Transcript Adaptations & Comparative Anatomy

Adaptations &
Comparative Anatomy
For a sticker:
1. Speciation occurs when a population diverges
because a ______ limits ____ ____.
2. Name the four prerequisites or conditions for natural
selection. (Hint: they start with V, I, O, and L.)
Homework
Recap
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Variation + Inheritance + Overreproduction +
Limited Resources = Natural Selection,
individuals bearing traits providing
reproductive advantage have more babies.
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Natural selection produces cumulative effects
over time.
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Evolution = allele frequency changes over time.
Sufficient cumulative effects can yield speciation.
Populations diverge (speciate) when gene
flow is limited by a barrier.
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Three kinds of speciation.
How rapidly speciation occurs depends on amount
of selective pressure, and generation length.
Objectives
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Learn three methods scientists use in
determining a species’ ancestry
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Differentiate between different kinds of
anatomical patterns that evolution produces
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Be able to explain how you can use anatomy to
determine descent
Determining Descent
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Scientists have many approaches to
studying evolution. The rest of the unit,
we will focus on three of those
approaches.
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We will study how scientists use anatomy,
molecular evidence, and the fossil record.
We will skip biogeography, biochemistry,
behavior, and active experimentation,
unfortunately.
Determining Descent
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These are all different methods of
determining descent.
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Organisms have common descent if they
come from a common ancestor.
The most recent ancestor that they all
have in common is called the last
common ancestor.
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Common Descent
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The more recent the last common ancestor,
the more closely related and the more similar
the organisms are.
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Today, we will focus on anatomy: morphology and
embryology.
Adaptations
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An adaptation is a trait that
developed/spread because it conferred a
survival advantage.
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What adaptations have you learned about in
previous lessons?
Adaptations
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An adaptation is a trait that
developed/spread because it conferred a
survival advantage.
What adaptations have you learned about in
previous lessons?
 Adapatations can include features like
camouflage, limb shape, tooth size, bone
density, scales, etc.
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Adaptations and Structures
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An adaptation that arises in a
species will be passed down to its
descendents, perhaps with
modification, unless the
descendents inhabit an environment
that makes the adaptation
disadvantageous.
We can track common ancestry
by tracking who inherited the
same adaptations.
Adaptations & Structures
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Homologous structures are anatomical features
that are similar between two organisms because
they came from the same ancestral form.
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They don’t necessarily have the same function
anymore.
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“Homo” root, remember what that means?
A great example is the shape of the forelimb in
animals.
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Homology
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Same structure, diverse functions: 1 bone, then 2, then a
wrist, then radiating digits.
The common ancestor for all these organisms had the same
limb pattern, which was modified in descendents depending
upon the pressures of their environments.
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Homology vs Analogy
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This is different from an analogous structure.
Analogous structures: Perform the same function but
evolved separately.
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Remember what an analogy is?
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For instance, the wing of a butterfly, bat, and bird. All
structures enable flight, none of these organisms inherited
the structure from one of the others. Consequently, they
have similar physics but obviously different underlying
anatomy.
• Analogous structures are useful to ecological study, but not to
studying descent.
Vestigiality
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Also very useful are a type of homologous structure
called a vestigial structure.
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When a structure becomes disadvantageous to an
organism, it’s rarely instantly lost. Instead, its
significance gradually diminishes, just like its significance
originally gradually appeared.
The structure may never go away entirely. If it gets small
and insignificant enough that it no longer affects the
organism, selection can’t act on it anymore so that last
tiny shard of it may remain in the species for a very long
time.
Vestigiality
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A homologous structure that has lost part
of all of its original function is called a
vestigial structure.
A “vestige” is a “remnant.” A vestigial
structure is a leftover remnant of a structure
that was whole and important in an ancestor.
 Examples…
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Vestigiality Examples
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The human appendix. Our primate and
other vertebrate ancestors had a
caecum at the end of their intestines,
which held a colony of specialized
bacteria that increased the efficiency of
cellulose digestion.
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Over the history of human evolution, it
shrank and now is quite useless at its
former job. All it does is get infected 7% of people get acute appendicitis in
their lifetimes.
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Vestigiality Examples
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Flightless birds haven’t yet lost their
wings. The wings are vestigial.
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Vestigiality Examples
Whales have tiny vestigial hind legs,
remnants of their terrestrial ancestry.
 Some snakes also have vestigial legs
from their lizard ancestors.
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Vestigiality Examples
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Human erector pilli (the muscles that make
your hair stand on end), the human coccyx,
and human wisdom teeth are more examples
from our species.
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Our primate ancestors had larger jaws than us that
could accommodate more teeth. But our jaw size
has been shrinking faster than our wisdom teeth, so
we still have them. Given another few million
years, maybe we’ll be rid of the nuisance!
Determining Descent
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So one way that scientists track descent by examining
physical characteristics is by seeing which organisms:
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Have the same characteristics
• All mammals have hair, so it’s logical to conclude that their last
common ancestor also had hair.
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Have homologous characteristics
• Amphibians, reptiles, birds, and mammals all have homologous limb
structures, so it’s logical to assume that their last common ancestor
had that limb structure.
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Have vestigial structures, and which organisms have the
functional versions of those structures
Determining Descent
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Another comparison of physical characteristics is
made by examining the embryos of organisms.
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Genes don’t just code for how an adult is built, they also
code for how an embryo is built.
An organism’s instructions for how to build an
embryo are inherited by its descendents just like
any other genes. Therefore, the more structures
two species’ embryos share, the more closely
related the species are.
Determining Descent
What’s more, because evolution is much more
likely to proceed by modifying existing
structures than by making whole new ones,
embryos will often have the same structure that
an ancestor did, but the structure will change or
disappear by the time of birth.
 For instance…
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For instance, birds have limbs with three fused digits.
Their reptile ancestors had five digits. It would require
a significant genetic change for a bird to only make
three digits, but it’s a relatively minor change to make
five digits and then, while the bird is still in the egg,
bind them together while they’re still cartilage.
And so, bird embryos have five digits, indication of
their reptilian ancestry.
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Determining Descent
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Dr. Kenneth Miller on some lines of
descent that were in the news when the
movie was made (2006). Ch. 9-13
Recap
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Definitions: adaptation, homologous structure,
analogous structure, vestigial structure
Scientists can determine common ancestry by
comparing anatomy.
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Identical structures
Homologous structures
• Adult homologous structures
• Adult vestigial structures
• Embryo homologous structures
Coming Up
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Learning how to make and read a
phylogenetic tree (a map or “family tree” of
ancestry)
More sources of evolutionary evidence:
molecular (DNA & proteins), fossils
Putting it all together with the example of
human evolution
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After MCAS, we may come back and address
biogeography and applications of evolutionary
principles (poisons, antibiotics, crime scenes,
agriculture…)