Transcript The Mechanics of Natural Selection

The Mechanics of Evolution
Interaction of Natural Selection and
Inheritance (Genetics)
Lifeforms are not
immutable; they change.
Natural Selection and
how it works
The basics are pretty simple:
In each species, there is variation in traits.
For example, some beetles are green and some are brown.
There is differential reproduction.
Since the environment can't support unlimited population
growth, not all individuals get to reproduce to their full
potential.
In this example, green beetles tend to get eaten by birds and
survive to reproduce less often than brown beetles do.
There is heredity.
The surviving brown beetles have brown baby beetles
because this trait has a genetic basis.
End result:
The more advantageous trait, brown coloration, which
allows the beetle to have more offspring, becomes more
common in the population.
If this process continues, eventually, all individuals in the
population will be brown.
If you have variation, differential reproduction, and heredity,
you will have evolution by natural selection as an outcome.
It’s as simple as that.
We can make it more complicated.
... the outward physical manifestation of
internally coded, inheritable, information.
That is, it is comprised of both a phenotype and a genotype.
The "outward, physical manifestation" of the organism
The physical parts, the sum of the atoms, molecules,
macromolecules, cells, structures, metabolism, energy
utilization, tissues, organs, reflexes and behaviors.
Anything that is part of the observable structure, function or
behavior of a living organism.
This is the "internally coded, inheritable information" carried by all
living organisms.
This stored information is used as a "blueprint" or set of instructions
for building and maintaining a living creature.
These instructions are found within almost all cells (the "internal"
part), they are written in a coded language (the genetic code), they are
copied at the time of cell division or reproduction and are passed from
one generation to the next ("inheritable").
These instructions are intimately involved with all aspects of the life of
a cell or an organism.
They control everything from the formation of protein macromolecules,
to the regulation of metabolism and synthesis.
The relationship between the two is simple!
The
codes for the
We can make it more complicated.
Fitness describes how good a particular genotype is at
leaving offspring in the next generation relative to how
good other genotypes are at it.
So if brown beetles consistently leave more offspring than
green beetles because of their color, you'd say that the
brown beetles had a higher fitness.
Fitness is a relative thing.
A genotype's fitness depends on the environment in which
the organism lives.
The Fitness Concept
Fitness is a handy concept because it lumps
everything that matters to natural selection
(survival, mate-finding, reproduction) into one
idea.
The fittest individual is not necessarily the
strongest, fastest, or biggest.
A genotype's fitness includes its ability to
survive, find a mate, produce offspring — and
ultimately leave its genes in the next generation.
An adaptation is a feature that is common in a
population because it provides some improved
function.
Adaptations are well fitted to their function and are
produced by natural selection.
Adaptations can take many forms: a behavior that
allows better evasion of predators, a protein that
functions better at body temperature, or an
anatomical feature that allows the organism to
access a valuable new resource — all of these might
be adaptations.
Many of the things that impress us most in nature
are thought to be adaptations.
Adaptation
Microevolution and its processes
Microevolution is evolution on a small scale — within a
single population.
That means narrowing our focus to one branch of the tree
of life.
Mutation
Some "green genes" randomly
mutated to "brown genes" (although
since any particular mutation is rare,
this process alone cannot account for
a big change in allele frequency over
one generation).
Migration (or gene flow)
Some beetles with brown genes immigrated
from another population, or some beetles
carrying green genes emigrated.
Genetic drift
When the beetles reproduced, just by random luck
more brown genes than green genes ended up in the
offspring.
In the diagram at right, brown genes occur slightly
more frequently in the offspring (29%) than in the
parent generation (25%).
Natural selection
Beetles with brown genes escaped predation and survived to
reproduce more frequently than beetles with green genes, so
that more brown genes got into the next generation.
Macroevolution and its processes
Macroevolution is evolution on a grand scale — what we see
when we look at the over-arching history of life: stability,
change, lineages arising, and extinction.
Macroevolution generally refers to evolution above the species
level.
So instead of focusing on an individual beetle species, a
macroevolutionary lens might require that we zoom out on the tree
of life, to assess the diversity of the entire beetle clade and its
position on the tree.
Natural Selection: A Summary
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All species produce offspring at a faster rate than food supplies
can increase.
Except for identical twins, no two individuals are exactly alike.
In each generation, more individuals are produced than can
survive.
Individuals that possess certain traits have an advantage over
those that do not.
The environment determines whether a trait is beneficial.
Traits are inherited and passed on to the next generation.
Over time, successful variations accumulate in a population, and
a new species may appear.
Geographical isolation may lead to the formation of a new
species.
Mechanism of Evolutionary Change by
Natural Selection
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A trait must be inherited in order to have importance in natural
selection.
Natural selection cannot occur without variation in inherited
characteristics.
Fitness is a relative measure that will change as the environment
changes.