SPECIES SPECIATION Option D 2 IB Biology Miss

SPECIES & SPECIATION Option D. 2 IB Biology Miss Werba

OPTION D - EVOLUTION D. 1 ORIGIN OF LIFE ON EARTH AHL D. 5 PHYLOGENY AND SYSTEMATICS AHL D. 4 THE HARDY– WEINBERG PRINCIPLE J WERBA – IB BIOLOGY D. 2 SPECIES AND SPECIATION D. 3 HUMAN EVOLUTION 2

THINGS TO COVER � Allele frequency & the gene pool � Barriers between gene pools � Polyploidy & speciation � Allopatric & sympatric speciation � Adaptive radiation � Convergent & divergent evolution � Pace of evolution – gradualism & punctuated equilibrium � Transient & balanced polymorphisms J WERBA – IB BIOLOGY 3

ALLELE FREQUENCY & THE GENE POOL D. 2. 1 Command term = DEFINE � Allele frequency: the proportion of all copies of a gene that is made up of a particular variant (allele) � Gene pool: The total collection of different alleles in an interbreeding population J WERBA – IB BIOLOGY 4

ALLELE FREQUENCY & THE GENE POOL D. 2. 1 Command term = DEFINE � If a recessive allele b made up 40% of the pig population… � …then the dominant allele B would make up 60%. BB bb Bb Bb BB � The allele frequency for b would be expressed as 0. 40 and for B 0. 60. � For characteristics determined by two alleles, the sum of the frequency of the recessive allele and the dominant allele will add up to 1. 00 (b + B = 1) J WERBA – IB BIOLOGY 5

ALLELE FREQUENCY & EVOLUTION D. 2. 2 Command term = STATE � Evolution involves a change in allele frequency in a population’s gene pool over a number of generations. � By natural selection, alleles coding for beneficial adaptations will confer a survival advantage and lead to improved reproduction. � These alleles are more likely to be inherited and thus the population's gene pool will change over generations. J WERBA – IB BIOLOGY 6

D. 2. 3 WHAT IS A SPECIES? Species Command term = DISCUSS J WERBA – IB BIOLOGY Breeding Group of organisms capable of breeding naturally to produce fertile offspring Ecological Group of organisms adapted to share the same ecological niche Genetic A group of organisms with the same karyotype or gene pool Evolutionary A group of organisms that shares a common ancestor, and have developed a unique collection of structural & functional characteristics Cladistic A group of organisms that shares a common ancestor (parent species goes extinct) 7

D. 2. 3 WHAT IS A SPECIES? Command term = DISCUSS � The genetic definition is most widely used. � As always, there are some exceptions… 1) The species could be reproductively isolated, as it is mechanically not possible for them to mate. 2) Hybrids can be produced (eg. mules) but are usually infertile. horse (64 chroms) + donkey (62 chroms) = mule (63 chroms) 3) The genetic definition does not apply to organisms that reproduce asexually. J WERBA – IB BIOLOGY 8

BARRIERS BETWEEN GENE POOLS D. 2. 4 Command term = DESCRIBE � If a species is somehow separated by an isolation mechanism or barrier, one species could potentially diverge into two. � If the environments on either side of the barrier are different, each environment will select for a different set of features. � The two isolated groups cannot interbreed, so there is no gene flow between them. � After a long period of isolation and selection, the groups on either side of the barrier may become so different that they can no longer interbreed when put together. J WERBA – IB BIOLOGY 9

BARRIERS BETWEEN GENE POOLS D. 2. 4 Command term = DESCRIBE � Circumstances that prevent interbreeding are known as isolating mechanisms or barriers. � These mechanisms can be: ◦ Pre-zygotic barriers �Prevention of mating or fertilisation �Gametes don’t meet so zygote never forms ◦ Post-zygotic barriers �Prevention of hybrids �Zygote forms �Hybrids are infertile, inviable (not strong) or die in utero J WERBA – IB BIOLOGY 10

BARRIERS BETWEEN GENE POOLS D. 2. 4 Command term = DESCRIBE Pre-zygotic barriers: temporal isolation � the gametes never meet because the two groups are separated in time � eg. diurnal vs noctural � eg. seasonal J WERBA – IB BIOLOGY 11

