Download Principles of Evolution

Natural Selection
So which changes stick around and which ones
don’t??
Whichever traits provide a benefit are selected
• Nature selects those organisms that are
adequate enough to grow up (survive),
mate and produce offspring.
• That’s why this process, Natural Selection
is a non-random process of sorting out
genotypes and resulting phenotypes that
don’t work.
Is nature the only thing that selects traits?
• In artificial selection,
people are the
choosers
This is why some ridiculous traits (by nature’s
standards) may show up in a populations
In Natural Selection the
environment is the
chooser
Natural selection is a
nonrandom process
by which traits are
selected for by nature
because they provide
some survival benefit.
Natural selection is a mechanism of
evolution.
• There are four main principles to the theory of natural
selection.
– Variation of heritable traits
– overproduction and struggle for survival
– adaptations arise over many generations
– descent with modification
Fitness is the measure of survival ability and
ability to produce more offspring in the
population, directly tied to the unequal # of
produced by breeding pairs of the
•offspring
.
same species. Differential reproduction.
Natural selection acts on existing
variation.
• Natural selection can act only on traits that already exist.
• Structures take on new functions in addition to their
original function.
five digits
wrist bone
How do we get variation?
All that selection pressure can alter the
normal distribution of traits. The outcomes of
this are called “modes of selection.”
3 types of modes of selection:
1) Directional-one of the 2 extreme phenotypes is
favored by selection pressure.
3 types of modes of selection:
1) Directional-one of the 2 extreme phenotypes is
favored by selection pressure.
2) Stabilizing-the middle phenotype is favored by
selection
3 types of modes of selection:
1) Directional-one of the 2 extreme phenotypes is
favored by selection pressure.
2) Stabilizing-the middle phenotype is favored by
selection
3) Disruptive/Diversifying: Both extreme
phenotypes are favored by selection
Another outcome of fitness is Sexual
selection It occurs when certain traits
increase mating success.
• Occurs due to higher
cost of reproduction
for females.
– males produce many
sperm continuously
– females are more
limited in potential
offspring each cycle
There are two types of sexual selection.
– Intra-sexual selection: competition among males
– Inter-sexual selection: males display certain
traits to females
Species can shape each other over
time.
• Two or more species can
evolve together through
coevolution.
– evolutionary paths become
connected
– species evolve in response
to changes in each other
– Two types: beneficial &
competitive
beneficial
Competitive
https://www.youtube.com/watch?v=vOzI8O-oUgo
Why is nature so cruel??
• Limited resources means that organisms compete
with themselves and others for survival. Because
of this, nature weeds out inadequate individuals
from the group so that only the most able produce
diverse offspring.
• Descent with modification allows each generation
to try and produce “better” offspring in terms of
survival.
• But even those offspring won’t all live and
reproduce
• Biotic potential versus
environmental resistance.