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Evolution Review
Darwin’s Evidence for Evolution
• Various finches, whose beaks were shaped
depending on what they ate
• Giant tortoises, whose shells were specific to
their home island
• Snake embryos with leg buds
• Whales with teeth as embryos and not as
adults
• Gill slits in Human embryos
Descent with Modification
• Beginning with a common ancestor, over time,
across generations, species could change
dramatically. Some might add new body
features, others might drop them.
• Ultimately one type of creature could be
transformed into something utterly different
Dog Breeding
• Darwin recognized the artificial selection
happening with dog traits and wondered if
there was a similar process going on in nature
without human interference
Variation
• He realized that not every individual was the
same, stamped out like a toy from a press, but
there was variation.
• Darwin realized that variation must be the
starting point for change in nature. In any
generation, the animals in a litter are never
quite the same. And in the wild, such a tiny
variation might make all the difference
between life and death.
Natural Selection
• The pattern that Darwin saw was that the
creatures that survived were those best adapted
to the specific environments they lived in.
• Darwin suggested, over many, many generations,
these tiny variations would allow the fit to get
fitter, and the unfit would vanish.
• These variations accumulate and eventually new
species branch off. This is evolution by natural
selection.
DNA, Genes, Proteins and Appearance
• And that is the key. D.N.A. is a code, and its
double strand contains all the information to
make living things grow and develop. Lined
along each D.N.A. molecule arranged special
sequences of this code that form our genes.
• Many genes get translated into proteins, and
these proteins make the stuff of our bodies.
One protein makes hair; another makes
cartilage; others make muscle.
DNA Changes
• When a baby is conceived, the fertilized egg
receives half its D.N.A. from the mother and
half from the father, creating wholly new
combinations. It's why we look a bit like our
parents, but also different.
• Another way that D.N.A. can change is
mutation.
Mutations
• Mutation is a critical ingredient in the recipe for
evolution. Without mutation, everything would
stay constant, generation after generation.
Mutation generates variation, differences
between individuals.
• Mutation seems to mean that something bad has
happened. Well, mutations are neither good or
bad. Whether they are favored, or whether they
are rejected, or whether they're just neutral,
depends upon the conditions an organism finds
itself in.
Human Genes
• There were estimates that humans would have
between, let's say, 80,000 and 120,000 genes.
• So when the final answer came in 2003, it was a
shocker: 23,000 genes, the same number as a chicken,
less than an ear of corn.
• People were freaked out by the relatively small number
of genes. It's down to something like 22- or 23,000
protein-coding genes in a human genome.
• And it wasn't just that we had so few genes, but many
of our key genes were identical to those of other
animals.
Embryos
• In terms of animal diversity; it is the platform for
diversity.
• What fascinates modern biologists is that all
these different animals don't just look the same,
they are using virtually the same set of key genes
to build their bodies.
• This has led scientists to a crucial insight about
how animal bodies have evolved. It's not the
number of genes that counts.
• It's not the genes you have but how you use them
that creates diversity in the animal kingdom.
Junk DNA
• The vast bulk of the double helix, some 98
percent of it, doesn't code for proteins, which
make the stuff of our bodies. The genes which
do comprise just two percent.
Switches
• Switches are not genes. They don't make stuff
like hair, cartilage or muscle, but they turn on
and off the genes that do.
• Switches are very powerful parts of D.N.A.,
because they allow animals to use genes in
one place and not another; at one time, and
not another; and so, choreograph the spots
and stripes and blotches of animal bodies.
Hox Genes
• These are a different type of gene; they're the genes that boss
other genes around.
• Hox genes have been found in all complex animals, dating back
some 600 million years. And in all that time, the letters of their
D.N.A. have remained virtually unchanged.
• They are aristocrats of the gene community, near the very top of
the chain of command. They give orders that cascade through a
developing embryo, activating entire networks of switches and
genes that make the parts of the body. They are absolutely critical
to the shape and form of a developing creature.
• These genes determine where the front and the back of the
animal's going to be; the top, the bottom; the left, the right; the
inside, the outside; where the eyes are going to be; where the legs
are going to be; where the gut's going to be; how many fingers
they're going to have.
Hox Genes
• There are genes that make the stuff of our
bodies, switches that turn them off and on,
and still other genes that give those switches
orders (Hox). Together, in a complex cascade
of timing and intensity, they combine to
produce the amazing diversity of life on this
planet.
Chimps vs Humans
• 99% identical
• Differences in thumbs and brains (ours is 3
times bigger and more organized)
Hands / Thumb
• A sequence that was different in 13 places,
compared to chimp D.N.A.
• Turned out to be a switch that helps form that
key human attribute, our thumb, the part of
our hand that gives us so much power and
precision.
Brain Development
• The skulls of apes and humans are made of
several independent bone plates. They let our
heads get bigger as we grow. The muscles for
chewing pull against these plates, and in an
ape, these forces can be enormous.
• All this muscle power forces an ape's skull
plates to fuse together at an early stage, and
this puts limits on how much the brain can
grow.
Brain Development
• In a chimpanzee, gorilla or orangutan, those
growth plates are pretty much shut down, closed
for business, by about three, four years of age. In
a human, they remain open for growth to
perhaps the age 30.
• A mutation in our jaw muscle allows the human
skull to keep expanding into adulthood, creating a
bigger space for our brain. And so our most
important organ is able to grow.
Brain Switches Chimp vs Human
• material charting the differences between
humans and chimps. Importantly, many of those
differences were not in the actual genes. They
were in switches.
• They're pieces of D.N.A. that turn a nearby gene
on or off, that tell it where, in what cells in our
body, in what tissue, at what time or at what level
to be operating.
• A large number of them, more than half, were
nearby a gene that was involved in the brain.