BIOL 4160 Evolution Phil Ganter 301 Harned Hall 963-5782
Darlingtonia californica - this insectivorous plant has a very limited distribution in California and Oregon

Evolution and Geography

Email me

Link to a list of Specific Objectives for lectures

Back to:

• Biogeography as evidence for evolution
• Large-Scale Patterns
• History and Distributions
• Testing the Relationship
• Ecology of Distributions
• Latitudinal Diversity Gradient

Biogeography as evidence for evolution

Species have restricted geographical distributions that are both the result and cause of evolutionary events

• Darwin and Wallace view the distribution of a species as the outcome of:
  • local origin of a species (not everywhere at once)
  • subsequent spread of the species through dispersal, which depends on
    • dispersal abilities of the organism
    • presence of barriers to and corridors for migration

As distributions enlarge, populations become isolated and opportunities for speciation arise, which starts the process over

Convergence of phenotypes in geographically isolated species when faced with similar environmental challenges/opportunities demonstrates the importance of natural selection to evolutionary processes

• Easiest to see in environments where some physical parameter is at or near a limit with respect to living organisms
  • Deserts are familiar example of an extreme environment (low moisture)
    • Very different lineages of plants produce stem succulence
    • Thorns or spines to protect plants from macroherbivores

Large-Scale Patterns

General descriptors of distributions:

• Ubiquitous species are those that are found world-wide (where their habitat exists)
• Endemic species are those with restricted distributions
• Native species are those found in a location that are present because they originated there or migrated there long enough ago that they have become a member of the local community
  • Autochthonous species are those that originated in the region of interest
  • Allochthonous speciesoriginated elsewhere and dispersed into the region of interest
• Exotic species are those that have arrived recently
• Introduced species are exotics that have been brought to a locality as a result of human activity
• Exotic species may not become part of the community or may become long-term residents (eventually they must be considered native)
  • Those that become permanent residents may have little effect or great affect on total community composition

Biogeographic Realms - large regions set apart because many of the species within each are confined to the realm (Palearctic, Nearctic, Neotropical, Ethiopian, Oriental, Australian, Antarctic) - first defined by Wallace

Disjunct Distributions - species that have gaps between populations - gaps can be very large

Relict Populations - populations no longer part of the larger species distribution due to extinction of intervening populations

The fir trees in the Smokies are a relict population of firs left at high elevation, where it is cool and moist enough for the firs, while the main population distribution is in northern North America

here the change in climate since the last glaciation has warmed the lower elevations and the firs went extinct between Canada and the Smokies

History and Distributions

Three historical processes that affect contemporary distribution

• Extinction - extinction can turn a continuous distribution into a disjunct distribution or can move the center of a  distribution in a single direction
• Dispersal - dispersal is obviously important in shaping the distribution of s species over time
• it can result in constant range expansion (Gypsy Moth in North America), disjunct distributions, or re-colonization
• Vicariance - separation of populations through the rise of a geographic barrier (mountain formation, continental drift, lake formation, etc.) or by a climate or habitat change

These processes are not mutually exclusive and the history of a species distribution can be complex

Testing the Relationship

Above, we used distribution data to demonstrate evolution.  Here, we will use phylogenetics to explain distributions

Given the current distribution of a species, how can one assess the events and processes that produced it?

• Geological history of the area is the starting place (this includes not only continental drift but historical climate studies, etc.)
• Fossil distribution is very important if available as it can date when a species appeared in an area and when it became extinct
• Distribution of other species in the community (if a gap in one species is not found in the distributions of other members of its community, it is hard to argue vicariance is responsible)
• Phylogenetic relationship among populations or species can aid in understanding distributions
• Related species may occur in a series, due to origin of new species after range expansion
• Clusters of related species in one region may indicate colonization by a single ancestor species

Phylogeography is an inter-disciplinary field that combines historical and phylogenetic data to explain species distributions.

Ecology of Distributions

Range limits are the outer edge of a distribution, either in space or in an ecological variable

• Example - many plants cannot tolerate freezing and so are limited to regions in which the coldest temperature of the year is above freezing

The idea of limitations seen in terms of ecological variables is tied to the idea of the Niche from ecology

• Fundamental Niche - the set of environmental and biotic conditions necessary for the existence of a species (edges are fuzzy because individual organisms differ)
  • The niche can be seen as a space (or volume) in "resource space" , which (unlike real space) has as many dimensions as resources
• Environmental Space - the set of conditions actually available within the resource space
• Potential Niche Space - the intersection of the fundamental niche space with the environmental space (no intersection, no potential niche!)
• Realized Niche Space - this is the portion of the niche actually occupied by the species
  • some potential niche space may not be occupied due to historical effects (species has not dispersed far enough north to experience range limitation based on temperature, for instance)

The idea of niches becomes more complicated (and realistic) of biotic factors are included as resources

