 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

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

› 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 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

 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

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

Chicken5539 Chimp16785 Dog12852 Fly2208 Human16735 Mosquito4019 Mouse19119 Worm2445

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