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Evidence for Evolution in Living Things
Sometimes a discussion of evolution needs to consider how evolution does NOT happen.
A question often posed by folks who do not understand evolutionary mechanisms is:
"If humans evolved from monkeys, then why are there still monkeys?"
All living things give birth to things that are--more or less--similar to themselves.
- Mushrooms don't evolve into pine trees.
- Fish don't evolve into amphibians.
- Frogs don't evolve into reptiles.
- Reptiles don't evolve into birds.
- Birds don't evolve into mammals.
- Monkeys don't evolve into humans.
Evolution proceeds by changes to the traits already present in a population.
It is a process of gradual "remodeling" or "tinkering" with a body plan by
- random events (mutations, genetic drift)
- non-random forces (natural selection)
HUMANS DID NOT EVOLVE FROM MONKEYS
- Today's monkeys are descended from successful ancestors that had monkey-like traits.
- Today's humans are descended from successful ancestors that had human-like traits.
- But humans and monkeys do share many traits in common.
- Somewhere back in time, humans and monkeys share a common ancestor.
- That ancestor was not a monkey, nor was it a human.
- But that ancestor had traits still shared by today's monkeys and humans.
- forward-directed eyes/binocular vision
- shoulders/pectoral anatomy adapted for tree climbing/swinging
- opposable thumbs
- trichromatic color vision
- dry noses (!)
- primarily diurnal
- single pair of nipples (fewer babies per cycle)
- Go back far enough, and all species share a common ancestor.
- We determine species' shared ancestry by comparing shared traits.
Evidence for Evolution: Homology
Recall our four lines of evidence:
- distribution in space and time
- recent examples of the evolutionary process
What observable evidence exists that life on earth has
evolved its diversity through a long process of descent with modification?
One of Darwin's most profound realizations was that all species on earth are related to each other, somewhat like the people on a family tree.
He noticed that structures that superficially looked very different had underlying similarities that could be traced all the way back to embryos.
These similarities are known as HOMOLOGIES.
In biology, a homology is a character (trait) shared by two species (or other
taxa) because of common ancestry. Species descended from a common ancestor show...
- morphological homology - similar anatomy
- ontogenetic homology - similar embryo development
- molecular homology - similar DNA, RNA and proteins
Homology vs. Analogy
A homologous character found in two different species
- may or may not perform the same function in those two species
- but is inherited from the common ancestor of those two species
A classic example of homology is seen in the skeletal components of vertebrates.
(photo link will help!)
Evolution can be considered a process of "remodeling" a population over
the course of many generations, with one of the main the driving forces being the natural
selection factors that favor one form over another in specific
Scientists have several criteria for determining whether structures are homologous (<--required link).
Let's have a look at some of our own homologies in The Zoo that is YOU.
Of course, structures can look the same for reasons other than common ancestry.(<--required link)
An analogous structure found in two different species
- serves the same function in two species
- but is not derived from a common ancestral structure
Example: The wing of a butterfly and the wing of a bat.
How do we know the common ancestor of the butterfly and the bat didn't pass on the wings to both species?
You'll have to trust me on this for now, but the most recent common ancestor of a butterfly and a bat existed billions of years ago,
and probably looked something like this:
(A microscopic organism called a gastrula.)
No eyes. No head. No wings.
The wings of the butterfly and the wings of the bat evolved independently, long after their ancestral lineages diverged.
More examples of analogous structures:
- wings of various species (though the bones supporting the wing may be homologous, in some cases)
- walking limbs of insects and vertebrates
- cranium of vertebrates and exoskeleton head of insects
- fusiform shape of fish and cetaceans (whales and dolphins)
Just as physical similarities among siblings provide evidence that they are related to each other,
physical similarities between species tell us that they are related.
Just as humans change across generations, so, too, do natural populations of other species.
By comparing the traits of species that are both closely and distantly related to each other, we can begin to understand their evolutionary history.
Primitive and Derived Characters
We can learn about the process of evolution from studies of both extant and extinct species, and comparing their physical (and sometimes chemical) characteristics. In any organism you're studying...
A primitive character is one that is relatively unchanged from its
original, ancestral form.
Example: All vertebrates have a bony tail posterior to the anus.
It was present in the ancestral vertebrate, and though different vertebrates have evolved differently shaped tails, they are all evolved from the common ancestral tail. The PRESENCE of the tail is primitive.
