Option D: Evolution D2: Species and Speciation

D 2.1 Define allele frequency and gene pool. gene pool – sum of all the genes of all the individuals in a population including all the alleles for all the genes present in the population allele frequency – the percentage of a specific allele of a given gene locus in the population

D 2.2 State that evolution involves a change in allele frequency in a population’s gene pool over a number of generations. evolution of populations is best understood in terms of allele frequencies if the allele frequencies remain constant from generation to generation, then the population is not undergoing any evolutionary change and is in genetic equilibrium evolution can be defined as changes in gene frequencies that occur in a gene pool over time (change in the genetic makeup of populations over time)

Biological species concept ▫defined by Ernst Mayr ▫population whose members can interbreed & produce viable, fertile offspring ▫reproductively compatible Western MeadowlarkEastern Meadowlark Distinct species: songs & behaviors are different enough to prevent interbreeding D 2.3 Discuss the definition of the term species.

Species are created by a series of evolutionary processes ▫populations become isolated  geographically isolated  reproductively isolated ▫isolated populations evolve independently D 2.4 Describe three examples of barriers between gene pools.

- obstacle to mating or to fertilization if mating occurs PRE-reproduction barriers behavioral isolation geographic isolation ecological isolation temporal isolation mechanical isolation gametic isolation D 2.4 Describe three examples of barriers between gene pools.

Geographic isolation Species occur in different areas ▫physical barrier ▫allopatric speciation  “other country” Harris’s antelope squirrel inhabits the canyon’s south rim (L). Just a few miles away on the north rim (R) lives the closely related white-tailed antelope squirrel D 2.4 Describe three examples of barriers between gene pools.

Ecological isolation Species occur in same region, but occupy different habitats so rarely encounter each other ▫reproductively isolated 2 species of garter snake, Thamnophis, occur in same area, but one lives in water & other is terrestrial lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest lions & tigers could hybridize, but they live in different habitats:  lions in grasslands  tigers in rainforest D 2.4 Describe three examples of barriers between gene pools.

Temporal isolation Species that breed during different times of day, different seasons, or different years cannot mix gametes ▫reproductive isolation ▫sympatric speciation  “same country” Eastern spotted skunk (L) & western spotted skunk (R) overlap in range but eastern mates in late winter & western mates in late summer D 2.4 Describe three examples of barriers between gene pools.

Behavioral isolation Unique behavioral patterns & rituals isolate species ▫identifies members of species ▫attract mates of same species  courtship rituals, mating calls  reproductive isolation Blue footed boobies mate only after a courtship display unique to their species D 2.4 Describe three examples of barriers between gene pools.

firefly courtship displays courtship display of Gray-Crowned Cranes, Kenya courtship songs of sympatric species of lacewings Recognizing your own species

Mechanical isolation Morphological differences can prevent successful mating ▫reproductive isolation Even in closely related species of plants, the flowers often have distinct appearances that attract different pollinators. These 2 species of monkey flower differ greatly in shape & color, therefore cross-pollination does not happen. Plants D 2.4 Describe three examples of barriers between gene pools.

Mechanical isolation For many insects, male & female sex organs of closely related species do not fit together, preventing sperm transfer ▫lack of “fit” between sexual organs: hard to imagine for us… but a big issue for insects with different shaped genitals! Animals I can ’ t even imagine! D 2.4 Describe three examples of barriers between gene pools.

Gametic isolation Sperm of one species may not be able to fertilize eggs of another species  biochemical barrier so sperm cannot penetrate egg  receptor recognition: lock & key between egg & sperm  chemical incompatibility  sperm cannot survive in female reproductive tract Sea urchins release sperm & eggs into surrounding waters where they fuse & form zygotes. Gametes of different species— red & purple —are unable to fuse. D 2.4 Describe three examples of barriers between gene pools.

POST-reproduction barriers Prevent hybrid offspring from developing into a viable, fertile adult ▫reduced hybrid viability ▫reduced hybrid fertility ▫hybrid breakdown zebroid D 2.4 Describe three examples of barriers between gene pools. liger

Reduced hybrid viability Genes of different parent species may interact & impair the hybrid’s development Species of salamander genus, Ensatina, may interbreed, but most hybrids do not complete development & those that do are frail. D 2.4 Describe three examples of barriers between gene pools.

Mules are vigorous, but sterile Reduced hybrid fertility Even if hybrids are vigorous they may be sterile ▫chromosomes of parents may differ in number or structure & meiosis in hybrids may fail to produce normal gametes Donkeys have 62 chromosomes (31 pairs) Horses have 64 chromosomes (32 pairs) Mules have 63 chromosomes!

Hybrid breakdown Hybrids may be fertile & viable in first generation, but when they mate offspring are feeble or sterile In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species. In strains of cultivated rice, hybrids are vigorous but plants in next generation are small & sterile. On path to separate species. D 2.4 Describe three examples of barriers between gene pools.

