Download The Basic Process of Evolution

The Basic Process of Evolution
The basic theory of evolution is surprisingly simple. It has three essential parts:
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It is possible for the DNA of an organism to occasionally change, or mutate. A mutation
changes the DNA of an organism in a way that affects its offspring, either immediately or
several generations down the line.
The change brought about by a mutation is either beneficial, harmful or neutral. If the
change is harmful, then it is unlikely that the offspring will survive to reproduce, so the
mutation dies out and goes nowhere. If the change is beneficial, then it is likely that the
offspring will do better than other offspring and so will reproduce more. Through reproduction,
the beneficial mutation spreads. The process of culling bad mutations and spreading good
mutations is called natural selection.
As mutations occur and spread over long periods of time, they cause new species to form.
Over the course of many millions of years, the processes of mutation and natural selection have
created every species of life that we see in the world today, from the simplest bacteria to
humans and everything in between.
Billions of years ago, according to the theory of evolution, chemicals randomly organized themselves
into a self-replicating molecule. This spark of life was the seed of every living thing we see today (as
well as those we no longer see, like dinosaurs). That simplest life form, through the processes of
mutation and natural selection, has been shaped into every living species on the planet.
In the book "The Dragons of Eden," Carl Sagan summarized the theory of evolution in this way:
Accidentally useful mutations provide the working material for biological evolution -- as, for
example, a mutation for melanin in certain moths, which changes their color from white to black.
Such moths commonly rest on English birch trees, where their white coloration provides
protective camouflage. Under these conditions, the melanin mutation is not an advantage -- the
dark moths are starkly visible and are eaten by birds; the mutation is selected against. But when
the Industrial Revolution began to cover the birch bark with soot, the situation was reversed, and
only moths with the melanin mutation survived. Then the mutation was selected for, and, in time,
almost all of the moths are dark, passing this inheritable change on to future generations. There
are still occasional reverse mutations eliminating the melanin adaptation, which would be useful
for the moths were English industrial pollution to be controlled. Note that in all this interaction
between mutation and natural selection, no moth is making a conscious effort to adapt to a
changed environment. The process is random and statistical.
Can such a simple theory explain all of life as we know it today? Let's start by understanding how life
works and then look at some examples.
How Life Works: DNA and Enzymes
Evolution can be seen in its purest form in the daily evolution of bacteria. If you have read Here's a
quick summary to highlight the most important points in How Cells Work:
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A bacterium is a small, single-celled organism. In the case of E. coli, the bacteria are about onehundredth the size of a typical human cell. You can think of the bacteria as a cell wall (think of
the cell wall as a tiny plastic bag) filled with various proteins, enzymes and other molecules,
plus a long strand of DNA, all floating in water.
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The DNA strand in E. coli contains about 4 million base pairs, and these base pairs are
organized into about 1,000 genes. A gene is simply a template for a protein, and often these
proteins are enzymes.
An enzyme is a protein that speeds up a particular chemical reaction. For example, one of the
1,000 enzymes in an E. coli's DNA might know how to break a maltose molecule (a simple
sugar) into its two glucose molecules. That is all that that particular enzyme can do, but that
action is important when an E. coli is eating maltose. Once the maltose is broken into glucose,
other enzymes act on the glucose molecules to turn them into energy for the cell to use.
To make an enzyme that it needs, the chemical mechanisms inside an E. coli cell make a copy
of a gene from the DNA strand and use this template to form the enzyme. The E. coli might
have thousands of copies of some enzymes floating around inside it, and only a few copies of
others. The collection of 1,000 or so different types of enzymes floating in the cell makes all of
the cell's chemistry possible. This chemistry makes the cell "alive" -- it allows the E. coli to
sense food, move around, eat and reproduce. See How Cells Work for more details.
You can see that, in any living cell, DNA helps create enzymes, and enzymes create the chemical
reactions that are "life."
Sexual Reproduction
Bacteria reproduce asexually. This means that, when a bacteria cell splits, both halves of the split are
identical -- they contain exactly the same DNA. The offspring is a clone of the parent.
As explained in How Human Reproduction Works, higher organisms like plants, insects and animals
reproduce sexually, and this process makes the actions of evolution more interesting. Sexual
reproduction can create a tremendous amount of variation within a species. For example, if two parents
have multiple children, all of the children can be remarkably different. Two brothers can have different
hair color, different heights, different blood types and so on. Here's why that happens:
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Instead of a long loop of DNA like a bacterium, cells of plants and animals have chromosomes
that hold the DNA strands. Humans have 23 pairs of chromosomes, for a total of 46
chromosomes. Fruit flies have five pairs. Dogs have 39 pairs, and some plants have as many as
100.
