Evolution Flashcards
What are influenza strains named after?
H-haemagglutinin (virus binds to cell)
N-neuraminidase (virus released from cell)
E.g. The H3N2 strain predominated in 2018
What is evolution?
Accumulated, heritable changes within a population, over generations, giving rise to new species
How do changes in the genotype arise?
Mistakes occur during DNA or RNA replication:
Small duplications, insertion, deletions and base substitutions are examples of little mistakes
Large deletion, DNA duplications, chromosome rearrangements, viral insertions and insertion of transposable elements are examples of big mistakes
Natural selection- organisms better adapted to environment survive to produce more offspring
Genetic drift- variation in relative frequency of different genotypes in small populations due to chance
Gene flow- transfer of alleles from one population to another
Applied selection:
Plant or animal breeding
Antibiotic, drug, pesticide resistance
How can mutation rates act as molecular clocks?
Mistakes/mutations accumulate over time
Species that share a recent common ancestor will have fewer differences than species that are more distantly related
We can use molecular differences to construct phylogenetic trees
What does the study of phylogenetics refer to?
Since new species arise from earlier species, they must share relationships
All organisms are derived from a distant common ancestor
Phylogeny refers to the history of a species
Phylogenetic relationships can be represented using phylogenetic trees
the last common ancestor is called LUCA
What are some evolutionary definitions?
Living organisms can be classified, based on molecular or morphological criteria
Systematics deal with classifying living things
Taxonomy- naming of groups of organisms
Classification- arranging taxa into an ordered, hierarchical system
Phylogeny- determination of ancestral relationships of organisms and their evolutionary history
Describe the E.coli experiment used for observing evolution.
12 colonies of E coli from a single clone
Sub-culture every day for 31 years
Freeze culture samples every 500 generations (75 days)
Over 73,000 generations have been observed
Differences between cultures have emerged over the years
Genetic analysis of ancestors is now possible
Test for ability to grow in citrate in the presence of oxygen
The ability to use citrate (in oxygen ) evolved after about 30,000 generations in one group in particular E.coli can’t usually use citrate in oxygen
Earlier generations from this same ancestral E coli were repeatedly able to evolve the ability to utilise citrate at about the same time
The result:
The region with the citrate gene and RNK promoter are duplicated in tandem so now the RNK promoter which is normally on in the presence of oxygen means that the citrate gene is also turned on in the present of oxygen
Large-scale genome sequencing allows us to track evolution in real time
What is one example of how DNA mutations prevail through certain circumstances?
Sickle cell anaemia (SCA)
Mutation in beta-globin gene by a single base substitution
Glu6Val
Mutant Hb molecules
Aggregate and form crystals when deoxygenated
Cells with characteristic “sickle” shape
SCA is autosomal recessive and causes early death
So why is SCA common in tropical regions
Correlates with regions of endemic malaria
Heterozygotes have improved malaria survival rates
Lower parasitaemia
Fewer severe complications
Even though it’s a harmful mutation, it confers heterozygote advantage
What are homology and analogy?
Homology: two structures from a common ancestral version
E.g. Tetrapod limbs
Because these structures are derived from a common ancestor, they form by a common mechanism
Analogy: no common ancestor- similar structures produced by selection to meet similar function (convergent evolution)
E.g. insect and bird wing
Although different developmental mechanisms, some molecular components may be the same
What is molecular analogy? What is a prominent example?
Also known as molecular convergence
Same function of proteins with no structural identity e.g. the crystallins (lens proteins)
There are three main types of crystallins in the lens- a and b/g family
Alpha crystallins are related to HSPs (heat shock proteins) so have recruited to a lens function
Beta/gamma crystallins are homologous
But many other different lens proteins exist in different species
These tend to be enzymes/ have enzymatic activity
In addition to being soluble, stable, transparent & refractive
Modification of a regulatory region allows expression in lens as well as other tissues
What is molecular homology and its subcategories?
Molecular homology determined from amino acid or nucleic acid sequence
Two genes from same organism (by duplication)- paralogs
Two genes from different organisms (common ancestor)- orthologs
Gene duplication drives evolution
Gene/regulatory region duplication allows the ancestral activity to be maintained
Duplicated genes can take on other functions
Complementary functions (redundancy)
Non-overlapping functions (novel)
How do homologues related to Hox genes?
Homologues, paralogues and orthologues
Homologues are related by descent from a common ancestor
Paralogues are homologues within the same species e.g. mouse Hox a1, a2, a3, a4 etc
Orthologues are homologues between species e.g. the fruit fly Antennapedia gene is the orthologue of mhttps://www.brainscape.com/decks/9587595/cards/quick_new_cardouse Hox a6, b6, c6
What are Hox genes?
Hox genes are developmentally important transcription factors
Hox genes bind DNA in a sequence-specific fashion and regulate the expression of adjacent/nearby genes
Confer positional identity along the Anterior-Posterior (craniocaudal) axis
Paralogues duplicated within ancestral animals
Present in clusters along chromosome
Present in vertebrates
Chromosomal organisation and function conserved
How do Hox genes cause antennapedia?
The thoracic segment T2 and T3 and T1 are where the antennapedia genes are normally expressed
Its function there is to tell those segments to be thoracic and take on a thoracic characteristic; and that’s where the legs of the adult fly would grow
So the antennapedia mutation causes the genes to be expressed in the head region
NOT a disruption in sequence
There are 8 hox genes in the drosophila embryo at this locus
How do Hox genes evolve?
An ancestral hox gene, so something that’s able to regulate the expression of other genes, will have emerged
And that has been duplicated, then again and again with some additional genes in there as well
So (the one above) is the organisation that we see in invertebrates such as Drosophila
In vertebrates, that whole cluster has been further duplicated and that duplication has then occurred again so that vertebrates actually have four Hox gene clusters (a, b, c and d)
But the grouping remains the same so genes on the left-hand side would be more expressed in the anterior part of the organism while genes on the right would be more expressed on the posterior
