Note: This show first aired July 25, 2015.
Natural selection is the mechanism by which evolution takes
place, and by evolution, I mean the change in genetic material in a population
over time. That change over time is thought to make populations more robust,
more “fit”, fitness meaning more successful reproductively; and successful
reproduction is the name of the game when it comes to life on Earth.
Darwin’s theory of evolution by natural selection is based
on three simple premises: that more individuals are produced than can survive,
that there is variation in the traits of those individuals, and that the
individuals with the traits best suited for survival and reproduction in the
environment will survive. The genes that code for the successful traits get
passed on. Genes that code for less successful traits get passed on far less
frequently, and eventually disappear from the gene pool. That is how over time,
the genetic makeup of a population changes.
We’ve been talking the last couple of weeks about an ill
fated pair of baby birds, they were discovered tumbled out of a nest that was
blown out of a tree, and birds and nest were replaced into said tree as best an
anxious adult and curious child could do. Upon first check up, the baby birds
were discovered to be out of said nest again, and replaced a second time. Upon
second check up, both babies were gone, as were the parents. In some ways the
experience feels like failure, but as I talked about last week, the odds for
those babies were never very good even in the best circumstances, which brings
us to that first tenet of Darwinian evolution: more individuals are produced
each generation than can survive. So why didn’t these birds survive?
The second tenet of natural selection states that there is
variation in traits among individuals in a population. Among all of the
chipping sparrows in the northeast, some make very strong well protected nests,
others make flimsier ones. Some choose nest sites out of the prevailing wind,
others choose more exposed shrubbery. Nest building is a very instinctual
behavior in birds (though there are studies that show that as some birds
practice, their nests get better, indicating that there is a significant
learning component to nest construction as well). The instinctual part of nest
building results from genetics, some birds have genes that code for proteins
that end up directing cells to do X, other birds may have genes that code for
proteins that direct cells to do Y. If X causes the nest to be built in a very
secure location, and Y causes the nest to be built in a more exposed location
the genes have significant implications for the fitness of the individuals. Those
birds may survive, but where they locate their nests impacts the survivorship
of their young, and whether or not the genes of the parents get passed on into
the chipping sparrow gene pool or not. If they can’t successfully reproduce
because they keep building their nest in an unsuitable place, they are a genetic
dead end, which brings us to Darwin’s third tenet of natural selection. Genes
that code for successful traits get passed on, genes that code for less
successful traits eventually die out.
There is one more factor beyond how successful or not the
traits your genes code for are, and that factor is simple bad luck. Sometimes
organisms are doing all the right things, have robust traits, and high
survivorship, when bad luck strikes. In the case of birds, they could be in a
tree that gets cut down, or in a tree that gets damaged by lightening or a
falling nearby tree, things that don’t have anything to do with their nest
location and building skills. And that is the other hard truth about natural
selection, there is a large and strong random component to it. Just as the
mutations that give the gene pool variation are random, so are the incidents of
“bad luck” that can change the distribution of genes in a population without
respect to their impact on fitness. These shifts in gene frequency due to bad
luck (or being in the wrong place at the wrong time) are called genetic drift,
and the smaller the population size, the bigger an impact it can have.
The mechanism of natural selection cuts a broad and
unforgiving swath through populations every reproductive cycle, and while there
can be bad luck, there is rarely good luck. The modern human experience of the
world is so different from this that it is hard for us to remember. In nature,
the default is failure. Keep this in mind as you look around you and recognize
what you see for what they are-the chosen few, the rare success stories amidst
nearly infinite attempts at life.
References:
The basics of Darwinian evolution: https://www.ndsu.edu/pubweb/~mcclean/plsc431/popgen/popgen5.htm
On birds, nests, instinct and learning—this well publicized
study came out in 2011 and challenged the long held assumption that bird nest
building is entirely instinctual http://www.sciencedaily.com/releases/2011/09/110925192704.htm
More about those chipping sparrow nests: http://www.allaboutbirds.org/guide/Chipping_Sparrow/lifehistory#at_nesting
Genetic drift: http://evolution.berkeley.edu/evolibrary/article/evo_24