BARRIERS BETWEEN GENE POOLS D. 2. 4 Command term = DESCRIBE Pre-zygotic barriers: ecological isolation � the gametes never meet because the two groups are separated in their habitats � eg. arboreal vs non-arboreal Hello up there! � eg. akaline vs acidic soils � eg. separated by river J WERBA – IB BIOLOGY 12

BARRIERS BETWEEN GENE POOLS D. 2. 4 Command term = DESCRIBE Pre-zygotic barriers: behavioural isolation � the gametes never meet because the two groups are separated by different mating and courtship rituals J WERBA – IB BIOLOGY 13

BARRIERS BETWEEN GENE POOLS D. 2. 4 Command term = DESCRIBE Pre-zygotic barriers: reproductive isolation � the gametes never meet because the two groups are separated because of reproductive incompatibility J WERBA – IB BIOLOGY 14

D. 2. 5 POLYPLOIDY & SPECIATION Command term = EXPLAIN � Polyploidy: Condition where the cells of an organism contain more than two homologous sets of chromosomes. eg. salmon � eg. ◦ Triploid (3 n) ◦ Tetraploid (4 n) eg. kiwifruit ◦ Pentaploid (5 n) ◦ Hexaploid (6 n) ◦ Decaploid (10 n) eg. strawberries J WERBA – IB BIOLOGY 15

D. 2. 5 POLYPLOIDY & SPECIATION Command term = EXPLAIN � Polyploidy doesn’t add new genes to the gene pool, but gives rise to new combinations of genes. � It involves a single organism or hybridisation between organisms of a different species. � Very common in plants. � Polyploidy in a species results in very quick changes to gene structure & gene expression. � Two versions of polyploidy are: ◦ autopolyploidy ◦ allopolyploidy J WERBA – IB BIOLOGY 17

D. 2. 5 POLYPLOIDY & SPECIATION Command term = EXPLAIN Autopolyploidy : � Multiple chromosome sets derived from a single species � Result from the fusion of 2 n gametes (2 n+2 n = 4 n) � Autopolyploids with odd number ploidys (eg. tetraploid) have trouble reproducing sexually. � But can still be good crops (if they can be propagated asexually). J WERBA – IB BIOLOGY 18

D. 2. 5 POLYPLOIDY & SPECIATION Command term = EXPLAIN Allopolyploidy : � Multiple chromosome sets derived from different species � A sterile F 1 hybrid doubles all of its chromosomes and becomes fertile! J WERBA – IB BIOLOGY 19

D. 2. 5 POLYPLOIDY & SPECIATION Command term = EXPLAIN Allopolyploidy : � eg. Triticale ◦ An allopolyploid ◦ A hybrid of wheat and rye ◦ Has 6 sets of chromosomes: 4 from wheat and 2 from rye ◦ The resulting species is infertile with both parent species. W+R = T J WERBA – IB BIOLOGY 20

ALLOPATRIC & SYMPATRIC SPECIATION D. 2. 6 Command term = COMPARE � Speciation: the process by which one or more species arise from previously existing species. � Two different species may give rise to a new species interspecific hybridisation OR � A single species may give rise to a new species intraspecific speciation ◦ If it occurs whilst the populations are geographically isolated allopatric speciation ◦ If it occurs while the populations are occupying the same geographical area or range sympatric speciation J WERBA – IB BIOLOGY 21

ALLOPATRIC & SYMPATRIC SPECIATION D. 2. 6 Command term = COMPARE Allopatric speciation: � Results from geographical isolation over a long period of time � Gene flow is restricted between the two populations and they can adapt to different circumstances. � They will diverge due to natural selection in their respective environments. J WERBA – IB BIOLOGY 22

ALLOPATRIC & SYMPATRIC SPECIATION D. 2. 6 Command term = COMPARE Allopatric speciation: � A famous example of allopatric speciation is that of Charles Darwin's Galápagos Finches J WERBA – IB BIOLOGY 23

ALLOPATRIC & SYMPATRIC SPECIATION D. 2. 6 Command term = COMPARE Sympatric speciation: � Results from reproductive isolation caused by genetic divergence. � The two variants inhabit the same geographic region. � Mating preferences may play a role in the isolation. � Gene flow will be restricted and speciation will occur. J WERBA – IB BIOLOGY 25