Chromosomes come in pairs. Each chromosome is a tightly packed strand of DNA. There are
two strands of DNA joined together at the centromere to form an X-shaped structure. One
strand comes from the mother and one from the father.
Because there are two strands of DNA, it means that animals have two copies of every gene,
rather than one copy as in an E. coli cell.
When a female creates an egg or a male creates a sperm, the two strands of DNA must combine
into a single strand. The sperm and egg from the mother and father each contribute one copy of
each chromosome. They meet to give the new child two copies of each gene.
To form the single strand in the sperm or egg, one or the other copy of each gene is randomly
chosen. One or the other gene from the pair of genes in each chromosome gets passed on to the
child.
Because of the random nature of gene selection, each child gets a different mix of genes from the DNA
of the mother and father. This is why children from the same parents can have so many differences.
A gene is nothing but a template for creating an enzyme. This means that, in any plant or animal, there
are actually two templates for every enzyme. In some cases, the two templates are the same
(homozygous), but in many cases the two templates are different (heterozygous).
Here is a well-known example from pea plants that helps understand how pairs of genes can interact.
Peas can be tall or short. The difference comes, according to Carol Deppe in the book "Breed your own
Vegetable Varieties":
...in the synthesis of a plant hormone called gibberellin. The "tall" version of the gene is normally
the form that is found in the wild. The "short" version, in many cases, has a less active form of
one of the enzymes involved in the synthesis of the hormone, so the plants are shorter. We refer
to two genes as alleles of each other when they are inherited as alternatives to each other. In
molecular terms, alleles are different forms of the same gene. There can be more than two
alleles of a gene in a population of organisms. But any given organism has only two alleles at
the most. Shorter plants usually cannot compete with the taller forms in the wild. A short mutant
in a patch of tall plants would be shaded out. That problem isn't relevant when a human plants a
patch or field with nothing but short plants. And short plants may be earlier than tall ones, or less
subject to lodging (falling over) in the rain or wind. They also may have a higher proportion of
grain to the rest of the plant. So shorter plants can be advantageous as cultivated crops.
Specific mutations or alleles are not good or bad in and of themselves, but only within a certain
context. An allele that promotes better growth in hot weather may promote inferior growth in
cold weather, for example.
One thing to notice in Deppe's quote is that a mutation in a single gene may have no effect on an
organism, or its offspring, or its offspring's offspring. For example, imagine an animal that has two
identical copies of a gene in one allele. A mutation changes one of the two genes in a harmful way.
Assume that a child receives this mutant gene from the father. The mother contributes a normal gene,
so it may have no effect on the child (as in the case of the "short" pea gene). The mutant gene might
persist through many generations and never be noticed until, at some point, both parents of a child
contribute a copy of the mutant gene. At that point, taking the example from Deppe's quote, you might
get a short pea plant because the plant does not form the normal amount of gibberellin.
Another thing to notice is that many different forms of a gene can be floating around in a species. The
combination of all of the versions of all of the genes in a species is called the gene pool of the species.
The gene pool increases when a mutation changes a gene and the mutation survives. The gene pool
decreases when a gene dies out.
One of the simplest examples of evolution can be witnessed in an E. coli cell. To get a better grip on
the process, we'll take a look at what happens in this cell.
The Simplest Example of Evolution
The process of evolution acts on an E. coli cell by creating a mutation in the DNA. It is not uncommon
for the DNA strand in an E. coli bacterium to get corrupted. An X-ray, a cosmic ray or a stray chemical
reaction can change or damage the DNA strand. In most cases, a particular E. coli cell with mutated
DNA will either die, fix the damage in the strand or fail to reproduce. In other words, most mutations
go nowhere. But every so often, a mutation will actually survive and the cell will reproduce.
Imagine, for example, a bunch of identical E. coli cells that are living in a petri dish. With plenty of
food and the right temperature, they can double every 20 minutes. That is, each E. coli cell can
duplicate its DNA strand and split into two new cells in 20 minutes.
Now, imagine that someone pours an antibiotic into the petri dish. Many antibiotics kill bacteria by
gumming up one of the enzymes that the bacteria needs to live. For example, one common antibiotic
gums up the enzyme process that builds the cell wall. Without the ability to add to the cell wall, the
bacteria cannot reproduce, and eventually they die.
