Lec 11 Flashcards
New species formation;
Isolate Diverge Sympatry Reinforce Repeat
background extinctions
Extintions are constantly occurring in nature
Species struggling for survival are displaced by other, better adapted species - competitors, predators or parasites
The background extinction process is responsible for 96% of the extinctions
Endemic species
Native to only one specific rea
VERY small populations
Endemic species and extinction
Studying extinction is easier in endemic species because extinction is common and you only need to focus on a small geographic area
Endemic hotspots are geographic regions with high numbers of species found nowhere else in the world
Drivers of extinction: Predation
Fossil records shows inoveramid clams were one of the most common bottom-dwelling creatures
Around 67 million years ago, they had mostly vanished
Fossils show increased evidence of predator attacks
The timing of this increased evidence of predation coincides with an adaptive radiation of brachyuran crabs
Crabs radiated, clams wen extinct
Evidence suggests that extinction of clams due at least in part to increase in crab predation
Drivers of extinction: Indirect effects
Cascading effects of ecosystem disruption can result in extinction
Predation can be particularly likely to lead to extinction when there is one dominant prey species
Why are island foxes an example of indirect drivers of extinction?
Because disruption of the ecosystem led to cascading impacts, culminating in (near) extinction
Drivers of extinction: Competition
We can look at how fossils replace each other for evidence of extinction via competition
there is always overlap between a dominant species in decline and an increasing species that will become dominant in the future
-Y axis shows percent of taxonomic representation in fossil record
Plant species destined to become dominant often have novel morphologies that are more efficient at gathering light and transporting nutrients
Suggests competition leads to community turnover
Extinction and disease
Infectious diseases can cause large population declines
Amphibian populations have dropped precipitously in the last 30-40 years
Tropical rainforest frogs are going extinct in Australia
Declines proceed in a south-to-north pattern, suggesting a disease front and have precipitous drops
Found to be due to chytrid fungus
Compounding extinction drivers
Typically multiple causes of exinctions
Since human arrival, 90-110 of the ~135 bird species endemic to Hawaii have gone extinct
-The vast majority of Hawaiian birds have gone extinct
Drivers include disease, hunting, predation, habitat destruction
WHICH species go extinct depends on the driver. In Hawaii, the first birds ot go extinct were large (due to hunting). Later extinctions were of insectivores and nectarivores, due ot habitat conversion for agriculture
Why are island and endemic species more likely to go extinct?
a) Population sizes are smaller on average
b) Evolving in small communities means less exposure to predators and pathogens and thus greater susceptibility to perturbations
c) High levels of specialization may mean higher extinction risk if the environment changes
d) All of the above
d) All of the above
Rates and patterns of evolutionary change
How quickly do new species arise?
how do changes accumulate over time?
The study of long-term evolutionary change is called MACROEVOLUTION
Microevolution
How allele frequencies change from one generation to the next
Asked for EACH population
Macroevoltution
How do large groups of organisms change over long periods of time
Rates of evolutionary change: 2 schools of thought developed in the 1970s: Phyletic Gradualism
Adaptations arise in populations due to slow, gradual process
Beneficial variants slowly increase in frequency in populations
New species arise from gradual transformation of ancestral species through slow, constant change
Example: Evolution of equines
-We have a very good fossil record for horses
New species appear in fossil records through:
- Cladogenesis: Branching speciation events
- Anagenesis: Modification over time WITHOUT branching (causes pseudoextinction)
Rates of evolutionary change: 2 schools of thought developed in the 1970s: Punctuated equilibrium
Most lineages are static for long periods of time, followed by a burst of rapid change of caldogenesis (branching events)
Speciation most frequent in small, isolated populations (allopatry + strong drift)
-Cut off gene flow; genetic drift occurs in SMALL populations
This suggests bursts of speciation - imaging island archipelagos with periodic migration to and from mainland
If migration is INFREQUENT, develop into separate species
However - islands are often ephemeral due to changing sea levels, which means island species may not fossilize. We have much better fossil records from continents
All that preserves in fossile record is MAINLAND, not little islands
The fossil record therefore shows punctuated “bursts” of new species due to non-random missing information in the fossil record
Cambrian Explosion + Punctuated Equilibrium
held up as example of punctuated equilibrium
Period when most of the major groups of animals first appear in the fossil record
Most fossils come from the Burgess Shale, which preserved soft-bodied organisms
Seems to be a time in history when evolutionary change was very rapid
This pattern occurs in bryozoans over last 20My
May be due to rising and falling sea levels
Punctuated equilibrium vs. phyletic gradualism
Evidence for BOTh in the fossil record
Current research focuses on conditions under which punctuation vs gradualism are mroe likely
Studies of punctruated equilibrium suggest that:
a) Rates of speciation vary widely through time
b) Rates of speciation may not vary that much through time, but our ability to detect speciation events is biased to make it look like rates vary
c) There is not enough information to determine how speciation rates vary over time
d) A and B
d) A and B
Evidence for phyletic gradualism and punctuated equilibrium in fossil record
Punctuated equilibrium: A whole bunch of forms appear out of nowhere in the fossil record
- Some of those cases are probably just artifacts of fossil record
- -I.e. we tend to see species diverging on islands, don’t usually have good fossil records on islands, when those island species go back on mainland, it LOOKS like rapid diversification
We also see some instances where it was just an artifact
Evolutionary trends
When do we see directional changes over time within groups?
Example: Species in mammalian clades increase in size over time (Cope’s Rule)
Why do trends occur?
Evolutionary trends: 3 possible outcomes
1) No evolutionary trend: Body size is as likely to increase as to decrease
2) Passive trend: There is a constraint on trait values, but away from that line of constraint, traits are as likely to increase as decrease; NO directional tendency, start with min trait value (i.e. you can’t be smaller than 3ft tall; as long as you’re bigger than constraint, you will be selected for
3) Active trend: each lineage tends to increase in body size
- -Favors increase in body size
Across 39 species and 854 traits, selection favors increased body size
Selection for increased body size associated with speciation in fishes
-Evolutionary trend for larger body size
Active trends
2 processes generate active trends in trait evolution
Distribution in morphology between 2 big groups of organisms
Within the CLADE, distribution has shifted over time
Process 1:
-Trait values could shift over time due to differential extinction and speciation (e.g. small species go extinct, large species survive and speciate)
Process 2: The distribution of trait values within a clade shifts over time because each species is changing in the same direction
- Parallel evolution WITHIN CLADES
- Individuals evolved to have bigger body sizes
Active trends in crustacean limb morphology
Adamowicz et al compiled data on limb complexity for 66 crustacean orders
-Suggests a lot of species
Looked at how complexity in limbs changes over time
-Found that limb complexity INCREASES over time
The absence of minimally complex orders in the present day suggests this is an active trend
To test if this is due to species selection or parallel change within clades, compared 12 pairs of fossil clades and their closest living relatives
Species selection: Differential extinction of groups with simple limbs
Parallel change: Groups that HAD simple limbs evolved more complex limbs over time
Within each clade, the modern relative had more complex limbs than the fossil species, indicating parallel changes within clades over time
there was also evidence for species selection: younger taxa had more complex limbs, suggesting a correlation between speciation rate and limb diversity
-Maybe species with more complex limbs more prone to speciation
Extinction rates where higher in clades with low limb diversity
-Results indicated a bit of both methods
