D.2.8 Compare convergent and divergent evolution

1. 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/

2. We all have complex camera* eyes. They evolved independently in organisms only very distantly related. They are an example of convergent evolution Complex eyes have evolved 50 to 100 times! *Camera means ‘room’

3. Convergent evolution describes the acquisition of the same biological trait in unrelated lineages. • 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! Little Auk http://commons.wikimedia.org/wiki/File:AlleAlle_2.jpg Little Penguin http://commons.wikimedia.org/wiki/File:Little_penguin_Eudyptula_minor.jpg

4. I’m including this image because I mentioned bats twice on the last slide and bats are awesome! * *Whatever he’s saying, its ultrasonic http://www.flickr.com/photos/furryscalyman/673915993/

5. 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/

6. Divergent Evolutionis 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 functions http://www.cals.ncsu.edu/course/ent425/library/labs/external_anatomy/intro.swf

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

8. Divergent evolution Convergent evolution Time Parent species Parent species Parent species (common ancestor)

9. 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

10. 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

11. Punctuated equilibrium was first proposed by palaeontologistsNiles Eldredgeand 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/ideonexus/4022727065/ http://www.flickr.com/photos/mrccos/288136783/sizes/m/in/photostream/

12. 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. http://bcs.whfreeman.com/thelifewire/content/chp23/2302001.html The periods of stasis may be explained by stabilising selection The punctuation could be explained by directional selection or disruptive selection

13. You should be able to understand and interpret these diagrams. Practise sketching them. The downward facing arrows indicate selection pressure against individuals with that morphology Stabilising Directional Disruptive Before After All images CC Andrew Colvin

14. 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/

15. 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

16. 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

17. After each extinction event, the number of genera has bounced back Phanerozoic_Biodiversity.svg

18. 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/

19. 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 temporary Polymorphismis 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/

20. 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/

21. 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

22. 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 (HbSHbS) are subject to a debilitating condition and have a shortened life expectancy

23. On the brighter side, while individuals who are heterozygous (HbAHbS) 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 (HbAHbA) have no sickle cells and no resistance to malaria. Historical distribution of malaria Distribution of the sickle cell trait

24. HbAHbA Haemoglobin: Normal RBCs: Normal O2 Capacity: Normal Malaria resistance: None Heterozygous: Sickle cell trait Heterozygous: Sickle cell trait HbA HbS Haemoglobin: 50% normal, 50% mutant RBCs: Usually normal, sickle when [O2] low O2 Capacity: Mild anaemia Malaria resistance: Moderate A S A S A A S A A S S S HbS HbS Haemoglobin: mutant RBCs: Sickle O2 Capacity: Severe anaemia Malaria Resistance: High Homozygous: ‘Normal’ 25% chance Heterozygous: Sickle cell trait 50% chance Homozygous: Sickle Anaemia 25% chance http://en.wikipedia.org/wiki/File:Autorecessive.svg

25. 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 http://en.wikipedia.org/wiki/File:Plasmodium.jpg http://en.wikipedia.org/wiki/File:Sicklecells.jpg http://commons.wikimedia.org/wiki/File:Simple_balance_scales-01.jpg

26. 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: • no more immigration, • modern medicine, • 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