"Well, it is a theory. It is a scientific theory only, and it has in recent years been challenged in the world of science -- that is, not believed in the scientific community to be as infallible as it once was." - Ronald Reagan, 1980
Is it true, or have they naively misinterpreted the methods of science, recent advances, or some combination? Let's see.
Other mechanisms of evolution: Mendel's work on genetics was rediscovered by the scientific community in 1900. When this occurred, biologists began synthesizing the fields of genetics and evolution in a movement called the New Synthesis. In this synthesis, several other evolutionary processes besides natural selection were discovered.
Now, play the genetic drift game.
- A deck of cards sorted into red and black.
- A bag or box.
- Either two or four "evolvers,"
- a "generator of diversity" (g.o.d. henceforth)
- and one or more score keepers.
- Each "evolver" represents an individual in a population (of two or eight). Each has a "genome" consisting of one gene whose alleles are represented by two cards. The alleles may be "black" or "red." The remaining cards are placed on a table in red and black stacks.
- Each "evolver" reproduces twice. To reproduce, pick one of your two card hand at random. Its color will be the genotype of your gamete. To make a gamete take a card of the same color as the one you picked from your hand. from the appropriate stack, and throw it into the bag. Do this twice.
- Die. The evolvers place their cards in a "dead" pile and designate their "offspring" to be new evolvers.
- The g.o.d. shakes the bag, thus mixing the gametes.
- New evolvers (offspring of the old ones) each pick two gamete cards to be their genome.
- Score keepers note the number of red and black "alleles" in each new generation.
- Repeat until all alleles are either black or red. Scorekeepers note how many generations this took.
- Nuclear DNA coalesces into chromosomes. These line up in homologous pair, each of which contain alleles, variants of equivalent genes.
- Crossing over: Chromosomes are fragile and tend to break, however the repair mechanisms of the cell relentlessly repair them. When homologous pair are lined up, however, they tend to become intertwined and break. The cell may then inadvertently mismatch the parts during repair. The result is that the information in homologous chromosome pairs is constantly being recombined.
- Meiosis: Normal body cells are diploid - that is, they contain two a full compliment of chromosomes and two alleles of every gene. Gametes are haploid. They contain half the normal compliment and only one allele of each gene. This happens because during meiosis, homologous chromosome pairs are separated into separate gametes.
Thus, genetic information gets shuffled during both meiosis and crossing over. Note, however that:
- During crossing over, genes that are close to one another on the same chromosome are more likely to travel together than ones that are far apart.
- During meiosis, genes on the same chromosome are more likely to travel together.
Suppose that a particular gene is highly favored by natural selection. Genes with no particular selective advantage that are close to it on the same chromosome are likely to be favored simply because they are fellow-travelers of the selectively favored gene. This "genetic parasitism" is termed genetic linkage. An example in humans is the linkage of nail-patella syndrome (a condition causing abnormalities of the limbs and kidney disease) and type-B blood.
Genetic linkage sometimes manifests itself during sexual selection. Suppose there were a human population in which females possessed a gene that cause a compulsion to mate with men with red hair? Clearly we would get a general shift across generations toward a kind of genetic linkage:
- Red hair in both their male and female offspring (even though females would be indifferent to female redheads).
- Prevalence of the "female red-hair mating preference" gene in both their male and female offspring (even though it would not be expressed in males).
And, any individual passing on one gene would likely pass on the other. This particular concatenation of genes can lead to "runaway sexual selection" in
which bizarre and otherwise maladaptive structures and behaviors evolve.
- Development is a process of differentiation from generalized to specialized form, with general structures appearing first.
- The embryo of any creature is never identical to the adult of any other.
- General patterns of heterochrony:
- Paedomorphosis: Heterchronic change in which the adult of a derived organism resembles a juvenile of the ancestor. Compare the derived, paedomorphic Axolotl to a "normal" relative, the tiger salamander: juvenile, adult,
- Peramorphosis: Heterchronic change in which the juvenile of a derived organism resembles an adult of the ancestor. Compare a derived peramorphic veiled chameleon to a normal panther chameleon.
- Peramorpic hindlimbs.
- Paedomorphic faces:
Cultural "evolution": Note that paedomorphosis and peramorphosis occur in popular cultural figures.
- Quite frequently, a logo or cartoon character will be juvenilized so that it will be "cute." E.G:
- Less frequently, we see peramorphosis:
Ernst Haeckel (1836 - 1919) Noted that the embryos of evolutionarily derived creatures may pass through stages in which they display ancestral traits. The human embryo, for instance, at certain stages has a tail and open gill slits. Proposed what he termed the biogenic law which maintains that "ontogeny recapitulates phylogeny." He attempted to integrate it with Darwinian evolution through the notion that evolutionary change is accomplished by the addition of new changes to the end of the embryological or developmental sequence. Haeckel's approach is called recapitulationism.
Karl Ernst von Baer (1792 - 1876) - Was an opponent of recapitulationism and set forth a series of developmental laws that held that:
Stephen J. Gould: (1941 - 2002) In 1979, Stephen Gould stepped to the plate with the terminology and organization of Heterochrony as we understand it. We have two basic forms of heterochrony, and several developmental parameters that can be tweaked to produce them.
A single orgamisn can incorporate both paedomorphic and peramorphic change. Consider humans: