There are a lot of steps in between the evolution of multicellular life and the evolution of the plants and animals as we think about them today. We can fill in that part of the story by talking about the meanings of vocabulary words from biology. Each of those words refers to a concept in biology that tells a story about some part of evolution. Each vocabulary word is a piece of the puzzle, and together they tell a much bigger story of the patterns of life.
ACT I
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Our story of the evolution of life has gotten up to multi-cellular life floating in the ocean.
You know how many more steps there are between them and us? Neither do I. So let’s think of another way to tell the story of biology.
You know how in science class you have to memorize vocabulary words for every chapter? But our story of Being Human on Planet Earth is all of your classes at once.
In English class you learn how to use words to tell stories. In science class the new words you learn refer to ideas that are critical for making and understanding scientific discoveries. Since science depends on math, each of those words relates to some kind of relationship among numbers. You probably don’t have philosophy classes in high school, but if you understand the ideas those words refer to, they change how you see the world in some way. Over the course of history, people who understood those ideas made different decisions in some way than people who didn’t understand them,.
So how much of the story of biology can we fill in just by talking about the meanings of words?
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The similarities among organisms that define a species are called traits. Traits are what all the members of a species have in common— or at least, all but a very rare few.
Characteristics are the differences among members of the species.
Traits are created by selection pressures, meaning factors that affect the survival or reproduction of the members of the species. If all the members of a species share a trait, it means they’ve depended on it to survive or reproduce. Penguins are good at swimming in cold water because all the ones who can’t die quickly.
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Characteristics are caused by variations among genes.
Those variations happen among genes that can occupy the same position on a chromosome. Variations among genes are called alleles, or alternates, or variants. I usually call them variants, or sometimes alternates.
There’s some gray area between traits and characteristics because of the way genes create characteristics.
Sometimes the result you see is caused by just one pair of genes, with multiple variations that could be in the pair.
Sometimes the result is caused by one pair of genes, where both genes are identical and there are no variations.
Other times the result is caused by an interaction among pairs of genes, all of which have multiple variations.
ACT II
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All animals have a number of genes that give them their colors.
There’s one pair of genes that starts the process, and if that pair doesn’t work the animal is albino, with no pigment at all.
The next pair starts the production of melanin, and if that pair doesn’t work the animal has some pigment but no melanin. That’s called leucism, which gives it enough pigment to give it blue eyes and make it a slightly darker shade of white than an albino.
Let’s say both of those pairs work and we’re talking about animals with normal colors.
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Ravens are solid black. That might mean there’s only one more gene pair in the process, and all ravens have a pair of the same genes. That would mean that if that pair malfunctions and doesn’t do anything, the raven would be white.
Tigers are orange with black stripes. If their orange pigment was created by only one pair of identical genes, that would make all tigers the same shade of orange. But if their pigment was created by multiple pairs of genes, where each had multiple variants, that would make them different shades of orange.
The larger the number of genes that contribute to their pigmentation, the more genes there are that can get variations, and the more each of those genes can vary without drastically changing the color. If there are ten genes pairs that contribute to the tiger’s pigment, then if it’s missing a pair it still gets 90% of its pigment. If a new variation made a gene pair double its part of the pigment, the tiger would only get 110% of the normal pigment, not double the normal pigment.
Things like that would make some tigers more red and some tigers more yellow. Some could be lighter, and some could be darker. But they’d all still be orange.
ACT III
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Genes are big molecules. They connect to each other to form chromosomes, which are gigantic molecules.
Also inside the cell is a gelatinous substance called cytoplasm. Cytoplasm is a mixture of other chemicals that the genes need to start their chemical reactions. That starts with the chemicals that are in a fertilized egg. Then it’s affected by things like air, water, nutrition, hormones, alcohol, drugs, poison, and any other chemical that can affect your health internally.
Genes react with the other chemicals and cause many different chemical reactions. The variations in the genes cause variations in those chemical reactions. Those chemical reactions are what create the anatomy and physiology of the organism.
Dogs have short hair or long hair because they have genes for one or the other. The chemical reactions that make their hair grow stop sooner for short haired dogs than for long haired dogs.
Differences in the chemicals in the cells cause differences in the chemical reactions also. That’s why changing your diet or starting or stopping smoking affects your health. Because whatever chemicals you put into your body go into your cells.
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Evolution works by the differential survival rate of genes.
