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Animal Body Plans and Evolution
Discuss some trends in animal
evolution.
Explain the differences among the
animal phyla.
Features of Body Plans Each animal phylum has
a unique organization of body
structures called its “body
plan.” The features of a body plan include
▶ levels
of organization: cells, tissues, organs, organ systems
▶ body
symmetry:
• radial symmetry: body
parts extend from a central point
• bilateral symmetry: left
and right sides are mirror images, with front and back ends
▶ differentiation
of germ layers:
• endoderm, the innermost
layer
• mesoderm, the middle
layer
• ectoderm, the outermost
layer
▶ formation
of a cavity, or fluid-filled space between the digestive tract and the body
wall:
• a true coelom (found in
most complex animal phyla) develops in the mesoderm and is
lined with tissue derived from
the mesoderm
• a pseudocoelom is only
partially lined with mesoderm
• Some invertebrates lack a body
cavity and some have only a primitive, jellylike layer
between the ectoderm and
endoderm.
▶ patterns
of embryological development
• Sexually reproducing animals
begin life as a zygote, or fertilized egg.
• The zygote develops into a
hollow ball of cells, the blastula.
• The blastula folds in on itself
and creates a tube that becomes the digestive tract; the tube
has a single opening, the
blastopore:
▷ In
protostomes (most invertebrates), the blastopore becomes the mouth.
▷ In
deuterostomes (chordates and echinoderms), the blastopore becomes the
anus.
▶ segmentation:
repeated parts, such as the segments of worms
▶ cephalization: the concentration of
sense organs and nerves near the anterior (head) end
▶ limb
formation: external appendages such as legs, flippers, and wings
The Cladogram of Animals The features of body
plans provide the evidence needed to
build a cladogram, or
phylogenetic tree, of all animals. Animal phyla are usually defined by
their adult body plans and
patterns of embryological development.
▶ The
characteristics of animals vary within each phylum.
▶ Each
phylum may be thought of as an “evolutionary experiment.” Phyla with successful
body
plans have survived.
Features of Body Plans
Complete the table of main ideas
and details about animal body plans.
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Main Idea: Feature of Body Plan
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Details: Important structures
or patterns
of development
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Levels of organization
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Cells, tissues,
organs, organ systems
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Body symmetry
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None, radial, or bilateral
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Germ layers
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Endoderm, mesoderm,
ectoderm
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Body cavity
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None, pseudocoelom,
or true coelom
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Patterns of embryological
development
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The zygote develops
into a hollow ball of cells, the
blastula.
• Protostomes: the
blastopore becomes the mouth.
• Deuterostomes: the
blastopore becomes the anus.
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Cephalization
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The concentration of
sense organs and nerves near the
anterior (head) end
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egmentation
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Repeated parts, such as the
segments of worms
Limb formation
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Limb formation
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External appendages
such as legs, flippers, and wings
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*Bilateral
explain an object that
can be divided into two matching halves along one axis.
*Radial
is an object that can be
divided into equivalent segments by drawing a line through its center.
Difference
between acoelomate, pseudocoelomate, and coelomate animals. Label ectoderm,
mesoderm, and endoderm.
The acoelomate
show mesoderm and a digestive cavity, as in a flatworm.
The pseudocoelomate
show a pseudocoelom and a digestive tract, as in a roundworm. The coelomate
sketch should have a coelom and a digestive tract, as in an earthworm.
Evolution
How Do We Study Evolutionary Relationships?
Systematics is the part of science that
deals with grouping organisms and determining how they are related. It can be
divided into two main branches:
- Taxonomy focuses on classifying,
naming, and grouping organisms. A group, or taxon, can be a
population, a species, a genus, or a higher-level grouping, such as
family, order, class, phylum, kingdom, or domain. The plural of taxon is taxa.
- Phylogenetics is the study of determining
evolutionary relationships, or patterns of descent of organisms.
All of
the species of organisms that are alive today have descended from ancestral
species. This is due to evolution, or simply change over time. The
evolutionary relationships of ancestral species and their descendants can be
diagrammed using branching evolutionary trees. Just like your family
tree, an evolutionary tree indicates which ancestors gave rise to which
descendants.
