Evolution

Evidence that Prokaryotic Cells Existed Before Eukaryotic Cells

• Prokaryotic cells have simpler metabolism and structure.

• Eukaryotic cells would not have been able to survive in the hostile conditions of Earth’s early atmosphere.

• Stromatolites have been found in some of the oldest fossils known. These are structures made up of ancient bacterial mats in which sediment has become trapped and compressed to form rocks.

Endosymbiotic Theory

When all life consisted of prokaryotic cells, some of the larger cells may have engulfed some of the smaller cells. In some cells, the smaller engulfed cell was able to respire aerobically or photosynthesise, like mitochondria and chloroplasts do in eukaryotic cells. Evidence for this theory is that mitochondria and chloroplasts:

• Have their own circular DNA.

• Contain their own ribosomes.

• Are able to self-replicate, resembling the binary fission of prokaryotes.

• Have two membranes, an outer membrane similar to the host cell’s membrane suggesting that the inner membrane was part of the original prokaryotic cell and the outer membrane was derived from the host cell.

Abiogenesis

Experiments have shown that organic molecules, such as amino acids, could have formed in conditions similar to those of primitive Earth. Because of their chemistry, lipids can self-organise into primitive vesicles, called protocells.

Possible Roles of RNA and Ribozymes in the First Simple Cells

It is likely that the first simple cells used RNA. Evidence for this includes:

• RNA is a simpler molecule than DNA, so it is more likely that RNA formed first and that its role was eventually replaced by DNA.

• RNA is able to self replicate.

• Some RNA molecules, called ribozymes, are able to catalyse simple chemical reactions. Ribozymes can also evolve.

Species

A group of organisms that can interbreed to produce fertile offspring, in their natural environment.

Levels of Ecological Organisation

• Organism - a single living thing.

• Population - all the individuals of a species living within a specified area.

• Community - all of the interacting populations in a localised area.

• Ecosystem - all of the living organisms interacting with their non-living environment in a specific area.

• Biosphere - All the environments on Earth inhabited by life.

Other Criteria used to Define a Species

• Morphological similarity - a group of organisms that conform to certain characteristics.

• Biochemical similarity - a comparison of the proteins or genetic material in different organisms.

• Gene pool - sharing a common gene pool.

Reproductive Isolating Mechanisms

Pre-zygotic (preventing zygote formation):

• Temporal isolation - producing gametes at different times.

• Behavioural isolation - differences in mating behaviour.

• Mechanical isolation - anatomical differences prevent fertilisation.

• Gamete isolation - biochemical differences prevent fertilisation.

Post-zygotic (prevention of fertile offspring):

• Hybrid inviability - zygote or embryo fails to develop.

• Hybrid sterility - hybrid is infertile.