From Biol301

Contents 1 Making Tracks though Animal Space – new Darwin 1.1 5 important questions: 1.2 The Continuous Variable 1.3 Repetition of Trends

Making Tracks though Animal Space – new Darwin

5 important questions:

• 1. Could the human eye have arisen directly from no eye at all in a single step?*

• A:* “It would need a gigantic and vanishingly improbable leap across genetic hyperspace.” So…no.

• 2. Could the human eye have arisen directly from something slightly different from itself, something that we may call x?*

• A:* “If the answer to question 2 for any particular degree of difference is no, all we have to do is repeat the question for a smaller degree of difference.” So…yes. “By interposing a large enough series of Xs, we can derive the human eye from something not slightly different from itself but very different from itself.”

• 3. Is there a continuous series of Xs connecting the modern human eye to a state with no eye at all?*

• A:* “Obviously the available time imposes an upper ceiling on this game, for there can be only one X per generation. In practice the question therefore resolves itself into: ‘Has there been enough time for enough successive generations?’ We can’t give a precise answer to the number of generations that would be necessary. ……the number of generations that separate us from our earliest ancestors is certainly measured in the thousands of millions. Given, say, a hundred million Xs, we should be able to construct a plausible series of tiny gradations linking a human eye to just about anything.” “We have concluded that there is a series of imaginable Xs, each sufficiently similar to its neighbors that it could plausibly turn into one of its neighbors, the whole series linking the human eye back to no eye at all.”

• 4. Considering each member of the series of hypothetical Xs connecting the human eye to no eye at all, is it plausible that every one of them was made available by random mutation of its predecessor?*

• A:* “The smaller the change you postulate, the smaller the difference between X’’ and X’, the more embryologically plausible is the mutation concerned. … Whatever problems may be raised by Question 4, then, we can at least see that the smaller we make the difference between any given X’ and X’’, the smaller will be the problems.”

• 5. Considering each member of the series of Xs connecting the human eye to no eye at all, is it plausible that every one of them worked sufficiently well that it assisted the survival and reproduction of the animals concerned?*

• A:* “An ancient animal with 5 percent of an eye might indeed have used it for something other than sight, but it seems to me at least as likely that it used it for 5 percent vision. And actually I don’t think it is an excellent question. Vision that is 5 percent as good as yours or mine is very much worth having in comparison with no vision at all.” “To put it another way, ancestors of stick insects that did not resemble sticks did not leave descendants.”

• Ex:* Stick insects, page 81, how little had to change on the stick insect to be marginally helpful: page 83, squid eyes page 85, beetles shooting boiling hot chemicals page 86/7,

The Continuous Variable

• it’s all about that fact that continuous variables can be bettered by very little amounts. There’s not big jump, just small advantages for a mutated individual that gives it a better chance.
  • First example talks about surface area of the lungs: page 87 / 88.
  • Jumping -> flying capability: page 89.
  • “The idea of tiny changes cumulating over many steps is an immensely powerful idea, capable of explaining an enormous range of things that would be otherwise inexplicable.” Page 90
• “Wherever we have an X in a real live animal, where X is some organ too complex to have arisen by chance in a single step, then according to the theory of evolution by natural selection it must be ate case that a fraction of an X is better than no X at all; and two fractions of an X must be better than one, and a whole X must be better than nine-tenths of an X.” Page 91.
• C. Darwin said “If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. Page 91
• An example of successive steps gone wrong –kind of.
  • Fish on the sea bottom: some went flat vertically, some horizontally. Page 91-92.
    • “No sensible designer would have conceived such a monstrosity if given a free hand to create a flatfish on a clean drawing board. … But evolution never starts from a clean drawing board. It has to start from what is already there.”
  • Photocells in our eyes point toward the back of our head, not toward the light. This doesn’t make much sense. Page 93.
    • “Any engineer would naturally assume that the photocells would point toward the l9giht, with their wires leading backwards to the brain. He would laugh at any suggestion that the photocells might point away from the light, with their wires departing on the side nearest the light. Yet this is exactly what happens in all vertebrate retinas.”

Repetition of Trends

• “There is no reason why general trends in evolution shouldn’t be reversed.” Page 94
• “Such superficially convergent resemblances are often extremely striking, and I shall devote the rest of the chapter to some of them.” Page 95
• “The basic rational is that, if a design is good enough to evolve once, the same design principle is good enough to evolve twice, from different starting points, in different parts of the animal kingdom.” Page 95
  • Ex 1: echolocation fond in bats, and dolphins. Page 95/96/97
    • “At least two groups of bats then, two groups of birds, toothed whales, and probably several other kinds of mammals to a smaller extent, have all independently converged on the technology of sonar, at some time during the last hundred million years. We have no way of knowing whether any other animals now extinct – pterodactyls perhaps? – also evolved the technology independently.”
  • Ex 2: Electrolocation in fish and sharks, page 98
  • Ex 3: periodical cicadas: page 99 / 100.
  • Ex 4 – x: pages 100 – 109 are dedicated to examples of larger animals that have convergent evolution. Specifically this focuses on old world, and South America and Australia –the three big old real continents. Interesting stuff.