Transcript The Origin of Species - Scituate Science Department

The Origin of Species
What is a Species?
• “All the populations of organisms that are
potentially capable of interbreeding under
natural conditions and that are
reproductively isolated from other
populations.”
• So, essentially members of a species can
mate and produce viable offspring with
each other but NOT with members of any
other species
This is a lion
This is a tiger
Both are totally badass. Both are considered to be “big cats.” They can
reproduce and have offspring with each other, but…
No Matter How Closely Related
They Are The Offspring Can’t
Reproduce Any Further
This liger is pretty BA
So is this tigon
But, they are sterile. This means that lions and tigers are closely
related but are two different species
Same goes for the jackass
I just realized that this jackass is a jackass. Well
maybe this guy’s the jackass, I mean who loaded
this cart anyway?
How Do New Species Form?
• Speciation requires that two populations
be isolated from gene flow between them
and develop significant genetic divergence
• There are two basic ways that this
happens.
Allopatric Speciation
• This occurs by geographical isolation (this
means an ocean or something like the Grand
Canyon separates the populations) which leads
to divergence of the separated populations
through genetic drift or natural selection
Kaibab Squirrel
Abert Squirrel
Sympatric Speciation
• This occurs by ecological isolation (this means
the same geographical area has more than one
distinct habitat) and subsequent divergence or
by rapid chromosomal change such as through
polyploidy (this happens mostly in plants)
A Bit About Polyploidy
• It happens in plants when a fertilized egg
copies its DNA but doesn’t divide
• The resulting cell is now tetraploid (4
copies of each chromosome)
• If all resulting cell divisions are normal
then the organism will be tetraploid
• Most tetraploid plants are healthy and
vigorous
Its Okay for Plants To Do This
• …because many can self fertilize and/or
reproduce asexually
• If a tetraploid self-fertilizes the offspring
will be tetraploid, so no problem
• Asexually derived offspring are identical to
the parent so again, no problem
• In fact almost 50% of flowering plants (the
most successful plants on Earth) are
polyploid
The Only Problem is…
• If a tetraploid mates with a diploid the offspring
will be triploid
• These guys are weak and flawed and
experience many problems during development
• Oh yeah, they are always sterile too
• This is why animals cannot be tetraploid. All
animals are sexual so if an individual is a
tetraploid it would have to mate with a diploid
and the resulting offspring would be triploid
The Fossil Record Can Be
Tricky
• It is fairly easy, these days, to determine if
two individuals are from the same
species…
• But what about the fossil record?
• How do you tell if two extinct, fossilized
specimens are different species or just two
different individuals but no speciation has
occurred?
Which one is the T. rex?
Or, are they both T. rex?
How about now?
Really...
• …there’s no way to tell with the fossil
record
• so, paleontologists usually just call it a
different species when they’re not sure
• this is why the names keep changing
• Ever notice that? When I was a kid there
was just brontosaurus, now its
brachiosaurus and apatosaurus and
supersaurus
Something to think about
• Speciation can occur without major
anatomical change and
• Significant anatomical change can occur
without speciation occurring
How Is Reproductive Isolation
Maintained Between Species
• Genetic isolation is a necessary condition
for speciation to occur but it is not
sufficient unless part of the genetic
divergence causes a mechanism that
ensures you stay isolated
• The structural and/or behavioral
modifications that prevent interbreeding
are called isolating mechanisms
Premating isolating
mechanisms
• Incompatibilities that prevent mating
between species
Post-mating isolating
mechanisms
• Incompatibilities that prevent the formation
of vigorous, fertile hybrids between
species.
Geographic Isolation
• Simply prevents members of different
species from ever meeting
• It is not yet determined…well it wasn’t
determined when Geri and Teri wrote this
book, whether geographically isolated
populations constitute distinct species
Ya, vee cootent determine dat.
Ya.