How to Read an Evolutionary Tree
An
evolutionary tree can also be called a phylogenetic tree, or a just a phylogeny.
There are many different ways to draw phylogenies, but they do all have certain
parts that you must understand before you try to interpret them.
The root
of a phylogeny represents the common ancestor of all the descendants in the
tree. The descendant taxa are labeled at the tips of the tree. A node
splits into two branches and indicates a divergence or speciation event.
The node itself represents the common ancestor of any descendants that branch
off of it. The two taxa that branch off at a node are called sister taxa.
They share an immediate common ancestor. In this phylogeny, taxa B and C are
sister taxa. They are both equally related to taxon A.
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The
branches of a phylogeny can be rotated around a node without changing
evolutionary relationships. If you want to determine how two or more taxa are
related, it is important to look at the nodes and branches in a tree and not
just the ordering of descendant taxa. Sometimes the evolutionary relationships
between taxa cannot be determined. This results in a phylogeny with a polytomy,
or a node from which more than two groups split. A clade is a group that
includes an ancestor and all of its descendants. Clades, also called monophyletic
groups, can be nested in larger clades. For example, mammals are a
monophyletic group because they all descended from a common ancestor. Within
the mammals, there are also many smaller clades, such as primates or bats.
Not all
groupings of organisms qualify as monophyletic. A paraphyletic group
consists of an ancestor and only some of its descendants. Reptiles are
animals like crocodiles, lizards, and snakes. This is actually a paraphyletic
grouping because the ancestor that gave rise to all reptiles also gave rise to
birds. If birds are added to the definition of reptiles, then it could be
considered a monophyletic group.
A polyphyletic
group is made up of various descendants with no recent common ancestor. Marine
mammals are polyphyletic. Whales and seals are both marine mammals, but
they are not closely related at all. Seals are more closely related to bears
than they are to whales. Whales share a more recent common ancestor with deer
than they do to seals.
Humans and Their Relatives
Now that
you know how to read an evolutionary tree, let's look at a simple example using
humans and their living relatives.
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From this
tree, you can see that humans and chimpanzees are sister taxa. Remember this
means that they share a recent common ancestor with each other. Humans did not
evolve from chimpanzees! Gorillas share a common ancestor with both humans and
chimpanzees, but it is not as recent. Gorillas are just as closely related to
chimpanzees as they are to humans. Of the species shown, orangutans are the
most distantly related to humans.
Terminological difference
There are
two approaches, (1) Evolutionary taxonomy and (2) Phylogenetic systematics
derived from Willi Hennig. They differ in the use of the
word eg "monophyletic". In evolutionary
systematicists, "monophyletic" means only that a group is derived
from a single common ancestor. In phylogenetic
nomenclature, there
is an added caveat that the ancestral species and all descendants should be
included in the group.
The term
"holophyletic"
has been proposed for the latter meaning. As an example, amphibians are monophyletic under evolutionary taxonomy,
since they have arisen from fishes only once. Under phylogenetic taxonomy,
amphibians do not constitute a monophyletic group in that the amniotes (reptiles, birds and mammals) have evolved from an amphibian ancestor and yet
are not considered amphibians. Such paraphyletic groups are rejected in
phylogenetic nomenclature, but are considered a signal of serial descent by
evolutionary taxonomists.
Hierarchical Classification
Now that
we know why scientific names are used and how they are written, let's look at
the hierarchical system of creating these names. Linnaeus developed a system
that went from broadest to most specific. The levels of classification he used
are: kingdom, phylum, class, order, family, genus, and species. You can see
that genus and species are the two most specific categories, which is why they
are used in binomial nomenclature to identify an organism. A good way to
remember the order of classification from broadest to most specific is to use a
mnemonic device. While there are countless mnemonic devices out there, I always
remember the order using: Kings Play Chess On Fat
Guys' Stomachs. The first letter of each word corresponds to the
first letter of each level of organization, making it easy to remember.
