IB BiologyOption D

D2 Species and SpeciationAll syllabus statements ©IBO 2007All images CC or public domain or link to original material.

Jason de Nys

“…if I really have as bad an expression, as my photograph gives me, how I can have one single friend is surprising."

http://www.flickr.com/photos/limowreck666/171979083/

D.2.1 Define allele frequency and gene pool

al·lele fre·quen·cyAllele frequency is the proportion of all copies of a gene that is made up of a particular gene variant (allele).

gene poolNoun: The total collection of different alleles in an interbreeding population.

ExampleSay if a recessive allele h made up

2% of the total in a human population…

…then the dominant allele H would make up 98%.

The frequency for h would be expressed as 0.02 and for H 0.98Recessive allele frequency + dominant allele frequency = 1

(for characteristics determined by two alleles)

D.2.2 State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations

New combinations of alleles lead to new phenotypes that can then be selected for or against by the environment. This leads to

evolutionary change in the species

D.2.3 Discuss the definition of the term species

Ecological speciesA set of organisms adapted to a particular set of resources, called a niche, in the environment.

Genetic speciesBased on similarity of DNA of individuals or populations. Having a common gene pool.

Cladistic SpeciesA group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, members may diverge from one another: when such a divergence becomes sufficiently clear, the two populations are regarded as separate species. (This differs from the Evolutionary definition in that the parent species goes extinct when two new species are recognised).

Evolutionary speciesA group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, some members may diverge from the main population and evolve into a subspecies.

Breeding SpeciesTwo organisms that are able to reproduce naturally to produce fertile offspring of both sexes.

There are many definitions, here are five!

Lots to discuss if you get a question about this!

http://en.wikipedia.org/wiki/Species

The genetic definition is most widely used and works well for most multicellular organismsThe following 4 slides outline the exceptions:

…is anything ever simple in Biology?

It could be (ahem) physically impossible for members of the

same species to mate.

Therefore they are genetically isolated.

http://www.flickr.com/photos/lepetitblonde/2247963815/

Hors d’oeuvre?

1 : Lesser Black-backed Gull2 : Siberian population Black-backed gull3 : Heuglin's gull4 : Birula's Gull5 : East Siberian Herring Gull6 : American Herring Gull7 : Herring Gull

1 and 7 cannot produce offspring.

Ring Species:Adjacent populations can interbreed but the populations at the “end of the line” cannot.

http://commons.wikimedia.org/wiki/File:Ring_species_seagull.svg

So close…

…and yet so far :’(

The liger is a hybrid cross between a male Panthera leo (lion), and a female Panthera tigris (tiger) and is denoted scientifically as: Panthera tigris × Panthera leo.

The tiglon is a hybrid cross between a female Panthera leo (lion), and a male Panthera tigris (tiger) and is denoted

scientifically as: Panthera leo × Panthera tigris.…conversely…

Ligers and tiglons sometimes produce offspring when mated back with a parent species

e.g. The hybrid of a male lion and a female tiglon is a li-tiglon!

MADNESS!!

Hybrids are usually infertile and can not produce offspring together, for example the mule (63 chromosomes): a cross between a Male horse (64 chromosomes) and a female donkey (62 chromosomes)

http://xkcd.com/419/

Why are 63 chromosomes a

problem when reproducing?

The genetic definition only applies to sexually reproducing organisms and doesn’t apply to single-celled organisms

Rats!

http://www.flickr.com/photos/microagua/3721497804/

Additionally:

Fossil remains can’t tell us whether species were able to interbreed or

produce viable offspring so palaeontologists tend to use the

cladistic definition

?

http://www.flickr.com/photos/bruce_mcadam/1393218574/

D.2.4 Describe three examples of barriers between gene pools

The circumstances preventing different species from interbreeding are known as reproductive isolating mechanisms

Temporal isolation

J F M A M J J A S O N D

MAXPinus radiata (Monterey Pine)Pinus attenuata (Knobcone pine)

