evolution and fossils

• Life has existed on earth for 3.5 billion years
• fossils are the preserved remains and traces of past life, many found in sedimentary rocks
• only a small percentage of organisms become fossilised: due to natural processes or being unable to (did not live in a suitable environment for making fossils)
• fossils show there has been a change over time from simple to complex organisms, which is evidence for evolution
• evolution is the process of cumulative, inheritable change in a population over many generations
• A geological time scale divides Earth's history so we can place the major changes in the history on a scale
• a major change of Earth is continental drift- the movement of continents over millions of years from pangea to now
• the 5 types of evidence for evolution:

  • paleontology
  • biogeography
  • developmental biology (embryology)
  • morphology
  • comparative genomics
• Biogeography: the study of distribution of species and ecosystems over time
• comparative genomics: the study of genes and their interactions within a genome, heredity and variation in living organisms.
• the more similar in a sequence the genes and genome of two species are, the more closely related they are, because less time has passed for mutations and other genetic changes to accumulate
• DNA hybridisation was used in the past to analyse the relatedness of pairs of species. Their resistance to separating ('percentage hybridisation') is used to work out evolutionary relatedness
• a downside to DNA hybridisation is that it can be unreliable when comparing closely related species
• bioinformatics is the digital storage, analysis, retrieval and organisation of biological data such as nucleotide sequences from different species
• bioinformatics has produced what?

  significant advancements in our knowledge of genomes and providing more evidence for evolution
• fossilisation requires very specific and rare conditions, meaning the remains of many organisms may never be found. this makes the fossil record incomplete and biased toward organisms more easily fossilised
• principle of superposition- oldest rock layer found at the bottom of the rock. used to estimate relative age of fossils
• archaeopteryx- transitional organism between dinosaur and bird
• steps for fossilisation:

  • organic matter is quickly deposited, covered in sediments and in an environment lacking oxygen, preventing decomposition
  • minerals from seduments replace the bone and harden the fossil
  • organisms covered in sediments turn to sedimentary rock
  • long period of time
• Relative dating is used to determine the age of a rock, or a fossil contained in the rock, relative to other rocks or fossils found nearby
• Absolute dating assigns a numerical age in years to a fossil or rock, usually through radiometric dating
• Life hasn't always been the same on earth, as:

  • organisms change and evolve over time
  • one species may give rise to multiple species over time
  • evolution is a process that takes thousands of years and many generations
  • species change due to climate or changing environments
  • rate of evolution can fluctuate
• adaptive radiation: species rapidly diversify into many taxa with differing adaptations, due to many factors e.g changing resource availability. It is a type of divergent evolution
• greater similarity in molecular sequences of DNA and proteins reflects greater closeness of relationships and demonstrates progression of form and complexity over time
• Molecular Evidence – e.g DNA polymerase
• relative dating vs absolute dating:

  used to determine the age of a fossil or rock relative to other rocks/fossils nearby VS used to assign a numerical age

  • relative: based on strata they are found
  • absolute: radiometric dating (carbon 14)
• radiometric dating: uses the half life of an element to determine age; the time taken for the original amount to decay to the present amount
• genetic drift: random changes in allele frequency
• genetic variation arises from processes within meiosis, random fertilisation/mating, gene flow and mutation
• There is a struggle for existence – being able to survive long enough to reproduce passing the alleles onto the next generation
• Isolation is no gene flow between similar gene pools (populations) due to geographic, cultural, behavioural barriers.
• examples of selective pressures:

  food availability, predators, climate, disease
• selective pressures are often due to overpopulation in a population; more than the environment can sustain
• when the probability of survival for a particular phenotype is greater than others in the population, more survive leading to an increased number of their alleles in a population
• individuals with a particular allele, that result in a survival and reproductive advantage, will produce more offspring with the favourable allele.
• over generations the favourable allele frequency in the population increases, resulting in changes in the gene pool.
• In addition to environmental selection pressures, what contributes to changes in allele frequency in a gene pool?

  sexual selection, mutation, gene flow and genetic drift
• Evolution definition?

  the change in allele frequency in a population over time/generations.
• what may lead to allopatric speciaton?

  differing selective pressures between geographically isolated populations
• Natural selection occurs when selective pressures in the environment create a specific (survival and reproduction) advantage on a particular phenotype, changing gene pool frequencies. (to have more favourable alleles)