A good
way to think of how this system helps us identify groups of related organisms
is to compare it to your address. You could identify very broadly where you
live by just providing the continent, but you could also very specifically
provide your house or apartment number. Let's look at an example of this
comparing an address to the levels of organization used by Linnaeus. We can
compare kingdom with the continent, phylum with the country, class with the
state, order with the zip code, family with city, genus with street, and
species with apartment number. We see here how this all lines up and helps to
identify different levels. We could talk about all of the members of the same
class just as we could talk about all of the people within one state, while you
could more specifically talk about one species like you could talk about the
people living in the same house.
The Six Kingdoms
A long
time ago, scientists used just two kingdoms for the classification of living
things: plants and animals. As our understanding of life has changed over the
past few centuries, we have revised the kingdoms to reflect this. Currently, we
use a six-kingdom setup. The kingdoms are: Archaebacteria, Eubacteria,
Protista, Fungi, Plants, and Animals. Each kingdom has specific characteristics
allowing taxonomists to accurately group organisms.
We will
look at the importance and evolutionary history of these six kingdoms later.
For now, let's just distinguish between the six different kingdoms. They are
classified based on their cell type and number as well as how they get food.
- Archaebacteria are known as
ancient bacteria. The prefix 'archae' means 'ancient,' making this one
easy to remember. They are prokaryotic and unicellular.
- Eubacteria are what you
generally think of when you think of bacteria - such as E. coli and
salmonella. The prefix 'eu' means 'true,' so these are true
bacteria. They too are prokaryotic and unicellular but have different
genetic compositions than their ancient predecessors.
- Protista are small
eukaryotic organisms that are grouped in somewhat of a hodge-podge group.
They are generally broken up into three groups within the kingdom:
plant-like, animal-like, and fungi-like.
- Fungi were once grouped with
plants but, among several other differences, are not capable of
photosynthesis and are therefore not plants. They are eukaryotic and
heterotrophic, meaning they have to consume food.
- Plants are capable of
photosynthesis, which is one of their defining characteristics, and are
eukaryotic.
- Animals are the largest
kingdom and are all eukaryotic, multicellular, and heterotrophic.
The
animal kingdom ranges from simple organisms like sponges to complex organisms
like humans. We will look at some defining characteristics of animals as well
as examples of both invertebrates and vertebrates.
Characteristics of Animals
Out of
all six kingdoms, the animal kingdom is most likely the one that you already
know the most about. After all, we are a part of this vast group. While the
characteristics of animals vary greatly, there are a few things that they all
share. Animals are eukaryotic, multicellular organisms that do not have
cell walls, get their nutrients by ingestion, and are capable of movement.
With so
many shared qualifications, it may be hard to imagine all the possible
differences between the more than 1.3 million living species of animals.
Because they are made of eukaryotic cells - those with a nucleus and
membrane-bound organelles - they are part of the Eukarya
domain. 'Multicellular' means that they are made of many cells - not just one
cell like their unicellular protozoan ancestors.
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Unlike plant cells, animal cells do not have cell
walls.
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Unlike
the other kingdoms, animal cells do not have cell walls. While cell walls help
protect cells, they also limit flexibility - something that is essential to the
animal cells since these organisms are capable of movement. This movement is
also permitted by the presence of both muscles and nerves.
As for
reproduction, most animals reproduce sexually, though some species are capable
of asexual reproduction. Sexual reproduction involves the formation of gametes
- egg and sperm - as well as the union of these two cells to create genetically
unique offspring. Most animals have sperm that are capable of movement due to
the flagella, while the larger egg is not
capable of movement.
Scientists
are able to study embryos and have found that different animal species have
very similar early stages of development - even though the mature organisms may
be nothing alike. We can see below that the early embryos of several species
look relatively the same but the mature organisms are completely different.
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In the early stages of development, animals
species look the same.
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Four main characteristics
classifying animals
There are
four main characteristics that are used to classify animals: symmetry, body
cavity, tissue, and vertebral column. Let's quickly take a look at some of
these before we look at some examples of animals. The first characteristic is
symmetry. Animals generally fall into two categories - they either have radial
or bilateral symmetry. Organisms like sea stars have radial
symmetry, while organisms such as humans have bilateral symmetry - as we can
see here:
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The two categories of symmetry in animals are
radial and bilateral.
