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/
Jan 02, 2016
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
http://www.flickr.com/photos/furryscalyman/673915993/
I’m including this image because I mentioned bats twice on the last slide and
bats are awesome!
*
*Whatever he’s saying, its ultrasonic
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
Mouthpart functionshttp://www.cals.ncsu.edu/course/ent425/library/labs/external_anatomy/intro.swf
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
Gradualism is the older idea. Darwin is one of the
originators of the concept, borrowing from his friend
Charles Lyell. Darwin recognized however that not all species evolve at the same rate all of the time
"I think case must be that one generation should have as many living as now. To do this and to have as many species in same genus (as is) requires extinction . Thus between A + B the immense gap of relation. C + B the finest gradation. B+D rather greater distinction. Thus genera would be formed. Bearing relation" (next page begins) "to ancient types with several extinct forms" http://commons.wikimedia.org/wiki/File:Darwin_tree.png
Punctuated equilibrium was first proposed by palaeontologists Niles Eldredge and Stephen Jay Gould in 1972. They were the first to suggest that species did not change for long periods of time but were in stasis until events punctuated (disrupted) the equilibrium (balance)
Richard Dawkins is a prominent critic of the theory
TOK: Find out more: • What evidence are the two theories based on?• Gould (deceased) and Dawkins have both become
popular writers. How does this affect the weight of their opinion:• In the scientific community?• In the wider community?
http://www.flickr.com/photos/mrccos/288136783/sizes/m/in/photostream/http://www.flickr.com/photos/ideonexus/4022727065/
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
http://bcs.whfreeman.com/thelifewire/content/chp23/2302001.html
All images CC Andrew Colvin
Befo
reAft
er
Stabilising Directional Disruptive
You should be able to understand and interpret these diagrams. Practise sketching them.
The downward facing arrows indicate selection pressure against individualswith that morphology
Darwin’s Finches (again!) are an example of disruptive selection
A B
Short-beaked birds (A) and long-beaked birds (B) were able to exploit different food sources and this selection pressure led to the evolution of two species
http://www.flickr.com/photos/kookr/2917861361/
Lake Turkana (Kenya, Ethiopia) contains several species of snails that have a fossil record showing long periods with little change followed by sudden change (punctuated equilibrium)
The periods of change coincide with times when the water level of the lake dropped and it became a series of smaller lakes.
What happens then?
http://en.wikipedia.org/wiki/File:Lake_turkana_satellite.jpg
That’s right: geographic isolation
Smaller gene pools are more susceptible to directional selection
So evolution of the isolated populations may be faster than when they were one big happy gene pool
By the time lake levels recovered and the populations were united, isolating mechanisms were in place that prevented hybridisation
Phanerozoic_Biodiversity.svg
After each extinction event, the number of genera has bounced back
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
Transient means temporaryPolymorphism 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
Balanced polymorphism means stable and on-going)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 hemoglobin (hemoglobin S) to crystallize 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 hemoglobin. 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
Heterozygous:Sickle cell trait
Heterozygous:Sickle cell trait
Heterozygous:Sickle cell trait
50% chance
Homozygous:Sickle Anaemia
25% chance
Homozygous:‘Normal’
25% chance
S
AA A A
AA
SSSS
S
HbA HbA
Haemoglobin: NormalRBCs: NormalO2 Capacity: NormalMalaria resistance: None
HbA HbS
Haemoglobin: 50% normal, 50% mutantRBCs: Usually normal, sickle when [O2] lowO2 Capacity: Mild anaemiaMalaria resistance: Moderate
HbS HbS
Haemoglobin: mutantRBCs: SickleO2 Capacity: Severe anaemiaMalaria Resistance: High
http://en.wikipedia.org/wiki/File:Autorecessive.svg
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 anemic and have less chance of surviving to reproduce.
Likewise individuals homozygous for normal hemoglobin 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
Hypotheticals:
France has the greatest number of sickle cell sufferers in Europe because of immigration from its former African and Caribbean colonies.What do you expect will happen to the sickle cell allele in France over time given:
1. no more immigration,2. modern medicine,3. and the absence of malaria?
What do you expect will happen to the sickle cell allele in West Africa over time if:
• We eradicate malaria?Or
• We develop medication that helps all sickle cell sufferers live normally