UWCSEA IB Biology 2023 - Topic A4.1 - Evolution and Speciation

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A4.1.1 What is evolution? What is the mechanism of evolution?

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Evolution is a change in the heritable characteristics of a population; (of a particular species);

over time;

Natural selection is the mechanism of the evolution;

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A4.1.2 What is the evidence for evolution from base sequences of DNA/RNA and amino acids?

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More closely related species share more similar DNA sequences; and therefore RNA sequences;

Comparing a particular gene between two species shows that the more closely related they are, the fewer differences in base sequence there are;

The smaller the differences in DNA/RNA and amino acid sequences, the more recently the organisms shared a common ancestor;

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14 Terms

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A4.1.1 What is evolution? What is the mechanism of evolution?

Evolution is a change in the heritable characteristics of a population; (of a particular species);

over time;

Natural selection is the mechanism of the evolution;

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A4.1.2 What is the evidence for evolution from base sequences of DNA/RNA and amino acids?

More closely related species share more similar DNA sequences; and therefore RNA sequences;

Comparing a particular gene between two species shows that the more closely related they are, the fewer differences in base sequence there are;

The smaller the differences in DNA/RNA and amino acid sequences, the more recently the organisms shared a common ancestor;

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A4.1.3 What is selective breeding? What is the evidence for evolution from selective breeding of animals? Or crop plants?

Selective breeding is when humans have selected for particular desired traits;

E.g. for milk production in cows;

humans bred/mated those individuals that possessed the desired characteristic; e.g. more milk being produced;

this is done repeatedly, each time selecting and breeding only the organisms who best display the trait;

causing rapid changes in heritable characteristics through the generations;

this is evolution; through artificial selection;

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A 4.1.4 What are homologous structures? What is the evidence for evolution from homologous structures?

Homologous structures are structures which may appear different, but have a common ancestral form;

E.g. forelimbs of a human, mole, horse, porpoise and bat;

have different functions;

but all came from the pentadactyl limb;

from a common ancestor;

each form evolving due to different environmental pressures;

this also explains vestigial structures;

which have no obvious function;

e.g. small thigh bone in whales;

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A4.1.5 What are analogous structures? What is convergent evolution? How does convergent evolution explain the existence of analogous structures?

Analogous structures seem similar as they share the same function; e.g. wings in birds and insects are both used for flying;

however, they are not from closely related species; and often have very different structures;

Convergent evolution is the idea that the similar environments and selection pressures led to the development of the same adaptation;

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A4.1.6 What is speciation?

Speciation is when one species splits into two new species;

this can happen if two populations of the same species are separated;

e.g. by geographical reasons;

different environments will cause different selection pressures;

leading to changes in the two populations;

eventually they will be unable to interbreed; and are now two different species;

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A4.1.7 What is reproductive isolation? What is differential selection? What are their roles in speciation?

Reproductive isolation and differential selection are both required for speciation;

Reproductive isolation is when two populations of a species are prevented from interbreeding;

Commonly this is due to geographical separation; e.g. a mountain range separating them;

Differential selection is when there are different selection pressures in different environments, leading to particular adaptations being selected for; by natural selection;

Over time large differences in the adaptations selected for can occur, so that the populations become different species;

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What are some examples of differential selection that could occur to two populations which are separated geographically?

Different climates, different predators or prey, competition for resources, types of food sources available;

e.g. different seeds which suit different shaped beaks;

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AHL Material - A 4.1.8 - What is allopatric speciation? How does it occur?

Allopatric speciation is when two populations of a species become geographically separated; e.g. by a large river or ocean;

they can no longer interbreed to form fertile offspring;

and so are reproductively isolated;

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AHL Material - A 4.1.8 - What is sympatric speciation? How does it occur?

Sympatric speciation is when two populations of a species are still in the same location, but can no longer interbreed;

they are reproductively isolated;

this can be due to behavioural differences or temporal differences;

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AHL Material - A 4.1.9 - What are adaptations? What is adaptive radiation? How is it a source of biodiversity?

Adaptations are characteristics that make an organism suited for its environment;

Adaptive radiation is where species that have evolved from a common ancestor adapt to different ecological niches or roles;

e.g. the ancestor of vertebrates had a pentadactyl limb;

in different environments this has been modified through natural selection into a wing, a flipper, the hand etc.

because it allows different species to fulfil different roles, it is a source of biodiversity; leading to new species being formed;

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AHL Material - A 4.1.10 What is hybridization? What is sterility? How do barriers to hybridization prevent mixing of alleles between species?

Hybridization is when members of two different species cross-breed;

e.g. a horse and donkey; creating a hybrid;

However the hybrid mule is sterile;

This means is cannot reproduce and therefore the alleles are no longer passed on and mixed;

This, in evolutionary terms, is a waste of energy and resources;

therefore animals have evolved barriers to mating with other species, such as specific courtship behaviours;

Creating a barrier to hybridisation that would mix alleles;

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AHL Material - A 4.1.11 What is polyploidy? How can polyploidy cause abrupt speciation? how can hybridization?

Polyploidy is when an organism has more than one set of chromosomes;

it is caused when chromosomes are duplicated but then not separated or divided correctly in meiosis (formation of sex cells) or mitosis;

for example, sometimes all the chromosomes are copied, leading to 4 sets of chromosomes rather than 2;

This is called autotetraploidy;

These individuals can then breed with a normal individual, causing sterile hybrids;

This would then stop breeding, leading to new species;

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AHL Material - A 4.1.11 What are allotetraploids? How can their creation lead to abrupt speciation?

Allotetraploids are formed when two different species cross-breed creating interspecific hybrids;

which are sterile;

but then can undergo polyploidy, forming 4 sets of chromosomes;

which can then interbreed with other allotetraploids;

but not with the parent species; making them a new species abruptly;