By Christie Wilcox
We view this as the natural progression of life. Truth is, there was no
guarantee that some big brained primates in Africa would end up like we
are now. It wasn’t inevitable that we grew taller, less hairy, and
smarter than our relatives. And it certainly wasn’t guaranteed that
single celled bacteria-like critters ended up joining forces into
multicellular organisms, eventually leading to big brained primates!
Evolution isn’t predictable, and randomness is key in determining how
things change. But that’s not the same as saying life evolves by
chance. That’s because while the cause of evolution is random (mutations
in our genes) the processes of evolution (selection) is not. It’s kind
of like playing poker – the hand you receive is random, but the odds of
you winning with it aren’t. And like poker, it’s about much more than
just what you’re dealt. Outside factors – your friend’s ability to bluff
you in your poker game, or changing environmental conditions in the
game of life – also come into play. So while evolution isn’t random, it
is a game of chance, and given how many species go extinct, it’s one
where the house almost always wins.
Of course chance is important in evolution. Evolution occurs because
nothing is perfect, not even the enzymes which replicate our DNA. All
cells proliferate and divide, and to do so, they have to duplicate their
genetic information each time. The enzymes which do this do their best
to proof-read and ensure that they’re faithful to the original code, but
they make mistakes. They put in a guanine instead of an adenine or a
thymine, and suddenly, the gene is changed. Most of these changes are
silent, and don’t affect the final protein that each gene encodes. But
every once in awhile these changes have a bigger impact, subbing in
different amino acids whose chemical properties alter the protein
(usually for the worse, but not always).Or our cells make bigger
mistakes – extra copies of entire genes or chromosomes, etc.
These genetic changes don’t anticipate an individual’s needs in any
way. Giraffes didn’t “evolve” longer necks because they wanted to reach
higher leaves. We didn’t “evolve” bigger brains to be better problem
solvers, social creatures, or hunters. The changes themselves are
random*.
The mechanisms which influence their frequency in a population,
however, aren’t. When a change allows you (a mutated animal) to survive
and reproduce more than your peers, it’s likely to stay and spread
through the population. This is
selection, the mechanism that
drives evolution. This can mean either natural selection (because it
makes you run faster or do something to survive in your environment) or
sexual selection (because even if it makes you less likely to survive,
the chicks dig it). Either way the selection isn’t random: there’s a
reason you got busier than your best friend and produced more offspring.
But the mutation occurring in the first place – now
that was luck of the draw.
Mistakes made by genetic machinery can lead to huge differences in
organisms. Take flowering plants, for example. Flowering plants have a
single gene that makes male and female parts of the flower. But in many
species, this gene was accidentally duplicated about 120 million years
ago. This gene has mutated and undergone selection, and has ended up
modified in different species in very different ways. In rockcress (
Arabidopsis),
the extra copy now causes seed pods to shatter open. But it’s in snap
dragons that we see how the smallest changes can have huge consequences.
They, too, have two copies of the gene to make reproductive organs. But
in these flowers, each copy fairly exclusively makes either male or
female parts. This kind of male/female separation is the first step
towards the sexes split into individual organisms, like we do. Why? It
turns out that mutations causing the addition of a single amino acid in
the final protein makes it so that one copy of the gene can only make
male bits. That’s
it. A
single amino acid makes a gene male-only instead of both male and female.
Or, take something as specialized as flight. We like to think that
flight evolved because some animals realized (in some sense of the word)
the incredible advantage it would be to take to the air. But when you
look at the evolution of flight, instead, it seems it evolved, in a
sense, by accident. Take the masters of flight – birds – for example.
There
are a few key alterations to bird bodies that make it so they can fly.
The most obvious, of course, are their feathers. While feathers appear
to be so ideally designed for flight, we are able to look back and
realize that feathers didn’t start out that way. Through amazing fossil
finds, we’re able to glimpse at how feathers arose, and it’s clear that
at first, they were used for anything but airborne travel. These
protofeathers were little more than hollow filaments, perhaps more akin
to hairs, that may have been used in a similar fashion. More mutations
occurred, and these filaments began to branch, join together. Indeed, as
we might expect for a structure that is undergoing selection and
change, there are dinosaurs with feather-like coverings of all kinds,
showing that there was a lot of genetic experimentation and variety when
it came to early feathers. Not all of these protofeathers were selected
for, though, and in the end only one of these many forms ended up
looking like the modern feather, thus giving a unique group of animals
the chance to fly.
There’s
a lot of variety in what scientists think these early feathers were
used for, too. Modern birds use feathers for a variety of functions,
including mate selection, thermoregulation and camouflage, all of which
have been implicated in the evolution of feathers. There was no plan
from the beginning, nor did feathers arise overnight to suddenly allow
dinosaurs to fly. Instead, accumulations of mutations led to a structure
that happened to give birds the chance to take to the air, even though
that wasn’t its original use.
The same is true for flying insects. Back in the 19
th
century, when evolution was fledging as a science, St. George Jackson
Mivart asked “What use is half a wing?” At the time he intended to
humiliate the idea that wings could have developed without a creator.
