Where did the first life come from?

1924 - Alexander Oparin (Russian biologist) published The Origin of Life. Never translated from Russian, it had little impact at the time.

1929 - J.B.S. Haldane (British biologist) published similar ideas, but they were still essentially ignored.

1936 - Oparin published The Origin of Life as a book, which was translated and widely read...and the race was on!

Stay tuned!

Artificial Selection is evidence for Natural Selection.

e.g. - Brassica oleracea has been selected by humans to become the cultivars we know as broccoli, cauliflower, kale, chard, brussels sprouts, kohlrabi, etc....

This suggests an important fact:

* Natural selection/evolution occurs at the level of the population--NOT the individual! (Individuals do not evolve; populations do.)

* Change in an individual in response to environmental pressures is more accurately termed adaptation.

Evolution is a change in the genetic composition of a population in response to environmental selective pressures.

microevolution: genetic change in a species over time (without speciation)

macroevolution: the formation of two reproductively isolated species from a single ancestral species.

Some economically important examples of microevolution (due to anthropogenic factors) are occurring all around us, as we speak!

1. antibiotic resistant bacteria

2. pesticide resistant insects (and other pests)

So...you might ask, why can't larger animals evolve resistance to poisons and other selective factors. (Why couldn't birds, say, evolve the ability to NOT have DDT cause them to lay thin-shelled eggs?)

1. Generation time!

The shorter the cycle time between generations, the more opportunities there are for genetic change and mutations to be incorporated into a population. (Vertebrates have very long generation time in comparison to bacteria!)

2. Preadaptation

Populations cannot simply evolve a character because they "need" it. The genetic machinery to create a phenotypically beneficial trait (in a particular environment) must already exist in the gene pool in order to be selected. If the "needed" trait isn't there--the species may just go extinct.

***EVIDENCE FOR NATURAL SELECTION***

Biogeography - The study of the geographical distribution of species.

Darwin could be considered one of the earliest biogeographers:

His questions:

* Why were the species on the Galapagos islands so similar among the islands and so similar to those on the mainland of South America?

* Why should it not be that islands with similar environmental, yet on opposite sides of the globe, should not share the most similar species?

Central to the study of biogeography is the concept of continental drift. The present day continents were once joined in one landmass known as Pangaea.

Pangaea broke into a southern segment known as Gondwanaland and a northern segment known as Laurasia.

Gondwanaland present day derivatives: Africa, South America, India, Antarctica, Australia

Laurasia present day derivatives: North America, Europe, Asia

Regions of volcanic/geologic activity between the earth's crustal plates (see Fig. 23.9 of your text):

subduction zones: where one plate is forced beneath the adjacent plate, due to pushing from...

sea floor spreading zones: where the earth's crust separates, and molten substance from beneath bubbles up, spreading the plates farther apart.

Biogeographic events that separate populations can be global in scale (such as continental drift), or much smaller, such as the local flooding of one area of a continent during times of warmer climate and higher ocean levels.

Groups of species may be isolated from other groups by such local calamities, and survive only in localized, physically separated refugia, where they evolve independently of one another.

A species which is found only in one geographic region, to the exclusion of all others, is known as an endemic species. A geographic region populated by a large number of endemic species is known as an area of endemism.

Examples: Galapagos islands; Hawaiian islands; Madagascar, Papua, New Guinea etc.

By charting the relatedness of taxa in various geographic regions (via comparative anatomy and molecular genetics, for example), biogeographers can trace the evolutionary path from ancestral to extant taxa.

(Prediction: the longer ago the geographic split, the greater the genetic distance between related species on the two geographic regions.)

***COMPARATIVE ANATOMY***
Primitive character (aka - plesiomorphy) is one which is relatively unchanged from an ancestral form.

symplesiomorphy: shared, primitive character found in several taxa

Derived character (aka - apomorphy) is one which is relatively modified from an ancestral form.

synapomorphy: shared, derived character.

Kingdom Animalia

Phylum Chordata

Subphylum Urochordata (tunicates)

Subphylum Cephalochordata (lancelets)

Subphylum Vertebrata (vertebrata)

All chordates (no matter which subphylum) all have the following character at some point during their development

* notochord (dorsal, cartilaginous support rod)

* dorsal, hollow nerve tube

* pharyngeal gill slits

* tail posterior to the anus

* muscle bundles arranged segmentally

The above five characters are symplesiomorphies, shared by all chordates.

HOWEVER--remember that the terms symplesiomorphy and synapomorphy are RELATIVE terms.

* The five characters above are symplesiomorphies with respect to the CHORDATES ONLY.

* But if you suddenly bring in the entire Kingdom Animalia, these five characters become synapomorphies (new evolutionary features) which link all the chordates to their ancestral chordate, but which separate them from all animals which "never evolved" the five characters above.

To take this a bit further...

Larval ("tadpole") Urochordates exhibit all five primitive characters listed above. Once they metamorphose into adults, however, the notochord and segmental muscle bundles are lost and the organism becomes sessile (attached to a substrate).

The secondary loss of the notochord and segmental muscles in the adult Urochrodate is a synapomorphy common to all species of Urochordates.

***********

...and another example:

Embryonic vertebrates have all five primitive chordate characters listed above. However, various vertebrate classes show varying degrees of specialization upon reaching adulthood.

These are the vertebrate classes:

Agnatha (jawless fishes)

Osteichthyes (bony fishes)

Chondrichthyes (cartilaginous fishes)

Amphibia (amphibians)

Reptilia (reptiles/birds)

Mammalia (mammals)

Of these six classes, only Agnatha retains the notochord as an adult. In all other vertebrate classes, the notochord is secondarily lost as the organism reaches adulthood.

This means that although the notochord itself is a symplesiomorphy found in all vertebrates (and all chordates), the secondary loss of that notochord can be considered a synapomorphy linking all vertebrate classes beyond Agnatha.

And one final point...

All vertebrates have a bony cranium and a bony, articulated spinal column. No other chordates share this characteristic, which is considered to be an evolutionary innovation unique to the vertebrates.

Therefore, the bony cranium and articulated spinal column are synapomorphies separating the vertebrates from all other chordates--but symplesiomorphies linking all vertebrates together (if you don't consider the rest of the chordates).

Now, go take a couple of aspirin and see me next Monday. Cheers!