When the antibiotic enters the dish, all of the bacteria should die. But imagine that, among the many
millions of bacteria living in the dish, one of them acquires a mutation that makes its cell-wall-building
enzyme different from the norm. Because of the difference, the antibiotic molecule does not attach
properly to the enzyme, and therefore does not affect it. That one E. coli cell will survive, and since all
of its neighbors are dead, it can reproduce and take over the petri dish. There is now a strain of E. coli
that is immune to that particular antibiotic.
In this example, you can see evolution at work. A random DNA mutation created an E. coli cell that is
unique. The cell is unaffected by the antibiotic that kills all of its neighbors. This unique cell, in the
environment of that petri dish, is able to survive.
E. coli are about as simple as living organisms can get, and because they reproduce so rapidly you can
actually see evolution's effects on a normal time scale. In the past several decades, many different types
of bacteria have become immune to antibiotics. In a similar way, insects become immune to
insecticides because they breed so quickly. For example, DDT-resistant mosquitoes evolved from
normal mosquitoes.
In most cases, evolution is a much slower process.
The Speed of Mutations
As mentioned in the previous section, many things can cause a DNA mutation, including:
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X-rays
Cosmic rays
Nuclear radiation
Random chemical reactions in the cell
Therefore, mutations are fairly common. Mutations happen at a steady rate in any population, but the
location and type of every mutation is completely random. According to Carl Sagan in "The Dragons of
Eden":
Large organisms such as human beings average about one mutation per ten gametes [a
gamete is a sex cell, either sperm or egg] -- that is, there is a 10 percent chance that any given
sperm or egg cell produced will have a new and inheritable change in the genetic instructions
that make up the next generation. These mutations occur at random and are almost uniformly
harmful -- it is rare that a precision machine is improved by a random change in the instructions
for making it.
According to "Molecular Biology of the Cell":
Only about one nucleotide pair in a thousand is randomly changed every 200,000 years. Even
so, in a population of 10,000 individuals, every possible nucleotide substitution will have been
"tried out" on about 50 occasions in the course of a million years, which is a short span of time in
relation to the evolution of species. Much of the variation created in this way will be
disadvantageous to the organism and will be selected against in the population. When a rare
variant sequence is advantageous, however, it will be rapidly propagated by natural selection.
Consequently, it can be expected that in any given species the functions of most genes will have
been optimized by random point mutation and selection.
According to the book "Evolution," by Ruth Moore, it is possible to speed up mutations with radiation:
So Muller put hundreds of fruit flies in gelatin capsules and bombarded them with X-rays. The
irradiated flies were then bred to untreated ones. In 10 days thousands of their offspring were
buzzing around their banana-mash feed, and Muller was looking upon an unprecedented
outburst of man-made mutations. There were flies with bulging eyes, flat eyes, purple, yellow
and brown eyes. Some had curly bristles, some no bristles...
Mutations fuel the process of evolution by providing new genes in the gene pool of a species.
Then, natural selection takes over.
Natural Selection
As you saw in the previous section, mutations are a random and constant process. As mutations occur,
natural selection decides which mutations will live on and which ones will die out. If the mutation is
harmful, the mutated organism has a much decreased chance of surviving and reproducing. If the mutation
is beneficial, the mutated organism survives to reproduce, and the mutation gets passed on to its offspring.
In this way, natural selection guides the evolutionary process to incorporate only the good mutations into
the species, and expunge the bad mutations.
The book "Extinct Humans," by Ian Tattersall and Jeffrey Schwartz, puts it this way:
...in every generation, many more individuals are produced than ever survive to maturity and to reproduce themselves. Those
that succeed -- the "fittest" -- carry heritable features that not only promote their own survival but are also passed along
preferentially to their offspring. In this view, natural selection is no more than the sum of all those factors that act to promote the
reproductive success of some individuals (and its lack in others). Add the dimension of time, and over the generations natural
selection will act to change the complexion of each evolving lineage, as advantageous variations become common in the
population at the expense of those less advantageous.
Let's look at an example of natural selection from How Whales Work.
The ancestors of whales lived on land -- there is evidence of the evolution of the whale from life on land to
life in the sea (read How Whales Work for details), but how and why did this happen? The "why" is
commonly attributed to the abundance of food in the sea. Basically, whales went where the food was. The
"how" is a bit more perplexing: Whales are mammals, like humans are, and like humans, they lived and
walked on solid ground, breathing air into their lungs. How did whales become sea creatures? One aspect
of this evolution, according to Tom Harris, author of How Whales Work, is explained as follows:
To make this transition, whales had to overcome a number of obstacles. First of all, they had to contend with reduced access to
breathable air. This led to a number of remarkable adaptations. The whale's "nose" moved from the face to the top of the head.
