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Jul 27 2007

Evolution-15: How species evolve

(Please see here for previous posts in this series.)

The final feature that needs to be addressed is the probability of mutations cumulating to produce new organs and species.

This question lies at the heart of many people’s objections to evolutionary ideas. They cannot envisage how infinitesimal changes, each invisible to the eye, can add up to major changes. That is because they tend to think that the two foundations for this to occur (the occurrence of successful mutations and the mutations then spreading throughout the population) are both highly unlikely, and so that the chance of a whole sequence of such processes occurring must be infinitesimally small.

What this series of posts has shown is that given large population sizes and the long geological times available, not only are those two things not unlikely, they are almost inevitable. The mathematical results discussed in the previous posts have shown two very important results. The first is that when the large sizes of populations and the long times involved are taken into account, the probability of favorable mutations occurring is quite high. The second is that the probability of a single favorable mutation spreading to every organism in the population is also high. These two results undercut the arguments of those who simply throw their hands up in the air and declare that evolution is extremely unlikely to have occurred.

Once that is understood, it is not hard to see how successive mutations that each have a selection advantage can pile one on top of another to produce new species leading to the diverse and complex biological system that we see today. The history of twentieth century biology is the discovery of case after case of the evolutionary history of organisms. Richard Dawkins’s book Climbing Mount Improbable describes many such cases.

In order to understand how speciation comes about, we need to understand better what constitutes different species. As pointed out by biologist Steve Jones, “Species are divided from each other in many different ways – by space, by time, by mating preference, by the inability to fertilize an egg or produce healthy young, or by the sterility of the offspring.” (Almost Like a Whale, p. 231)

The most famous historical example is that of the finches of the Galapagos Islands. During his five year trip on the Beagle Darwin had periodically shipped various specimens back to England to specialists to study them, and a respected ornithologist John Gould had agreed to study the birds. Soon after his return to England in 1836, Darwin met with Gould who gave him the surprising news that birds that Darwin had thought were different species like wrens and finches and orioles were actually all different kinds of finches. Furthermore, Gould had also found among Darwin’s specimens three mockingbird species, each set of which came from a different island in the Galapagos. (The Reluctant Mr. Darwin, David Quammen, (2006), p. 24)

This fueled Darwin’s suspicions that these birds had evolved from a single species that had somehow made the difficult journey to the Galapagos from the South American mainland, and after spreading out to each island had then become isolated from the others and evolved into separate species under the different selection pressures they experienced on the different islands.

Darwin decided to devote himself to the study of speciation and in order to do so, started with the breeding of animals in captivity. He extensively studied another bird (the pigeon) that was highly popular among breeders in England at that time and for which there existed a huge number of breeds. By studying pigeons (the first chapter of On the Origin of Species deals exhaustively with them), Darwin satisfied himself that all the species of pigeons were descended from a single type, the rock pigeon. “Great as the differences are between the breeds of pigeons, I am fully convinced that the common opinion of naturalists is correct, namely, that all have descended from the rock-pigeon (Columba livia).” (On the Origin of Species, Charles Darwin, (1859), p. 23)

In an interesting follow up to the Darwin finch story, Rosemary and Peter Grant spent twenty years in the Galapagos studying the finches and they actually observed the evolution that Darwin could only speculate about. For example, Darwin had noticed that the finches’ beaks “resembled different kinds of pliers: heavy duty lineman’s pliers, high-leverage diagonal pliers, straight needle-nose pliers, curved needle-nose pliers, and so on. Darwin eventually reasoned that one kind of bird was blown to the islands and then differentiated into thirteen species because of the demands of different ways of life on different parts of the island, such as stripping bark from trees to get at insects, probing cactus flowers, or cracking tough seeds.” (How the Mind Works, Steven Pinker (1997), p. 163)

Darwin felt that such changes would occur too slowly to be observed. But what the Grants did 150 years later was to show that such changes could in fact be observed in real time. They “painstakingly measured the size and toughness of the seeds in different parts of the Galapagos at different times of the year, the length of the finches’ beaks, the time they took to crack the seeds, the numbers and ages of the finches in different parts of the islands, and so on – every variable relevant to natural selection. Their measurements showed the beaks evolving to track changes in the availability of different kinds of seeds, a frame-by-frame analysis of a movie that Darwin could only imagine.” (Pinker, p. 163)

Evolution is a fact. Natural selection is far and away the only theory that comes even close to explaining in detail how it happened, although will always be many things as yet unexplained. As biologist Steve Jones writes, “Although biologists still argue about how the process works, fossils make it impossible for anyone, biologist or not, to deny that it happened.” (Almost like a Whale, 1999, p. 306)

Next in the series: The evolution of the eye

POST SCRIPT: The Darwin Year 2008-2009

This series of posts was inspired by the fact that 2009 will be the 200th birth anniversary of Charles Darwin and the 150th anniversary of the publication of On the Origin of Species. Case is planning a year long celebration of this event for the 2008-2009 academic year.

As part of this, the Common Book Reading committee has selected David Quammen’s The Reluctant Mr. Darwin as the book that all incoming first-year students in fall 2008 read, and will also try to expand the program to make it a campus-wide reading experience.

Quammen has agreed to be the fall convocation speaker in September 2008. There are also plans to have E. O. Wilson (one of the most eminent evolutionary biologists and author of Sociobiology), Sean B. Carroll (The Making of the Fittest), and Rosemary and Peter Grant also give talks during that year.

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