ALLOPATRIC & SYMPATRIC SPECIATION D. 2. 6 Command term = COMPARE Parapatric speciation: � Results from ecological isolation. � The two variants inhabit different niches within the same geographical area. � Again, gene flow will be restricted and speciation will occur. J WERBA – IB BIOLOGY 26

D. 2. 7 ADAPTIVE RADIATION Command term = OUTLINE � Adaptive radiation: the diversification of a group of organisms into forms filling different ecological niches. � Think of Darwin's Finches again! ◦ They originated from one population of finches that migrated to the Galapagos islands. ◦ They colonised the islands and evolved independently from one another due to geographical isolation. ◦ Their beaks adapted to suit the food on their island. ◦ Their beaks are homologous structures – structures that have evolved from a common ancestor to have different functions. J WERBA – IB BIOLOGY 27

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE Divergent evolution: � Another way of describing adaptive radiation � When two related species evolve similar features due to their shared ancestry. � These features may have evolved for different functions due to differences in selection pressures in their environments. � Features that come about by divergent evolution are known as homologous structures. J WERBA – IB BIOLOGY 28

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE Divergent evolution: � eg. The pentadactyl limb of all vertebrates J WERBA – IB BIOLOGY 29

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE Convergent evolution: � When two unrelated species evolve similar features if placed in similar environments � Due to similar selection pressures in their environments. � Features that come about by convergent evolution are known as analogous structures. J WERBA – IB BIOLOGY 30

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE Convergent evolution: � eg. The streamlined bodies and fins of sharks (fish) and dolphins (mammals) � eg. ant eaters, aardvarks, echidnas and numbats have all developed claws and sticky, long tongues to open up termite nests and eat them J WERBA – IB BIOLOGY 31

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE Convergent evolution: � eg. The camera eye of cephalopods (eg. box jellyfish) and vertebrates. Their last common ancestor had at most a very simple photoreceptive spot, but a range of processes led to the progressive refinement of this structure to the advanced camera eye – with one subtle difference; the cephalopod eye is "wired" in the opposite direction. J WERBA – IB BIOLOGY 32

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE J WERBA – IB BIOLOGY 33

CONVERGENT & DIVERGENT EVOLUTION D. 2. 8 Command term = COMPARE Divergent evolution Convergent evolution Common ancestor Different ancestor Diverge to produce homologous structures Converge to produce analogous structures Appearance becomes more different over time Appearance becomes more similar over time Species are closely related Species are unrelated eg. pentadactyl limb structure in vertebrates eg. wings in insects, birds and bats eg. streamlined bodies of sharks and dolphins J WERBA – IB BIOLOGY 34

D. 2. 9 THE PACE OF EVOLUTION Command term = DISCUSS � Evolution is a slow process. � Most of our ideas about evolution are supported by fossil records, which are incomplete. � There are two ideas about the pace of evolution that need to be discussed: ◦ gradualism ◦ punctuated equilibrium J WERBA – IB BIOLOGY 35

D. 2. 9 THE PACE OF EVOLUTION Command term = DISCUSS Gradualism: � Continuous change at a slow, but steady pace over a long period of time Punctuated equilibrium: � Species are stable most of the time � Disruptive events prompt periods of rapid change J WERBA – IB BIOLOGY 36

D. 2. 9 THE PACE OF EVOLUTION Command term = DISCUSS Gradualism: � The older of the two ideas � Slow but steady change � Results from the gradual accumulation of mutations / variations � Darwin proposed this concept � He did however recognise that all species do not evolve at the same rate all of the time J WERBA – IB BIOLOGY 37

D. 2. 9 THE PACE OF EVOLUTION Command term = DISCUSS Gradualism: � Supported by intermediate forms found in the fossil record � eg. Fossils found demonstrating the gradual evolution of the modern horse J WERBA – IB BIOLOGY 38

D. 2. 9 THE PACE OF EVOLUTION Command term = DISCUSS Punctuated equilibrium: � Long, stable periods disrupted by events causing periods of rapid change � Stability causes organisms to become well suited to the environment due to natural selection (eliminates extreme phenotypes) � The rapid changes could include volcanic eruptions, meteors, etc � This leads to directional selection towards the mean optimal phenotype. J WERBA – IB BIOLOGY 39