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The next
thing is body tissue. The more complex an organism is, the more distinctive
layers of tissue it has. There are three main layers:
- Ectoderm - the outermost
layer
- Endoderm - the innermost
layer
- Mesoderm - the middle layer
As for
vertebral column, there are two key groups: invertebrates and vertebrates. Invertebrates
are animals without a backbone, while vertebrates are animals with a
backbone. As we look at the evolution of animals, we will first look at the
more simple invertebrates and then consider the more complex vertebrates.
Evolution of Invertebrates
Let's
look at some key evolutionary characteristics of invertebrates. Most of the
pattern of evolution is from simple to more complex. Examples of simple
invertebrates include things like mollusks and nematodes. Mollusks have a soft
body and protective shell, such as snails, sea slugs, oysters, and squids.
Nematodes are simple worms - not earthworms. Nematodes are organisms like
hookworms and roundworms that often cause illnesses in humans. Nematodes are
actually the most widespread group of animals, meaning they live in many
different places.
Arthropods
are more complex invertebrates that have a segmented body, hard exoskeleton,
and jointed appendages. This group is often called the 'insects' but contains
more than just bugs. There are more than 1 million different identified species
of arthropods ranging from horseshoe crabs to spiders to centipedes to
butterflies.
Another
important group of invertebrates is the echinoderms. The name means 'spiny
skin,' which is easy to remember due to some classic examples including
starfish and sea urchins.
Evolution of Vertebrates
Remember
that vertebrates are organisms that have a backbone. Like the invertebrates,
the vertebrates have evolved to be more complex over time. Let's look at a few
key groups of vertebrates, starting with the simplest forms of vertebrates -
the lampreys. These organisms have a simple body plan and mouth. They are
mostly parasites and latch on to other animals.
Evolutionary Biology:- The study of how
evolution occurs; is
the study of the origin of species as well as their change and diversity.
Diversity: The variety of species in a sample, community, or area. over time. Evolutionary biologists synthesize the evidence generated by all of the other fields of biology (paleontology, ecology, anatomy, developmental biology, and genetics) to create a unified understanding of process of evolution as it occurred on Earth.
Diversity: The variety of species in a sample, community, or area. over time. Evolutionary biologists synthesize the evidence generated by all of the other fields of biology (paleontology, ecology, anatomy, developmental biology, and genetics) to create a unified understanding of process of evolution as it occurred on Earth.
Evolutionary biologists use a method called phylogenetic analysis.
Phylogenetic
Analysis:- Analytical method used to
find a hypothesis of relationships among species, by coding the various states
of homologous characters; also called cladistics. to determine a phylogeny.
Phylogeny: Sequence of ancestors of a particular lineage. (the history of an organism's lineage through time). This phylogeny is a hypothesis based on the taxa.
Phylogeny: Sequence of ancestors of a particular lineage. (the history of an organism's lineage through time). This phylogeny is a hypothesis based on the taxa.
Taxon:- An organism or group of organisms of the same
rank, e.g., members of an order, family, genus, or species. (pl. taxa) (the organisms under study, usually
species) and characters
Character:- A single attribute of an organism.
(the traits and sequences for each organism) used in the analysis. The
phylogeny is represented by a “tree” (also called a cladogram. Cladogram:- A tree diagram depicting patterns of shared characteristics and
relationships of organisms, generated through phylogenetic analysis.),
very similar in many aspects to a family tree or genealogy. The branching
pattern indicates which organisms are closely related and which are more
distantly related. The phylogeny is considered a hypothesis because its
branching pattern can (and often does) change when taxa or characters are
added, deleted, or changed. Each branch and its member taxa are called a clade.
A phylogenetic tree suggests the relationships among taxa and the
pathways of character evolution.
What Are Body Plans?
Think different types of animals found on
Earth. The diversity of animal life on Earth is indeed quite vast to be studied.
We classify animals based on certain body structures as well as how those body
structures are combined to make a whole animal. Together the organization and
combination of body structures describes an animal's body plan. For
example, you have legs, which is one of your body structures. But you are quite
different from a crayfish, which also has legs! Our overall combination of body
structures - our body plan - is not even close to that of a crayfish, even
though we share some of the same structures. And this combination and
organization of structures is how we differentiate between different types of
animals.
