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THEORIES OF EVOLUTION

Traditional, classical view: GRADUALISM - Large changes (reproductive isolation and morphological differentiation) occur due to the gradual accumulation of many genetic changes.

New hypothesis was put forth in 1972 by N. Eldredge and Stephen J. Gould: PUNCTUATED EQULIBRIUM.
They suggested that major changes occur very suddenly, and are "punctuated" by periods of relatively little change. (examples include polyploidy in plants and Founder Effect in various species). (NOTE: "very suddenly" is a relative term, geologically speaking. This can mean over thousands of generations instead of over millions!)
Eldredge and Gould suggest that this could explain how "awkward" intermediate forms such as the reptile-->flying bird and the terrestrial tetrapod-->swimming cetacean could have been "skipped".

Speciation is a temporal process. Populations exist in various stages of this process at any given time, and present day populations are even now undergoing microevolutionary processes which may eventually give rise to macroevolution.

SOCIOBIOLOGY: What's Genetics Got to Do with It?

You all owe yourselves the enrichment of reading E. O. Wilson's works. In 1975 he published Sociobiology: The New Synthesis. Now E.O. is nothing if not controversial. And this book has been the center of a great deal of controversy that has spread from biology to many other disciplines, including those in the Humanities and the Social Sciences.

What could cause such a ruckus? E.O. Wilson has said the unthinkable! The politically incorrect. To wit,

SOCIAL BEHAVIOR IS UNDER GENETIC CONTROL.

Now it's amazing that although the book consists of 26 chapters, the only one that makes people blink is the one that applies the theory to Our Favorite Ape: Homo sapiens.

Back in 1962, V.C. Wynne-Edwards published Animal Dispersion in Relation to Social Behavior, in which he suggested that animals regulated their own population density via behavior called ALTRUISM.

Altruism - risking the loss of fitness in an act that could improve the fitness of another individual.

Example: Under crowded conditions, many animals (birds, mammals, etc.) cease to reproduce. Wynne-Edwards interpreted this behavior as altruism: as "good for the species." He suggested a term called "GROUP SELECTION," saying that groups in which individuals exhibited altruistic behaviors that improved the survival of some of its members would have a survival advantage over groups that did NOT have altruistic members.

This, of course, is hogwash. An individual that gave up its own fitness, martyring itself to the group, would be selected against. And if that "altruistic behavior" were genetically based, it, too, would be a HUGE selective disadvantage to have.

So how do we explain such phenomena as alarm calls (which call attention to the caller and allow its conspecifics to silently escape) or sterile worker hymenoptera (which never reproduce, but live their entire lives supporting the queen who gave birth to them and who continues to produce more sterile workers)?

W.D. Hamilton was the first (in 1964) to develop ideas that explained apparently altruistic acts without resorting to the illogical "group selection" idea. Perhaps his most profound concept was that natural selection would favor an allele that promoted altruistic behavior toward relatives, since relatives share the alleles of the altruistic organism. By being altruistic to a relative, you are actually promoting some of your alleles' being passed on to future generations.

INCLUSIVE FITNESS, INDIVIDUAL FITNESS AND KIN SELECTION, OH MY.

We already know that the FITNESS of a particular phenotype/genotype is its reproductive contribution to subsequent generations relative to an alternative phenotype/genotype.

An individual's inclusive fitness may be comprised more of individual fitness or kin selection, depending on the species natural history/behavior.

The degree to which each of these two factors contributes to inclusive fitness depends to a great degree on whether a species is SOLITARY or SOCIAL.

Why is kin selection not altruism?

An example...Marmosets. Tiny, New World monkeys who live in social groups consisting of...

  • Group living is critical to the survival of these monkeys.
  • Queen supresses ovulation in her daughters by behavioral bullying/stress.
  • Aunts help rear their siblings.
  • What is the use to the aunt monkeys? (Of course, the monkeys don't know this! The idea is that genes that foster kin selection promote their own passing on to future generations simply by fostering the 50% likelihood that they'll be passed along in any given individual.)

    Why not take the chance to contribute all of your genes to future generations (In the form of multiple offspring, as the queen does)?

    Honeybees: A Social Hymenopteran Insect with HAPLODIPLOID populations.

    The kin selection advantage is even greater in this case.

  • Females (queen and workers) are diploid.

  • Males (drones) are haploid. (produce sperm via mitosis)

  • So gametes produced by the queen share 50% of her genes.

  • Gametes produced by a drone contain 100% of his genes.

  • Each female bee gets 50% of her alleles from dad and 50% from mom (queen).

  • This means that all sister (worker) bees have 100% of their drone parent's genes in common, and 50% of their queen parent's genes (Each gets 50% of queen's; on average, they'll be 50% different each time, due to crossing over.) in common.

  • Thus, sister worker bees share 75% of their alleles! The are more closely related to each other than they would be to their own offspring, who would share only 50% of their alleles.

