As you likely know, humans evolved from a species of monkey. Whales evolved from a deer-like animal.
Birds evolved from our favorite extinct animal, the dinosaur. And all life began from a simple
single cell four billion years ago, evolving into the mind-boggling diversity we see today.
How did this all happen? Evolution, obviously, but what exactly is evolution and how does it work?
What is evolution? Evolution “is the net change
in organisms or a population over the span of many generations.” We can break the definition into two
parts—the net change in organisms or a population, one that occurs over the span of many generations.
The first part reveals what and where these “changes” happen—the mechanisms of evolution like
natural selection. The second part tells us two things—the time it takes for organisms to change,
and, less obviously, that the process involves reproduction.
DNA, heritability and change The core of evolution is the
change in organisms, and the first place to investigate this change is in the DNA.
DNA is the blueprint of all life. Its sequence is composed of four letters
that bond in specific ways: A to T and C to G. Small sections of the DNA sequence, called genes,
code for certain traits like eye color or blood group. Genes can come in a variety of “flavors”,
meaning that the gene for eye color can code for blue eyes, or brown eyes, or even green eyes! Each
of us possess a unique combination of these genes, giving some of us brown eyes with blond hair,
or blue eyes with brown hair. These traits are passed down to offspring when organisms mate.
Now, these different flavors of genes get all mix-matched in a process called recombination,
which happens when organisms mate. In the nucleus of a cell,
long twisted ladder-like strands of DNA are formed into compact and butterfly shaped chromosomes.
Chromosomes come in pairs. Humans, for example, have 23 pairs of chromosomes,
one from the mother and one from the father. In the process of creating the egg and the sperm,
the chromosomes pair up and then swap parts of their DNA. This is recombination.
Once this recombination happens, the paired up chromosomes are separated, and the resulting
eggs and sperm have only one half of the chromosome pair. Then, when two people mate,
a random egg and a random sperm, will fuse together to create a new life! Recombination
creates combinations of genes that are unique to each one of us, creating the diversity and
why you and your sibling don’t look identical. Recombination isn’t the only way that variety
develops. Another process is mutation, which is a change in the DNA sequence.
Mutations can either modify traits, like a new eye color, or create a completely new trait.
The sad news is that most mutations won’t make you Jean Grey or Wolverine.
In fact, most mutations don’t cause any change at all. They are neutral. In a few cases,
mutations can be good, like a mutation that allows animals to camouflage and hide from predators,
or produce a scent that makes it extremely appealing to the opposite sex,
whereas bad mutations are those that cause diseases or weaken an
organism, which would make it less likely to survive long enough to reproduce.
Mutations can occur randomly. It can happen when DNA divides and the cell’s machines make mistakes.
Or it could happen from external factors, such as radiation, or even when viruses infect us.
Natural selection and genetic drift “Natural selection is a process where
the environment selects traits that allow the individual or group to survive best.”
Consider a hypothetical organism that gets a mutation one day that gives it heightened
hearing. This better hearing allows it to detect and flee from predators faster, so it lives
longer and can therefore produce more offspring. It passes its better hearing to its offspring,
who also survive longer than those organisms that don’t hear as well.
Soon, there are many better-hearing individuals. The
better hearing was selected for. On the other hand, if an individual can’t hear as well,
it is less likely to survive long enough to reproduce and pass its genes on to its offspring.
Natural selection will select a trait that increases an organism’s fitness. Here, fitness
doesn’t mean strength; it means anything that will aid the animal in survival and reproduction.
This is how the modern dog evolved from wolves. However, instead of the environment doing the
selecting, it was humans. Humans selected those traits in wolves that made them better companions,
including loyalty and a more docile temperament.
The wolves that showed these traits were bred. Over time, humans selected and mixed these traits,
resulting in the wide variety of dog breeds we see today. This is called “artificial selection”.
Sometimes though, luck also plays a role in how animals evolve.
For example, a natural catastrophe can destroy the majority of a given population. The individuals
that remain are there by mere chance, so the traits that remain in the “gene pool”
aren’t necessarily any better suited for their environment. Or perhaps an organism or population
somehow finds itself in a completely new place, separated from its normal reproductive population.
The individuals in this new place will now evolve and spread in that new place.
This is called “genetic drift”. Though natural selection chooses traits that
enable survival, it isn’t perfect. Diseases and certain disorders aren’t necessarily eliminated,
even though they cause pain. Furthermore, traits that no longer serve a purpose will not be
actively selected against, unless they become detrimental for survival. They will similarly
be passed down to future generations if the animal manages to live long enough to mate.
Speciation Thus far, we’ve only spoken about evolution
within a single group, but life is a tree, with thousands of branches to different species.
How do these different species evolve? This occurs through a long process of speciation.
Over time, mutations, recombination events, natural selection, luck and other factors
create a gulf of differences within a population that was once homogenous.
These changes might happen because of a literal geographic barrier that separates a population.
The environments on either side of the barrier might be different,
and over many generations, the two populations become different species.
This happened with Darwin’s finches on the Galapagos Islands. At one point,
the finches’ ancestors from the mainland must have migrated to the islands. The islands were rich in
food resources and each had a slightly different food profile. Due to variation, the birds with
longer beaks could survive on insects, whereas those with blunter beaks fed on seeds and leaves.
Over many generations, evolution took over and 15 different finch species evolved.
Once a new species comes into being, it doesn’t necessarily mean that the old species vanishes.
Just as single-celled bacteria didn’t go extinct just because multicellular life evolved,
similarly, old life does not simply go extinct because a new species appears on the scene.
Each can tap a different resource and exist in their
new habitats. Conclusion
The evolution of life doesn’t only act through one mechanism. The main debate revolves around
how much one mechanism contributed to the changes we see today. Do deer have antlers to look sexy,
or is it simply a byproduct of some other change? Did insects get smaller
because of a lack of oxygen or predators or both? Scientists study evolution in the lab and through
the fossil record. We know how whales evolved through the existence of intermediate fossils.
In the lab, we breed bacteria, flies and worms to see how they evolve under different conditions.
This evidence gives us some hint at how life reached such an astonishing level of diversity.
With all that in mind, take a moment to notice the diversity on our incredible planet
and appreciate that it took 4 billion years of evolution to lead
not only to you, but to the birds outside your window, the fly buzzing around you face,
the dog curled up at your feet, and every other magnificent variety of life on Earth.