Download Mechanisms Of Evolution - Transcript

Evolution Transcript
Part 2. The Mechanisms
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305 MechanismIntro
In Part One of our story of evolution we demonstrated the Facts of Evolution.
In Part Two we will explore the Mechanisms
And to do that, we need to ask and answer just two questions:
How does variety arise in the genomes of individuals in a group?
…and…
How does that variety give rise to new species?
310 FirstQuestion
The first question is probably easier to answer, so let’s work on it first.
Let’s start with the genomes of two individuals who are clones of one another.
Their genomes are as close to identical as the real world allows.
As these individuals live their lives, their genomes encounter viruses, which
implant their genetic material into that of the host, thus making the two genomes
no longer identical.
…the two genomes will also encounter radiation and mutagenic chemicals in the
environment which further modify their genomes differently.
… transposons already resident in each genome will modify the genomes
differently as well
...the process of DNA replication isn’t perfect and the mistakes that arise in this
process will further diverge the genomes.
If these modifications happen in the DNA of the egg or sperm cell, then they are
passed on to the children.
315 Recombination
In addition to those mechanisms for introducing variety into a genome, sexual
reproduction has its own mechanisms to ensure diversity.
This man has two complete sets of chromosomes – one from his mother and
one from his father. But when it comes time to give a single set to each
offspring, his two set recombine in different combinations.
The same is true for this woman.
So when they have children, each child gets a unique combination from each
parent.
This greatly diversifies the genome of a population every generation.
320 Fusion-Duplication-Deletion
Other DNA errors can cause variations as well.
Chromosomal fusion can cause two chromosomes to join together resulting in a
change in chromosome count.
Or genomes can increase and decrease in size and complexity by duplication
and deletion. Duplication can involve large sections of DNA becoming copied -which is a major source of raw material for evolving new genes. From tens to
hundreds of genes are duplicated in animal genomes every million years.
The result is that over time and generations, identical individuals will have
descendants whose genomes contain a great deal of variety.
340 SecondQuestion
Now that we understand how variety can arise in individual genomes, we are
prepared to tackle the second question.
How does this variety give rise to new species.
There are two major answers to the second question… Natural Selection and
Genetic Drift
345 DarBabies
These creatures are DarwinBabies – DarBabies for short, and they have
volunteered to be test subjects while we study how they evolve in their
environment.
They are not strong
They are not fast
They are not poisonous
They are not smart.
All they can do is hide.
Predators exist in the environment and a DarBaby that stands out is more likely
to get eaten than one that is camouflaged.
Their genomes consist of 8 pieces of DNA…each piece contributing to the color
of the DarBaby. Like this…
(Show Darbabies with the DNA going from
00000000 -- all White
10000000 – Gray
to
11111111 – All Black)
In each generation, 25% of the Darbabies get eaten by predators.
Another 25% perish for random acts of misfortunes – just unlucky.
Mating is completely random and each DarBaby Baby has a 50% chance of
getting one random mutation.
As you can see, almost all of this environment is random. The only tiny
nonrandom factor is the predators eating the 25% that are easiest to spot.
Will evolution really work here??? Perhaps – Evolution is a very robust
phenomenon.
351 Generation1
We start with all the DarBabies matching the environment…gray
10000000 MID-GRAY -- 16 of them
But the environment changes (to black) and now they stick out!
Predators come to feed (just eyes in the dark)
Some are just unlucky
The survivors mate – mating is totally random…producing the next Generation
352 Generation2
Notice all the mutations which are indicated by the GREEN DNA bits.
Some mutations are bad – notice the DarBaby that is completely WHITE
(00000000)
Many do almost nothing…
…and some are actually GOOD
Predators come to feed – notice how the really bad mutation causes that poor
DarBaby to stick out and get eaten.
While the really good mutation prevents that DarBaby from being seen – keeping
him alive – for now
Some are just unlucky…oops the DarBaby with the really beneficial mutation was
unlucky and still died. How can evolution EVER work?
The survivors mate…producing the next generation
353 Generation3
(Let it run until mating and point out this guy)
…This guy gets to mate SIX times.
While this guy gets to mate only two times…remember mating is a random
variable.
354 Generation4-13
Now sit back and watch the rest of the evolution unfold – thirteen generations
total --surely that won’t be enough?!
This guy survived to mate, and just because of bad luck never found a partner.
Remember, mating is a random variable.
364 Gen14
Did you see it happen? In an environment filled with random variables, a tiny bit
of non-random competition imposed by the predators was able to drive evolution.
380 NaturalSelection
If we repeat the simulation but with a light background, this is what we get…
(show light result)
(show graph comparison)
The environment biases the competition – thus steering the evolution.
And…THAT is NATURAL SELECTION.
In an environment full of random variables, a tiny bit of competition is all it takes.
Notice how the evolution is not smooth.
Sometimes evolution appears to stop.
Other times it moves fast.
It can even go backwards.
Yet, over time, the net effect is a directed process making the organisms more fit
for their environment.
385 GeneticDrift
The element of misfortune – unlucky DarBabies dying without regard to their
fitness is an example of Genetic Drift – the OTHER prime ingredient in the
mechanisms of evolution.
400 Conclusion
There are many more issues that the scientists must pursue… going by names
such as
Punctuated Equilibrium
or
Bottlenecks
or
FounderEffects
But you have just seen how it happens. It really works.
Common Descent is a Fact
Species Evolve
Natural Selection and Genetic Drift are the primary mechanisms