Ecological Isolation
• Confines members of different species to
different habitats
• Examples: The white-throated sparrow
and the white-crowned sparrow have
extensively overlapping ranges
• They rarely interact because the whitethroated sparrow frequents dense thickets
and the white-crowned frequents open
fields and meadows
Temporal isolation
• Different species that inhabit the same
habitat cannot mate if they have different
mating seasons
• Bishop pines and Monterey pines can
grow right next to each other and never
interbreed. Even though in the lab they
can make viable offspring
• The Monterey Pine releases pollen in the
early spring and the Bishop pine in the
summer
Behavioral Isolation
• Many animal species employ fairly
elaborate courtship rituals
• This may include bright colors and specific
mating calls
• They attract members of the opposite sex
but members of different species are
indifferent to them
• Ex. Male birds are usually very brightly
colored and have a specific call to attract
females
continued
• Don’t forget the wing-beating frequency of the
fruit fly
• and frogs…lets talk frogs
• Their mating behavior may be the closest to
humans
• Males simply jump on any frog that is around
them, regardless of species
• Females, however, are only attracted to the
“ribbet” from their species
• If mauled by a member of another species, they
utter a “release” call and the male lets go
BOOM!
Mechanical incompatibility
• Yes, this means sometimes the parts just
don’t fit together
• Some species use internal fertilization and
some employ external fertilization
• Individuals of these two different types
would have trouble combining their DNA
• Even among plants, flower size and
location can make the different
Post-mating Isolating
Mechanisms
Gametic Incompatibility
• This is when, even if a male inseminates a
female, the sperm may not fertilize the
eggs
• The fluids in the female reproductive tract
may weaken or kill sperm from other
species
• This is probably how plants prevent
pollination from a different species
Hybrid inviability
• If fertilization does occur, the offspring may
be unable to survive
• They can be highly uncoordinated due to
their attempting to use behaviors of the
two different parent species
Hybrid Infertility
• Is exactly what it says
Hybrid infertility
• Exactly what it says
• This prevents weird hybrids from passing
their weird genes on to another generation
What is the Rate of Speciation?
• Over the history of our planet, species
have continually formed, existed for a time
and then go extinct.
• All the ideas we have discussed tend to
ensure that speciation never stops
• However, the rate at which this happens
varies throughout evolutionary history
Most biologists agree...
• ..that one random mutation can make
significant anatomical changes but it is
highly unlikely that it will contribute to
evolutionary change
• Think about it like this, if you randomly
picked a car part off the shelf of a store
and then replaced an existing part in your
car with it, would it improve the cars
performance?
No, it wouldn’t.
• It would probably disrupt the finely tuned
machinery already in place
• Evolutionary changes happen due to many
small changes that accrue over long
periods of time
• So, instantaneous speciation is highly
unlikely in animals
In Some Cases…
• …a species gives rise to many in a short
time
• This is called adaptive radiation
• this results due to one of two possible
causes
Species encounters a wide
variety of unoccupied habitats
• This is when a species arrives in an area
and the only competition is from other
members of that species
– examples: marsupial invasion of Australia
– Darwin's finches
A species’ superior adaptations
displaces less adapted species
from a variety of habitats
• Example: When the isthmus of Panama
rose above sea level it became a bridge
between North and South America.
• Species from both continents migrated
and interacted
• The fossil record shows that the North
American species were able to outcompete the South American ones
Punctuated Equilibrium
• The fossil record is incomplete and
ambiguous in regards to the rate of
speciation
• Organisms have to die in the right way to
become a fossil and then a human has to
find that fossil
• It is widely believed that the intermediate
forms (missing links) existed and may
have been fossilized but we haven’t found
them yet, and we may never
A New Idea
• In 1972, Niles Eldridge and Peter Jay
Gould suggested a different interpretation
of the fossil record
• They said what if there aren’t any gaps
• What if evolutionary change didn’t occur
slowly over time, what if there are sudden
bursts of speciation and then a rather long
periods of stasis. This is punctuated
equilibrium
This idea can easily be
misinterpreted
• Here are some ideas that ARE NOT stated by it
• 1. Does not imply instantaneous speciation
(offspring so different from their parents that they
are a new species), rather it implies
instantaneous in the context of geologic time.
• 2. The theory is not concerned with how species
arise but the distribution of these events over the
course of evolutionary time
• This idea caused a lot of debates and
misinterpretations
They All Agree
• That speciation takes many generations
• Evolution does not always proceed at the
same rate