Latitudinal Diversity Flashcards
Explain Alfred Russel Wallace’s hypothesis for the LDG
Time and Stability:
The temperate areas have been covered in ice during ice ages, giving less time for speciation in these areas than in the tropics, which were not covered by glaciers
Also, in temperate areas, the main competition is between species and the environment, which facilitates the evolution of a few species that do very well and the rest don’t do well. In contrast, in the tropics, the main competition is between species, which facilitates the evolution of many species, as all species are able to survive in the environment, and competition for niches doesn’t repress the number of species.
Describe Dobzhansky’s addition to Wallace’s hypothesis
Harsh conditions only favour species with the specific traits to be able to survive in these conditions, making variations between species rare (because these variations will often lead to reduced fitness). Since the main selective pressure in the tropics is competition between species, there can be lots of adaptations to try to out compete others.
Niche specialization is a good strategy in the tropics, while generalizing is a better strategy in temperate climates
What was Fischer’s problem with Dobzhansky and Wallaces hypotheses?
They are circular in nature, so while high diversity drives competition, which increases biodiversity, they don’t explain why there were so many species (driving up competition) in the first place
Describe Fischer’s view
He doubted that speciation rate would be faster in the tropics than in temperate areas, therefore he supported the idea that the biodiversity is higher in the tropics because the tropics have had fewer climatic upheavals than temperate areas, so there has been more time for speciation to happen
What are the four parts of MacArthur’s Theory?
1) The dimensionality of the environment (the number of types of resources in the environment for the species to use)
2) The extent of different types of resources (e.g. if there are lots of types of seeds then there can be lots of types of birds)
3) The average amount of a resources used by a species (i.e. if a species can use a smaller amount of resources then there can be more species with a smaller amount of resource)
4) The overlap in the resources used by different species (i.e. if there is not much overlap, then fewer species from less resources, but if lots of overlap then there can be more species)
Explain how the four factors of MacArthur’s Hypothesis apply to the LDF
In the tropics, there are more and more types of resources, and species are more tightly packed into resource space because there is less resource use per species, and high overlap in resource use between species. Overall, this means that the tropics have a higher carrying capacity for species than temperate areas
What was Rohde’s problem with the time hypothesis?
If species were able to extend their range towards the equator as the climate cooled, they should have been able to rapidly repopulate lower latitudes as the climate warmed again
Explain Rohde’s hypothesis
That species diversity in the tropics is due to higher speciation rate, which is linked to higher energy inputs in the tropics
What mechanisms did Rohde propose as a link between speciation rate and energy input of an area?
1) higher levels of solar radiation means higher mutation rates
2) higher temperatures mean faster physiological processes which means shorter generation time
Describe gamma statistic
Looks at the node heights along the time axis. Is negative if there is a slow down and positive if there is a speed up
Describe the hypotheses put forward for why diversification rate could be higher in the tropics
1) there is more solar radiation at the tropics because of its angle with the sun, which has a mutagenic effect on DNA, possibly creating more phenotypic variation for natural selection to act on
2) Warmer temperatures mean faster growth rate and less time to maturity, leading to a shorter generation time, speeding up speciation events.
3) Larger geographic space in the tropics makes it more likey that a population will be separated by geographic barriers, leading to allopatric speciation.
Describe the cradle versus museum hypothesis
Cradle: That there is elevated speciation in the tropics
Museum: That there is reduced extinction in the tropics
What is revealed by taking away the biases in the fossil record?
That there are more first appearances of marine invertebrate species in the tropics. Jablonsky interpreted this as meaning that evolutionary novelty, leading to the formation of higher taxa, happened more frequently in the tropics
What are some of the factors that influence the quality of the fossil record?
Movement of continents, preservation potential of different types of organisms, and sampling bias
What did Weir and Schluter’s result indicate?
That the tropics are a museum for biodiversity rather than a cradle.
