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
Which is TRUE of active trends?
Trait values change in the same direction across clades over time
2 types of reproduction:
Asexual: Genetically identical; present in bacteria
Sexual: 2 genetically distinct come together; most eukaryotic organisms
Sexual reproduction
Genetically distinct from family -> Allows species to EVOLVE = improves survival
hemoglobin gene: 2 abnormal copies = sickled cells; 1 abnormal cop = mild disease, resistance to malaria
how does sexual reproduction generate genetic diversity?
Meiosis
1) Homologous recombination
2) Independent assortment (geneticaly distinct)
Sexual mating is:
Energy consuming
Slow
Risky
Generation of gametes + gestation exhausting
Advantages of Asexual Reproduction
Energy efficient
Fast (some as fast as 20 minutes)
Genetically identical offspring
Rare random mutations = genetic diversity (SLOW process)
Defining asexual reproduction
Asexual reproduction is the production of offspring from unfertilized gametes
unfertilized gametes are produced by mitosis-like cell division, producing identical daughter cells with same number f chromosomes
Produce haploid gametes, with restoration of diploidy by fusion of haploid nuclei from same meiosis. Offspring are not genetically identical to parents and siblings, but much less genetic variation is produced
Defining Sexual Reproduction
Joining together of genetic material from 2 parents. This is characterized by alternating stages of meiosis and gamete fusion
1) Recombination between homologous chromosomes
2) Gamete production: haploid gametes produced by diploids
3) Gametic fusion: haploid gametes fuse to create a diploid offspring
Identifying sexual vs. sexual reproduction
Classic approach: Morphology, behavior
Now: Genetics! (identification of genes involved in sexual reproduction, phylogenies of nuclear vs mtDNA, microsatellite studies)
Female reptile asexually produced fertile eggs via parthenogenesis
Able to reproduce sexually or asexually
Phylogenetic History of asexual and sexual reproduction
Of 42,000 recognized vertebrate species, only 74 are exclusively asexual
Phylogenetic evidence shows that asexual taxa go extinct more quickly than sexual taxa
There are NO asexual clades - asexual reproduction appears randomly and rarely across the broader vertebrate tree
Genes associated with meiosis are found in asexual species, suggesting sexual reproduction is the ancestral state
Why is sexual reproduction so great?
Most multicellular eukaryotes reproduce sexually
Even those that have asexual reproduction will often reproduce sexually on occasion
-Facultative sexual reproduction when conditions warrant it
Sexual reproduction is COSTLY
You only transmit HALF your genes to your offspring in sexual reproduction vs 100% in asexual reproduction
How can this be favored by natural selection?
The Twofold Cost of Sex
John Maynard Smith’s model
1) Imagine a population with asexual females and sexual males and females
2) The asexuals will increase at twice the rate of sexuals
3) Males do not produce offspring - they just contribute gametes to females (“cost of males”)
Two-fold cost of sex:
1) Only transmit half of genes
2) Producing males is waste of reproductive effort because they only contribute gametes to someone else, do not produce any offspring
Evidence for the twofold cost of sex (“cost of males”)
New Zealand snails have asexual and sexual lineages
Experimental tanks were made of 120 sexual snails and 65 asexual snails (35% asexual)
After one year (2-3 snail generations), frequency of asexuals was on average 62%
- Asexual snails are outcompeting sexual snails
- Shows clear benefits of asexuality
Other costs of sexual reproduction
Search costs for mates
Courting mates takes time
Searching for and courting mates exposes you to predators
Sexual reproduction can also lead to transmission of parasites and diseases
Sexual reproduction breaks up co-adapted gene complexes by breaking down linkage disequilibrium
Can break up beneficial haplotypes with sexual reproduction
What are the two costs of sex under the Maynard Smith model?
You only pass on 50% of your genes, and half of your offspring don’t produce their own offspring
Why is sexual reproduction so great?
There must be some individual benefit to sexual reproduction for it to be favored by natural selection
2 main categories of hypotheses
1) Sexual reproduction purges deleterious mutations
2) Sexual reproduction generates novel genetic variants for natural selection to act on
Deleterious mutations and Muller’s rachet
When a deleterious mutation arises in a sexual population, offspring without that mutation can be produced
If a deleterious mutation appears in an asexual population, it cannot be removed except by lethality
Muller’s rachet: The irreversible accumulation of deleterious mutations over time. Assumes that mutations do not mutate back to the wild type state
Asexually reproducing lineages will just accumulate deleterious mutations
A rachet can only turn in one direction
Can only go in one direction, can’t go backwards
Now all chromosomes have 2 deleterious mutations. Over time, more mutations will accumulate
Why would genetic recombination reverse the rachet?
Because it generates new haplotypes with fewer deleterious mutations
Recombination __________ the rachet by creating chromosomes (haplotypes) with fewer deleterious mutations
REVERSES
Tests of the “rachet” hypothesis
Asexual populations of freshwater snails have more mutations that similar-aged sexual populations
In Daphnia, deleterious mutations accumulate 4 times faster in asexual vs sexual lineages
The rachet and the y chromosome
Y chromosomes do NOT recombinne because they have no homologs
-There is no YY
Comparing the Y chromosome to other chromosomes in sexual species shows Y is degenerated - it is smaller, has fewer functional genes, and has more non-functional genes
Suggests deleterious mutations accumulate more quickly on the Y
The Fisher-Muller Hypothesis
Imagine 2 large populations, one sexual and one asexual
Imagine 3 different beneficial mutations, A, B, and C, emerge in 3 different individuals
How do these beneficial mutations spread in asexual vs sexual populations?