When you look at all the members of the species that have one gene, and follow their descendants that inherit that gene for many generations, does the number of individuals with the gene increase, stay the same, or decrease each generation? How much bigger or smaller does the number get compared to the previous generation? That’s the survival rate of the gene.
Now do the same thing with all the variants of that gene, and compare the survival rates to each other. That’s the differential survival rate. That’s the math behind the Theory of Evolution.
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All the genes of the species are called the gene pool. We can also talk about the gene pool in terms of one gene and all of its variants.
If a gene has a higher survival rate than all of its variants for enough generations, until all the variants die out of the gene pool and only the one gene is left, we can say that gene has taken over the gene pool. That characteristic has become a trait.
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Ravens could’ve been many different colors once.
If one raven got a gene that turned it completely black, and that kept all of its predators from seeing it from a distance, so it lived twice as long as the other ravens and had twice as many offspring, and it passed that gene on to half of them, that would mean that gene had a much higher survival rate than its variants.
If those offspring lived twice as long as the other ravens, had twice as many offspring, and half of them got the black gene, that higher survival rate would continue.
If that went on long enough, the ravens that were easy to see would keep getting eaten, until eventually the only ones that hadn’t gotten eaten would be the black ones— meaning, the descendants of the one with that first black gene.
That might sound strange at first. But if this process took 20 or 30 generations, ravens could mate with each other safely even though they all had a common ancestor. Because at that point they’re fifteenth cousins or something like that.
Alternately, ravens could’ve been many different colors but their environmental selection pressures slightly favored darker colors. If the genes for darker colors had higher survival rates, in each generation there would be more ravens with darker colors. The ones with light colors would die out over the generations. Meanwhile, new variations of pigment genes that made the ravens even darker would get even higher survival rates.
If ravens evolved their black color gradually, over many generations, they could have many variations on their pigment genes, all of which make them black.
ACT IV
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New variations appear for a few different reasons.
The one you hear about the most is mutation. For some reason, like being hit by radiation from outer space, atoms get moved around in a gene or knocked out of it completely, and the gene starts functioning differently.
Some other, more common but less exciting reasons, happen when the chromosomes break up and reform when egg and sperm cells are produced.
Two genes can stick together and function as one new gene.
A gene can split in half and function as two new genes.
Or two genes can swap places on the new chromosome and function as new genes for whatever those parts of the chromosome do.
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Most of our genes don’t contribute anything to us. They used to, somewhere back in our evolution. But now the chemical reactions they start get stopped or undone by chemical reactions that are started by newer genes.
That’s why have tailbones. We don’t need tails now, but we used to. We didn’t evolve into not having tails by our tail genes disappearing, but because we got new genes that make our tails stop growing soon after they start.
Those non-functional genes keep making copies of themselves every generation because they’re mixed in with the genes that create us. When we reproduce, we reproduce all of our genes, including the non-functional ones.
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The branch of biology that studies how genes start the chemical reactions that make us and all other animals grow the way we do is called evolutionary development, or evo devo.
A good introductory book about that is called Endless Forms Most Beautiful, by Sean Carroll. He got that title from the last sentence of The Origin of Species.
ACT V
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Animals and people have mental traits and characteristics too. Traits are the similarities we’re born with. Characteristics are the differences we’re born with.
We also learn ideas and feelings from our life experiences. Those make us different from each other because no two people live identical lives. But they also make us the same when we live similar lives.
Everyone who grows up in the US learns English. But that’s not because we were born to learn English. It’s because we’re all born to learn languages, and English is the one we learned.
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Our mental traits are our instincts and our basic abilities.
Our mental characteristics are our individual talents and individual personalities.
We have instincts for learning.
We have abilities for learning.
We have individual talents for learning.
Our personalities affect what we’re interested in learning and what we pay attention to.
The different things that happen to us make us learn different things.
People with similar life experiences learn the same things, like languages, stories, religions, or life skills.
With traits, characteristics, and learning we have three different sources of information processing, and they’re all intertwined.
That’s why human psychology is the most complicated branch of science by far. But at the same time, we see examples of human behavior all around us all the time.
We watch movies and TV shows, and human behavior is the foundational artistic medium of the film industry. It’s also the foundation of literature.
If we stop thinking about science as something that only happens in science class, we can see human traits and characteristics everywhere. If we think about psychology in terms of understanding people, rather than training for jobs as psychologists, we have plenty of material to use to tell the story of Being Human on Planet Earth.
Chorus
There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.