Natural Selection and Evolution Flashcards
Unity of Life
- all life has a common ancestor
- there is a diversity of form, but all are linked
- sharer common molecules and chemistry
- central dogma, DNA->RNA->protein
- evolutionary and cell theory are the basis for the interrelation of all living things
Cell Theory
- all life is based on cells
- life forms composed of cells can be classified as either eukaryotic or prokaryotic
Cell Theory
Multicellularity
-multicellularity was a key step in the evolution and development of plants and animal as it lead to the specialisation of cells
Evolution
General Trend
-increasing size and complexity over time
Classification
Definition
-theories about the basis of natural order
Classification
Historically
- before the mid 1900s anything that wasn’t an animal was classified as a plant
- plant and animals were not seen to be related
Classification
Today
-fungi are now known to be closer related to animals than plants
Phylogenetics
Definition
-study of evolutionary relatedness among groups of organisms
What are the three domains of life?
- Eubacteria
- Archaebacteria
- Eukaryota
Classification Hierarchy
Life Domain Kingdom Phylum Class Order Family Genus Species
Archaebacteria
- discovered in extreme environments e.g. acid pools and volcanic springs
- single cell
- prokaryotic
- initially identified as extremophiles but now known to be present in a wide range of environmental conditions
- recognised as having major roles in life processes on Earth e.g. nitrogen cycle
Era of Genomes
- two years ago, 200 different genomes had been sequenced
- that number is rapidly increasing as genome sequencing gets cheaper
Genome Sequencing
First Generation
- Sanger Sequencing
- chain termination method
Genome Sequencing
Second Generation
- also known as next generation sequencing
- massively parallel sequencing
- millions of DNA fragments from a single sample are sequenced together at the same time
Genome Sequencing
Definition
nucleic acid sequencing to determine the exact order of nucleotides in DNA
What has analysis of genomes led to?
- refined classification of eukaryotes into supergroups
- precise relationships are still debated
- but we are getting closer to a clearer view
When was the Earth formed?
4.6bya
When did the first prokaryotic cell form?
~3.75bya
When was oxygen first present in the atmosphere?
~2.75bya
When did the first eukaryotic cell form?
~2bya
When did the first multicellular eukaryotic cell form?
~1.5bya
When did the first animal form?
0.75bya
When did the Cambrian Explosion occur?
535-525mya
When was the land colonised?
less than 500mya
When did mammals originate?
~180mya
When did the Cretaceous Mass Extinction occur?
65.5mya
What proportion of all existing species are now extinct?
Nearly all of the species that have ever been present on Earth are now extinct
Important Macroevolutionary Changes
- origin of photosynthesis
- emergence of complex, compartmented cells
- multicellularity
- emergence of terrestrial organisms
- long term impacts of mass extinctions
How Life Began
chemical and physical processes lead to the formation of very simple cells:
1) Abiotic synthesis of small organic molecules
2) joining of these molecules to form macromolecules
3) packacging of molecules into protobionts
4) origin of self replicating molecules
Miller and Urey
Showed in a lab experiment in 1952 that abiotic synthesis of molecules in the early atmosphere was possible
Protobiont
Definition
- collection of abiotically produced molecules surrounded by a membrane or membrane-like structure
- they exhibit simple reproduction and metabolism and maintain an internal chemical environment
What was the first genetic material?
probably RNA
What is a ribozyme?
RNA molecules that can catalyse many different reactions including forming a complimentary RNA strand
Self Replicating RNA in Protobionts
early protobionts that had self replicating RNA were able to be more effective at using resources
they quickly increased in frequency by natural selection
Stromatolites
- oldest known fossils
- ~3.5bya
- rocklike structures
- layers of bacteria and sediment formed in shallow water
- prokaryotic
Prokaryotes
- only inhabitants of Earth 2.1 - 3.5bya
- structurally simple and metabolically complex
Hypothesis of Endosymbiosis
-mitochondria and plastids used to be small prokaryotes living within larger host cells
Endosymbiont
Definition
cell that lives within a host cell
How did the endosymbionts enter the host cells?
- as undigested prey or internal parasites
- in the process of becoming interdependent they became a single organism
Serial Endosymbiosis
supposes that mitochondria evolved before plastid in a series of endosymbiotic events
Evidence for the Endosymbiotic Origins of Mitochondria and Plastids
- similarities in inner membrane structure and function with prokaryotes
- division similar in these organelles to some prokaryotes
- transcribe and translate their own DNA
- ribosomes are more similar to prokaryotes than eukaryotes
Natural Selection
- only mechanism to explain adaptation
- most dominant mechanism underpinning evolutionary change
Allele
Definition
different versions of the same gene that diverged due to mutation, they can differ in fitness
Directional Selection
selection away from the mean
Diversifying Selection
selection that increases the number of different phenotypes
Stabilising Selection
selection that increases the proportion of the population with a trait that is already the most common
this type of selection leads to the repression of change
Are evolution and natural selection the same thing?
- no, natural selection is only one mechanism by which evolutionary change can occur
- in the absence of natural selection trait can and do still change over time via migration and genetic drift etc.
Requirements for Evolution
- populations of animals which are variable
- at least some variation that is heritable
- differential survival and reproduction, i.e. differences in fitness
Fitness of Genotype
Definitions
average per capita lifetime contribution of individuals of that genotype to the population after one or more generations
Relative Fitness
for evolution by natural selection it is the relative fitness of genotypes that is important
if everything else is equal, the fitter genotype will increase in frequency over time
Components of Fitness
- Viability - probability of survival to reproduce
- Mating Success - number of mates
- Fecundity - average number of viable offspring per female
- Gamete viability
Coefficient of Selection, s
- amount by which fitness of a genotype differs from a reference genotype
- fitness of the wildtype is set as 1
- fitness of other genotypes is measured as a proportion relative to this
Darwins Finches
- Galapogos Islands
- one colonising event
- small original group of birds, 30-100
- 0.9 - 1.5 mya
- there is now wide variation in beak form related to feeding preference
- a drought in the 1970s lead to an increase in seed hardness resulting in directional selection for increasing beak size
Peppered Moths
- two forms
- melanic, black form and ‘normal’ speckled form
- black first seen in 1848
- black made up majority of urban populations by 1901
- due to better camouflage
Soapberry Bug
Decrease in beak length as they started feeding on thinner fruit
Fitness and Environment
Fitness is not fixed, it is dependent on environmental factors
-some genotypes are more advantageous in some locations than others
Natural Selection and Variation
- natural selection can only act on variation that exists
- it is not predictive or foresighted
Preadaptation and Exaptation
- adapting existing body parts for new functions
e. g. limbs to wings in bats and wings to flippers in penguins - but there is a cost as by doing this they lose the function of the original body part
Does natural selection lead to perfection?
- not necessarily
- e.g. bird beak length , width and length controlled by one genes so one cant be changed without the other, short and wide is most advantageous but beaks become long and wide, advantages of increased width outweigh the disadvantages of increased length, i.e. the selection pressure on width is stronger than on length
