Unit 3 Exam

Species and speciation

• formation of “new” species is a primary focus of evolution

• but defining species can be difficult

• no single definition of a species is sufficient for our needs

Biological Species Concept

• the BSC is maybe the most widely accepted

• a species is a group of individuals that can produce reproductively viable offspring

• definition makes logical sense

• works in many cases

• does not work in all cases:

  • asexual reproduction

  • ring species

  • species that can interbreed but are definitely species

Ecological Species Concept

• species are a group of individuals that occupy the same ecological niche

• where the niche is an n-dimensional representation of the necessary resources for an individual to survive

• by definition two species cannot occupy the same, exact niche (Principle of Competitive Exclusion)

• defining a niche is nearly impossible (n-dimensional)

• How different much niches be before once can say they are different enough?

• maybe more difficult with extinct species

• also, not an easy definition to explain to a non-scientist

Phylogenetic Species Concept

• a species is defined by having a distinct evolutionary history

• removes difficult to answer questions (e.g., niche)

• accounts for parallel evolution

• not easy to explain

• How unique is unique?

• maybe more difficult to use with inly morphometric characters

A single definition may not do

• there is a no reason we must only use one definition

• being unable to breed is pretty obvious evidence

• but if two species can interbreed but have obvious evolutionary differences they can still be valid species

• evolutionary significant unit

Speciation

• Let’s pretend that we have a workable definition of what a species is

• now we want to know how species arise

• this is classes speciation

• there are three main routes to speciation:

  • allopatric

  • parapatric

  • sympatric

Allopatric speciation

• most widely accepted mode of speciation

• requires geographic isolation: two sub-populations that do not occur in the same area

• since the populations are separate gene flow is eliminated

• overtime the population become different species

Vicariance

• a geographic barrier arises between the two populations

• isolation leads to new species

• don’t thinking current geography

Peripheral Isolate

• population isolated on the edge of the main population

• most obvious example: an island population, in modern times this could be an urban island

• the island population is isolated enough that gene flow eliminated/heavily curtailed

• in both cases: each population evolves independently

• overtime reproductive isolating mechanisms will arise, or the niches will diverge enough etc

• the two populations then can be considered two species

Allopatric speciation in spotted skunks

• glaciers divided population

• diapause extended in western, so breeding season is different

Parapatric speciation

• speciation without geographic isolation BUT with gradient in phenotypes

• the two (or more) phenotypes are isolated from each other by a hybrid zone

• over time the isolated groups can deviate

• ring species may be an example of parapatric speciation

• the different phenotypes are in the process of speciation

• just haven’t gotten there quite yet

Sympatric speciation

• speciation without geographic isolation

• a little more controversial

• nothing prevents deneflow, so most modules would predict that speciation unlikely due to genetic changes being swamped

• sympatric can happen in plants with polyploidy, causing rapid genetic isolation

• disruptive selection also could lead to two phenotypes relatively rapidly

• can be reenforced through sexual selection

Reproductive isolating mechanisms

• reproductive isolation is important in speciation

• even if not total the isolation can be practical (e.g., wolves and coyotes)

• isolation can be broken down into prezygotic and postzygotic isolation

• prezygotic isolation either prevents mating or successful fertilization

  • these can be behavioral, physical, or physiological

  • mating rituals (song or dance) can act as a barrier

  • many species have induced ovulation

  • requires a specific stimuli to occur (sometimes called lock and key)

  • plants can also have pollinator mediated isolation

  • if a flower A is pollinated by moth X and flower B by moth Y interbreeding would be eliminated/severely reduced

  • prezygotic methods may 100% eliminate breeding, but often are less than that

  • this accounts for some of the issues with the BSC, yes two groups can interbreed, but in the wild they do not

• postzygotic isolation is considered more complete

  • generally, these are genetic incompatibilities

  • ploidy in plants

  • chromosome number or alignment

  • these may prevent cell duplication or salt in lethal traits

  • hybrids can be produced, but fitness is very low

Sexual selection

• talked about natural selection, thought of as effects on survival

• but fitness is measured in offspring, so one has to survive to reproduce, but just because one survives does not mean that one will have high fitness

• this is where sexual selection comes in

Why sex?

