Biology 1B Evolution Lecture 12 species selection macroevol transitional forms evolution of novel phenotypes Species selection Traits that arise via microevolution can also act to increase net speciation rate Speciation rate Extinction rate Traits that increase speciation rate and or decrease extinction rate in a clade of species can be idenitifed through comparison of closely related clades especially if trait arises repeatedly and is consistently associated with high species diversity in those clades relative to sister clades that lack this trait see Figure from Rabosky McCune 2010 The power of species selection relative to individual based selection within species is clearest when traits that are disadvantageous within species are associated with increased net speciation among clades ie opposing effects Example evolution of self compatibility SC ability to self fertilize in monoecious plants vs self incompatibility SI no self fertilization SI can be a disadvantage within species if pollinators are limiting in this circumstance SC allows reproduction But in the longer term SC increased rate of inbreeding loss of genetic diversity etc higher extinction rates SC has arisen repeatedly in flowering plants eg Figure for Solanaceae tomatoes relatives but analysis of phylogenies suggest that it leads to higher extinction rates of species despite short term advantage to individuals Thus clades with SI have more species surviving than those with SC The fossil record transitional forms Comparison of extant living forms predictions of evolutionarily transitional phenotypes in the fossil record Major Transitions in Phenotypes e g Vertebrate Fish see next page Amphibian Tetrapods Biology 1B Evolution Lecture 12 species selection macroevol transitional forms evolution of novel phenotypes Point of transition from exclusively aquatic organisms to terrestrial vertebrates is important Figure 34 20 page 711 8th edition There is a huge jump between these groups particularly in phenotypes which cannot be constructed from modern fauna this is where the fossil record comes into play The transition from ancestors of lungfish to ancestors of tetrapods and key insights are coming from fossils Acanthostega has a moving neck lungs limbs but no wrists and a weak rib cage that would have made breathing on land very difficult so it probably lived in a swamp Tiktaalik however has the evolved traits of Acanthostega plus wrists and a stronger rib cage see image below The extinct forms are not necessarily direct ancestors but they do allow us to see how and when certain traits adaptations evolved Evolution of Novel Structures and Exaptation The hammer and anvil bones of the middle ear of mammals that act as transmitters from the outer ear into the inner ear There is a complete set of fossil transitions that takes us from the original function of the bones articulation points of the jaw in cynodonts and synapsids Biology 1B Evolution Lecture 12 species selection macroevol transitional forms evolution of novel phenotypes Figure 34 31 page 721 8th edition see also Figure 25 6 page 513 8th edition Co opting structures from one function to another is known as exaptation don t need a new structure to arise can simply modify an existing one Feathers are another example their original purpose may not have been for flight but rather for attracting mates or thermoregulation new studies have found dinosaurs with feathers that were colored but not for flight suggesting their use in display Evolutionary Novelties evo devo Re programming of developmental pathways can cause changes in timing location of expression of key regulatory genes Within sets of genes there are often duplicates allowing one copy to be kept and others to be used for new functions Biology 1B Evolution Lecture 12 species selection macroevol transitional forms evolution of novel phenotypes Figure 25 22 page 527 8th edition In insects the Ubx gene suppresses leg development to 6 legs but it can have a different effect in other species Very different but homologous structures typically have a common site of developmental genes These structures have very different anatomy yet they have similar patterns of Hox gene expression
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