- It is because of the concept of species changing?
- Or, changing into a new species?
- Can a species undergo significant changes?
Few people still doubt it based on micro-evolution examples such as resistance to penicillin and pesticides developed by bacteria and insects respectively.
However, what about new species?
- Doubling, tripling genomes (see the example with Primrose)
It is becoming harder and harder to argue that evolution is limited to only small changes.
Dr. Rogers used the example of the eye to discuss complex adaptation.
- lens vs. retina
- retina useless without lens
- how would selection favor a partial eye?
Charles Pritchard (1866)
- He was the first one to argue that the eye could not evolve.
Charles Darwin (1872)
- He was the first one to refuse Pritchard's argument. However this argument would last.
Weakness of Pritchard's argument:
- The claim is about plausibility. To refute such claim you need to invent a plausible story.
- There is no need for evidence.
- There is no need for the story to be true.
Hypothetical steps in the evolution of the eye:
A. Eye spot - simple eye, but very sensitive to light from all directions
B. Eye cup - light prevented from extreme angles.
C1. Pin-hole camera eye - forms image, not much light, very dim.
C2. Primitive lens - secrete mucus (easy to produce), denser than H2O, will refract light, only sensitive to direct light found straight ahead of the eye.
Next: improvement of the eye --> Need a vertebrate eye.
Are the steps plausible?
- Yes, they are all found in living organisms today.
- Eye evolution is possible. Pritchard was incorrect.
How did the eye evolve?
- Darwin's prediction: retina evolved earlier then came the lenses --> we can test this hypothesis by looking at living animals.
- We all share common characteristics because we have inherited genes from common ancestors.
We need to start at the basis, the protein level. Opsins are light sensitive proteins. Every animal that sees does so because of the presence of opsins. Similar species have similar opsins.
When cells divide, DNA duplicates. Sometimes the machinery stutter and the DNA is duplicated twice. When you have two copies of the same gene, the new copy may provide new functionality.
We need to observe what humans have in common with their closer relatives, the apes and old world monkeys. We have one opsin that adapted to dim light and three for color vision. Most mammals only have two for color and this is why they are colorblind. The reason humans, apes and monkeys have three for color is due to the fact that our common ancestor experienced a duplication in the DNA sequences thus resulting in one extra copy for that particularly opsin.
Evidence of common descent in opsin: closely related species have similar opsin molecules. Humans have similar opsin proteins to insects and cephalopods.
Crystallins: transparent proteins used in lens and cornea.
- If lenses evolved early, humans and insects should have similar crystallins. But they don't because the lens came later. Insects and cephalopods have very different crystallin proteins from humans.
What about eye morphology?
Vertebrates have eyes that work like cameras. All arthropods have compound eyes (including trilobites). Snails however have a great variety of eyes (they evolved in different varieties). Heteropod sea snail has eyes like slits, with a field vision of 180 degree wide, but just few degrees high. Eye scans are done with rapid movements up and down.