ORIGIN OF SPECIES & MODERN THEORY OF
EVOLUTION
Red Queen Hypothesis (Theory of Natural Selection)
The environment is constantly changing
Natural selection operates to enable the organisms to maintain their state of adaptation (not to improve it)
For a species to remain in existence in a constantly changing environment, it must have enough of the right kind of heritable variation to change along with the environment
If genetic variation is inadequate, the species will go extinct
Selection Pressure
The degree to which the environment plays a role in survival
Low Selection Pressure› Low mortality› Lots of survival, more variability› New combinations can be tested› Includes any new genes and mutations
High selection pressure› High mortality› Only the most fit survive› Eliminates all but the “best” genes› Population becomes specialized
Environmental Change› Factors that determine our environment are
constantly changing› Change occurs as a result of high selection
pressure Environment change may make all individuals less
fit and extinction possible
› Change occurs as a result of low selection pressure More forms of gene present due to increased
variability Environment change may favour the different gene
MODERN THEORY OF EVOLUTION
Modern evolutionary synthesis› The modern theory of evolution that
takes into account all branches of biology Biologists today define evolution as
changes in the gene pool of a species over time.
Gene pool: the complete set of alleles contained within a species or population
Mutations: The Source of Variation
All species exhibit genetic variation New or altered traits arise when new
alleles and genes are produced by mutation and acted upon by natural selection
FACTOR INFLUENCE ON EVOLUTION
Beneficial mutations Relatively rare but are favoured by natural selection and tend to accumulate in populations over time
Harmful mutations More common than beneficial mutations but are selected against and therefore have no influence on populations
Duplication mutations Often neutral and so do not immediately benefit the individual, but provide a source of new genetic material with the potential to evolve into new genes
Mutation rates Relatively low for individuals but can be numerous in populations overall
http://evolution.berkeley.edu/evolibrary/article/1_0_0/eyes_10
Homologous Genes and Pseudogenes
Homologous All mammals have similar numbers and
arrangements of bones because they have inherited this pattern from a common ancestor
Similarly closely related species inherit homologous genes, which mutate and evolve over time
The degree of similarity between homologous genes provides good evidence for the degree of relatedness between species
The more closely related two species are, the more similar we would expect their homolgous genes to be
Chicken 5539
Chimp 16785
Dog 12852
Fly 2208
Human 16735
Mosquito 4019
Mouse 19119
Worm 2445
http://tables.pseudogene.org/collection.py?id=15
Pseudogenes pseudogene
› A vestigial gene that no longer codes for a functioning protein; genes that have undergone mutations and no longer serve a useful purpose
Like vestigial anatomical features, a pseudogene is the remaining part of a gene that once serves a useful purpose
These genes are found virtually in all species