Types of Symmetry
One of the most distinguishing features of
animals is their body symmetry. This describes how an animal looks from
one side to the other. Some animals have radial symmetry, which means
that they look the same on all sides from the centre. Think of a bike wheel or
pumpkin pie. No matter where you slice it, each piece looks the same as it
radiates outward from the centre. Animals like sea anemones, corals and jellies
have radial symmetry.
Most of the animals have bilateral
symmetry. This is when each side of the animal looks like a mirror image of
the other side. Mammals (including humans) have bilateral symmetry - if you
folded us in half, one side would fit perfectly over the other as a mirror
image. Crayfish has bilateral symmetry. Despite of these similarities, there
are important differences between crayfish and man. Symmetry is important in
animal function, and it tells us quite a bit about the animal's lifestyle.
Animals with bilateral symmetry tend to have their brains, mouths and other
sense organs (like eyes and ears) up in their heads. This allows them to have
mobility - the head goes through the environment first with the eyes and ears ready
to take in all of the activity around them.
In contrast, radially symmetric animals are
designed for a sedentary lifestyle. Because they are the same on all sides,
they're ready for whatever comes at them - no matter the direction of attack.
Body Cavities and Tissues
Another important feature that
differentiates animals is whether or not they have a true body cavity, called a
coelom. This is a space between the body tissues and internal organs,
and it allows for independent movement and growth of those organs. In animals
that don't have a coelom, the internal organs are attached to the tissues of
the body wall, so they are not independent. Some animals have a pseudocoelom, which
is a body cavity not completely lined by tissue. Pseudocoeloms work just like
true coeloms, even though this name meaning 'false cavity' may suggest
otherwise.
Most bilaterally symmetrical animals
(including us) are coelomates. Animals, such as roundworms, fall into the
pseudocoelomate group, while other worms, such as flatworms, do not have a
coelom at all.
In addition to body cavities, we can also
differentiate animals by whether or not they have true tissues. Animals that
have true tissues are known as eumetazoans. What this means is that they
have body tissues that are specialized, like cells that specifically form
muscle, ligaments and different organs.
Location of Body Structures
The location of an animal's body structures
is one of the most important factors. An animal's body plan has sides such as
'right,' 'left, 'up' or 'down' leaves much open to the viewer's interpretation,
so instead we have more descriptive terms that clearly describe the location.
Origin of Invertebrates
Invertebrates
are certainly the most abundant animals on earth, but how did they get here and
how did all of their diverse forms come about? In one of the more popular
hypotheses in the evolutionary story, organisms
the single-celled supposedly began to form colonies. Colonies offer
these organisms an unknown advantage; so natural selection leads to more and
more colonies. As time passes, these colonial organisms start to develop specialized cells and then form into a ball with an indentation
that can ingest things and take
advantage of a new food source. Exploiting this new food source leads to more
advances, and eventually there are thousands of different invertebrate life
forms swimming in the ancient seas. One problem—there is no fossil evidence for the story. The story is based on the
comparison of DNA sequences (which
relies on many assumptions between organisms and a presumed common ancestor.
The
simplest animals, sponges, are supposed to have evolved from ancient
choanoflagellate protists based on the fact that the larval stage of sponges
looks similar to other choanoflagellates that exist today. This could be
explained as convergent evolution, but it is used as proof of common descent in
this case.
Within
the evolutionary sequence of events, the mechanisms that made the changes are
not known, and they cannot be known. Again, the lack of testability and
repeatability demonstrates how this subject lies in the realm of historical
science, if it is science at all. In order for the colonial protists, or even unicellular
protists, to become truly multicellular
organisms, an amazing change must happen extremely rapidly. Unicellular
organisms cannot produce more than one type of cell. In order to become a
multicellular organism, new information must be available that tells the cells
to develop in new ways to perform different functions. The source of this new
information cannot come from random mutations, but even if it could, a
multitude of new functions must be simultaneously added to the genome. The hormonal control of development and cell coordination must be present with
the information to code for the hormones and new cell structures. It is
insufficient to say that old hormones and proteins get used in a new way
because those old hormones and proteins must still perform their original
functions or the cell is not as fit and would be removed by natural selection.
