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.

Mutation

A permanent change in the sequence of DNA nucleotides and is the ultimate source of genetic variation in a species. Mutations in gametes and cells that divide to produce gametes can transfer to offspring.

Sources of Genetic Variation in a Species that Reproduces Sexually

In asexual reproduction, the only source of genetic variation is mutation. In sexual reproduction, genetic variation results from not only mutation, but also crossing over (during prophase I, homologous chromosomes pair up and non-sister chromatids exchange segments of DNA, resulting in a mixture of maternal and paternal genetic information on each chromatid) and independent assortment (homologous pairs line up randomly during metaphase I of meiosis, leading to gametes with a mix of paternal and maternal chromosomes) during meiosis, and the random fertilisation of gametes.

Comparative Genomics

If two species have evolved from a common ancestor and their separation was recent, it is likely that there will not have been enough time for more than a few mutations in each species to have taken place. Their DNA sequences should therefore be very similar. Genetic analysis techniques are:

• Sequencing of common proteins - comparing the amino acid sequence of proteins from different species. Cytochrome c is often used for this as it is found in all organisms that respire aerobically. The more similarities, the more recent the species have evolved from a common ancestor.

• DNA-DNA hybridisation - DNA from one species is heated to separate complementary strands. The single stranded DNA from the other species is mixed with the separated strands, and the mixture is cooled. Closely matched strands of DNA will bond more tightly than those that are not well matched. The newly-formed hybrid double helix is reheated to see how readily the strands separate. Poorly matched strands separate more easily than well matched ones.

• Comparing DNA sequences - determining the base sequence of a segment of DNA to directly compare the sequences from two different species. Electrophoresis is used to achieve this.

• rRNA gene sequencing - ribosomes are in all cells, therefore the genes that code for ribosomes (made from rRNA) are needed in all species.

Phylogenetic Tree Diagrams

Represent evolutionary relationships, showing the divergence of species from a common ancestor over a period of time.

Gene Pools

The sum of all the gene variations (alleles) of all the individuals in an interbreeding population. A large gene pool indicates considerable genetic diversity and is found in populations that are more likely to survive selection pressures.

Natural Selection

The process by which heritable traits that are best adapted to the environment increase in frequency in a population over many generations. The process of natural selection is:

1. A population contains genetic variation.

2. Not all organisms will survive and reproduce.

3. Individuals with favourable heritable traits are more likely to survive and reproduce, and the genes for these traits are passed to the next generation.

4. The frequency of the favourable trait increases in the population.

Adaptations

• Structural adaptations - how the organism’s body functions or looks like on the outside. Body parts (like feet and ears) and body coverings (like fur and scales) are structural adaptations.

• Physiologic adaptations - how the organism’s body functions on the inside. This includes changes in the cells, chemicals, and processes inside the organism’s body.

• Behavioural adaptations - how an organism acts. This includes actions like hibernating and communicating.

Factors Besides Natural Selection that Affect Evolutionary Changes

• Sexual reproduction - increases genetic variation of offspring.

• Genetic drift - changes in the frequency of alleles in a population due to chance events, decreasing the gene pool and resulting in evolution.

Allopatric Speciation

Speciation due to physical separation. When a population is divided into geographically isolated subgroups, after many generations, a new species can form. This will only occur if interbreeding between subgroups (gene flow) cannot occur. The two populations will remain as separate species even when they cohabit a community, due to reproductive isolation.

Sympatric Speciation

The evolution of two species living in the same geographical area. Examples of this are rare, and most commonly occur in plants and bacteria.

Convergent Evolution

The evolution of similar characteristics in species that are not closely related, as a result of similar selection pressures in their environment. An example of this are dolphins and sharks, or echidnas and hedgehogs.

Divergent Evolution

The slow and gradual accumulation of new characteristics from different selection pressures, culminating in the formation of a new species from a shared ancestral species.

Adaptive Radiation

A special case of divergent evolution in which there is a sudden (compared to normal divergent evolution) emergence of new species from a common ancestor. An example of this are Darwin’s finches, which inhabit the Galapagos islands, and different species of finch evolved different beak shapes for different types of food.

Succession

The change in composition and structure of communities over time. Bare surfaces become inhabited by pioneer plants, and over time will develop into a climax community. The two types of succession are:

• Primary succession - the process of change beginning with no soil. For example, on a new volcanic island or on the rubble left by a retreating glacier.

• Secondary succession - occurs after an existing community has been cleared by a disturbance that leaves the soil intact. For example, a fire or a flood.

Humans can cause disturbance that leads to succession, for example through deforestation, trawling, or farming.

Low Genetic Diversity

Species or populations that have a reduced genetic diversity have a higher risk of extinction, as they are more susceptible to selection pressures. An example of a species with low genetic diversity is the Tasmanian devil. Once distributed widely across mainland Australia and Tasmania before the two were split due to rising sea levels, it now only inhabits Tasmania. About 400 years ago, the Tasmanian devil disappeared from mainland Australia, possibly due to competition and predation by dingoes that were introduced about 4000 years ago. The remaining small population was reduced even further in the 19th and 20th century by hunting and eradication by humans. This greatly reduced genetic diversity of the population, meaning that all Tasmanian devils are now very similar genetically. Another example is the cheetah, which had its population reduced greatly in a mass extinction. Human activity has now reduced its habitat, and therefore reduced the population. Cheetah populations are genetically isolation, meaning there is limited gene flow between them, and inbreeding has further increased harmful genetic defects.

Human Impact on Climate

Increasing carbon dioxide levels in the atmosphere as a result of increased fuel usage, resulting in rising sea levels from the melting of polar ice and changes to the world’s weather patterns.

Human Impact on Environment

• Burning of fossil fuels causing acid rain, and resulting in many lakes and waterways becoming sterile.

• Clearing of land for crops reducing natural communities.

• Removal of mangroves in coastal areas to create harbours or recreation areas decreasing surrounding communities by removing their safe, sheltered habitat.

Human Activities and Extinction of Species

Human activities can create new and significant selection pressures on a gene pool, leading to species extinction. This may be through:

• Hunting - many species have been hunted to extinction by humans, such as the Tasmanian tiger.

• Introduced species - imported pests compete with and prey on native flora and fauna. An example is the rabbit and fox being introduced to Australia, which compete with and prey on bilbies, one species of which is now presumed extinct.

Life on Earth

• The origin of Earth was approximately 4.6 billion years ago.

• The oldest prokaryotic fossils are 3.5 billion years old.

• The oldest eukaryotic fossils are 2 billion years old.

The Ethics of Maintaining Biodiversity

Humans have an ethical obligation to prevent species extinction. Conserving species is best achieved through conserving their habitat, both abiotic and biotic factors.