These can be graphically represented as:

Yes. You're responsible for reading and understanding those web pages. Don't worry. They're small.

THE PACE OF EVOLUTION.

How fast 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.

Known examples:

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

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

HOW DO SPECIES CHANGE INTO DIFFERENT SPECIES?

anagenesis (= phyletic evolution) - the conversion of an entire population, over tiem, 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 creation of two new species from a single ancestral species. (Net increase in species diversity). An ancestral species that gives rise to a variety of diverse species through repeated cladogenesis (as its descendant species "radiate" into new ecological niches, which helps to drive their diversification) is said to have undergone adaptive radiation. The diversification can be a result of one or more of any of the factors that can alter allele frequencies: mutation, genetic isolation, natural selection.

WHAT PHENOTYPIC CHARACTERS TELL US ABOUT EVOLUTION

The goal of the modern biosystematist (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 biosystematist 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.

REPRODUCTIVE 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 help define separate species by considering the mechanisms that restrict successful mating/gene flow between them.

Two related species may be separated by one or more of these types of reproductive isolating mechanisms.

PREZYGOTIC ISOLATING MECHANISMS prevent the formation of viable zygotes.

• environmental/spatial isolation - the geographic ranges of two species overlap, but their microhabitats or breeding conditions differ enough to cause reproductive isolation.

  e.g. - Rana aurora (Red-legged frog) breeds in fast-moving, ephemeral streams, whereas its relative Rana catesbiana (Bullfrog) breeds in permanent ponds. (The metamorphosis times of the tadpoles are correspondingly different.)
  

  

• temporal isolation - two species whose ranges overlap have different periods of sexual activity (or breeding season)

  Rana aurora - breeds January - March
  

  

  Rana boylii - breeds late March - May
  

  

  another example -

  Drosophila persimilis - breeds in early morning

  Drosophila pseudoobscura - breeds in the afternoon
  

• behavioral isolation - In species with courtship rituals (breeding calls, mating dances, etc.), there is usually a complex, give-and-take "ritual" before actual mating takes place. This prevents "wasted effort" with a partner who will not produce fertile offspring with you!

• mechanical isolation - morphological differences prevent mating/pollination.

• gametic isolation - sperm and ova are chemically incompatible, and will not fuse.
  

POSTZYGOTIC ISOLATING MECHANISMS prevent hybrids from passing on their genes.

• hybrid inviability - zygote forms, but dies after a few series of cell divisions (the genetic information from male and female parent were insufficient to carry the organism through morphogenesis)
  

• hybrid sterility - viable hybrid is produced (often physically more vigorous than either parent), but is unable to reproduce due to meiotic problems.
  Examples: liger or mule

• hybrid breakdown - successive generations of hybrids suffer greatly lowered fertility --> sterility. Eventually, they are selected out of the population.