Evolution is Complex
2026 Aug 10
See all posts
Evolution is Complex
As someone who never seriously studied life sciences, I've always
thought that the story with evolution was basically one of "random
mutations can sometimes increase the likelihood of reproduction." A bird
who happens to get a genetic mutation for, say, a longer beak may have
an easier time feeding, surviving and crucially, mating. Since it's more
likely to reproduce and this gene can be passed on, the bigger beak
shows up more frequently in subsequent generations. This process is
called natural
selection and it drives evolution which turned amoebas into fish
into lizards into apes into humans. It turns out that while this
description is on the right track, it leaves out big parts of the
story.
For example, today we obviously co-exist with many other species
including apes with whom we share a common ancestor. If apes developed
some random human-oriented mutation that spread through generations,
wouldn't at some point the apes just be completely gone? A big missing
piece of the story is the details of speciation, the
process by which species separate out into branches of the evolutionary
tree that we see today. The primary cause of speciation is geographic
isolation. While Darwin meticulously documented (non-genetic)
evidence of natural selection, he didn't really understand how the
conditions for divergence first arose. That was worked out later by
Ernst Mayr, Leon Croizat and others in the mid-20th century. Mayr showed
how a subset of a bird population can migrate and form a new colony on
an island. Once geographically separated, reproduction stops between the
groups and they then evolve into distinct species through natural
selection. Croizat showed how changes in Earth's geography like a new
mountain range can physically separate a species and lead to the same
effect.
While geographic isolation is the primary mechanism for speciation,
it isn't the only one. The apple maggot
fly originally laid its eggs on hawthorn fruit in North America.
After Europeans introduced apples, some flies began laying their eggs on
apples instead. These flies also tend to mate on the fruit where they
lay their eggs, so apple flies mostly encounter other apple flies while
hawthorn flies mostly encounter other hawthorn flies. Apples also ripen
earlier than hawthorns, causing the two groups to emerge and mate at
different times. Reproduction between them has slowed and yet they live
in the same geographic area. They are in the process of sympatric (same
place) speciation today. Anything that causes at least some reproductive
isolation can lead to speciation because then the gene pools can evolve
independently.
DNA mutations or "copying errors" are far from the only source of
variation in phenotypes. The cells in your body have 46 chromosomes, 23
pairs from your mother/father. Your sperm/eggs (gametes) are produced by
meiosis, which
splits normal cells into specialized cells which only have 23
chromosomes. In the process of splitting, a maternal or paternal copy
from each chromosome pair is randomly selected. That "independent
assortment" or reshuffling can produce different traits in the
species without any DNA mutation. When a new selective pressure arises
due to a changing environment, the first thing that happens is natural
selection upon the best shuffling of existing genes, rather than new
variants created through mutations. The randomness inherent in
independent assortment and mutation contribute to what's known as random
genetic drift.
Each subsequent generation is primarily like the reshuffling of a deck
of cards. Occasionally a brand new card is added to the deck,
corresponding to a new mutation. In any given sequence of generations,
it's possible that an unlikely set of shuffles appears. The average beak
size of a bird population may shift randomly, without any new selective
pressure at all.
What if a mutation shows up when the environment hasn't changed much?
The environment usually changes much slower than new generations of
organisms are produced. If a bird has a random genetic mutation that
creates an extreme physical trait like a beak that is way too small or
way too large, that bird will have a hard time surviving and
reproducing. So the variant will be naturally selected out of existence.
Most of natural selection is "stabilizing
selection" in this sense. Non-extreme phenotypes better suited
towards the existing environment are favoured in the process. An
environmental change creates the conditions for a new adaptive trait,
but even a static environment involves constant natural selection to
maintain the existing adaptive traits. There can also be genes that
simply have no selective impact at all. After sequencing the human
genome, we've discovered that large parts of it are "junk" DNA that doesn't
seem to get involved in any downstream processes. If it doesn't impact
any traits, then mutations on this junk DNA have no selective pressure
one way or another. In fact, the neutral
theory of molecular evolution argues that most of the DNA
differences we see between species are from this neutral variation.
Some aspects are still debated even today. How much of the DNA
difference between two birds is really from adaptation to their given
environments vs just random differences that drifted there because it
didn't matter either way? Under what conditions does junk DNA later
become useful (exaptation)?
