From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Fri Oct 03 2003 - 15:11:13 EDT
>I failed to see any substantial evidence of this massive evolution of intermediate life forms in either the fossil record or in life forms, as we know them today.<
Remember the selectivity of the fossil record. Things have to be preserved to start with, not destroyed by erosion, metamorphism, etc. in between, and then studied by someone in order for us to know about them. If you take something with a very complete fossil record, such as marine microfossils, you can generate a relatively continuous series of transitional forms. Conversely, intensely studied taxa such as humans often have a lot of known transitions. However, most things do not have a continuously preserved, accessible habitat. E.g., we know more about shallow-water marine organisms from times when sea level was high than when it was low. A pattern of gradual change may appear punctuated if you have several small gaps. This problem is exacerbated by the frequent pattern of paleontologists focusing on a particular time and place. They may not realize how similar their species is to something that occurred somewhere else or at some earler time. Another factor is th!
e fact that not everything changes at once. Rather, something new appears in one individual and gradualy spreads, if it is successful. The apparent suddent appearance of a new form may just mean that it developed somewhere else. For example, an archaeologist working in New Guinea could go from a layer containing only stone tools to one containing bottles and cans. However, one working in the near east could observe gradual development of metalworking and glassmaking.
As for modern transitional forms, we cannot see any because we do not know the future. Who knows what odd variation of today may be the root of the next era's new kind? We do see some transitions taking place; for example, some hawthorne flies are evolving into apple flies (involving mutations).
>Gould noted that species have largely not developed along the lines suggested by Darwin. Instead the evidence shows the relative abruptness of new species.<
The original examples cited by Gould and Eldredge actually involved known transitional forms; the difference from the traditional picture of continuous gradual change was the pattern of long intervals of little change punctuated by more rapid change, but the rapid change was observed.
Further study has turned up examples of both gradual and punctuated changes (the one specific study I did had one example of each). It has been suggested that gradual change may take place when the environment is very favorable and the organism is free to vary a bit without difficulty. On the other hand, a more constraining environment makes changes more likely to cause problems. In that case, things are likely to go on a while without changing much. "If it ain't broke, don't fix it!" Only a significant innovation or a change in the environment will spur change in the population. The level of study also plays a role. We can detect continuous shifts in insignificant molecular characters in modern populations, but these would leave no fossil trace.
>Even though Gould was an atheist, it certainly leaves room open for the possibility that God stepped in and changed things.<
Although I don't think it was your intent, this wording suggests that God steps out during the ordinary course of evolution.
>The development of sexual reproduction is an interesting case to ponder. What step-by-step process would produce an egg and sperm, and all the other equipment that goes along with being male and female? Is this irreducibly complex (IC)? What good is half an egg and half a sperm, or half a penis and half a vagina, or a fertilized egg with no uterus or no unbilical cord, or a baby that doesn't know how to suck, or a mother with no mammary glands?<
None of these features are present in all sexual reproduction. The volvocine series, cited by Glenn, provides a good example of a stepwise development of distinctive reproductive cells (more or less like eggs and sperm). These are microscopic, planktonic green algae. If you remember from biology class looking at things that looked like tiny green soccer balls, that's Volvox. However, closely related forms are much simpler. Some are unicellular. Two cells can meet, fuse, mix DNA around, and split again. Some have a few cells associated. These can reproduce asexually or sexually on a single-cell basis. Others have more cells, and cannot just break up to reproduce asexually. Volvox has a hollow ball of cells, and new colonies are formed by specialized reproductive cells that develop into little balls inside the big parent ball.
Sex (exchange of genetic material) provides good opportunities to get novel useful DNA, so developing some form of sex is evolutionarily advantageous in many circumstances. The close association of sex and reproduction is associated with multicellularity. It is not practical for an animal to mix new DNA into all its cells, but it can try incorporating the new DNA into the production of offspring.
There are two main strategies for improving reproductive success: more offspring and better care for the offspring. Sperm minimize the extras and concentrate on maximizing the potential number of offspring. Eggs involve more parental effort to produce, but individual eggs have a much better chance than individual sperm. In either case, specialized reproductive cells can do better than just any spare cell that is handy. Thus, with increased development of specialized sexual reproduction, there is increased differentiation of male and female cells and systems.
Likewise, there is a wide range in development of other structures associated with reproduction. Most birds do without a penis, presumably to minimize weight; it is also lacking in the many organisms that simply release sperm into the water and hope it reaches an egg. However, if there is contact between males and females, then specialized devices for transferring and receiving sperm can be advantageous. Even relatively simple devices provide some advantage, so there is no irreducible complexity there. Also, there is wide range in female adaptations, from simply releasing unfertilized eggs into the water to elaborate structures for protecting the developing young inside various parts of a parent (usually but not always female). The specific transition from egg laying to live young that get nourishment from the female can be seen in sharks and snakes, with some species that simply lay eggs, some that keep the eggs inside the female until they hatch, and some that have eq!
uivalents of umbilical cords. The transition to milk can be seen in monotremes, in which the female secretes a milk-like substance but without a specialized nipple or juvenile sucking ability. A better delivery system has obvious evolutionary advantages, and anything evolving such a system could milk it for all its worth.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487-0345 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa
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