Download divergent evolution

Patterns of
Evolution
Patterns of Evolution
Measuring the amount of
change…
• Macroevolution: large
evolutionary changes; takes
a long time (ex: Reptiles
evolving into birds)
• Microevolution: small
evolutionary changes (ex:
the peppered moth changing
color)
Macroevolution: Is it Possible?
Patterns of Evolution
Measuring the amount of time it takes…
• Gradualism: evolution as the slow accumulation of many
small changes (ex – sharks today are basically the same as
they were before the dinosaurs)
• Punctuated Equilibrium: sudden change in a group after years
of no change (ex- mammals evolved very quickly to become
large after the dinosaurs went extinct)
Patterns of Natural Selection
Natural Selection on
Single Gene Traits
Reminder: natural selection on single gene traits can lead to
changes in allele frequencies and thus to evolution
Natural Selection on
Polygenic Traits
Natural selection can affect a population in any
of three ways: (leave space between each to
draw a small graph!!!)
1. Directional Selection – one extreme is favored (ex: smallest
survive best)
2. Stabilizing Selection – the average trait is favored (ex:
medium survive best)
3. Disruptive Selection – both extremes are favored (ex:
smallest AND largest survive best)
Directional Selection
- individuals at one end of the curve have higher fitness than those in the
middle/the other end
- the entire curve moves as the character trait changes
- evolution causes an increase in the # of individuals with the trait at one
end of the curve
Stabilizing Selection
- when individuals
near the center of the
curve have higher fitness
than individuals at the
ends
- the center of the curve
at its current position, but
it narrows the overall
graph
Disruptive Selection
- the individuals at the upper and lower ends of the curve have higher
fitness that individuals near the middle
- acts most strongly against individuals of an intermediate type
Divergent Evolution
Disruptive Natural Selection often leads to…
• divergent evolution: when two related groups
develop more and more differences
– examples:
polar bear – white fur for camouflage
grizzly bear – brown fur for camouflage
• If the disrupted population is completely split ,
it leads to speciation – the formation of new
species.
Divergent Evolution
Careful! Don’t confuse divergent with convergent
evolution…
• convergent evolution: when two UNRELATED species
evolve similarities ONLY because they live in the
same habitat and NOT because they are related (ex:
sharks and dolphins)
Speciation
Isolating Mechanisms
Definition of “Species” = group of similar individuals that can
successfully and naturally interbreed
- Preventing individuals from interbreeding (isolating) can cause
speciation; the following can cause reproductive isolation
1. Behavioral Isolation – not attracted to one another (ex:
different bird songs)
2. Geographic Isolation – don’t live in the same area (ex: island
species are separated from the mainland)
3. Temporal Isolation – reproductive timing is off (ex:
reproducing with the full moon instead of the half moon)
Behavioral Isolation
Two populations are capable of interbreeding but have
differences in courtship rituals or other types of behavior
Geographic Isolation
Two populations are separated by
geographic barriers (rivers,
mountains, bodies of water)
Temporal Isolation
Two or more species reproduce at different times
Temporal
Isolation
Careful! Speciation can split a population into more than just two groups
-
Adaptive radiation: one ancestral group gives rise to many different
species all at once (ex- Darwin’s finches evolved from the same
ancestor, but were adapted to MANY different food sources)
Organizing Evolution
Phylogenetic Trees
• Timeline/family tree of
evolution created using
ALL the evidence of
evolution/relatedness
we can find; the closer
together two species
are on the tree, the
more closely related
they are
Cladogram
• Chart of species based on derived (shared)
characteristics only; just used to show one
type of evidence, like physical similarities;
every species below the character lacks that
trait, every species above the character has it