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Through this ppt, you will learn about patterns of Evolution, Natural Selection, and different types of Natural Selection. You will also learn about Stabilizing Selection, Disruptive Selection, and Directional Selection
2.
Patterns of Evolution
Evolution can result in changes that
are small or large
•Macroevolution: large evolutionary
changes; usually takes a long time
and occurs as the gradual
accumulation of small changes (ex:
Reptiles evolving into birds)
•Microevolution: small evolutionary
changes that occur faster and are
easy to see within a lifetime (ex: the
peppered moth changing color)
3.
Macroevolution:
Is it even Possible?
4.
Patterns of Evolution
Evolution can occur (relatively) quickly or it can be a slow process.
•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, result of STRONG selective pressure and/or big mutations (ex-
mammals evolved very quickly to become large after the dinosaurs
went extinct)
5.
Results of Natural Selection
6.
Results of Natural Selection
Reminder: natural selection leads to changes in the GENE
POOL of the GROUP
7.
Results of Natural Selection
Natural selection can change a population
in three ways:
1. Directional Selection
2. Stabilizing Selection
3. Disruptive Selection
8.
Results of Natural Selection
Natural selection changes are
based on bell curves. A bell
curve shows the number of
individuals within the group that
have a particular trait. Since
every individual is unique, some
may have different versions of
the trait. The highest point of the
bell curve is the most common
version of the trait in the group.
Bell curves taper off in each
direction as the variations on the
trait become less common.
9.
Directional Selection
- One extreme is favored; individuals at one end of the curve have higher
fitness than those in the middle/the other end
- the entire curve shifts to the left or right as the extreme trait becomes
more common
10.
Directional Selection Example
Generation after generation
favored the largest of the horses.
Over time, they gradually
became larger and larger.
11.
Stabilizing Selection
- the average is favored;
individuals near the
center of the curve have
higher fitness than
individuals at either end
- the center of the curve
remains at its current
position, but the graph
narrows as the “sides” are
eliminated
12.
Stabilizing Selection
Most human babies are between 7 and 9 pounds because babies smaller
OR larger than that range have a higher risk of complications. Smaller
babies typically have difficulty living outside of the mother (pre-mature
birth). Large babies typically have difficulty being born.
13.
Disruptive Selection
- BOTH extremes are favored; individuals at the upper and lower ends of
the curve have higher fitness than individuals near the middle
- being in the middle is bad in this case; population might split into two
separate species given enough time.
14.
Disruptive Selection
Light and dark colored butterflies are better able to survive than
medium colored butterflies in this species. Perhaps light can blend
in with the light gray trees and dark can hide in the shadows while
medium has no where to hide!
15.
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
– To “diverge” means to split apart
16.
Convergent Evolution
Careful! If two RELATED species can become more
DIFFERENT, then two UNRELATED species can certainly
become more SIMILAR if they live in the same environment
•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
18.
Speciation
ANY of the natural selection patterns (directional, disruptive, or
stabilizing) can change the group so much that they end up looking very
different than their ancestors. At some point, they must be classified as a
new species. The formation of a new species is called speciation.
- Who or what defines a species?
• Scientists decide when enough changes have occurred to re-
categorize the organisms as a new species.
• A species is generally defined as a group of similar
organisms (plants, animals, fungi, cells, etc) that can
successfully and naturally interbreed in the wild.
19.
Isolating Mechanisms
The key to maintaining a species is the ability to interbreed and keep
the gene pool mixed. If something splits a group and stops them from
interbreeding, then they will form separate gene pools and separate
species. This is called reproductive isolation.
1. Behavioral Isolation – individuals are not attracted to one
another (ex: different bird songs)
2. Geographic Isolation – individuals don’t live in the same
area (ex: island species are separated from the mainland)
3. Temporal Isolation – the reproductive timing is off (ex:
reproducing with the full moon instead of the half moon)
20.
Behavioral Isolation
Two populations are capable of interbreeding but have
differences in courtship rituals or other types of behavior
21.
Geographic Isolation
Two populations are separated by
geographic barriers (rivers,
mountains, bodies of water)
22.
Temporal Isolation
Two or more species reproduce at different times
23.
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 resulting in
more than just two species)
25.
Phylogenetic Trees
• Tracing all the different pathways of
evolution results in an image that looks
similar to a tree
• This phylogenetic tree of evolution
must be created using ALL the evidence
of evolution/relatedness we can find;
the result is that the closer together
two species are on the tree, the more
closely related they are to each other
– Try it: Who is the dog most closely related
to?
26.
Cladogram
• Sometimes we don’t want to trace EVERYBODY’s path. Sometimes we just want to
focus on a specific group and their similarities. This can be done through a cladogram.
• Cladograms are based on derived (shared) characteristics only; only showing one type
of evidence (like physical or DNA similarities); to read them, know that every species
below the character lacks the trait while every species above the character has it
• Try it: Who has dry skin, but
no hair?
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