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哲學功課: Does natural selection explain why you and I have opposable thumbs?


Does natural selection explain why you and I have opposable thumbs?

In the debate between negative and positive natural selection, Sober argues that natural selection does not contribute to the existence of individual traits within the population. Neander argues that the cumulative natural selection process shapes the genetic properties of the population. Nanay backs up Neander’s claim and argues both arguments do not take limitation of environment resources in the account. In this essay, I will further enhance Nanya’s argument by stressing how direct competition over resource among individuals with different traits can be affected the process natural selection. I will present an argument that cumulative selection process could decisively alter the genetic trait of the population in the presence of direct competition; thus explain the why you and I have opposable thumbs.

In [1], Neander summarize Sober’s argument for the negative view of natural selection. Sober argue that natural selection cannot explain why you and I have opposable thumbs, natural selection can only explain why all our distance cousins without opposable thumbs are eliminated by natural selection. The negative view states that natural selection cannot create new traits; it can only destroy traits that are not fit for survival. The traits of individual either come from inheritance or random mutation. Natural selection did not create the tree of life: it just determines which branches were removed and which remained. [1, p68]. Sober illustrate the negative view of natural selection using an example of a class of third grade students who can read and write at the level of third grade. Every student in the third grade class can read and write at the third grade level because that is exactly the selection criteria of the class. No student who cannot read or write at third grade level is selected into the class in the first place. Therefore, merely the fact that the student is in the third grade class does not explain how each student acquires his reading and writing skill.

In [4], Walsh makes a distinction between two types of explanations national selection, the wide scope and narrow scope. The wide scope explanation explains the frequency of traits within a population is biased by cumulative selection. The narrow scope explanation explains why a given individual possesses a particular trait. Walsh agrees with Neander that natural selection is two-stage process by taking into account of the reproduction factor. In the first stage, the new traits from random mutation are selected to pass down to the offspring. The second stage then has a biased population base to start the next iteration of reproduction. Traits that affect reproductive success will cumulatively alter the population make up in the long run. Walsh argues that the two-stage process only address the wide scope explanation by answering how a trait type is arise within a population from the selection advantage of that trait, but it does not answer the narrow scope question that explains how the trait token occurs in each individual of the population.

In [5], Nanay tries to address the narrow scope question by bringing in another factor into the equation, namely the limitation of resource in the environment. Nanay argues if individuals with different traits are compete for the same environment resources and the total population the environment can support has a limit, then elimination of individuals of one trait alter the chances of survival of other individuals with a different trait. The trait is more fit to survive in the competition of scare resource will have a selection advantage. In short, the distribution of different traits within the population is a zero sum game. The adaptation of one trait is in the expense of another mutually exclusive trait that is inferior in term of survival fitness. The elimination of the later trait is the other side of the same coin of the survival of the former trait. Nanay provides an indirect explanation to the narrow scope question by making the claim that individual with a certain trait exists because its parent with the same trait survive and reach reproductive age thanks to other individual with a different trait in the previous generation failed to survive and reproduce.

In this long debate started by Sober and Neander, we can observe a trend that is going on. The negative view camp first posed a question that cannot be answered by natural selection. The positive view camp counter attack by introducing new parameters into the equation of natural selection and claim there is a causal relationship between the survived trait is the selected trait. For example, Neander introduces the reproduction factor and Nanay introduces the limitation of environmental resources. The negative view camp defense by narrowing the definition of the natural selection question and focus on how the the inheritance linage of an individual occurs. I am not satisfy with Nanay’s indirect explanation because there are still rooms for the negative view camp to further narrow down the natural selection question and dodge the bullet. The elimination of trait B does not necessarily imply the inheritance of trait A, given that there are numerous other traits out there that are also completing for survival. I would like to introduce a parameter into the natural selection equation and seal the escape route of the negative camp once for all. The new parameter I would like to introduce is direct competition of reproduction of individual with different traits within the population. Nanay argues that indirect competition of scared resource in environment for survival and reproduction can indirectly explain why how a particular trait arises in an individual. Then the trait contributes to the direct competition of reproduction and survival can directly explain how a particular trait arises in an individual. Let me illustrate what direct competition is with a real world example, the moose horn. Male moose fight using their horn for the right to mate with female moose. Suppose in one generation, there is a random mutation of the moose horn genes and create two different genetic traits. Trait A is the normal horn gene that grows horn with regular strength. Trait B is the super horn gene that grows much stronger horn. The individual with super horn gene will win the fight and able to mate and reproduce offspring. Those offspring with super horn gene will further displace the normal horn gene in the next generation. By applying mathematical models, given that the survival fitness of individuals with the super horn gene is equal to individuals with normal horn gene in all other aspect, the super horn gene will spread over the entire population following the equation geometric series. In the case of direct competition in reproduction, we can answer the narrow scope question decisively. Individual trait token is inherited because the trait token has a reproduction advantage in the population. The other trait token lose the reproduction completion to this trait token in the previous generation. Instead of explaining the natural selection in terms of how the survival of the fitness affect probability distribution of trait token in the population, my explanation explains the inherence linage of individual traits in a winner takes all survival games.