No way for beneficial alleles to quickly move onto the same genetic background in asexual organisms
No way for all individuals in the populations to get all 3 beneficial mutations
In sexual populations, recombination quickly brings together beneficial alleles
Predicts that sexual recombination increases speed at which evolution operates
-Can much more quickly move beneficial mutations through the population
In yeast, sexual strains improve their fitness in a harsh environment faster than asexual strains
No difference in benign environments
How would predict asexual vs. sexual lineages respong to new environments?
Sexual lineages are able to adapt faster to new environments
The Red Queen Hypothesis and Sexual Reproduction
organisms must constantly evolve to escape parasitism
Organisms must constantly evolve to stay in current niche
The minute you stop running/adapting as fast as you can, you LSOT
Red Queen comes from scene where running as fast as they can but are not moving (staying in place)
The Red Queen Hypothesis
Organisms must constantly evolve to escape parasitism
Sexual reproduction allows organisms to generate new genetic backgrounds and ways to resist parasites and diseases
Asexual species may occasionally have higher fitness if they are initially resistant to a parasite
However parasites will quickly overcome host defenses
-Much more challenging for ASEXUAL lineages to respond
Which do you think is NOT a predicted outcome of the Red Queen hypothesis?
a) Oscillations in the relative frequency of asexual lineages within populations when parasites are present
b) Time lags between the emergence of a host defense and the evolution of pathogen traits that counter that defense
c) Correlation between parasite load and sexual reproduction
d) Maintenance of linkage disequilibrium and co-adapted gene complexes in sexually reproducing hosts
d) Maintenance of linkage disequilibrium and co-adapted gene complexes in sexually reproducing hosts
Red Queen: biological imperative
Pass on genes
Most species use sexual reproduction
When you mate with another individual, forfeit half your genes
40% minnows heavily infected with parasite that causes black spot disease
- Asexual minnows much more affected by parasites
- Sexual minnows had LESS parasites
Value of males outlines in Red Queen hypothesis:
- Lee can Halen: Does evolution stop when things are perfectly adapted? NO, keeps going fast
- Run as fast as you can to stay in the same place = adapt as fast as you can to occupy the same niche
Cloned fish stop evolving easy target for parasite
Sexual fish are genetically unique, parasite must attack each one differently
The Red Queen Hypothesis
Sexual reproduction is beneficial because it is a moving target for parasites
Asexual can spread genes more quickly due to cloning; if clone is resistant to local pathogens, it will increase; HOWEVER, as this asexual clone lineage increases in frequency, selection will favor pathogens that can exploit this new lineage; creates strong selective pressure on parasites
Once parasite has figured out a way in, the asexual can’t get it out, it just clones same immune system; if new beneficial mutation doesn’t crop up soon enough, this lineage will go extinct
The Red Queen hypothesis: evidnce in snails
Intitially, a clone reaches high frequencies ina population because it ahs some resistance ot parasites over time, parasites evolve to exploit the immune system of clone; Percentage of individuals with phenotype increases, frequency of genotype
in which kind of environment might sexual reproduction be favored?
Unpredictable or variable environments, because it is more likely that one or a few offspring does well in that environment
Environmental unpredictability and sexual reproduction
Environments are VARIABLE and UNPREDICTABLE
These environments may favor sexual reproduction
- Increases the chance that at least one offspring will be a good match to the environment in which it finds itself
- Individuals with similar genotypes likely compete, so allows offspring to find new niches and reduce competition among siblings
Evidence:
Cyclical parthenogens switch between sexual and asexual reproduction
–Shift from asexual to sexual reproduction when environment changes
–Daphnia switch to SEXUAL reproduction when there are new predators and decreases in food quality
Anisogamy
Difference in gamete size between males and females
Imagine a marine organism that sheds gametes in the water
Gametes fuse to produce zygotes
There is a wide range of gamete sizes
There is a tradeoff between gamete size and number
-Many small gametes, or few large gametes, or intermediate number of intermediate-sized gametes
Larger gametes are less mobile and larger zygotes survive better
-2 large gametes will produce larger zygotes more likely to survive
Most zygotes will be small, because individuals can produce many small gametes
Survival of these zygotes is lower than zygotes produced by fusion of rarer, but larger, gametes
If a small gamete can mate with a large gamete, it will produce larger offspring
It is better to be a large gamete because your zygote survives better
However, large gametes are rare, and it might be hard for you to find another one to fuse with, lowering your fitness due to search costs
Selection favors larger and smaller gametes (eggs and sperm) that fuse with each other
There is strong selection against intermediate gametes
-Intermediate gametes cannot disperse as well
Gametes differ in investment of resources
-Large gametes invest a LOT of resources ot zygote; if you produce many small gametes, hope fo rht ebest
Better to produce few large or many small than any intermediate
Small gametes (sperm) evolved to parasitize the investment of large gametes (eggs)
If you are a SMALL gamete what size gamete should you want to fuse with?
A large gamete
You will produce large zygotes with higher fitness
If you are a LARGE GAMETE WHAT SIZE GAMETE SHOULD YOU want to fuse with?
A small gamete
Avoid spending too much time searching for large gamete
