tail
feathers
Replica of Archaeopteryx fossil;
half bird half reptile
Reptile-like features
Bird-like features
Evolution all around us
Darwin said, “It is not the strongest of the species that
survives, nor the most intelligent, but rather the one most
responsive to change.”
The environment today
is changing rapidly as
a result of habitat
destruction, pollution
and the artificial
control of populations.
How is environmental change affecting the evolution of
animals, plants and microbes?
What happens when habitats
change?
Adaptive evolution ensures that individuals within a species
have traits allowing them to survive and reproduce in their
habitat.
If the habitat changes, however, successful traits can
become a disadvantage.
For example, if global warming
caused arctic snow to melt, brown
rabbits living in arctic regions might
be better camouflaged and so
more likely to survive than white
rabbits.
Individuals that fail to reproduce, compete effectively for food
or survive against new predators will eventually die out. If the
last individual of a species dies, the species is extinct.
Why did the dodo become extinct?
The dodo was a large flightless bird
that lived on the island of Mauritius.
It nested on the ground in forests,
producing one egg at a time.
When human settlers arrived on the
island in the mid-1600s, they brought
animals such as rats and dogs to the
island, which ate the dodos’ eggs.
The settlers chopped down the forests in which the dodos
lived, and may have even hunted the dodo for food.
The dodo became extinct sometime in the late 1600s.
What traits might have helped the dodo to survive longer?
Why does extinction happen?
Extinction has always taken place, even before humans
evolved. For example, the dinosaurs are thought to have
become extinct due to a comet striking the Earth.
How does human activity
cause extinction?
 destruction of natural
habitats
 over-hunting
 climate change
 pollution.
Do you know of any extinct animals?
Countdown to extinction
Lack of sustainability threatens many organisms with
extinction.
Over 1.5 million different species of organisms have been
identified, but scientists think there are many millions still
to be discovered.
However, extinction
rates are the highest
since the death of the
dinosaurs. On
average, one species
of plant or animal
becomes extinct every
20 minutes.