D. 2. 9 THE PACE OF EVOLUTION Command term = DISCUSS Punctuated equilibrium: � Supported by the gaps in the fossil record and the lack of intermediate forms for many species. � Supported by the fact that the fossil record shows that many new species appeared following a mass extinction. J WERBA – IB BIOLOGY 40

TRANSIENT AND BALANCED POLYMORPHISMS D. 2. 10 D. 2. 11 Command term = DESCRIBE � Polymorphism: The existence of two or more different forms of a species � Polymorphisms can be: ◦ Transient �If two different variants of a phenotype are in the process of replacing each other ◦ Balanced �If two different variants of a phenotype coexist in the same population J WERBA – IB BIOLOGY 41

TRANSIENT AND BALANCED POLYMORPHISMS D. 2. 10 D. 2. 11 Command term = DESCRIBE Transient polymophism: � eg. The peppered moth ◦ Light coloured moth was once prevalent. ◦ Industrial revolution changed the colour of the trees directional selection. ◦ Lead to increased frequency of dark variant in polluted areas. ◦ Clean air acts have begun to return the trees to their original colour. ◦ Black moths now exist in polluted areas and light moths exist in rural areas. ◦ They are not distinct species – they can still interbreed! J WERBA – IB BIOLOGY 42

TRANSIENT AND BALANCED POLYMORPHISMS D. 2. 10 D. 2. 11 Command term = DESCRIBE Balanced polymophism: � eg. Sickle-cell anaemia ◦ Sickle-cell anaemia causes a mutant variety of haemoglobin to form ◦ The mutant haemoglobin crystallises at low oxygen concentrations and can block capillaries. ◦ Affected individuals (homozygous recessive) have a low quality of life and a shortened life expectancy ◦ Heterozygotes still have some mutant haemoglobin. They can lead normal lives and are resistant to the malarial parasite. ◦ Unaffected individuals (homozygous dominant) have no sickle cells and no malarial resistance. J WERBA – IB BIOLOGY 43

TRANSIENT AND BALANCED POLYMORPHISMS D. 2. 10 D. 2. 11 Command term = DESCRIBE Balanced polymophism: � eg. Sickle-cell anaemia ◦ There is a balancing or stabilising selection occurring Lower survival chances because they are LIKELY TO CONTRACT MALARIA Lower survival chances because they are SEVERELY ANAEMIC J WERBA – IB BIOLOGY 44

TRANSIENT AND BALANCED POLYMORPHISMS D. 2. 10 D. 2. 11 Command term = DESCRIBE Balanced polymophism: � eg. Sickle-cell anaemia ◦ There is a balancing or stabilising selection occurring Heterozygotes have an advantage. They are most likely to survive and reproduce in areas affected by malaria. This maintains the sickle cell allele in the population. J WERBA – IB BIOLOGY 45

Sample questions Q 1 (6 min) a) i. Distinguish between transient polymorphism and balanced polymorphism. [2] ii. State an example of transient polymorphism. [1] b) Describe an example of a barrier between gene pools. [2] J WERBA – IB BIOLOGY 46

Sample questions Q 2 (5 min) Outline allopatric and sympatric speciation. J WERBA – IB BIOLOGY [4] 47

Sample questions A 1 a) i. transient when one allele gradually replaces another / directional selection; balanced when allele frequencies do not change / stabilizing selection; [2] ii. eg. gradual increase in darker peppered moths due to environmental change/industrial melanism [1] Accept any other valid example. a) name of type of barrier; description; [2] eg. behavioural isolation; different courtship behaviour so no mating between two populations; Accept only the first type of barrier in the answer. Accept geographical separation, hybrid infertility, difference in chromosome number or breeding time. J WERBA – IB BIOLOGY 48

Sample questions Q 2 speciation is the formation of a new species by the splitting of an existing species; allopatric speciation caused by geographical separation; sympatric speciation occurring within the same habitat caused by different niches / caused by courtship/feeding differences/behavioural differences; both processes lead to isolation of sub-populations; isolation favours certain genetic variations (within a species); over time this leads to genetic barriers/speciation; Both allopatric and sympatric speciation must be mentioned. [3 max]if only one mentioned. J WERBA – IB BIOLOGY 4 max 49