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2.3: "Forces" of Evolution

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  • One of the elegant things about evolutionary theory is it can describe phenomena on both the small scale and the large scale. We can use these same forces to explain microevolution ­– the change of an allele frequency of a population of the same species from one generation to the next; and we can use them to explain macroevolution ­– the change of one species into another species over long periods of time. This is similar to the way the theory of gravity can be used to describe the motion of molecular particles or large galaxies.

    We use the word "force" to refer to a process that drives change, but thinking about evolution as a set of forces can be dangerous because it's easy to fall into the trap of thinking of evolution as a directional agent, pushing organisms towards an ultimate goal.


    Mutation is the prime mover, the creator of all new alleles. We'll learn more about how mutations happen in the section on cellular biology.

    Exercise \(\PageIndex{1}\)

    Review the last section on Darwin.

    Read Dennis O'Neil's description of Natural Selection with an Emphasis on Population Genetics.


    • Article on carcinogenic traditional medicine, notice the kinds of mutations this plant causes


    Migration (also called "gene flow") is where someone physically moves alleles from one population to another. When people move from one population to another they pack all 23 pairs of chromosomes inside the nuclei of their cells, and bring it all with them. If this changes the allele frequency of either population then it is by definition a kind of evolution. It can bring new alleles to a population that hadn't had that mutation recently, or just bring or remove a significant quantity of a certain allele to change the frequency of either population.

    The individual doesn't actually have to migrate to the new population, they can just leave or pick up a few alleles. The stereotype that sailors have kids in every port, is probably better represented by today by the traveling businessman, soldier on leave, sexual tourist, or the sex trafficked. Migration is the geographic movement of alleles from one population to another. We'll come back to migration in future sections as an important point in understanding human origins and human variation. It is why there is only one species of hominid on the planet today.


    Article on coywolves and other hybrids which can be understood as a kind of gene flow and loosening of the species concept.

    Genetic Drift

    Random genetic drift, or genetic drift, is about statistics. The "drift" part has nothing to do with geographical movement (that would be migration/gene flow), what drifts is the allele frequency, like when you look at a graph of a complex system changing over time, and from a distance it looks like a straight line, but as you zoom in, the line becomes jagged, jumping up and down; the smaller your field of view, the more drastic the changes become.

    A good way to understand genetic drift is to plan two trips to Viejas Casino, the first with $1,000,000,000 and the second with $100. Sit down at the cheapest table or slot machine you can find and start playing. For your first trip, your money will go up a little ($1,000,000,135) and down a little (down a little more because the House sets the odds $999,999,564) but after a few hours, you'll get bored and go home with around $1,000,000,000. Ok now go back with $100, your money will go up a little ($135) and down a little ( $64), then up a little ($68), then down a little ($24) then up a little ($26) then down a little ($4) then up a little ($6) then down... whoops! no more money ($0), time to go home broke. The analogy here has to do with population size and alleles. Every generation alleles are shuffled and with a huge population statistically the allele frequency will stay pretty much the same, but with a small population, the random fluctuations are more drastic, and allele frequencies can drop to zero. If an allele frequency drops to zero, the game's over, and it's gone from the gene pool.

    Here's a statistics exercise called the Gambler's Fallacy that also shows the difference between flipping a few coins and flipping a thousand coins.

    A bottleneck is where the population shrinks to the point where lots of alleles drop out like this. The founder's effect is where a small group of people move to a new area and start a new population. The new population may grow quickly, but even though the number of people grows, if there is no other force of evolution, the allele frequencies of the new population is determined by the small number of founders who might happen to not represent the population they left. There's no way a small number of people can represent the diversity of a large population. In statistics this is known as sampling error. When comparing the old and new population, they have different allele frequencies, so by definition, evolution has occurred, and we attribute this kind of evolution to genetic drift.

    Exercise \(\PageIndex{2}\)

    • Read Dennis O'Neil's Small Population Size Effects
    • Read Stanford's Encyclopedia of Philosophy: Random Drift

    The "drift" in genetic drift comes from statistics, called stochastic drift. Stochastic just means "random", so stochastic drift is a fancy way to say that random stuff tends to happen with a small sample size.

    Vocabulary For 2.3

    • founder's effect
    • gene flow
    • genetic drift
    • migration
    • mutation
    • stochastic drift