Chapter 4 - Taxonomy and Speciation

Taxonomy

science of classification, especially of biological organisms

• based on evolutionary relationships, not just physical appearance (like how early naturalists thought)

Taxonomic hierarchy of humans

Domain - Eukarya

Kingdom - Animalia

Phylum - Chordata

Subphylum - Vertebrata

Class - Mammalia

Subclass - Eutheria

Order - Primates

Superfamily - Hominidea

Homologous traits

AKA shared derived traits

• similarities based on common descent from a common ancestor

analogous traits

similarities based on common function with no assumed common evolutionary descent

AKA Homoplasy

• ex. birds and bats don’t have a common ancestor, but have wings with similar function (flying)

homologous traits in tetrapods

• all tetrapods have the same basic limb bone structure (with modification) derived from a common tetrapod ancestor

• tetrapod = four footed vertebrates

convergent evolution

takes species without a common ancestor and evolves them to have a similar function

• ex. birds, insects that can fly, and bats

primitive (ancestral) traits

• passed on from an ancestor without evolutionary modification

• if your ancestor has the same trait as you, it’s primitive

• doesn’t help sort out taxonomic groups

derived (modified) traits

• modified by evolution from the ancestral condition

• if you have a trait that your ancestor doesn’t, it is derived

• helps sort out taxonomic categories

can traits be both derived and primitive

yes depending on the context

ex. 4 limbs

• comparing mammals with mammals

  • it is primitive since even if mammal species evolved from other mammals, they share the same trait

• comparing mammals with fish

  • derived since mammals evolved from fish, and fish don’t have 4 limbs

Linnean Taxonomy

• system that classifies organisms based on shared physical traits

• only classification, no concept of evolutionary relationships

evolutionary systematics

traditional approach that interprets evolutionary relationships based on a wide range of evidence (like evolutionary traits)

cladistics

• groups organisms based on evolutionary relationships

  • specifically derived homologous characters

grades vs clades

evolutionary systematics - uses evolutionary traits, is more intuitive, but doesn’t make biological sense all of the time

• based on grades

• ex. birds are not grouped with reptiles since they don’t look physically similar (even though they evolved from dinos)

cladistics - based on derived homologous characters

• based on clades

• ex. clade: Amniotes (birds reptiles and mammals)

  • grouped together because they share a common ancestor which layed eggs

grades

groups of species that share physical similarities

clades

groups which share ancestry, even if they are not physically similar

phylogenetic tree

diagram that shows evolutionary relationships based on evolutionary systematics

problems with evolutionary systematics

closely related organisms sometimes have unique differences and distantly related organisms sometimes have primitive similarities

cladogram

diagram that shows evolutionary relationships based on cladistics

• branching tree with nodes that represent shared ancestors

• based on shared derived characters

• doesn’t take into account of time and doesn’t suggest ancestor-descendant relationships

types of clade groups (cladistic categories)

• Monophyletic clade - includes all descendants from one ancestor

  • the only evolutionarily meaningful cladistic category

• paraphyletic group - does not include all descendants of a common ancestor

• polyphyletic group - includes two groups that are not closely related

• outgroup - distantly related group that is used to establish primitive and derived traits

synapomorphies

traits shared in common between two life-forms and their common ancestor

• most useful for making evolutionary interpretations

• ex. shared presence of feathers in birds and dinos

which is better cladistics or evolutionary systematics?

neither

• cladistics - helps us understand evolutionary relationships more accurately

• evolutionary systematics - works better intuitively with our understanding of life, but not biologically all the time

species

group of organism’s that can interbreed to produce offspring, but can’t produce viable offspring with other species

• concept that scientists made up to help understand biological diversity

biological species concept + problems with it

members of a species are interfertile but reproductively isolated from other species

• problem - asexual organisms don’t interbreed, so BSC doesn’t work here

speciation and ways it can happen

process where new species evolve from existing species

• one species can change over time into a single very different species

• or population can diversify into two or more distinct species (through isolation and other selective pressures)

allopatric speciation

speciation by geographic isolation

• something causes population to be divided by geography

• natural selection occurs if there is little to no gene flow

• eventually they become different species