Lecture #27 Eye evolution 5/7/13 With acknowledgements to Dr Richard Payne
The end is in sight • Wiki’s due Thursday at midnight • Final exam on Thurs 5/16 1:30-3:30 pm • 3 questions - pick 3 of 5 • 10 Q are posted on web • 1 question - pick one of 4 student Q • Your question is due tonight and study Q will be • posted by tomorrow • 1 question - last two lectures (today and Thurs)
Charles Darwin 1809-1882 • 5th youngest of 6 children • Father Robert Waring Darwin was a physician • Mother, Susannah Wedgewood, died when Charles was 8 • Father sent him to Edinburgh to study medicine • Couldn’t bear sight of blood - gave up on medicine • Developed naturalist interests • Enrolled in Christ’s College, Cambridge to study biology, geology
Voyage of HMS Beagle: Dec 1831 - Oct 1836 • Professor Henslow got him position as naturalist and companion to captain • Father objected but uncle interceded • Collected many specimens • Interpreted what he saw in terms of Charles Lyell • Small gradual changes over long times could make immense changes
The Origin of Species by Charles Darwin Darwin wrote a treatise to explain his theory of natural selection and how that could lead to new species.
Mystery of mysteries - where do new species come from? Cichlid fishes of Lake Malawi
Selection leads to evolution of different way to feed to fill different niches
Mechanisms of evolution • Natural selection • Changes that result because of increased fitness, survivability • Sexual selection • Changes that result because of increased mating success • Genetic drift • Random effects because genes sampled at random from small population
Difficulties of the Theory • Tried to address arguments against his theory • Acknowledge them • Come up with counter arguments
Darwin’s theory of natural selection • Why is the eye a problem? • What is Darwin’s solution? • What is his evidence?
“Organs of extreme perfection” – the problem “To suppose that the eye with all its inimitable contrivances - for adjusting the focus to different distances, - for admitting different amounts of light, and - for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree”
The solution • Reason tells me, that - if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; - if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and - if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection … should not be considered as subversive of the theory.
How to gather evidence In searching for the gradations through which an organ in any species has been perfected, we ought to look exclusively to its lineal progenitors; but this is scarcely ever possible, and we are forced to look to other species and genera of the same group, that is to the collateral descendants from the same parent-form, in order to see what gradations are possible, and for the chance of some gradations having been transmitted in an unaltered or little altered condition. But the state of the same organ in distinct classes may incidentally throw light on the steps by which it has been perfected.
To understand how something evolves, would like to follow ancestral path
Some of data • ..aggregates of pigment-cells, apparently serving as organs of vision, without any nerves..Eyes of the above simple nature are not capable of distinct vision, and serve only to distinguish light from darkness. • The simplest organ which can be called an eye consists of an optic nerve, surrounded by pigment-cells and covered by translucent skin. • In starfishes, small depressions in the layer of pigment which surround the nerve are filled with transparent gelatinous matter..to concentrate luminous rays • With insects it is now known that numerous facets on the cornea of their great compound eyes form true lenses
Conclusion When we reflect on these facts, here given much too briefly, with respect to the wide, diversified, and graduated range of structure in the eyes of the lower animals; and when we bear in mind how small the number of all living forms must be in comparison with those which have become extinct, the difficulty ceases to be very great in believing that natural selection may have converted the simple apparatus of an optic nerve, coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the Articulate class.
He who will go thus far, ought not to hesitate to go one step further, if he finds on finishing this volume that large bodies of facts, otherwise inexplicable, can be explained by the theory of modification through natural selection; he ought to admit that a structure even as perfect as an eagle's eye might thus be formed, although in this case he does not know the transitional states.
What are some key questions regarding eye evolution? • What order to parts evolve? • Which kinds of eyes evolved first • How did parts start and then change with time • Molecular basis of these changes causative mutations • Number and complexity of layers • What was environment? Water or land? • How many times did it happen? More than once? • How long it took between changes • How many steps does it take to make part more efficient • Were parts ever lost • When color vision • Predicting future eye evolution • Trial and error of gains and losses
Questions in eye evolution • What was the proto-eye? • How many times have eyes evolved? • How did sub-components evolve? • Photoreceptors • Neural cells • Lenses • What is the evolutionary history of the vertebrate eye?
Diversity of eyes • 33 animal phyla • 1/3 have no specialized light detector • 1/3 have light sensitive organs • 1/3 have eyes
6 phyla have image forming eyes Cnidaria - coral, sea anemone, jelly fish Mollusca - snails, muscles, squid Annelida - worms, leeches Onychophora - velvet worms Arthropoda Chordates
Porifera Ctenophora ANCESTRALCOLONIALFLAGELLATE Metazoa Cnidaria Eumetazoa Acoela Echinodermata Figure 32.11 Chordata Deuterostomia Bilateria Platyhelminthes Rotifera Evolution of eyes Ectoprocta Lophotrochozoa Brachiopoda Mollusca Annelida Nematoda Ecdysozoa Arthropoda
Visual system constraints • Energy to make and operate an eye • Make space for eye • Neural system to interpret • Sensitive conditions of night and day • Environmental – amt light, wavelengths • Life history traits – visual tasks • Pressure / temperature • Shape / ability to focus • Form transparent cells • Maintain eye (eyelids / tears)
What unique parts are needed to make an eye? • Photoreceptor cells • Optic nerves – get message to brain • “retina” = neural processing • Pupil – control light levels • Lens • Cornea • Vasculature to support it
How easy is it to make an eye? • How long does it take to evolve an eye? • Start with light sensitive patch on the skin - photoreception already exists • Assume can change 1% in some dimension each generation • Small change
How easy is it to make an eye? Takes 1829 steps: if a step is only 1 yr = 1829 years However, 1% change / year is actually pretty big
If only 0.005% per step then it would take <400,000 generations Protective layer Receptors Pigment cells Fig 1.6
Are there any flaws to this argument? • Selection acts on lots of things • Selection may not be constant (stagnant) • May be some trade offs • Are you really stuck on these peaks • Deux ex machina - photoreceptors and lens appear
How many times have eyes evolved? • What kind of data can we use to decide this? • Rods and cones evolved at diff times • Conservation of genes • Organization of retina • Loss of eyes – mechanisms shared? • Compare
Have eyes evolved once or many times? • Ciliary and microvillar photoreceptors Salmander Drosophila Human
Have eyes evolved once or many times? • Source of eye tissue during development • Epidermal epithelium • vs • Neural epithelium (brain)
Have eyes evolved once or many times? • Photoreceptor orientation
Have eyes evolved once or many times? Phototransduction