selection and evolution Flashcards

Question

Every individual within a species population has the potential to reproduce and have offspring which contribute to population growth what would occur if the offspring for every individual survived to adulthood and reproduced?

Answer 1

- then the population would experience **exponential** growth This type of growth only happens when there are no environmental factors or population checks acting on the population (for example, when there are plentiful resources and no disease) In reality, there are several environmental factors that prevent every individual in a population making it to adulthood and reproducing ## Footnote One well known but rare example of exponential growth in a population is the introduction of 24 European rabbits into Australia in the 1800s. The rabbits had an abundance of resources, little or no competition and no natural predators. This meant the population increased rapidly and they became a major pest

Answer 2

limit population sizes by reducing the rate of population growth whenever a population reaches a certain size

Answer 3

biotic or abiotic Biotic factors involve other **living** organisms This includes things like predation, competition for resources and disease Abiotic factors involve the **nonliving** parts of an environment Examples of abiotic factors include light availability, water supply and soil pH When biotic and abiotic factors come into play **not all individuals within a population will survive** For example, if a food source is limited some animals within a population will not get enough to eat and will starve to death For most populations in the wild, the number of offspring produced is much higher than the number of individuals that make it adulthood

Answer 4

**1. Competition for food** There is a limited supply of prey: other lions and carnivores will also be hunting the same prey. If a lion is not able to hunt and feed then they will die from starvation **2. Competition for a reproductive mate** Female lions will often outnumber male lions in a population. This means the males compete with each other to mate with the females. When one male is in a contest with another male one (or both) could be injured or killed. Whoever loses the contest won’t be able to mate with the females in a pride and so won’t pass on his genes to any offspring **3. Supply of water** African habitats can be very arid during the dry season. The water sources that the lions drink from can be miles apart. If a lake or source of water dries up then they can die due to dehydration **4. Temperature** The extreme heat experienced in the lion’s African habitat can cause them to overheat and die. It can also prevent them from hunting for long periods during the day, meaning they are less likely to get the food they need to survive The combined effect of all these environmental factors leads to a decrease in population growth as fewer individuals survive to adulthood and reproduce

Answer 5

This means that some individuals within the population possess different phenotypes (due to genetic variation in the alleles they possess; remember members of the same species will have the same genes)

Answer 6

selection pressure

Answer 7

surviving and reproducing over others

Answer 8

having a higher fitness

Answer 9

the ability to survive and reproduce (pass on its alleles to offspring) Organisms with higher fitness posses adaptations that make them better suited to their environment

Answer 10

the frequency of alleles in a population through natural selection

Answer 11

Natural selection is the process by which individuals with a fitter phenotype are more likely to survive and pass on their alleles to their offspring so that the advantageous alleles increase in frequency over time and generations ## Footnote EG: Rabbits have natural predators like foxes which act as a selection pressure Rabbits with a white coat do not camouflage as well as rabbits with brown fur, meaning predators are more likely to see white rabbits when hunting As a result, rabbits with white fur are less likely to survive than rabbits with brown fur The rabbits with brown fur therefore have a selection advantage, so they are more likely to survive to reproductive age and be able to pass on their alleles to their offspring Over many generations, the frequency of alleles for brown fur will increase and the frequency of alleles for white fur will decrease

Answer 12

Environmental factors that affect the chance of survival of an organism - Orgnaisms with 1 phenotype are more likely to survive and reproduce than those with a diff phenotype - can have different effects on the allele frequencies of a population through natural selection ## Footnote For example, there could be high competition for food between lions if there is not plentiful prey available; this environmental factor ‘selects’ for faster, more powerful lions that are better hunters

Answer 13

Stabilising Disruptive Directional

Answer 14

Stabilising selection is natural selection that keeps allele frequencies relatively constant over generations This means things stay as they are unless there is a change in the environment A classic example of stabilising selection can be seen in human birth weights Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights - bell shaped curve | see one note graphs/save my exam graphs

Answer 15

Directional selection is natural selection that produces a gradual change in allele frequencies over several generations This usually happens when there is a change in environment / selection pressures or a new allele has appeared in the population that is advantageous ## Footnote For example: A recent finding has shown that climate change is having an effect on fish size in certain habitats The increase in temperature is selecting for a smaller body size and against a larger body size Warmer seas cause fish metabolism to speed up and so increases their need for oxygen; oxygen levels are lower in warmer seas Larger fish have greater metabolic needs than smaller fish, and so they feel the effect of increased temperatures more strongly Organisms are sensitive to changes in temperature primarily because of the effect that temperature can have on enzyme activity Fish with a smaller body size are therefore fitter and better adapted to living in seas experiencing increased temperatures Fish body size is determined by both genetic and environmental factors Fish of a smaller size are more likely to reproduce and pass on their alleles to offspring Over generations, this leads to an increase in the frequency of alleles that produce a small body size and a decrease in the frequency of alleles that produce a larger body size