This blowhole makes it easy for whales to breathe in air without fully surfacing. Instead, a whale swims near the surface, arches
its body so its back briefly emerges and then flexes its tail, propelling it quickly to lower depths.
Odd as it seems that the whale's "nose" actually changed positions, the theory of evolution explains this
phenomenon as a long process that occurs over perhaps millions of years:
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Random mutation resulted in at least one whale whose genetic information placed its "nose"
farther back on its head.
The whales with this mutation were more suited to the sea environment (where the food was) than
"normal" whales, so they thrived and reproduced, passing on this genetic mutation to their
offspring: Natural selection "chose" this trait as favorable.
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In successive generations, further mutations placed the nose farther back on the head because the
whales with this mutation were more likely to reproduce and pass on their altered DNA. Eventually,
the whale's nose reached the position we see today.
Natural selection selects those genetic mutations that make the organism most suited to its environment
and therefore more likely to survive and reproduce. In this way, animals of the same species who end up in
different environments can evolve in completely different ways.
Creating a New Species
Imagine that you take a group of Saint Bernards and put them on one island, and on another island you
put a group of Chihuahuas. Saint Bernards and Chihuahuas are both members of the species "dog" right
now -- a Saint Bernard can mate with a Chihuahua (probably through artificial insemination) and create
normal puppies. They will be odd-looking puppies, but normal puppies nonetheless.
Given enough time, it is possible to see how speciation -- the development of a new species through
evolution -- could occur among the Saint Bernards and the Chihuahuas on their respective islands.
What would happen is that the Saint Bernard gene pool would acquire random mutations shared by all
of the Saint Bernards on the island (through interbreeding), and the Chihuahuas would acquire a
completely different set of random mutations shared by all of the Chihuahuas on their island. These
two gene pools would eventually become incompatible with one another, to the point where the two
breeds could no longer interbreed. At that point, you have two distinct species.
Because of the huge size difference between a Saint Bernard and a Chihuahua, it would be possible to
put both types of dogs on the same island and have the exact same process occur. The Saint Bernards
would naturally breed with only the Saint Bernards and the Chihuahuas would naturally breed with
only the Chihuahuas, so speciation would still occur.
If you put two groups of Chihuahuas on two separate islands, the process would also occur. The two
groups of Chihuahuas would accumulate different collections of mutations in their gene pools and
eventually become different species that could not interbreed.
The theory of evolution proposes that the process that might create a separate Chihuahua-type species
and Saint Bernard-type species is the same process that has created all of the species we see today.
When a species gets split into two (or more) distinct subsets, for example by a mountain range, an
ocean or a size difference, the subsets pick up different mutations, create different gene pools and
eventually form distinct species.
Is this truly how all of the different species we see today have formed? Most people agree that bacteria
evolve in small ways (microevolution), but there is some controversy around the idea of speciation
(macroevolution). Let's take a look at where the controversy comes from.
Holes in the Theory
The theory of evolution is just that -- a theory. According to "The American Heritage Dictionary," a
theory is:
A set of statements or principles devised to explain a group of facts or phenomena, especially
one that has been repeatedly tested or is widely accepted and can be used to make predictions
about natural phenomena.
Evolution is a set of principles that tries to explain how life, in all its various forms, appeared on Earth.
The theory of evolution succeeds in explaining why we see bacteria and mosquitoes becoming resistant
to antibiotics and insecticides. It also successfully predicted, for example, that X-ray exposure would
lead to thousands of mutations in fruit flies.
Many theories are works in progress, and evolution is one of them. There are several big questions that
the theory of evolution cannot answer right now. This is not unusual. Newtonian physics worked really
well for hundreds of years, and it still works well today for many types of problems. However, it does
not explain lots of things that were eventually answered by Einstein and his theories of relativity.
People create new theories and modify existing ones to explain the unexplained.
In answering the open questions that still remain unsolved, the theory of evolution will either become
complete or it will be replaced by a new theory that better explains the phenomena we see in nature.
That is how the scientific process works.
Here are three common questions that are asked about the current theory of evolution:
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How does evolution add information to a genome to create progressively more complicated
organisms?
How is evolution able to bring about drastic changes so quickly?
How could the first living cell arise spontaneously to get evolution started?