• If these are not included, it may be hard to explain why the realized niche is smaller than the potential
• Perhaps the example above, in which a species has not reached its low temperature range limit, is not due to history but because its prey has a higher low-temperature limit and is not found in the north, so the original species is not found in the north

Phylogenetic Niche Conservatism - the similarity in fundamental niche between related species

• PNC can be a strong influence on the ecology of species, so strong that it is possible to predict the niche of ancestral species from the similarity of the niches in the species in the clade to which the ancestor gave rise

Community Convergence - niche effects can be seen beyond the species level

• Species that originate in  similar (but physically separated) environments tend to exploit those environments in similar ways
  • Entire communities of organisms can converge into a similar set of species
• Rodents in different grasslands often divide food resources by size.  Larger rodents eat larger food items and, when the range of food items is similar, the sizes of the rodents in different grasslands is also similar, even though the grasslands have no species of rodent in common

Latitudinal Diversity Gradient

A feature of the biosphere many have documented for different kinds of organisms, is a decline in the number of species in the group (or in the type of habitat) with increasing distance from the Equator

Tropical forests have many more tree species than do boreal (northern) forests

The reason for the gradient may be multi-factorial and some factors may be more important for some types of organisms than for others.

• Some hypotheses about the reason for the gradient are based on ecological differences only
  • e. g., the shortening of the growing seasons for temperate and boreal environments means less productivity and fewer species
• Some hypotheses involve historical considerations and this means that evolution is involved:
  • Tropics have more species because the rate of origination of new species is larger there than in northern latitudes (or lower extinction rate, or a combination of both)
  • Time and Area Hypothesis - Cenozoic has mostly been warm, and so most of the continental and shallow marine environments were tropical
    • thus the area of the tropics has been larger than the area of the temperate or boreal zones for a long time
    • The larger the area, the greater the chance that new species will originate, thus the tropics have more species

Human Biogeography - to be added

Biodiversity through Phanerozoic

Measuring the diversity of life

• Species Richness is the statistic (number of species), which differs from diversity (richness and evenness)
• Richness is dependent on sample size (larger sample gets more rare species)
• Should not use extant taxa in the analysis (Bias of the Recent) as many taxa today might never have even a single fossil occurrence and we must assume that many taxa at each point in the past will not be in the fossil record, so modern species are counted only if they have a fossil record.
• Data restricted to skeletonized marine animals (shallow sea) families as these are the most complete records and span the entire Phanerozoic

Patterns

• Steep increase from Cambrian to Ordovician, then a plateau until great Permian extinction at end of Paleozoic
• Steady increase throughout Mesozoic and Cenozoic, no evidence of a plateau, although the K-T extinction is evident
• Terrestrial life begins in the Devonian and insects, plants and land vertebrates all show increases with no evidence of plateaus
• Plants and insect families have a more-or-less steady increase but land animal families take off only during the Cenozoic

Extinction and Origination rates (not speciation, as the data are families)

• Except during mass extinctions and plateaus, origination rate is greater than extinction rate

Why the most recent increase in diversity?

• Extinction rates have declined over the Phanerozoic (both for absolute rates and per family rates)
• Within lineages, the trend is reversed (extinction rates increase)
  • Not sure why the trend within is an increase in per capita rate
  • Origination rates decrease within lineages
  • Trends are correlated such that lineages with higher origination rates are also those with higher extinction rates

So, how can the overall trend be a decrease but the trend within individual lineages is an increase?

• Both can be true if, at first, there is a mixture of lineages with high extinction and origination rates ("volatile lineages") and fewer "sedate lineages"
• Correlation of extinction and origination rates within lineages produces the volatile versus sedate lineages
  • Over time the volatile lineages disappear and the sedate lineages persist, so the total rate of extinction declines, even though extinction rates increase in each lineage
• If this is so, then we would expect the more volatile lineages to be extinct than sedate lineages, which is so

Where does this volatility come from?  Are lineages inherently different in extinction rates or are external factors responsible for the differences in extinction rates?

• Correlation between origination and extinction rates may have ecological origins
  • Smaller population size may result in both increased extinction rate (see chapter on Genetic Drift) and increased speciation rate (easier to isolate small populations than geographically widespread populations)
• So, ecological characteristics like geographic range and habitat specialization should correlate with "volatility"
  • Van Valen's Red Queen Hypothesis predicts that lineages must constantly evolve to changes in external conditions to survive
  • remember here that external conditions include not just physical factors but also changes in species with which the lineage interacts
  • evolution of food species, predators, and pathogens likely to be more rapid than changes in physical conditions
• External factors controlling extinction rate would mean that clades should lose species at a constant rate (dependant on conditions) and not that some lineages within the clade are inherently more "long-lived" than others
  • Data seems to agree with external control of extinction rate