Answer 16

Disruptive selection is natural selection that maintains high frequencies of two different sets of alleles In other words, individuals with intermediate phenotypes or alleles are selected against Disruptive selection causes polymorphism: the continued existence of two or more distinct phenotypes in species This can occur in an environment that shows variation ## Footnote For example, birds that live on the Galapagos Islands use their beaks to forage for different sized seeds The size of the bird's beaks are either small or large with the intermediate medium-sized beak selected against The reason for this is that the different types of seed available are more efficiently foraged by a shorter or longer beak

Answer 17

involves changes in allele frequencies over time - can be caused by natural selection Selection pressures (caused by the environment) increase the likelihood that certain individuals with specific alleles survive to reproductive age, enabling them to pass on their alleles to their offspring -it is also possible for allele frequencies to change as a result of chance; this can occur due to a process known as genetic drift

Answer 18

the gradual change in allele frequencies in a small population, where some alleles are lost or favoured just by chance and not by natural selection. ## Footnote When a population is very small chance can affect which alleles get passed on to the next generation Meiosis results in haploid gametes, meaning that a fertilisation event only passes on half of the alleles of an individual; the half that gets passed on is the result of random fertilisation, and the other half of the alleles may be lost to the next generation Over time some alleles can be lost or passed on purely by chance; this is genetic drift

Answer 19

The founder effect The bottleneck effect

Answer 20

Directional selection - prduces gradual change in allele frq over several generations ## Footnote There is always phenotypic variation within a population There is a selection pressure in the environment, e.g. the presence of a predator Some individuals in a population may have a phenotype that aids their survival in the presence of a selection pressure The phenotype is produced by particular alleles Individuals with the favoured phenotype are fitter and so more likely to reproduce and pass on the advantageous alleles to their offspring Those who do not possess the advantageous allele or phenotype are less likely to survive and pass on their alleles to their offspring So over time and several generations the frequency of the advantageous allele increases and the frequency of other alleles decreases

Answer 21

small population ## Footnote E.g. if a coin is tossed 10 times it is reasonably likely that heads will not come up at all, whereas if a coin is tossed 100 times and heads didn't come up at all you would think you had a dodgy coin! In a similar way the chances of a certain allele simply being lost by chance as a result of random fertilisation is much greater if only 10 pairs of birds are breeding than if there were 100 pairs of birds breeding

Answer 22

In a small population of five plants growing near a playground with a rubber floor, three of the plants have blue flowers and two of the plants have pink flowers By chance most of the seeds from the pink flowered plants end up on the rubber floor of the playground while all the seeds from the blue flowered plants land on fertile soil where they are able to germinate and grow Note that the seeds from the pink flower do not fall on the impermeable surface because of any disadvantageous allele in the plant's genome, but purely by chance, e.g. because of a gust of wind or a passing animal If this happens by chance over several generations the allele for the pink flowers may be lost from this population

Answer 23

the reduction in a gene pool compared with the main populations of a species resulting from only 2 or 3 individuals (with onlt a selection of the alleles in the gene pool) starting off a new population ## Footnote The founder effect occurs when a small number of individuals from a large parent population start a new population The founder effect can come about as the result of chance E.g. a chance event such as a storm may separate a small group of individuals from the main population As the new population is made up of only a few individuals from the original population only some of the total alleles from the parent population will be present In other words, not all of the gene pool is present in the smaller population Because the population that results from the founder effect is very small it is more susceptible to the effects of genetic drift EG: The founder effect in lizards Anole lizards inhabit most Caribbean Islands and they can travel from one island to another via floating debris or vegetation A small number of lizards may be separated from the main population on a larger island and carried away to a smaller island by a chance event such as a large ocean wave or a storm The lizards arriving at a new island may only carry a small selection of alleles between them, with many more alleles present in the lizard population on the original island E.g. the lizards on the original island could display a range of scale colours from white to yellow and the two individual lizards that arrived on the island may have white scales This means that the whole population that grows on that island might only have individuals with white scales In comparison the original island population has a mixture of white and yellow scaled individuals. If the yellow allele were recessive and present as a single copy in the original two lizards that arrived on the island, the chance of it being lost as a result of genetic drift is increased due to the small size of the gene pool

