A3.2 classification and cladistics (HL)

Question 1

the need for classification of organisms

Answer 1

classification is needed because of the immense diversity of species.

reasons for a classification system:

• information storage and retrieval: classification makes it easier to store and retrieve information about discovered species.
• identification of an organism’s species name:
  biologists need to identify organisms that they do not recognize immediately e.g dichotomous key
• predicting characteristics:
  species in a classified group share many similar traits, so characteristics of a species can be predicted from its group
• researching evolutionary origins:
  groups species that share traits meaning they have evolved from a common ancestor. system can be used to generate hypotheses of evolutionary origins of a species

Question 2

disadvantages of traditional hierarchy of taxa (Kingdom, Phylum, Class, Over, Family, Genus, Species)

Answer 2

taxa are groups used to classify organisms (e.g phylum, class, order).

Taxonomy is assigning organisms to taxonomic groups.

• taxonomists can disagree over how to classify a species, arbitrary decision.
• oversimplified + makes assumption that organisms of e.g the same family belong to the same genus and therefore species.
• unclear how populations are grouped into species, e.g hybrids (introgression)

Question 3

introgression

Answer 3

a process by which hybrids form over many generations, but instead of having an equal share of the original two species’ genetic info. there is an unequal contribution from each species.

Question 4

advantages of classification using evolutionary relationships (phylogeny)

Answer 4

ideal classification follows evolutionary relationships, so all the members of a taxonomic group have evolved from a common ancestor. characteristics of organisms within the same clade can be predicted due to their common ancestor.

phylogeny is the study of the evolutionary past of a species where organisms are classified using molecular differences in protein sequences and DNA (molecular systematics).

• species that are more similar in their DNA sequences are more likely to be closely related, while species with a higher degree of differences are considered less likely to be closely related

phylogenetic trees show the evolutionary relationships between species by showing which species developed from a common ancestor

+ not based on arbitrary, subjective categories, instead reflects species’ natural gene sequences

Question 5

cladistics

Answer 5

groups of species that have evolved from a common ancestor are identified but not given a rank e.g order or family.

Question 6

clades as groups of organisms with common ancestry and shared characteristics

Answer 6

two types of characteristics:

• primitive traits: evolved early in the history of the organism
• derived traits: evolved more recently in the form of modifications of a previous trait

organisms assigned to clades based on DNA base sequences or amino acid sequences of proteins. If this data is not available, then organisms are grouped into clades based on morphological traits.

Question 7

the molecular clock

Answer 7

an estimate of when clades diverged from a common ancestor based on gradual accumulation of sequence differences (mutation rate).

only an estimate because mutation rates are affected by the length of the generation time, size of population, intensity of selective pressure and etc. (assumes that mutations happen at a constant rate)

• the larger the number of sequence differences between two species, the longer since they diverged from a common ancestor.

Question 8

constructing cladograms

Answer 8

base sequences of genes and amino acid sequences of proteins are the basis of constructing cladograms.

Question 9

all organisms can be classified into 3 domains using evidence from rRNA base sequences

Answer 9

extra taxonomic level above kingdom (domain) proposed in 1977.

3 domains:
- archae: single-celled organisms distinct from bacteria (extremophiles: thermophiles, methanophiles, halophiles)

• eubacteria
• eukarya

Question 10

what separates archae from bacteria

Answer 10

• differences in their ribosomal RNA
• archaea carry out metabolic reactions that no bacteria can perform
• different ways of transcription and translation
• different cell structure

Question 11

homologous structures

Answer 11

same structure, different function, descended from common ancestor, divergent evolution. e.g pentadactyl limb

Question 12

analogous structures

Answer 12

different structure, same function, no common ancestor, convergent evolution. e.g bats vs birds wings