A derived character is one that is relatively modified from its
original, ancestral form.
Example: the modified shapes of various vertebrate tails. An extreme example: The loss of the external tail in great apes (including humans). The baboon shows the primitive condition, whereas the gorilla and Very Handsome Man show the derived condition of the post-anal tail.
We great apes still have a tail; it's just very reduced and tucked where it doesn't show.
Though there is the occasional atavistic reminder of our ancestry.
Note that these are relative terms.
You can't call something
"primitive" or "derived" without comparing it to something else.
In other words, when you invoke the "primitive" or "derived" state of a particular character,
you are examining homology between two or more species derived from a common ancestor.
Stephen Jay Gould first coined the term exaptation:
the utilization of a structure or feature for a function other than that for which it was developed through natural selection.
Vestigial Characters: Derived and Mostly Non-functional
A vestigial structure is one that has marginal, if any use to the
organism in which it occurs.
These are some of the most interesting examples of homology.
Why do structures become vestigial? Is it, as Lamarck suggested, a case of "use vs. disuse"?
Modern hypothesis: It's all about natural selection.
The study of embryo development is one of the hottest areas in evolutionary biology, the study of the evolution of development (a.k.a., evo-devo).
ontogeny is the process of embryo development (from the Greek
onto, meaning "being" or "existence" and genesis meaning
phylogeny is a species' evolutionary history (from the Greek
phyla, meaning "tribe" and genesis meaning "origin")
Ernst von Haeckel was the first to notice that the embryonic forms of different species had amazing physical similarities.
He thought this similarity was because every living thing passes through its evolutionary history during its embryo development.
Well, he didn't get it quite right.
Actually, differences among embryos reflect changes in embryo development that sometimes divided one species into two new species.
Consider members our own phylum, Chordata.
These traits are unique to Chordates:
- gill pouches/slits in the pharynx (throat)
- a tail posterior to the anus
- dorsal, hollow nerve cord
- muscles arranged in bundles (sarcomeres)
- cartilaginous notochord (support rod along the back)
In the most primitive chordates (Cephalochordata, the
lancelets), the notochord is present not only in the larva, but also the adult:
In sea squirts (Urochordata), the
notochord present in the larva, but lost in the adult:
And in US (Vertebrata), the notochord
is seen only in early embryos, and is completely lost shortly thereafter.
As each group of chordates (Cephalochordates, Urochordates, Vertebrates) evolved, the notochord took on its own character state in each lineage.
All members in any one of the three lineages show the same state of the notochord, so it is most likely that...
- the ancestor of all lancelets had a notochord as an adult
- the ancestor of all sea squirts had a notochord as a larva, but not as an adult
- the ancestor of all vertebrates had a notochord only as an early embryo
Changes in Embro Development --> Changes in Adult Form
Recall the film you saw in the last class session...
Is the lancelet a "baby form" tunicate? Or is the tunicate a more complicated lancelet?
In embryo development,Timing is Everything! (<--required link)
Bottom line: More complex adult forms are the result of changes in embryo development. These can be
Changes in the timing of ontogenetic events (between related species) is known as heterochrony (<--required link).
- changes at various stages of development
- reduction of certain embryonic traits
- addition of greater complexity towards the end of development
The deepest level of homology: The molecules that carry the instructions for making and running our bodies, encoded in our DNA.
DNA stands for Deoxyribonucleic Acid, and we'll look at it a bit closer later.A (adenine), C (cytosine), G (guanine) and T (thymine)
For now, know that the language consists of only four "letters":
The four letters can be arranged in infinite combinations; DNA is millions and millions of "letters" long. And each slight variation changes the instructions.
The evolutionary history of a species can be seen in its DNA sequences. The more closely related two species are, the more similar their DNA sequences.
The very existence in of DNA every living thing on earth is, in itself, strong evidence of common ancestry. It would be highly unlikely for DNA to have evolved independently so many times, over and over.
We can see common ancestry in the chromosomes, themselves:
Molecular homologies also can be seen at the level of the finished protein
product encoded by the DNA, as shown in this comparison of amino acid sequence in
hemoglobin (the protein in your red blood cells that carries oxygen) of different species:
The study of molecular homology is the Evolutionary Biology of the Future.
And the future is now.
Next...Homologies as Tools for Re-creating Evolutionary Histories.