Answer 24

a period when the number of species fall to a very low level, resulting in the loss of a large number of alleles and therefore a reduction in the gene pool of the species ## Footnote The bottleneck effect is similar to the Founder effect It occurs when a previously large population suffers a dramatic fall in numbers A major environmental event can greatly reduce the number of individuals in a population which in turn reduces the genetic diversity in the population as alleles are lost The surviving individuals end up breeding and reproducing with close relatives Example of the bottleneck effect A clear example of a genetic bottleneck can be seen in cheetahs today Roughly 10,000 years ago there was a large and genetically diverse cheetah population Most of the population was suddenly killed off when the climate changed drastically at the end of the Ice Age As a result the surviving cheetahs were isolated in small populations and lots of inbreeding occurred This meant that the cheetah population today has a lack of genetic variation This is problematic for conservation as genetic variation within a species increases the likelihood that the species is able to respond in the event of any environmental changes Remember the environment exerts a selection pressure on organisms

Answer 25

Natural selection -> selection pressures produce a gradual change in allele freq over sevel generations Founder effect -> change in allele freq occur in a diff direction for the newly isolated small pop comparison to the larger parent pop due to chance Genetic drift -> gradual change in allel freq in a small pop due to chance and not natural selection Bottleneck effect -> reduction in the gene pool of a pop due to a dramatic decrease in pop size

Answer 26

Never - natural selec Fight - founder effect Giant - genetic drift Beasts - bottleneck effect

Answer 27

the complete range of DNA base sequences in all organisms in a species or population - is the collection of genes within an interbreeding population - can be thought of as the sum of all the alleles at all of the loci within the genes of a population of a single species or a population

Answer 28

Antibiotics

Answer 29

chemical substances that inhibit or kill bacterial cells with little or no harm to human tissue

Answer 30

naturally occurring substances that are harmful to prokaryotic cells (structurally or physiologically) but usually do not affect eukaryotic cells

Answer 31

is to aid the body’s immune system in fighting a bacterial infection Penicillin is a well-known example; it was the first antibiotic to be discovered in 1928 by Sir Alexander Fleming ## Footnote Antibiotics are either described as being bactericidal (they kill) or bacteriostatic (they inhibit growth processes), they target prokaryotic features but can affect both pathogenic and mutualistic bacteria living on or in the body However, like in all species, there exists genetic diversity within populations, and the same applies to disease-causing bacteria Individual bacterial cells may possess alleles that confer resistance to the effects of the antibiotic These alleles are generated through random mutation and are not caused by antibiotic use, but antibiotic use exerts selection pressures that can result in the increase in their frequency

Answer 32

- bacteria that are sensitive to penicilin die - this is the entire population of the disease-causing bacteria. - by chance there may be 1+ individual bacteria with an allele giving resistance to penicilin. - This allele arises by random mutation - one eg: Staphylococcus, where some individual bacteria have an allele that codes for the production of an enzyme, penicillinase, which inactivates penicilin. - As bacteria have only a single loop of DNA, they have only 1 copy of each gene, so the mutant allele will have an immediate effect on the phenotype of any bacterium possessing it. When an antibioitc is present: - Individuals with the allele for antibiotic resistance have a massive selective advantage so they are more likely to survive, reproduce and pass their genome (including resistance alleles) on Those without alleles are less likely to survive and reproduce Over several generations, the entire population of bacteria may be antibiotic-resistant Antibiotic resistance is an important example of natural selection

Answer 33

There are known populations of the bacterium Staphylococcus that possess alleles which make them resistant to the effects of penicillin These are known as resistant strains Due to the rapid reproduction rate of bacteria (generations of 20-30 minutes for some species in optimal conditions) a single resistant bacterium can produce 10 000 million resistant descendants within a day

Answer 34

Antibiotic-resistant strains are a major problem in human medicine New resistant strains are constantly emerging due to the overuse of antibiotics By using antibiotics frequently, humans exert a selective pressure on the bacteria, which supports the evolution of antibiotic resistance Scientists are trying hard to find new antibiotics that bacteria have not yet been exposed to, but this process is expensive and time-consuming Some strains of bacteria can be resistant to multiple antibiotics and they create infections and diseases which are very difficult to treat When antibiotics were discovered, scientists thought they would be able to eradicate bacterial infections, but less than a century later a future is being imagined where many bacterial infections cannot be treated with current medicines

Answer 35

if certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next ## Footnote can be useful when building models and making predictions, but the assumptions listed are very rarely, if ever, all present in nature

Answer 36

- Organisms are diploid - Organisms reproduce by sexual reproduction only - There is no overlap between generations, i.e. parents do not mate with offspring - Mating is random - The population is large - There is no migration, mutation, or selection This would mean no individuals entering the population (immigration) or leaving (emigration) Selection refers to both natural and artificial selection - Allele frequencies are equal in both sexes - The Hardy-Weinberg principle can be useful when building models and making predictions, but the assumptions listed are very rarely, if ever, all present in nature

Answer 37

for the calculation of allele and genotype frequencies within populations It also allows for predictions to be made about how these frequencies will change in future generations