Considering
this transformation, one cell from the colony has to “know” how to do all of
the jobs that the other cells were doing and gain that information from those
cells. After it has gained the information, it must then direct the different
cell types and orchestrate when the genes are turned off and on within each
cell, activities that are not required in unicellular life.
At least
95% of known fossils are of invertebrates, but the picture of invertebrate
evolution is still very fuzzy. The major reason for this is the sudden
appearance of fully formed invertebrate body plans in the fossil layer that
records what is known as the Cambrian
explosion. The ancestors to the variety of multicellular invertebrates are not known from fossils. Some traces of multicellular
life are found in Precambrian rocks
but not the types or abundant variety that would be expected from the diverse
organisms in the Cambrian rocks. The
cladograms and phylogenetic trees shown in the textbooks differ in
their major branching points, and the changing
nature of DNA and molecular evidence
means that by the time a text is published, the ideas about ancestry may have
changed. These models of evolutionary history are just speculative
interpretations, and different groups within the scientific community come to
different conclusions when they weigh the evidence with different assumptions
and biases.
This
illustration shows a representation of some of the forms that appeared very
suddenly in the fossil record during the period known as the Cambrian explosion. In the creationist
model, these animals represent descendants of original created kinds that
became extinct during the Flood of
Genesis. Gradual evolution
cannot account for the sudden appearance of so many types of life.
For
instance, evolutionists “know” cells must have formed in the ancient oceans
because we have cells today. The major advances in invertebrate body plans are
treated in this way. The development of soft-tissue features, like nervous
systems and body cavities, is rarely preserved in fossil specimens. A statement
like, “The development of bilateral
symmetry was an important evolutionary adaptation that allowed animals to
respond to their environment and compete better for resources” sounds logical,
but it is not based on evidence. This statement is not true in itself, but
it is a corollary to the evolutionary story. These are often referred to as evolutionary
“milestones” or “key adaptations” because they needed to happen for a new kind
of organism to form. When you read a statement like this or any statement that
expects you to accept something as fact, remember that these are
interpretations based on evolutionary assumptions. It is also important to
remember that these ideas have probably changed in the recent past and will
change in the near future as naturalistic scientists reinterpret the evidence
with ever-changing ideas. While change is expected in science as new knowledge
is gained, the fact that new data is constantly interpreted in the failed
framework of evolution, regardless of how well it fits, demonstrates the
importance of presuppositions and the prior commitment to a naturalistic explanation.
As
invertebrates diversified in “prehistoric” oceans, evolutionists suggest that
their nervous systems became more complex and that structures that sense the
environment (light, vibration, chemical detection, etc.) gave these new animals
an advantage. External skeletons developed in the ocean and allowed animals the
opportunity to invade the land where the plants that had already evolved to
survive on land could provide many open niches and food sources.
Another
interesting observation comes when you analyze the dates given in different
textbooks. Many of the evolutionary “milestones” and “first” fossils are given
extremely different dates in different textbooks. This is true in most cases,
from bacteria to the “most recent” divergent paths in evolution. The age of the
first animal fossils differs by 100 million years between textbooks, and many
other dates differ by varying degrees depending on which authority you use. The
variation in dates given by different experts exposes the speculative nature of
evolutionary storytelling, regardless of the supposedly objective nature of
science.
Evolutionary
Concept
1. All animals share a common
ancestor that evolved in the primitive seas over 600 (or 700) million years ago
from colonial protists.
2. Many adaptations represent key
stages in evolutionary history: nervous system, segmentation, body cavities,
bilateral symmetry, etc.
3.
Evolutionary
relationships of animal phyla are based on DNA and molecular evidence due to
the lack of fossil evidence of ancestral species.
4. Studying the embryonic
development of invertebrates reveals their evolutionary history.
5.
A wide
variety of body plans appears “suddenly” in the fossil record about 550 million
years ago—known as the Cambrian explosion.
6.
