The process of Evolution, Speciation and Extinction

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The process of Evolution, Speciation and Extinction

Chapters 23, 24, 25

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Objectives

a) What is evolution & speciation?

b) What factors affect they type of speciation occuring?

c) How do extinction events influence evolution?

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Types of Organisms

Chemoautotroph: Carbon and is gained from inorganic carbon = Carbon dioxide or HCO3- (abiotic environment)

Energy is needed for this process and is gained from inorganic molecules such as iron, sulfur or magnesium being oxidised, so is pre-photosynthesis. Occuring in ancient deep sea vents and in unique porkaryotes such as Sulfobus.

Photoautotroph: Energy is gained from light and is how photosynthetic prokaryotes (some cyanobacteria), plants and some protists function. Like above gains carbon from CO2 and HCO3.

Chemoheterotroph: Carbon comes from the breakdown of organic substances, how non-photosynthetic (consumers) life currently functions.

Like chemoautotrophs the energy required for this process, and comes from the oxidation of inorganic molecules.

Photoheterotroph: Carbon comes from breakdown of organic substances, whilst energy comes from life.

This is unique to certain aquatic and salt loving prokaryotes such as Rhodobacter and Chloroflexus.

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Early Prokaryotes underwent massive adaptive radiation

From 4,000 mya to 2,500 mya often known as the 'boring billions' as almost all organisms were prokaryotic and extremely similar in shape and function. However the genetic variation in different cells was massive as the environmental conditions (snowball Earth) meant that evolution was extremely important.

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What is evolution?

The change in allele/gene frequency in a population over time

Darwin's Observations:

•Plant and animal breeding (similarities and differences)

•Palaeontology (fossil records)

•Embryology

•Biogeography (similar species across different continents)

•Comparative anatomy (bone and organ structure)

This has been supported in more recent times by genetics, molecular biology and more advanced, comparative embryology

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BOOK CHAPTER 22 REVIEW - further reading into each observation

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Causes of evolution

The most driving factors that cause evolution to occur are:

Natural selection

Mutation pressure

Gene flow

Random genetic drift

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Natural Selection: The process by which organisms with heritable and beneficial characteristics survive and reproduce to pass on to their offspring.

Adaptation: A structure, process or behaviour that has evolved over time by natural selection, which affects it's reproductive success. (can be positive or negative to organism)

Gene flow: movement of alleles into or our of population

Genetic Drift: in small populations chance events cause allele frequencies to fluctuate unpredictably.

Mutation pressure: some parts of a gene

However it is genetic variation that makes evolution possible, otherwise their is no bias selection in the survival of organisms.

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Mutation Pressure

There are two main forces influencing the amino-acid composition of proteins: a diversifying one - mutational pressure, and a stabilising one - selection pressure according to the research of Sueoka in 1962

In a quantitative theory proposed by Sueoka, he explained the wide variety in bacterial individuals based on mutational pressure. He suggested that the genome was not randomly affected by mutation, but in fact had a directionality towards areas that were either high or low in Guanine-Cytosine content. These are often more neutral parts of the genome which don't affect the functionality of the organism.

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What is a species?

A species is a group of populations whose organisms can interbreed to create viable and fertile offspring.

This can be hindered by several barriers that can be prezygotic or postzygotic

Prezygotic includes isolation of some description. This could be a physical barrier, breeding only occurring during certain times or seasons, or not having the same mate recognition behaviour (courtship). It could also include not being able to mechanically complete sexual reproduction or having gametes that may not be able to fertilize each other (eg sperm cannot survive in reproductive tract)

Postzygotic includes having reduced hybrid viability, due to the mixture of genes having impairing characteristics. It could also include the offspring being sterile due to not having the same number or structure of chromosomes, or even having the second generation after the hybrid reproduces being feeble or sterile instead.

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Speciation

Allopatric (other country) speciation is the name given when the lack of breeding is due to a physical barrier. These organisms then have different genetic variety over time and as a result slowly evolve into different species.

Sympatric (same country) speciation is when it isn't a physical barrier, and is usually based on behaviours.

This can be a result of

• Polyploidy (more common in plants)

•Habitat differentiation in same area (some individuals exploit a new food source that matures at different time of year, so breeding time changes).

•Sexual selection such as Cichlid males being selected by females

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Extinction Events:

These can be either the result of a cataclysm (dinosaurs, ice age etc.) or a result of competition (usually over-population due to over abundance of food, so can be both inter and intra-species)

These cause a bottleneck effect, a form of the founder effect as the individuals are seperated, but also because the population size has been reduced, so there is less genetic variety.

This can both result in high and low evolutionary rates. Due to the low genetic variety, evolution is a far slower process as fewer new characteristics occur, however if the environment is still hostile this results in extreme environmental pressure towards evolving.