D 2.5 Explain how polyploidy can contribute to speciation. changes in chromosome number may cause instantaneous speciation ▫polyploidy – common speciation mechanism in plants – possession of more than two sets of chromosomes ▫may occur when a fertilized egg duplicates its chromosomes but does not divide into two daughter cells – all subsequent divisions may be normal and all cells are now tetraploid

D 2.5 Explain how polyploidy can contribute to speciation. ▫caused by nondisjunction – chromosomes do not separate completely/equally ▫most tetraploid plants are healthy and vigorous and can go through meiosis ▫gametes produced can only fuse with other gametes from tetraploid plants – cannot fuse with gametes from original parents ▫occurs in plants because plants can self-fertilize or reproduce asexually

D 2.5 Explain how polyploidy can contribute to speciation. Autopolyploids (auto= self) 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

D 2.5 Explain how polyploidy can contribute to speciation. Allopolyploids (allo= different) come about when a sterile F 1 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 + = Wheat Rye Triticale 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

Species are created by a series of evolutionary processes ▫populations become isolated  geographically isolated  reproductively isolated ▫isolated populations evolve independently Isolation ▫allopatric  geographic separation ▫sympatric  still live in same area D 2.6 Compare allopatric and sympatric speciation.

Adaptive Radiation  When a species gives rise to many new species in a relatively short period of time  Typically occurs when populations of a single species invade a variety of new habitats and evolve in response to the differing environmental selection pressures D 2.7 Outline the process of adaptive radiation.

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 colonized 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 2.7 Outline the process of adaptive radiation. Insect eaters Bud eater Seed eaters Cactus eater Warbler finch Tree finches Ground finches

Adaptive radiation Seed eaters Flower eaters Insect eaters Rapid speciation: new species filling new niches, because they inherited successful adaptations. D 2.7 Outline the process of adaptive radiation.

Warbler finch Woodpecker finch Small insectivorous tree finch Large insectivorous tree finch Vegetarian tree finch Cactus finch Sharp-beaked finch Small ground finch Medium ground finch Large ground finch Insect eaters Bud eater Seed eaters Cactus eater Warbler finch Tree finches Ground finches Darwin’s finches Differences in beaks ▫associated with eating different foods ▫survival & reproduction of beneficial adaptations to foods available on islands D 2.7 Outline the process of adaptive radiation.

Darwin’s finches Darwin’s conclusions ▫small populations of original South American finches landed on islands  variation in beaks enabled individuals to gather food successfully in the different environments ▫over many generations, the populations of finches changed anatomically & behaviorally  accumulation of advantageous traits in population  emergence of different species D 2.7 Outline the process of adaptive radiation.

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. -Flight/gliding in birds, pterosaurs, bats, insects and flying fish! Convergent evolution describes evolution towards similar traits in unrelated species. g Little Auk Little Penguin D 2.8 Compare convergent and divergent evolution.

Features that come about by convergent evolution are known as analogous structures D 2.8 Compare convergent and divergent evolution.

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. D 2.8 Compare convergent and divergent evolution. Divergent evolution describes evolution towards different traits in closely related species.

It gives rise to homologous structures, features that now look different or have a different purpose for each species that has evolved D 2.8 Compare convergent and divergent evolution.

Gradualism Gradual divergence over long spans of time ▫assume that big changes occur as the accumulation of many small ones D 2.9 Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium.

Punctuated Equilibrium Rate of speciation is not constant ▫rapid bursts of change ▫long periods of little or no change ▫species undergo rapid change when they 1 st bud from parent population Time D 2.9 Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium.

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 stabilizing selection. The punctuation could be explained by directional selection or disruptive selection. D 2.9 Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium.

All images CC Andrew ColvinAndrew Colvin Before After StabilizingDirectionalDisruptive The downward facing arrows indicate selection pressure against individuals with that morphology D 2.9 Discuss ideas on the pace of evolution, including gradualism and punctuated equilibrium.

Polymorphism is the existence of two or more different forms of a species (Poly = “many”; morphism = “shapes”) Transient Polymorphism temporary change in allele frequency ex: peppered moth melanism 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 D 2.10 Describe one example of transient polymorphism.

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. / D 2.10 Describe one example of transient polymorphism.

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 one example of transient polymorphism.

Balanced Polymorphism Two alleles are maintained in stable equilibrium Heterozygote has selective advantage 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. D 2.11 Describe sickle cell anemia as an example of balanced polymorphism.

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 (Hb S Hb S ) are subject to a debilitating condition and have a shortened life expectancy D 2.11 Describe sickle cell anemia as an example of balanced polymorphism.

On the brighter side, while individuals who are heterozygous (Hb A Hb S ) 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 (Hb A Hb A ) have no sickle cells and no resistance to malaria. Distribution of the sickle cell traitHistorical distribution of malaria D 2.11 Describe sickle cell anemia as an example of balanced polymorphism.

Heterozyg ous: Sickle cell trait Heterozyg ous: Sickle cell trait Heterozygou s: Sickle cell trait 50% chance Homozygous : Sickle Anaemia 25% chance Homozygo us: ‘Normal’ 25% chance S A A A A A A SSS S S Hb A Haemoglobin: Normal RBCs: Normal O 2 Capacity: Normal Malaria resistance: None Hb A Hb S Haemoglobin: 50% normal, 50% mutant RBCs: Usually normal, sickle when [O 2 ] low O 2 Capacity: Mild anaemia Malaria resistance: Moderate Hb S Haemoglobin: mutant RBCs: Sickle O 2 Capacity: Severe anaemia Malaria Resistance: High e.svg

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 D 2.11 Describe sickle cell anemia as an example of balanced polymorphism.