Let me illustrate my explanation further by going back to Sober’s classic analogy of a third grade class. In Nanay indirect explanation, there are limited seats in the third grade class, only students who passed the second grade can be promoted to the third grade. The twist is the mark scheme of the exam uses bell curve to determine who pass and who fail. Those at the bottom of the class that failed the second grade exam are equivalent to those individuals with traits do not survive to reproductive age. It is arguable that the indirect explanation still does not explain how each student in the third grade class acquires their third grade reading and writing ability, rather it is the third grade reading and writing standard is adjusted to meet the ability of last student who barely survive the exam cut off threshold. In my direct competition explanation, it is not a typical third grade class. It is a third grade class in a ninja assassin school. The ninja assassin school does not teach reading or writing, rather it teaches killing and self-defense. The second grade exam is not a pencil and paper test, it is literally a fight for survival. The second grade students are put inside a huge boxing ring and asked to have round robin fights using their best killing or self-defense skill. Those who remain standing at the end of the exam period got promoted to the third grade. It is irrelevant how the students acquire their killing or self-defense skill prior to the exam, because only killing or self-defense skill matter in the selection process.

In my conclusion remarks, I am drawing the significant of opposable thumbs in the direct competition of reproduction and survival. It is evident that without opposable thumbs, human cannot use tools and more important weapons. Imagine there are two tribes of pre-historical human; one tribe developed an opposable thumb while the other did not. For the first few millenniums before human invent stone tools and weapons, the opposable thumb did not give the former tribe neither survival advantage nor disadvantage. The two tribes populate the environment and multiply with roughly the same rate. Yet the balance of power is tipped once the tribe with opposable thumb developed tools and weapons. With the help of tools and weapons, the former tribe is able to genocide the later tribe when they are fighting for the control of the land. Having the opposable thumb and the ability to use weapon is a decisive winning factor in pre-historical warfare. Since the ancestors of our non-existed distant cousin without opposable thumbs are killed by our ancestors who have opposable thumbs, natural selection explains why you and I have opposable thumbs. Survival of the fitness does not only explain the survival of a particular genetic trait in relationship to the environment, it also explains the survival of a particular genetic trait in relationship to other traits. Natural selection is not a marathon that let all the traits develops on its own and then figure out which trait pass the finishing line first. Natural selection is like the Stanley cup play off, it makes different traits play head to head against each other and let the fittest traits be the winner.

1 . Neander (1995) “Pruning the Tree of Life” British Journal for the Philosophy of Science. Vol. 46, pp. 59-80.
2 . Sober (1995) “Natural Selection and Distributive Explanation: A Reply to Neander”, BJPS. Vol. 46, pp.384-397.
3 . Neander (1995) “Explaining Complex Adaptations: A Reply to Sober’s ‘Reply to Neander’”, BJPS. Vol. 46, pp. 583-87.
4 . Walsh (1998) “The Scope of Selection: Sober and Neander on What Natural Selection Explains” Australasian Journal of Philosophy. Vol. 76 (2) pp. 250-64.
5 . Nanay (2005) “Does Cumulative Selection Explain Adaptation?” Philosophy of Science, Vol 72, pp. 1099–1112.