Lecture 14 - Comparative methods part 1

Question 1

What is a comparative method?

Answer 1

A comparison of similarities and differences that seeks to explain their significance

Question 2

What are two key evolutionary mechanisms that cause us to see similarities and differences?

Answer 2

Common descent and divergence

Question 3

What was the comparative methods in 1991 by Harvey and pagel?

Answer 3

New synthesis of comparative approaches

Took into account the development of methods based on explicit evolutionary and statistical models

Techniques that exploited our improved knowledge of phylogenetics

Techniques that allow evolutionary hypotheses to be tested

Question 4

Relationship between body length and generation time

Answer 4

The larger the body length the larger the generation time

Question 5

What is the correlation between population density and body mass?

Answer 5

Negatively correlated

Maybe all populations are at maximum population density (same quantity of energy)

Question 6

How can phylogenetics help in comparative biology?

Answer 6

Can distinguish cause from effect

Removes the influence of confounding variables

Question 7

How does phylogenetics distinguish cause from effect?

Answer 7

Allows the timing of the evolution of a trait to be established

Does it occur before or after an event?

ALlows us to test whether a trait is adaptive

Question 8

How does phylogenetics remove confounding variables

Answer 8

Closely related taxa are likely to share many traits and vice versa

Therefore the significance of correlations between distantly related taxa may be misleading

Better to start comparisons with closely related taxa and move outwards

Question 9

What are the constraints on phenotypic diversity?

Answer 9

Phylogenetic niches conservatism

Phylogenetic time lags

Differences in adaptive response

Question 10

Phylogenetic niche conservatism

Answer 10

Current and ancestral taxa are likely to occupy similar niches because they are better adapted than and out-compete alternative taxa

Close phylogenetic relatives are likely to have similar phenotypes

Adaptive radiations may occur when competitors do not exist

Question 11

Phylogenetic time lags

Answer 11

Traits not lost immediatly

Therefore a trait might not appear to have adaptive significance

Comparison with other taxa can reveal the former use of a trait

Question 12

Examples of phylogenetic time lag in plants

Answer 12

Fruit trees in Central America produce lots of fruits with tough skins that are not all getting eaten

This was due to the co-evolution with extinct taxa

This is a less efficient dispersal

Question 13

Phylogenetic time lag in animals

Answer 13

Cetacean pelvic girdle

Pelvic girdle in whales derived from terrestrial ancestors

Different and more important use of pelvic girdle in the past

Question 14

Phylogenetic time lag in humans

Answer 14

Veniform in humans

Non-functional

But it is in the location of the lymphoid tissue which is important for developing immune response if anything reaches your appendix after ingestion

No strong selection pressure to lose it but some selection pressure to keep it for its immune function

Question 15

What are the differences in adaptive response?

Answer 15

Similar forces might act on different traits in different taxa because the phylogenetic history constrains the adaptive response

Question 16

Adaptive landscape

Answer 16

In a hypothetical landscape of selection pressures there are adaptive peaks

When climbing a peak and start adapting to a niche you can become perfectly adapted to that niche

However, there may be a higher peak

Hard to go down the peak and climb up as as you go down there a better adapted organisms that will outcompete you

Question 17

Examples of adaptive response

Answer 17

Tetrapod limbs

They all have basal conditions of tetrapod limbs

However the all have different limbs due to different ways of responding to the pressures around them

Question 18

What may adaptive response be caused by?

Answer 18

Genetic constraint

For examples, almost all mammals have 7 cervical vertebrae so they girraffes can’t really increase anymore in height

However key innovations as a result of mutation may remove these contraints and allow for adaptive radiations e/g/ cichlid jaws

Or irreversible evolution e.g. fully seperated pulmonary and systemic circuits in penguisn and cetaceans

Question 19

Homology

Answer 19

Similar due to common descnet

Question 20

Analogy

Answer 20

Similar due to common selection pressue

Question 21

Example of convergence

Answer 21

Dolphins resemble ichthyosaurs due to convergence

Both adapted to swimming

However they share synapomorphies with mammals such as TMJ, hair and mammary glands through common descent