• in general, multicellular species reproduce sexually while unicellular reproduce asexually

• BUT there is a lot of variation in this

• in singular-cells fission is the most common form of asexual reproduction

• much like mitosis

• in multicellular fission does not work, but gametes can be produced in either diploid (apomixis) or haploid ( automixis) form

• either way these produce, more-or-less, a clone of the parent

• in sexual reproduction gametes are produced through meiosis and the offspring are components of the two ( usually) parents

• but why choose one or the other (Some species can do both)

Asexual reproduction

• Advantages:

  • no two sexes, don’t have to find a mate

  • no worries about losing positive traits (outbreeding)

  • no “wasted” gametes

• Disadvantages:

  • slower to adapt without genetic recombination

  • no way to purge deleterious alleles

Sexual reproduction

• essentially the opposite of asexual reproduction

• Advantages:

  • genetic recombination

  • elimination of deleterious alleles (Muller’s ratchet)

• Disadvantages:

  • must find mates

  • waste of gametes

  • dilution of traits

The most significant benefit is accelerated adaptive evolution with sexual reproduction

• in asexual reproduction lineage must produce mutation separately

• in sexual reproduction mutations can be shared in a population

• this results in much faster adaptation

Pretend that ABC is the best genotype to have

• in an asexual population this can be accomplished with serial mutations

• one group gets the A mutation, then must wait for the B and then the C

• in sexual reproduction the ABC genotype could appear much more quickly

Although sexual selection has distinct advantages there are times that asexual is better

• sexual selection has an advantage in a variable environment

• whereas asexual selection can be “better” in a constant environment

• think the bacteria in the gut, the environment does not change

• genetic recombination is more likely to make one less fit

Sex ratio

• in most sexually reproducing species there are two sexes, one for each gamete type

• the sex ratio at birth is nearly 50:50

• but why would that be?

• selection pushes for a 50:50 ratio

In the short-term natural selection should favor having more males

• males can fertilize many females whereas females can only have so many offspring

• producing males will increase fitness much more than producing females

• BUT within a few generations there are so Manu males that the average fitness for a male decreases below that of a female

Sexual selection

• once a species “decides” on sexual reproduction there are other “decisions” to be made

• these include breeding system, mate choice and frequency of reproduction

• in general, sexual selection is driven by females due to a difference in incentives

Female are limited in their reproduction, there is a finite number of eggs and maximum that one can produce in a breeding season

• males do not have this issue, males produce infinite sperm (practically) and can inseminate multiple females

• so a female is pushed to be selective in who she breeds with

• a male is pushed to breed with as many females as possible

• this dichotomy is key in sexual selection

Breeding systems

• there are two main breeding systems: monogamy and polygyny

• monogamy is one female with one male, there are different subtypes (e.g., social monogamy)

• polygyny is one male with multiple females

• polyandry exists (more than one male per female) but this is rare

Monogamy

• monogamous breeding systems tend to be found under certain conditions

• resources are not defendable

• a make biased sex ratio

• reproduction is too difficult for one parent to succeed

• monogamy can be reenforced in some species

  • prairie voles (Microtus ochrogaster) receives hits of oxytocin and vasopressin when smelling their mate

  • has been called the neurobiology of love

  • but that does not mean they don’t mess around

Polygyny

• polygyny evokes under two general conditions

• resources are defendable

• male dominance hierarchy

Mate selection

• regardless of the mating system the females selects a mate

• females select their mate based on signals of quality

• this may e physical characteristics or behavioral queues (e.g., song or dance)

• this leads to sexual dimorphism and secondary sexual characteristics

• things like color spots, antlers or other ornaments advertise the quality of the male I

In male-dominance systems the choice may be made for the female

• when males fight the winner gains an increase in testosterone whereas the loser decreases testosterone

• continued losing can make a male less fertile

What are females choosing?

• the idea is that the secondary characteristics are indicating male quality to the female

• there are two similar, but competing ideas on what the signals mean

• sexy son hypothesis

• good genes hypothesis

Sexy son

• females select the “sexiest” male so that they will produce sexy sons that other females will choose

• females offsprings are kinda ignored, but they also would have the sexy traits to pass to their sons, and the females don’t have to be sexy

• does not directly relate to genetic, but likely associated

• maybe the size of antlers in moose is the sexy trait

• maybe it is interior design in bower birds

  • this can lead to runaway selection

  • a positive feedback between a trait and fitness

  • big antlers lead to higher fitness so antlers get bigger and fitness goes up which leads to bigger antlers…

  • can have bad consequences

Good Genes hypothesis

• the secondary sexual characteristics advertise the quality of the male

• usually, the characteristics is thought to advertise wastefulness

• the characteristics is expensive to produce, the males who produce the most are so awesome they can waste resources on these ornaments

it is hard/impossible to figure out what is correct

• it likely varies among species

• sexy characteristics also give some idea of quality

• a trait that singles good genes also may become “sexy”