Questions or comments? Email feedback@cstein.xyz
Evolution is Complex
2026 Aug 10 See all postsAs someone who never seriously studied life sciences, I've always thought that the story with evolution was basically one of "random mutations can sometimes increase the likelihood of reproduction." A bird who happens to get a genetic mutation for, say, a longer beak may have an easier time feeding, surviving and crucially, mating. Since it's more likely to reproduce and this gene can be passed on, the bigger beak shows up more frequently in subsequent generations. This process is called natural selection and it drives evolution which turned amoebas into fish into lizards into apes into humans. It turns out that while this description is on the right track, it leaves out big parts of the story.
For example, today we obviously co-exist with many other species including apes with whom we share a common ancestor. If apes developed some random human-oriented mutation that spread through generations, wouldn't at some point the apes just be completely gone? A big missing piece of the story is the details of speciation, the process by which species separate out into branches of the evolutionary tree that we see today. The primary cause of speciation is geographic isolation. While Darwin meticulously documented (non-genetic) evidence of natural selection, he didn't really understand how the conditions for divergence first arose. That was worked out later by Ernst Mayr, Leon Croizat and others in the mid-20th century. Mayr showed how a subset of a bird population can migrate and form a new colony on an island. Once geographically separated, reproduction stops between the groups and they then evolve into distinct species through natural selection. Croizat showed how changes in Earth's geography like a new mountain range can physically separate a species and lead to the same effect.
While geographic isolation is the primary mechanism for speciation, it isn't the only one. The apple maggot fly originally laid its eggs on hawthorn fruit in North America. After Europeans introduced apples, some flies began laying their eggs on apples instead. These flies also tend to mate on the fruit where they lay their eggs, so apple flies mostly encounter other apple flies while hawthorn flies mostly encounter other hawthorn flies. Apples also ripen earlier than hawthorns, causing the two groups to emerge and mate at different times. Reproduction between them has slowed and yet they live in the same geographic area. They are in the process of sympatric (same place) speciation today. Anything that causes at least some reproductive isolation can lead to speciation because then the gene pools can evolve independently.
DNA mutations or "copying errors" are far from the only source of variation in phenotypes. The cells in your body have 46 chromosomes, 23 pairs from your mother/father. Your sperm/eggs (gametes) are produced by meiosis, which splits normal cells into specialized cells which only have 23 chromosomes. In the process of splitting, a maternal or paternal copy from each chromosome pair is randomly selected. That "independent assortment" or reshuffling can produce different traits in the species without any DNA mutation. When a new selective pressure arises due to a changing environment, the first thing that happens is natural selection upon the best shuffling of existing genes, rather than new variants created through mutations. The randomness inherent in independent assortment and mutation contribute to what's known as random genetic drift. Each subsequent generation is primarily like the reshuffling of a deck of cards. Occasionally a brand new card is added to the deck, corresponding to a new mutation. In any given sequence of generations, it's possible that an unlikely set of shuffles appears. The average beak size of a bird population may shift randomly, without any new selective pressure at all.
What if a mutation shows up when the environment hasn't changed much? The environment usually changes much slower than new generations of organisms are produced. If a bird has a random genetic mutation that creates an extreme physical trait like a beak that is way too small or way too large, that bird will have a hard time surviving and reproducing. So the variant will be naturally selected out of existence. Most of natural selection is "stabilizing selection" in this sense. Non-extreme phenotypes better suited towards the existing environment are favoured in the process. An environmental change creates the conditions for a new adaptive trait, but even a static environment involves constant natural selection to maintain the existing adaptive traits. There can also be genes that simply have no selective impact at all. After sequencing the human genome, we've discovered that large parts of it are "junk" DNA that doesn't seem to get involved in any downstream processes. If it doesn't impact any traits, then mutations on this junk DNA have no selective pressure one way or another. In fact, the neutral theory of molecular evolution argues that most of the DNA differences we see between species are from this neutral variation.
Some aspects are still debated even today. How much of the DNA difference between two birds is really from adaptation to their given environments vs just random differences that drifted there because it didn't matter either way? Under what conditions does junk DNA later become useful (exaptation)?
Questions or comments? Email feedback@cstein.xyz