Polle

n Pr

oduc

tion

Pinus radiata and Pinus attenuata are prevented from hybridising because they have separate pollination times.They can be made to hybridise by pollinating them manually.

http://www.flickr.com/photos/alancleaver/4293345631/

*Random fact: The Monterey pine is at risk in it’s native range but is one of the most common plantation trees in the world. If you see a pine forest in Australia or NZ, it is probably Pinus radiata

Month

Ecological isolation

The two species are in the same area, but live in different habitats

http://www.flickr.com/photos/annetanne/3035068940/ http://www.flickr.com/photos/carinemily/644052381/

I love me some

CaCO3 in my soil

Blechhh!Acidic soils

are more my thing

Viola arvensis Viola tricolor

Behavioural isolation

Animals exhibit courting behaviour (song, dance etc.) or release pheremones to

attract mates. Individuals are only attracted to, and will only mate with, members of the opposite sex who perform the appropriate

ritual or release the correct chemical.

Yo! I don’t like your music!

Its like, totally

mutual!

http://www.flickr.com/photos/rowelbg/2895578034/http://www.flickr.com/photos/nrk-p3/2333221093/

Mechanical isolationAnimal example:Different species of bush baby (Galago) have particular shapes for their genitalia and they are physically incapable of copulation*.

It is like a lock and

key.

In plants, mechanical isolation occurs when different species have different pollinators that are not able to service

the flowers of other species

*Take care when Googling “Bush baby genitalia”!

http://www.flickr.com/photos/joachim_s_mueller/4113758487/

She says “We’re not a good fit”.

What is that supposed to

mean?

Hybrid Inviability

Remember: Male Horse + Female Donkey = Mule

Horse 2n = 64 sperm∴ n = 32

Donkey 2n = 62 ovum ∴ n = 31

Sperm + Ovum = Mule zygote32 + 31 = 63

Mule 2n = 63 gamete n = ?∴

Note: Sad eyes

http://www.flickr.com/photos/pirateparrot/301798478/

D.2.5 Explain how polyploidy can contribute to speciation

So far you’ve learnt that cells contain two homologous sets of chromosomes.Well….. that isn’t always the case.

It goes on:PentaploidHexaploidSeptaploidOctaploidEtc.up to:

84-ploid and 1260 chromosomes

Ophioglossum reticulatumA small fern.

The incredible thing is that this plant is able to carry out meiosis accurately with

1260 chromosomes to divvy up

http://commons.wikimedia.org/wiki/File:Haploid,_diploid_,triploid_and_tetraploid.svg

How it happens:

http://commons.wikimedia.org/wiki/File:Polyploidization.svg

Remember:When non-disjunction occurs during meiosis in humans, an individual can end up with an extra chromosome or missing chromosomes. E.g. An extra chromosome 21 means Downs syndrome (see 4.2.4)

Total non-disjunction, is when one of the two cells produced during Meiosis I gets all of the chromosomes. The other cell is not viable and is reabsorbed.This results in two (2n) daughter cells from meiosis instead of the usual four (n) daughter cells. See animation

Self fertilisation

Few polyploid organisms exist in the animal kingdom.Can you think of the reasons why not?

Animal polyploid species include salamanders, goldfish and salmon.

Polyploidy often leads to increased size, resistance to disease and overall vigour.Many plants used by humans are polyploid. Including cereal crops like wheat.Polyploid crops generally have bigger fruits, seeds and storage organs

However, polyploidy is a great source of speciation amongst plants.

Autopolyploidy (Auto = “self”)

• Autopolyploids are polyploids with multiple chromosome sets derived from a single species as described a couple of slides ago.

• Autopolyploids form following fusion of 2n gametes

• Autopolyploidy can be induced in plants using colchicine, a chemical extracted from the autumn crocus.

• Autopolyploids with odd ploidys eg triploid or pentaploid have trouble reproducing sexually WHY?

• That does not stop them from being good crops if they can be propagated asexually

Allopolyploidy (Allo = “different”)

Allopolyploids come about when a sterile F1 hybrid doubles all of its chromosomes and becomes fertile.

