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 budsWhales with teeth as embryos and not as

adultsGill 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

ARTIFICIAL SELECTION

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% identicalDifferences 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.