Mass Extinctions

• Characterized by loss within many different types of animals (but not all)
• Five recognized - Ordovician-Silurian (450-440 mya), Late Devonian (375-360 mya), Permian-Triassic (251 mya), Triassic-Jurassic (205 mya, and the Cretaceous-Tertiary (64.5 mya)
  • Extinction rates are high enough now that, without a matching increase in the origination rate and if the rate persists for several centuries, we are in the sixth mass extinction
  • Most common cause of current extinctions is habitat loss due to human activity
• Can cause major transitions in living forms
  • Gould discussed three different scales of evolutionary change 1. Microevolution at population level (within species), 2. Evolution at species level and above during normal geological periods and 3. Coincident changes in large portion of biota post mass-extinction

Permian-Triassic Mass Extinction most devastating

• 54% of all marine animal families, 84% of all genera, 90% of all species
• 57% of all Insect families, 83% of Insect genera
• 70% of Terrestrial Vertebrates lost
• Plants lost species but no single family of plants was lost!

Physical changes at the time of the Permian extinction

• Anoxia in oceans, acidification of oceans
• Change in Oxygen Isotope Ratios (¹⁸O²/¹⁶O)
• Drop in Carbon Isotope Ratio (¹³C/¹²C) in carbonate rocks deposited during P-T extinction (Permian-Triassic)

Causes of Permian-Triassic Mass Extinction (possible)

• Some evidence for two phases - a long, slow rise in the extinction rate then a sudden steep rise that quickly peaked and fell as rapidly as it rose so it lasted for only a short time
• Bolide Collision
  • bolides are large impactors and, although existence of a crater for the K-T extinction has been found, no good evidence for an impact 250 mya is known
  • Impact would eject mass of ash into atmosphere, cause volcanism in adjacent areas
  • However, most of the Earth is covered by sea floor and, due to subduction, no sea floor is older than 200 my or so, so the impact might have no crater now
• Volcanism/Ignition of coal deposits
  • Evidence of several large flood basalt events (flood basalt deposits on land are lava from many volcanoes coating a large area)
  • Volcanoes put particles and various gasses into atmosphere
  • When some of these wash out in rain they can acidify bodies of water
  • Particles in atmosphere reflect sunlight and lower energy for photosynthesis
  • Volcanoes release CO₂ with a lower ratio and this might have altered climate disastrously (not just warming, but changes in ocean as well)
  • documented amount of volcanism not enough to warm atmosphere but Stephen Glasby has found evidence of ash from massive burning of coal at this time
  • Coal combustion releases gasses including CO₂ that could intensify both greenhouse and acidification events sufficiently to alter atmosphere
• Methane Clathrate release
  • Methane is a very powerful greenhouse gas that can cause global warming
    • Clathrate methane is consistent with lower Carbon Isotope ratios
  • Clathrates (cages of ice that trap methane) form when methane (from both geological and biological sources) is mixed with water and cooled under pressure (doesn't have to be to 0 C, 50 atm at 4 C will cause formation)
    • This can happen in shallow ocean sediment deposit and, if shallow deposits are exposed to air or warm temperatures, the methane can be released
    • Clathrates once thought to be exotics that formed only at edges of solar system, now large clathrate deposits have been found on Earth (2x times the total carbon in ALL fossil fuel deposits)
  • Problem - Carbon isotope ratios returned to normal shortly after end of extinction period - no mechanism for sudden clathrate formation!

Diversification

• In total, we don't seem to be reaching an equilibrium species richness now, but there seemed to be equilibria in the past
  • Equilibrium occurs when origination rate equals extinction rate
    • But, is this a stable (the system returns to the equilibrium when perturbed) or unstable equilibrium?
      • Some evidence that species richness is negatively correlated with origination rates and positively correlated with extinction rates
    • This indicates "density-dependent" equilibrium
      • When richness is above equilibrium, origination rates are low and extinction rates are high, bringing species richness down to the equilibrium once again
      • When richness is below equilibrium, origination rates are high and extinction rates are low, increasing species richness back to the equilibrium once again
• Competitive Release might explain the correlation
  • When few species are present, room for expansion exists and when many are present, competition prevents successful origination
    • Lots of examples of correlated expansion of one lineage as another lineage of potential competitors increases
      • Gymnosperms/Ferns replaced by Angiosperms
      • Marsupial mammals replaced by Placental mammals
      • Bony fish replacing other fish lineages
  • Competitive Displacement occurs when one clade diversifies and out competes another, older and more diverse clade for the ecological "space" the older clade occupies
  • Incumbent Replacement occurs when, after an extinction event, the open space created is occupied by a different lineage than the previous lineage to occupy the space - no direct competition is necessary
• Species richness may suddenly increase when a Key Adaptation allows a group to enter a new Adaptive Zone (new ecological space)
• Provinciality may also increase species richness over time
  • Species in many lineages were found over a larger area in the past, giving rise to fewer biological realms in previous eras
  • As species become more localized, this opens up real space, not just ecological space
    • Hard to know if provincialism is a driver of speciation or if the reverse is true

Last updated February 20, 2011