• the key is that the traits must be heritable and honest

But of course it is really more complicated than this

• in species that have multiple offspring there are frequently multiple fathers

• in male dominance systems there are often sneaker males

• they do not breed as much but at least some

Monogamous systèmes are less Disney movie and more Soap Opera

• extra pair copulations (EPCs) are common, litters have multiple fathers

• make sense for both sexes

• males is obvious, but females increase genetic diversity and ensure pregnancy

Frequency of reproduction

• the last thing to “decide” is how frequently to reproduce

• there are two choices:

  • Iteroparous: reproduce multiple times in a lifetime

  • Semelparous: reproduce only once in a lifetime

Iteroparity is the most common in vertebrates

• having multiple years to breed allows risk migration

• if one breeds in a “bad” year and the offspring don’t survive you can try again the next year

• it can be less than years, small critters live on a monthly time scale

• but iteroparity requires that na individual has fewer than the maximum offspring

• one must balance current and future reproduction

• if one has a lot of younger this year they will produce fewer next year

• the future also is not guaranteed, one may bot be alive to reproduce next year

Semelparity is more common in invertebrates, but present in vertebrates

• has the advantage of no strategy, one would produce the maximum number of offspring

• does not allow risk mitigation, if it is a bad year that does not matter

• the hope is that at least a few offspring survive

Plants do a similar calculus

• plants can be annual, biennial, or perennial

• annual and biennial species reproduce once perennial multiple times

• the tradeoffs are the same, perennial must balance reproduction over a lifetime

Evolution of Sociality

• sociality is a bit of a conundrum in evolution

• being social has some advantages:

  • easier to find food (particularly if clumped)

  • easier to detect predators

• being social has some drawbacks too

  • easier for predators to find you

  • diseases spread much faster

• but the biggest issue is that social species must compromise

• evolution is basically selfish, I want to maximize my fitness, if I have to compromise then I cannot maximize fitness

it essentially boils down to altruism- doing something that benefits another with no direct expectation of a return

• examples of this could be warning others that a predator is near

• or letting other individuals know about a food source

• without this social behavior does not work

• but with them it costs the individuals who commit the altruistic behavior

Types of social species

• there is a continuum of social species

• on one extreme is something like heard animals, the association is a little loose and there may be relatively little interaction among members

• the other extreme is eusocial species

• in eusocial species only one member of the group reproduces

Eusociality

• eusociality has evolved multiple times

• most recently found in insects, particularly Hymenoptera

• but found in a variety of taxa

• the details vary but all have a single breeding female whereas other members of the group serve as workers of some sort

Eusociality and kin selection

• one model for its evolution is based on kin selection

• individuals maximize their inclusive fitness rather than individual fitness

• members of a eusocial “super organism” are all related, often by more than 50%

• raising the offspring of one sister still passes on your genes

• this model also suggest there are preconditions for eusociality

• characteristics like highly clumped and dispersed resources, large colonies (e.g., a beehive), or care for altricial young promote the evolution of a eusocial system

• this works for insects with haplodiploid (males develop from unfertilized eggs)

• in this case sisters are much more closely related than 50%

Eusocial evolution through natural selection

• kin selection does not work well without haplodiploidy

• an alternative mechanism through natural selection has been proposed

• a social species evolves eusociality through a series of steps promoted by natural selection

the basic model:

• a social nesting insect has a mutation. that silences dispersal

• increased colony size increases fitness of the nest

• further mutations follow that silence reproduction in all but one individual

• increased fitness due to increased size and reduced mating competition

• this model does not include explicit inclusive fitness

• the benefit is from the size of the colony and reduced competition

• but only works with clumped resources and a species that is already pretty social

• does work for any system and does not require ploidy shenanigans

Less social evolution

• less extreme sociality faces many of the same issues

• individuals must sacrifice something in order to be social

• in order to be social there must be some sort of benefit

• these cases are not rare, but are necessary conditions

predators can be benefited being social

• social predators (e.g., lions or wolves) can take down larger prey than a solitary predator of a similar size

• social foragers can spread to different locations to find ripe fruit

• being social, in and of itself, is not the issue, it is getting along with the other individuals in your group

basically it boils down to cheating

• if you share something you are hurt and the other is benefited

• so its pays to have others share with you and not share

• cheating would become more common and the world ends

• how can this be combated?

Prisoner’s dilemma

• two people (A&B) are arrested for a crime, the police do not have enough evidence to convict for this crime, only a lighter charge

• A and B are separated and interrogated

Change this to sharing in a social species

• there is no advantage, in a single event, to share

• there is a large advantage to a moderate cost to not sharing

• this suggest that altruistic behavior should not appear

but if you repeat the scenario and the players remember what happened previously then things change

• if A has some expectation that B will share then it is advantageous for A to be selfish

• BUT B should no longer share with A if A does not reciprocate

This is called the tit-for-tat strategy

• basically, treat another as they have treated you

• this makes altruistic behavior a viable strategy

• other games show that there can be a benefit to social behaviors

• but in all of them cheating (freeloading) must be punished

evolution and extinction

• extinction is an important part of evolution

• it is the ultimate fate of all species

• but also plays a critical role in diversification of a lineage

• new species arise when others go extinct

What is extinction?

• extinction is when all members of a species have died out

• it does not necessarily need to be at the species level

• it also could be localized- the species goes extinct in Kansas but not Missouri

• this is called extirpation

extinction is (or can be) a natural process

• it has long been recognized as a part of the evolutionary process

• each phylogeny that we see implies extinction of the majority of species

the fossils record is another obvious source of info on extinction

• the fact that we find skeletal parts from creatures that are no longer found makes that obvious

• important to not that the fossil record, while great, is an incomplete record of life on earth

We can date the rocks surrounding a fossil (can’t dat the fossil) to determine the age of the fossil

• this can help inform our phylogeny

• but the reverse can be true too

• we have a phylogeny and can predict at what age we should find specific fossils and then go find them

• Tiktaalik roseae, a 375 million year old transitional fossil often referred to as a “fish with legs”

Background extinction

• extinction, being a natural process, is happening all the time

• natural rate soft extinction are termed background extinction rates

• this does account for the vast majority of extinctions in history

• background extinction rates are estimated to be 0.1 to 1 extinction per million species per year

• around 2 million named species on earth

• estimated to be least 4 million total

• so this gives an estimate of 0.4 to 4 extinctions per year

• this is a really difficult number to estimate, but gives us an idea

• biased toward species with small geographic ranges

• also to species that are uncommon/rare

• difficult/impossible to “prove” that a species has gone extinct

• especially if you didn’t know that the species existed

What causes extinction?

• the short answer is change

• a species is adapted to a set of conditions, when those conditions change the species can be in trouble

• this is particularly true if the change is happening faster than the species can evolve

predation and competition

• for both of these it requires a new predator or competitor

• a new species moves into your spot and you are not adapted to the new species

• climate change (non-anthropogenic)

• the climate shifts, it gets colder or warmer and you cannot deal with it

mass extinction

• mass extinction occurs occasionally and are associated with geologic periods

• these are widespread events that significantly decrease the diversity of life on the planet

• usually wipe out more than 70% of the species

There are 6 widely accepted mass extinction events

• they are caused by different factors

• the great dying is at the end of the Permian (252 mya)

• 70-90% of species went extinct

• cause believed to be climate change associated with volcanic activity

The K-T(pg) extinction wiped out the cool dinosaurs

• due to climate change associated with asteroid impact in the Yucatan

• ~ 75% of species go extinct

Late Pleistocene extinction (50,000 ya)

• not recognized as one of the “Big 5’ but resulted in extinction of much of the megafauna worldwide

• thought to be related to climate change and human activity (particularly in North America)

Bias in mass extinction

• mass extinctions wipe out large numbers of species

• tends to bias towards large species

• T. rex went extinct but birds did not

• this is for several reasons

Large species require more land on which to live

• if geographic range is restricted that is a problem

• large species require more food

• food is limited due to change in climate

• large species have longer generation time

• reduced ability to adapt

The effect of extinction

• extinction is obviously bad for the species that go extinct

• but hose that survive are in different world than what was before

• extinction opens niches in the ecosystem

• basically, allows space for species to evolve

recall that the niche is all the resources a species needs to live

when niches overlap there is competition between species

when the ecosystem is intact there is little to no room for a species to evolve

• all the “spots” are taken

• moving to another resources puts you in competition

But if some species go extinct then niches open up

the surviving species can change to exploit the new, competition free areas

the effect is the same with any extinction but more profound in mass extinctions

• synapsids (mammal ancestors) went extinct at the end of the Permian, allowed Dinosaurs to take over

• dinos lost the lottery with the K-T allowing mammals and birds to take over