But studies on insects have shown that half a wing is actually quite
useful, particularly for aquatic insects like stoneflies (close
relatives of mayflies). Scientists experimentally chopped down the wings
of stoneflies to see what happened, and it turned out that though they
couldn’t fly, they could sail across the water much more quickly while
using less energy to do so. Indeed, early insect wings may have
functioned in gliding, only later allowing the creatures to take to the
air. Birds can use half a wing, too – undeveloped wings help chicks run
up steeper hills – so half a wing is quite a useful thing.
But what’s really key is that if you rewound time and took one of the
ancestors of modern birds, a dino with proto-feathers, or a half-winged
insect and placed it in the same environment with the same ecological
pressures, its decedents wouldn’t necessarily fly.
That’s
because if you do replay evolution, you never know what will happen.
Recently, scientists have shown this experimentally in the lab with
E. coli bacteria. They took a strain of
E. coli and
separated it into 12 identical petri dishes containing a novel food
source that the bacteria could not digest, thus starting with 12
identical colonies in an environment with strong selective pressure.
They grew them for some 50,000 generations. Every 500 generations, they
froze some of the bacteria. Some 31,500 generations later, one of the
twelve colonies developed the ability to feed off of the new nutrient,
showing that despite the fact that all of them started the same, were
maintained in the same conditions and exposed to the exact same
pressures, developing the ability to metabolize the new nutrient was not
a guarantee. But even more shocking was that when they replayed
that colony’s history, they found that
it didn’t
always develop the ability, either. In fact, when replayed anywhere
from the first to the 19,999th generation, no luck. Some change
occurring in the 20,000 generation or so – a good 11,500 generations
before they were able to metabolize the new nutrient – had to be in
place for the colony to gain its advantageous ability later on.
There’s
two reasons for this. The first is that the mutations themselves are
random, and the odds of the same mutations occurring in the same order
are slim. But there’s another reason we can’t predict evolution: genetic
alterations
don’t have to be ‘good’ (from a selection
standpoint) to stick around, because selection isn’t the only
evolutionary mechanism in play. Yes, selection is a big one, but there
can be changes in the frequency of a given mutation in a population
without selection, too. Genetic drift occurs when events change the gene
frequencies in a population for no reason whatsoever. A massive
hurricane just happens to wipe out the vast majority of a kind of
lizard, for example, leaving the one weird colored male to mate with all
the girls. Later, that color may end up being a good thing and allowing
the lizards to blend in a new habitat, or it may make them more
vulnerable to predators. Genetic drift doesn’t care one bit.
Every mutation is a gamble. Even the smallest mutations – a change of
a single nucleotide, called a point mutation – matter. They can lead to
terrible diseases in people like sickle cell anemia and cystic
fibrosis. Of course, point mutations also lead to antibiotic resistance
in bacteria.
What does the role of chance mean for our species? Well, it has to do
with how well we can adapt to the changing world. Since we can’t force
our bodies to mutate beneficial adaptations (no matter what Marvel tells
you), we rely on chance to help our species continue to evolve. And
believe me, we as a species need to continue to evolve. Our bodies store
fat because in the past, food was sporadic, and storing fat was the
best solution to surviving periods of starvation. But now that trait has
led to an epidemic of obesity, and related diseases like diabetes. As
diseases evolve, too, our treatments fail, leaving us vulnerable to mass
casualties on the scale of the bubonic plague. We may very well be on
the cusp of the end of the age of man, if random mutations can’t solve
the problems presented by our rapidly changing environment. What is the
likelihood that man will continue to dominate, proliferate, and stick
around when other species go extinct? Well, like any game of chance, you
have to look at the odds:
99.99% of all the species that have ever existed are now extinct.
But then again – maybe our species is feeling lucky.
* If you want to get into more
detail, actually, mutations aren’t completely random. They, too, are
governed by natural laws – our machinery is more likely to sub an
adenine for a guanine than for a thymine, for example. Certain sections
are more likely to be invaded by transposons… etc. But from the viewpoint of selection,
these changes are random – as in, a mutation’s potential selective
advantage or disadvantage has no effect on how likely it is to occur.
Originally posted Nov 1st, 2010.
References:
Airoldi,
C., Bergonzi, S., & Davies, B. (2010). Single amino acid change
alters the ability to specify male or female organ identity Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1009050107
XU Xing, & GUO Yu (2009). THE ORIGIN AND EARLY EVOLUTION OF FEATHERS: INSIGHTS
FROM RECENT PALEONTOLOGICAL AND NEONTOLOGICAL DATA Verbrata PalAsiatica, 47 (4), 311-329
Perrichot,
V., Marion, L., Neraudeau, D., Vullo, R., & Tafforeau, P. (2008).
The early evolution of feathers: fossil evidence from Cretaceous amber
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Marden, J., & Kramer, M. (1994). Surface-Skimming Stoneflies: A Possible Intermediate Stage in Insect Flight Evolution Science, 266 (5184), 427-430 DOI: 10.1126/science.266.5184.427
DIAL, K., RANDALL, R., & DIAL, T. (2006). What Use Is Half a Wing in the Ecology and Evolution of Birds? BioScience, 56 (5) DOI: 10.1641/0006-3568(2006)056[0437:WUIHAW]2.0.CO;2
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