Lecture 11

Question 1

Homology

Answer 1

• same structure shared by descendants of a common ancestor no matter how modified in descendent form
• important to evolution because we need to know if something we’re studying is related to something we already know
• comparision is vital–>is it the same thing?

Question 2

Analogs

Answer 2

• features that rose independently but look similar and don’t share a common ancestor
• convergence/parallel evolution

Question 3

Descent with modification

Answer 3

• descendants from same ancestor but results in branching pattern due to modification
• relates to characterization of homology (key idea)

Question 4

Homology vs. Analogy

Answer 4

• analogous structures have same function but aren’t evolutionarily linked (wings of fly and wings of airplane)
• homologous structures are highly conserved (forelimbs of vertebrates-same components or horns of antelopes-all very divers in structure but descendants of primitive horns)

Question 5

Evolutionary studies

Answer 5

• most based on homologous structures
• asking about think you can show to your satisfaction that they’re homologues.
• can start looking at new things including loss of elements, and changes for new function
• homology concept is used to trace evolution of homologous structures, including loss of element, or changes for new functions
• aids in identifying analogs in evolution

Question 6

Vertebrate forelimbs (birds)

Answer 6

• much different in structure but same components
• highly specialized for their function-know it’s doing something different, but you can’t see it.
• trace back history of bird forewing limb to an animal that has a different more basal structure as compared to that of the other vertebrates

Question 7

Levels of homology

Answer 7

• can be homologous at one level and analogous at the next

- bats and bird’s forelimbs are homologous but not at the level of wings those are analogous

Question 8

Artiodactyla

Answer 8

• started out with 5 digits but reduced to walking on 4
• then reduced in size to two
• lightweight running structure achieved

Question 9

Horses

Answer 9

• have one digit that evolved from 5
• went through stage of having three toes
• reduces weight of foot for running
• forelimbs are homologous but single toe is analog

Question 10

Criteria for homology

Answer 10

• shared spatial relationships in an anatomical system
• composition and properties of the structure
• historical continuity in evolutionary record (can you see earlier precursor, and earlier precursor of that?)
• e.g. vertebrate skull

Question 11

Vertebrate skull

Answer 11

• in evolution skulls of some bones lost, some bones fused together, and some adopted new function
• evolutionary trend you usually see: starts out complex, full of smaller bits that are eventually slimmed down to fewer components like in the case of the dog skull

Question 12

Jaw bone

Answer 12

• used to articulate behind the ear but now it articulates just in front of your ear
• the bones that used to be the jaw articulation that now no longer had a job were incorporated into the hearing apparatus (incus and malleus)

Question 13

Non-homology (homoplasy)

Answer 13

• convergence can produce similar solutions which can be considered false homologies–>homoplasy
• analogy: totally unrelated structures–>do direct relationship in evolutionary history
• homologous at basic structural level but analogous as independently derived structures

Question 14

Flight in vertebrates

Answer 14

• all use forelimb
• Pterodactyl: super long pinky with wing suspended from it
• bird: start with dinosaur ancestry; whole hand is fused to be sturdy support for feathers
• bat: umbrella method; suspended on all fingers
• HOMOLOGOUS as FORELIMBS; ANALOGOUS as WINGS
• insect wings are ANALOGOUS IN ALL WAYS to WINGS in VERTEBRATES

Question 15

Gene Homology

Answer 15

• organisms have homologous genes (the same gene which represents common descent of an ancestral gene in two lineages)
• e.g. Myoglobin-oxygen-binding protein found in tissues of all vertebrates so there are myoglobin genes

Question 16

How can we tell if a gene is homologous to a gene from another organism?

Answer 16

• homology criteria apply to gene sequences
• similarity=structural criterion
• differences in gene sequences represent the differences in the evolution of this gene in each of the species
• correct alignment of sequences is crucial
• positional criterion: overall molecular topology (built of similar domains)
• local sequence base positions-different base occupies homologous position although a mutation has occurred
• *look at example sequences
• share lots of same genes with other vertebrates and even share some with other phyla

Question 17

General Points about homology

Answer 17

• in evolution, pre-existing structures are often converted/co-opted to new uses
• history matters
• homologous structures don’t necessarily have the same function
• evolution is branching in pattern, not linear, so homologs diverge

Question 18

Gene Homology II

Answer 18

• certainty increases with length of DNA sequenced e.g. an entire stretch of DNA as in genomic sequencing. Often seen in gene families
• long sequence lets us see homologous genes and the suite of mutational differences accumulated over evolution
• useful tools in tracing evolution of both genes and organismal lineages