Saturday, May 14, 2011

WEIT: A Force to Be Reckoned With

Chapter 5 of WEIT is an overview of the force that has shaped all of life on earth: natural selection. When the less savvy creationists say they don't think we could have evolved purely by chance, they reveal a fundamental misunderstanding. Evolution contains an element of randomness (mutations), but natural selection is not random—and in fact, that's the very reason evolution works. (Coyne also briefly discusses genetic drift, a process that occurs more noticeably in small populations, which is in fact random.)

Coyne first lists the three components required for natural selection to take place: a trait must vary across individuals, it must be heritable, and it must affect the probability of reproduction. We can also make predictions based on this idea. As a general rule, natural selection predicts that a trait will:
  1. Evolve only by step-by-step processes.
  2. Raise the fitness of its possessor at each step.
  3. Increase reproductive output, but not necessarily survival.
  4. Never solely benefit members of a different species.
  5. Raise the fitness of the individual, but not necessarily some larger group.
  6. Be well-designed, but not necessarily optimally designed.
  7. Evolve regardless of any resulting suffering, however extreme.
We see these predictions fulfilled at every turn. Take #4, for instance: Coyne says that at first glance, the hollow thorns and nectar of certain acacia trees appear perfectly designed to house and feed ant colonies without benefit to the trees themselves. Could it be that nature isn't so impartial and pragmatic after all? Not so: having the ants around is useful because they attack animals that eat the trees' leaves and cut down rival seedlings.

Note also that most of these predictions (particularly the last two) are not necessarily what we would expect from a good and competent God. One example Coyne gives of cruelty in nature are the Asian giant hornets, which will invade a hive of bees and bite off their heads by the thousands. Japanese honeybees will respond by mobbing the hornets and cooking them alive through vibration. However, European honeybees are defenseless against them, having only been recently introduced.

Next Coyne moves on to observed evidence of natural selection and speciation, starting with bacteria and viruses. In Richard Lenski's famous experiment, E. coli mutated to grow 70% faster in conditions with varying amounts of available nutrients and gained the ability to eat citrate. In another experiment, E. coli evolved a new biochemical system for breaking down lactose in a series of three separate mutations. In a third, the bacteria strain Pseudomonas fluorescens formed two additional new species within just ten days. He also mentions drug resistance: for instance, after just 70 years, "more than 95 percent of staph strains are resistant to penicillin."

Finally, he goes over some examples of natural selection observed in plants and animals. A study of Galapagos finches found that during a drought which produced mainly hard-to-crack seeds, average beak size increased by 10 percent—astonishingly, all in a single generation. The length of the soapberry bug's proboscis (needed to penetrate fruit skins) changed by 25 percent within a few decades of colonizing three new plant species. And the wild mustard plant began blooming one week earlier in response to a five-year drought.

Coyne has a section on irreducible complexity as well, but I plan to cover that topic in more detail sometime in the future. Next up is a full chapter on sexual selection.

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