For example, Triticale is the hybrid of wheat (Triticum turgidum) and rye (Secale cereale). It combines sought-after characteristics of the parents, but the initial hybrids were sterile until doubling of the number of chromosomes occurred

Remember the poor sterile mule with 63 chromosomes?

Imagine if we could somehow induce sperm and ova with 126 chromosomes

Voila! The mule born would be fertile. Of course, it would need to be done a couple of

times to get a few mules to breed together

http://jonathanturley.org/2009/03/01/a-happy-mule/http://en.wikipedia.org/wiki/File:Wheat,_rye,_triticale_montage.jpg

+ =Wheat Rye Triticale

D.2.6 Compare allopatric and sympatric speciation

Allopatric speciation (Allo = “different”, patric = “fatherland”)

This arises when a species is subject to geographic isolation.This can occur when a population is split by:• A river• A mountain range• A desert• A road• The sea etc.

Gene flow is cut off between the two split populations and they can evolve in different directions (See animations below)

Remember Darwin’s finches?

http://commons.wikimedia.org/wiki/File:Darwin%27s_finches_by_Gould.jpg

Allopatric speciation of Drosophila in the lab

Even when the “geographic barrier” is removed, the populations are still

genetically isolated

Sympatric speciation (Sym = “same”, patric = “fatherland”)

The formation of two or more descendant species from a single ancestral species all occupying the same geographic location.

Whether it actually happens is still contested.Find a pair of species that are thought to have diverged by sympatric speciation

http://etc.usf.edu/clipart/2200/2288/salamander_1.htm

http://upload.wikimedia.org/wikipedia/commons/thumb/5/53/Speciation_modes.svg/500px-Speciation_modes.svg.png

Diagrammatic comparison

D.2.7 Outline the process of adaptive radiation

Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different morphological and physiological traits with which they can exploit a range of divergent environments.

Wikipedia

Think Darwin’s finches (AGAIN!)They originated from a population of an ancestral species that flew or were blown to the Galapagos islands from mainland South America.They colonised the islands and (while geographically isolated) evolved via natural selection to have beaks that suited the types of food available on their islands.

Their beaks are homologous structures in

that they have evolved from a common structure

to have different functions.

D.2.8 Compare convergent and divergent evolution

What do humans, octopi and box jellyfish have in

common?

http://www.flickr.com/photos/jlambus/2303592201/

We all have complex camera* eyes.They evolved

independently in organisms only very

distantly related.They are an example

of convergent evolution

*Camera means ‘room’

Complex eyes have evolved 50 to 100 times!

Other (random!) examples include:- Penguins in the southern hemisphere

and Auks in the northern hemisphere both use wings as flippers

- Echolocation in bats, toothed whales and shrews to capture prey. It even evolved independently twice amongst the bats

- Super strong jaws on different genuses of ants (Trapjaw )

- Flight/gliding in birds, pterosaurs, bats, insects and flying fish!

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.

http://commons.wikimedia.org/wiki/File:AlleAlle_2.jpg

http://commons.wikimedia.org/wiki/File:Little_penguin_Eudyptula_minor.jpg

Little Auk

Little Penguin

Features that come about by convergent evolution are known as analogous structures

http://www.flickr.com/photos/sniffette/6705872/http://www.flickr.com/photos/volk/1038089969/

http://www.flickr.com/photos/jaybock/4006029348/http://www.flickr.com/photos/martynr/76538849/sizes/o/in/photostream/

Divergent Evolution is another way of saying adaptive radiation (D.2.7). As natural selection acts on two or more species that have arisen from a common ancestor, they become phenotypically different.

It gives rise to homologous structures, features that now look different or have a different purpose for each species that has evolved

http://commons.wikimedia.org/wiki/File:Evolution_pl.png

Tim

e

Parent species(common ancestor)

Parent species

Parent species

Divergent evolution Convergent evolution

D.2.9 Discuss ideas on the pace of evolution including gradualism and punctuated evolution

Phyletic Gradualism, as the name suggests, is the idea that evolution occurs at a slow-but-steady pace.