    It is to each worker's genes' advantage to encode helping behaviors that allow the queen produce more workers (who are 75% genetically the same as the worker), rather than to produce her own offspring (which would be only 50% related to her)!

    The above scenarios make some rather big assumpitons, which are arguable:

  • The behaviors that foster these genetic events are heritable
  • The worker bees all have the same drone father

    Do the monkeys and the bees make this choice consciously? What do YOU think?

    Remember the Bottom Line: Behaviors that are genetically based (and if you believe E.O. Wilson and many others, all animal--including human--behaviors have at least some genetic component at their root) are either

    It's those interesting neutral ones that might some day become one or the other, depending on what happens in the environment of the organisms carrying the genes for that trait.

    The study of the interaction of genes and environment to produce phenotype is

    QUANTITATIVE GENETICS - The study of the interaction of genetic programming ("Nature") and environmental pressures ("Nurture")


    Key Ideas

  • In natural populations, phenotypic variation is usually continuous, not discrete (i.e., variation is better measured quantitatively than qualitatively)

  • Mendelian genetics extremely difficult to apply to such traits. Instead, statistical methods are used to analyze the variation, which can be due to such phenomena as variable penetrance and/or expressivity, and to other factors such as epistasis and pleiotropy.

  • The major task of the quantitative geneticist is to determine how genes/environment interact to produce a given trait distribution in a population. A trait produced by both environmental and genetic variables is known as a multifactorial trait.

  • Genetic variation in a continuous character may be due to
      - segregation at a single locus
      - segregation at numerous, interacting loci, causing cumulative effects

  • The estimated ratio of [genetic variation:environmental variation] is not equal to a measure of those things' relative contribution to a phenotype.

  • Important caveat: Such estimates made on a population are good only for the population under study! They cannot be applied across a broader range of populations!

    Traits controlled by multiple loci, each of which contributes equally to the phenotype exhibit

      - continuous (quantitative) variation
      - polygenic (quantitative) inheritance

    One genotype may give rise to several different phenotypes (depending on expressivity, penetrance, etc.), and several different genotypes may produce exactly the same phenotype.

    Usually, continuous traits are affected by many loci (they're polygenic), and so environmental effects on these loci create an even broader array of phenotypes.

    IMPORTANT QUESTIONS ASKED IN QUANTITATIVE GENETICS...

    • To what extent is phenotypic variation predicted by genotype variation?
    • To what degree does this variation reflect environmental influence on phenotype?
    • How many genes control a given trait?
    • To what degree does each of the genes affecting in a trait contribute to it?
    • How, and to what degree do the loci of a polygenic trait interact? (Are they additive?)
    • If a particular phenotype trait confers a selective advantage, how quickly does the trait change in response to selection? Do other traits change along with it?

    So...environment affects phenotype. How do we know if a phenotype is affected by genotype at all?

    Developmental processes governed by genes lie at the base of every character.

    For example, the morphological structures that make Homo sapiens capable of speech depends on the development of brain, vocal cords, and mouth and tongue structure. These are under genetic control, and few would argue that!

    However, variation in speech (languages) is almost entirely environmental.

    And Cow will never speak, except on Cartoon Network.

    If genes are involved in the development of a trait, then biological relatives should resemble each other in that trait more than non-relatives--but ONLY if relatives are no more likely to share common environments than non relatives!

    A familial trait is one shared by members of a biological family, for whatever reason.

    A heritable trait is one shared by individuals because of shared genotype.

    It is relatively (ha!) simple to determine familiality vs. heritability in controlled populations, but very difficult in wild populations of *any* organism--INCLUDING HUMANS.

    Because human families so often share a similar environment, the distribution of genetic vs. environmental effect on phenotype is often uninterpretable.

    Studies of monozygotic and dizygotic human twins shed some light on the issue, but even these are not entirely without confounding effects.

    Many of the traits in Homo sapiens are politically charged!

    • I.Q.
    • tendency towards alcoholism
    • tendency towards divorce/violent behavior
    • mental disorders
    • sexual orientation

    ...often show familiality.

    BUT CORRELATION DOES NOT IMPLY CAUSE AND EFFECT: NO SIGNIFICANT PREDICTABILITY HAS EVER BEEN SHOWN FOR THESE TRAITS.

    Norm of Reaction studies can be of use in such instances. However, norm of reaction studies show only small differences among naturally occurring genotypes, and those differences are not consistent over a wide range of environments. This means that "superior" genotypes in agricultural organisms (and heck, maybe the rest of us, too...) are "superior" ONLY IN CERTAIN ENVIRONMENTS.

    The Take Home Message: If human mental/emotional functions turn out to be under genetic influence, that variation is UNLIKELY TO FAVOR ONE GENOTYPE OVER ANOTHER, GIVEN A RANGE OF ENVIRONMENTS, as we already have hinted with the Balancing Model of Evolution by Natural Selection.
    Once again, "superior" is an entirely subjective term. Which traits are "superior" (i.e., adaptive) depends upon their environmental contexts.