These can be graphically represented as:

The Pace of Evolution

How quickly does evolution proceed? It depends.

PHYLETIC GRADUALISM
This is the classical, traditional view stating that large changes (reproductive isolation and morphological differentiation) occur due to the gradual accumulation of many genetic changes. The classic example put forth in many natural history museums in the form of a nice display is that of the evolution of the modern horse.

(NOTE: The modern systematist would not suggest that each of these species evolved into the next, more recent species. Rather, all these ancestral "steps" to the moder horse share a common ancestor that may have looked a lot like the "Dawn Horse," Hyracotherium (formerly known as Eohippus.

PUNCTUATED EQUILIBRIUM
This hypothesis was published in 1972 by Niles Eldredge and Stephen J. Gould.
They suggested that major changes can occur relatively suddenly, and that they "punctuate" long periods of relatively little change. Let's have a LOOK.

Remember: "suddenly" is a relative term, geologically speaking, and can mean over thousands of generations (quick!) instead of over millions (not so quick!)

Eldredge and Gould suggested that this could explain how "awkward" intermediate forms such as the reptile-->flying bird and the terrestrial tetrapod-->swimming cetacean might have been "skipped". A major genetic event could have produced a phenotype that was drastically different from the original, and that this trait could become modified and fixed in the population over relatively few generations.

Examples:

• transition from tetrapod ancestor to aquatic cetacean (whale)
• polyploidy resulting in relatively sudden reproductive isolation (many annual "wildflowers")

Speciation

Species that are on the verge of becoming separated are known as incipient species.

How do species change into different species?

There are two major competing hypotheses:

anagenesis (= phyletic evolution) - the conversion of an entire population, over time, to a new form so different from the original that the classical evolutionary taxonomist would consider it a new species. (No net increase in species diversity)

cladogenesis (= diversifying evolution) - the divergence of two new species from a single ancestral species. (Net increase in species diversity).

Adaptive Radiation

Example:
On the Hawaiian islands, a single, finchlike ancestor gave rise to about 40 different species of Honey Creepers. Each is specialized in bill shape and size, as selected by its particular microhabitat and diet. Colors may have evolved in response to sexual selection.

What do phenotypic characters tell us about evolution?
Phenotype (at many levels, including the level of the DNA) provides the the biologist with the most basic information s/he needs in trying to accurately reconstruct evolutionary relationships.

The goal of the modern systematist (i.e., a biologist who studies the evolutionary relationships between organisms) is to construct taxa (classification groups) that are monophyletic - derived from a single common ancestor.

In so doing, the systematist considers homologies, analogies, primitive and derived characters in the taxa under study at the level of morphology, ontogeny, and the biological macromolecules (DNA, RNA, proteins) themselves.

Reproducive Isolating Mechanisms

When one species gives rise to two new species (cladogenesis), what is it that determines whether or not the two can reproduce, if allowed to regain physical contact (i.e., if they become sympatric once again)? We can define separate species by considering the mechanisms that restrict gene flow between them.

Two related species may be separated by one or more of these types of reproductive isolating mechanisms, which may evolve--once again--due to the five factors already discussed that can change allele and genotype frequencies in a population.

Sociobiology

In 1975, Edward O. Wilson published Sociobiology: The New Synthesis. The controversy generated by one idea in his book spread from biology to many other disciplines, including those in the humanities and the social sciences.

Wilson's main tenet:

While it's no doubt true (as some critics have suggested) that one cannot apply the biology of social insects to the biology of humans, it is also quite likely that a great deal of human behavior is at least partly under the control of our genes, and hence, subject to natural selection and other evolutionary forces. (Thought it's a bit harder to study the connection in our species, due to ethical constraints!)

In 1962, British zoologist Verno Copner 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. defined as risking the loss of one's own fitness via an act that could improve the fitness of another individual.

For example, Wynne-Edwards noted that under crowded conditions, many animals' reproduction is severely reduced or ceased altogether. He interpreted this behavior as "altruism" that was for the "good for the group." He hypothesized that a group that restrained its reproductive output and did not "overeat" its food supply would be more likely to survive than a group that reproduced without restraint, to the point of destroying its food supply and then starving.