Punctuated Equilibrium is the idea that, for most of the time, species are stable. But every now and then there is a disruptive event that prompts rapid change.

The slope of the line indicates rate of change.• Vertical lines = little/no change• Horizontal lines = very rapid change

Revisiting the tree for punctuated equilibrium it should be noted that the “sudden” speciation events are only sudden in terms of geological time. They would still take many generations and possibly thousands of years.

The periods of stasis may be explained by stabilising selection

The punctuation could be explained by

directional selection or disruptive selection

The K/T extinction event (250 MA at the Cretaceous-Tertiary boundary) wiped out over half the genera, including most of the dinosaurs.

A layer of iridium has been found in sediments laid down at that time all over the globe. Iridium is in higher concentrations in meteorites than on Earth generally.Therefore it is postulated that a large meteor or comet hit the Earth and caused the extinction.

Individuals in the species that survived could move into the empty ecological niches and directional selection led to rapid evolution

http://www.flickr.com/photos/53402955@N08/4928503884/in/photostream/

D.2.10 Describe one example of transient polymorphism

Darwin's finches…. Have little to do with this point (for a change!).Instead, the peppered moths (Biston betularia) are the best known example

Polymorphism is the existence of two or more different forms of a species Poly = “many” morphism = “shapes”

Prior to 1840 peppered moths in Britain were light grey with dark spots to blend in with the grey lichen that grew on the trees in their habitat

http://www.flickr.com/photos/wildhastings/4720082589/

The first dark variant was reported in 1848 and by 1895 most of them were black.

The term industrial melanism was coined.

Soot and acid rain from the burning of coal changed the colour or the trees that the moths rested on.

Directional selection did the rest.

http://www.flickr.com/photos/naturalhistoryman/817332984/

Before long the majority were dark.

This situation reversed after 1956 when Britain instituted the clean air act. Less coal was burnt and most trees returned to their original colour.

Now in polluted areas most moths are dark and in rural areas most moths are light.

They are not distinct species because they still interbreed.

The theory that natural selection due to predation was the cause of these changes has been confirmed experimentally by Dr HBD Kettlewell

D.2.10 Describe sickle cell anaemia as an example of balanced polymorphism

Sickle cell anaemia occurs when a single-base mutation in the gene that codes for haemoglobin causes the amino acid valine to be produced in a particular spot rather than glutamic acid.

Valine is non-polar, unlike glutamic acid, and this causes the mutant variety of haemoglobin (haemoglobin S) to crystallise at low concentrations of oxygen.

This in turn pulls the red blood cell into a sickle shape. It is less able to carry oxygen and can get stuck in small capillaries, causing blockages, pain and damage.

Homozygous individuals (HbS HbS) are subject to a debilitating condition and have a shortened life expectancy

On the brighter side, while individuals who are heterozygous (HbA HbS) will have some mutant haemoglobin. They can lead normal lives. As a benefit, they are resistant to malaria as the plasmodium parasite that causes it is not able to use sickle cells to reproduce.

Individuals that are homozygous normal (HbA HbA) have no sickle cells and no resistance to malaria.

Distribution of the sickle cell traitHistorical distribution of malaria

http://commons.wikimedia.org/wiki/File:Simple_balance_scales-01.jpg

http://en.wikipedia.org/wiki/File:Plasmodium.jpg

http://en.wikipedia.org/wiki/File:Sicklecells.jpg

This is an example of balancing selection and balanced polymorphism

People who are homozygous for sickle cell are severely anaemic and have less chance of surviving to reproduce.

Likewise individuals homozygous for normal haemoglobin are likely to contract malaria and are less likely to survive.

Heterozygous individuals have what is termed heterozygote advantage. They are the most likely to survive

and reproduce.

Therefore both alleles are maintained in the population

Further information:

A PPT unaligned with the IB but with similar

information and different animations

embeddedOld syllabus