Sponges
and cnidarians were the first groups of animals to evolve over 650 million
years ago.
7.
Roundworms
and flatworms evolved from different ancestors.
8.
Segmented
worms and mollusks evolved in the ocean approximately 550 million years ago.
They are closely related based on embryology. The development of excretory
systems and eyes occur first in these groups.
9.
Lungs and
other organs have evolved independently in terrestrial animals, including
mollusks and arthropods, as some mollusks lost their shells in recent
evolutionary development.
10.
Arthropods
evolved from annelids over 600 million years ago in three distinct groups. Key
adaptations include jointed appendages and an exoskeleton. Exoskeletons allowed
arthropods to invade the land about 430 million years ago.
11.
Horseshoe
crabs have remained unchanged for 300 million years based on fossil evidence.
12.
Echinoderms
evolved over 650 million years ago. Based on deuterostome development pattern,
echinoderms share a recent common ancestor with vertebrates.
13.
Flight
evolved at least four different times, beginning with arthropods 100 million
years before pterosaurs.
International Code of Zoological Nomenclature (ICZN)
"Animal
naming"
The International Code of
Zoological Nomenclature (ICZN) is a widely accepted convention in zoology
that rules the formal scientific
naming of organisms
treated as animals. It is
also informally known as the ICZN Code, for its publisher, the same
applied to International
Commission on Zoological Nomenclature (which shares the acronym
"ICZN"). The rules principally regulate:- How names are
correctly established in the frame of binominal nomenclature
- Which name must
be used in case of name conflicts
- How scientific
literature must cite names
Principles
1.
Principle of
binominal nomenclature
This is the
principle that the scientific name of a species, and not of a taxon at any
other rank, is a combination of two names; the use of a trinomen
for the name of a subspecies and of uninominal names for taxa above the species
group is in accord with this principle. This means that in the system of
nomenclature for animals, the name of a species is composed of a combination of
a generic
name and a specific
name; together they make a
"binomen".
No other rank can have a name composed of two names.
Examples:
Species Giraffa camelopardalis
- Subspecies have a name
composed of three names, a "trinomen": generic name, specific name, subspecific name:Subspecies Giraffa camelopardalis rothschildi
- Taxa at a rank above species
have a name composed of one name, a "uninominal name".
Genus Giraffa, family Giraffidae
2.
Principle of Priority
This is the
principle that the correct formal scientific name for an animal taxon, the valid
name, correct to use, is the oldest available
name that applies to it. It is the most important principle—the fundamental
guiding precept that preserves zoological nomenclature stability. It was first
formulated in 1842 by a committee appointed by the British Association to consider the rules of
zoological nomenclature. Hugh Edwin Strickland wrote the committee's
report.
Example:
Nunneley 1837 established Limax maculatus (Gastropoda),
Wiktor 2001 classified it as a junior synonym of Limax maximus Linnæus 1758 from S and W
Europe. Limax maximus was established first, so if Wiktor's 2001
classification is accepted, Limax maximus takes precedence over Limax
maculatus and must be used for the species.
3.
Principle of
Coordination
The principle
of coordination is that within the family group, genus group and species
group, a name established for a taxon at any rank in the group is
simultaneously established with the same author and date for taxa based on the
same name-bearing type at other ranks in the corresponding group. In the
species-group, publishing a species name (the binomen)
Giraffa camelopardalis Linnaeus, 1758
also establishes the subspecies name (the trinomen)
Giraffa camelopardalis camelopardalis Linnaeus, 1758. The same applies
to the name of a subspecies; this establishes the corresponding species name. In
the genus-group, similarly, publishing the name of a genus also establishes the
corresponding name of a subgenus (or vice versa): genus Giraffa
Linnaeus, 1758 and subgenus Giraffa (Giraffa) Linnaeus, 1758. In
the family-group, publication of the name of a family, subfamily, superfamily
(or any other such rank) also establishes the names in all the other ranks in
the family group (family Giraffidae, superfamily Giraffoidea, subfamily
Giraffinae).
4.
Principle of the First
Reviser
This is the
principle that in cases of conflicts between simultaneously published divergent
acts, the first subsequent author can decide which has precedence. It
supplements the principle of priority, which
states that the first published name takes precedence.
Example:
Linnæus 1758 established Strix scandiaca and Strix noctua
(Aves), for which he gave different descriptions and referred to different types,
but both taxa later turned out to refer to the same species, the snowy owl.
The two names are subjective synonyms. Lönnberg 1931 acted as First Reviser,
cited both names and selected Strix scandiaca to have precedence.
5.
Principle of
Homonymy
This is the
principle that the name of each taxon must be unique. Consequently, a name that
is a junior homonym of another name must not be used as a valid name. It means
that any one animal name, in one particular spelling, may be used only once
(within its group). This is usually the first-published name; any later name
with the same spelling (a homonym) is barred from being used.
Primary homonyms are those with the same genus and same
species in their original combination. The difference between a primary junior
homonym and a subsequent use of a name is undefined, but it is commonly
accepted that if the name referred to another species or form, and if there is
in addition no evidence the author knew that the name was previously used, it
is considered as a junior homonym.
Examples:
Drury (1773) established Cerambyx maculatus
(Coleoptera) for a species from Jamaica. Fueßlin (1775) established Cerambyx maculatus
for a different species from Switzerland, and did not refer to Drury's name.
Fueßlin's name is a junior primary homonym.
Secondary homonyms can be produced if taxa with the same
specific name but different original genus are later classified in the same
genus (Art. 57.3, 59). A secondary synonym[clarification needed] is only a
temporary state, it is only effective in this classification. If another
classification is applied, the secondary homonymy may not be produced, and the
involved name can be used again (Art. 59.1). A name does not become unavailable
or unusable if it was once in the course of history placed in such a genus
where it produced a secondary homonymy with another name. This is one of the
rare cases where a zoological species does not have a stable specific name and
a unique species-author-year combination, it can have two names at the same
time.
Example:
Nunneley (1837) established Limax
maculatus (Gastropoda), Wiktor (2001) classified it as a junior synonym
of Limax
(Limax) maximus Linnæus, 1758 from S and W Europe. Kaleniczenko, 1851
established Krynickillus maculatus
for a different species from Ukraine. Wiktor, 2001 classified both Limax
maximus Linnæus, 1758 and Krynickillus maculatus Kaleniczenko, 1851
in the genus Limax.
This meant that L. maculatus Nunneley, 1837 and K. maculatus
Kaleniczenko, 1851 were classified in the same genus, so both names were
secondary homonyms in the genu Limax, and
the younger name could not be used for the Ukrainian species. So the Ukrainian
species can have two names, depending from its generic classification. Limax
ecarinatus, Limacus maculatus,
the same species.
Double homonymy (genus and species) is no homonymy: if the genera are
homonyms and belong to different animal groups, the same specific names can be
used in both groups.
Examples:
The name Noctua
Linnæus, 1758 was established for a lepidopteran subgenus. In 1764 he
established a genus Noctua Linné ,1764 for birds, ignoring that he had
already used this name a few years ago in Lepidoptera. Noctua Linné,
1764 (Aves) is a junior homonym of Noctua Linnæus, 1758 (Lepidoptera).
Garsault (1764) used Noctua
for a bird and established a name Noctua caprimulgus Garsault, 1764 (Aves).
Fabricius (1775) established a name Noctua caprimulgus Fabricius, 1775
(Lepidoptera), thus creating a double homonym. Double homonymy is no homonymy,
both names are available.
6.
Principle of
Typification
This is the
principle that each nominal taxon in the family group, genus group, or species
group has—actually or potentially—a name-bearing type fixed that provides the
objective standard of reference that determines what the name applies to. This
means that any named taxon
has a name-bearing type, which allows the objective
application of that name. Any family-group name must have a type genus, any
genus-group name must have a type species, and any species-group name can (not
must) have one or more type specimens (holotype, lectotype, neotype, syntypes,
or others), usually deposited in a museum collection. The type genus
for a family-group name is simply the genus that provided the stem to
which was added the ending "-idae" (for families). Example: The
family name Spheniscidae has as its type genus the genus Spheniscus
Brisson, 1760.