Wynne-Edwards believed that behaviors that improved the survival of some of a group's members would give that entire group an adaptive advantage over groups that did not have altruistic members.

The idea of "group selection" was subject to severe criticism for many reasons.

• Genetic variation is much higher between individuals than it is between groups, especially big groups. Alleles will likely be maintained in a population even if some individuals are removed.
• Many behaviors are not all that heritable, or show complex patterns of inheritance.
• Behavioral phenotypes are not usually controlled by a single gene. Selection for a behavior (or against it) would have to select for all the genes involved in its development.

• At the individual level, reproductive restraint is maladaptive (A genetic tendency towards martyrdom is likely to get you kicked out of the gene pool.)
• A few experiments (e.g., with populations of flour beetles) seemed to indicate that group selection could be possible. However, the results of those experiments could better be explained by another phenomenon (which we're about to discuss).

So how can such phenomena as

• reproductive "restraint" in response to crowding or other stimuli

• alarm calls (which might draw attention to the caller while allowing its conspecifics to silently escape)

• sterility in social insects (worker hymenopterans (ants, bees, etc.) 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 increasing the likelihood that some of your alleles will be passed on to future generations.

(Those interested in human medicine might take note that W.D. Hamilton was also well known for his support of the hypothesis that the AIDS pandemic in Africa might have involved a polio vaccine campaign. The idea has been dismissed by many as having been well refuted, but it persists in some circles.)

Inclusive Fitness, Individual Fitness and Kin Selection, oh my.

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

• individual fitness - the production of offspring by an individual

• kin selection - assisting relatives in rearing their offspring, which may share some of the assisting organism's genes.

• inclusive fitness - individual fitness PLUS fitness gained via kin selection activity.
  

• Solitary species tend to have a greater individual fitness component.

• Social species tend to have input from kin selection.

Consider The Marmoset.

This is a tiny, New World monkey who lives in social groups consisting of

• A Queen: reproductive female
• A King: reproductive male
• Immature males
• Adult females who serve as "aunts" and help their Queen Mum raise the babies.

• New sibling baby monkeys get 50% of the queen's genes
• and 50% of king's genes
• On the average, all sisters share 25% of queen genes and 25% of dad genes. It's not a total genetic loss.

Why should an "aunt" not take the chance to contribute all of her genes to future generations (In the form of multiple offspring, as the queen does)?

• Eventually, the queen may die, and one of the aunts will become the new queen. It's a "waiting game" and a gamble. But in the meantime, you are ensuring a greater rate of survival for at least some of your genes by helping to rear your own relatives.

• Staying with the group ensures a better chance of survival. A lone monkey would be monkey meatloaf very shortly after leaving the group, if it couldn't join a new group. That means it can no longer contribute to its own fitness even via kin selection.

Now consider the Honeybee.

These are social hymenopteran insects whose populations are haplodiploid.

The kin selection advantage is even greater in this case.

• Females (queen and workers) are diploid.
  
• Gametes produced by the queen share 50% of her genes.

• Males (drones) are haploid. (they produce sperm via mitosis)
  
• Gametes produced by a drone contain 100% of his genes.

• All the daughters of a single drone and queen get 50% of their alleles from dad and 50% from mom (queen).

• Because the drone is haploid, this means that (full sister) worker bees share 100% of their father's genes and (on average) 50% of their mother's genes

• Hence, a worker bee shares 75% of her alleles with her full sisters.

• A fertile worker would share only 50% of her genes with each of her offspring.
• Worker bees are actually more closely related to each other than they would be to their own offspring.

• Heritable behaviors that promote helping the queen to produce more 75% related sisters are adaptive. It's simple math.

But the probability of promoting one's own genes' survival is a much more logical explanation for apparently "altruistic" behaviors than Wynne-Edwards's idea of group selection.

Like physical traits, heritable behavioral traits (and if you believe E.O. Wilson and many others, all animal--including human--behaviors have at least some genetic component at their root) may be either

• adaptive
• maladaptive
• neutral

And recall that neutral traits may becoem adaptive or maladaptive if the organism's environment changes.

What makes a species a species?

It depends on the population. Speciation rate is affected by such factors as generation time, and what appears to be the poorly understood relative stability of certain species' genetic makeup.

But unanswered questions is what makes all this fun, right?