Answer 38

is the process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and many generations ## Footnote Humans have been selectively breeding organisms for thousands of years, long before scientists understood the genetics behind it Knowledge of the alleles that contribute to the expression of the desired traits are not required as individuals are selected by their phenotypes, and not their genotypes As the genetics is not always understood, breeders can accidentally enhance other traits that are genetically linked to the desirable trait These other traits can sometimes negatively affect the organism's health Examples of artificial selection include: Increased milk yield from cattle Faster racehorses Disease-resistant crops There are always biological limitations to how extreme a trait can become in an organism

Answer 39

1. The population shows phenotypic variation - there are individuals with different phenotypes / traits 2. Breeder selects an individual with the desired phenotype 3. Another individual with the desired phenotype is selected. The two selected individuals should not be closely related to each other 4. The two selected individuals are bred together 5. The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding 6. The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait ## Footnote Artificial selection in racing horses Selective breeding has been a major part of the horseracing industry for many years. Breeders have found that horses tend to have one of the three following phenotypes: Good at sprinting short distances Good endurance over long distances All-rounder If a breeder wanted to breed a horse for a sprinting event they are likely to do the following: Select the fastest sprinting female horse they have Select the fastest sprinting male horse they have Breed the two selected horses Allow their offspring to reach maturity and test their sprinting speeds to find the fastest horse (male or female) The breeder could then use this horse for racing, or they could continue the process of selective breeding by breeding this horse with another horse that is fast or descended from fast-sprinters Over several generations, it would be hoped that the offspring are all fast-sprinters (but remember there are biological limitations to this)

Answer 40

increasing the yield of a sellable product It is not done with the organism's survival in mind, and unlike natural selection, it can lead to organisms that are poorly adapted to their environments

Answer 41

- Disease-resistance in wheat and rice varieties - Hybridization in maize - Milk yield in cattle

Answer 42

- Wheat plants have been selectively bred for hundreds of years as a crop - Wheat crops can be badly affected by fungal diseases: Fusarium is a fungus that causes “head blight” in wheat plants - **Fungal diseases** are highly problematic for farmers as they destroy the wheat plant and **reduce crop yield** - By using selective breeding to introduce a fungus-resistant allele from another species of wheat, the hybrid wheat plants are **not susceptible to infection, and so yield increases** - Introducing the allele into the crop population can take many generations and collaboration with researchers and plant breeders - **Rice** is another crop that has been subject to large amounts of selective breeding - Rice plants are prone to different **bacterial and fungal diseases** Examples include “bacterial blight” and “rice blast” caused by the Magnaporthe fungus - These diseases all **reduce the yield** of the crop as they damage infected plants - Scientists are currently working hard to create varieties of rice plants that are resistant to several bacterial and fungal diseases

Answer 43

- Maize (also known as corn) is a staple crop in many countries around the world; it is grown to feed both livestock and people - In the past, maize plants have been heavily inbred (bred with plants with similar genotypes to their own) - This has resulted in small and weaker maize plants that have less vigour - This is inbreeding depression which: Increases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype Increases homozygosity in individuals (paired alleles at loci are identical) Leads to decreased growth and survivability - A farmer can prevent inbreeding depression by outbreeding - This involves breeding individuals that are not closely related - Outbreeding produces taller and healthier maize plants - It decreases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype - Increases heterozygosity (paired alleles at loci are different) - Leads to increased growth and survivability (known as hybrid vigour) - Crops of these plants have a greater yield - Uniformity is important when growing a crop: If outbreeding is carried out completely randomly, it can produce too much variation between plants within one field A farmer needs the plants to ripen at the same time and be of a similar height; the more variation there is, the less likely this is - In order to achieve heterozygosity and uniformity, farmers buy sets of homozygous seeds from specialised companies and cross them to produce an F1 generation - Different hybrids of maize are constantly being created and tested for desirables traits such as: resistance to pests / disease, higher yields and good growth in poor conditions

Answer 44

- Milk is a global food source, rich in calcium and protein (essential for growth) - Over many years and generations farmers have selected female cows that have the highest milk yield and crossed them with male bulls related to high yield females - Over time this selective breeding has resulted in cows with greater milk yields, which has been of great economical benefit to farmers - The selective breeding of cows for increased milk yield is a good example of how artificial selection (controlled by humans) does not take into account an organism's survival - Selective breeding usually focuses on only one, or a handful of, characteristics, often to the extreme. Little thought is given to other traits important to an organism's health In cows it has been observed that selectively bred individuals are much more prone to ailments such as mastitis (inflammation of the udder), milk fever and lameness compared to those that were allowed to breed at random

Answer 45

a group of organisms with similar morphological and physiological features that are able to interbreed and produce fertile offspring that are reproductively isolated from one another.

Answer 46

Natural selection Genetic drift The founder effect

Answer 47

the characteristics of the species will also change The change can become so great that a new species forms This is evolution

Answer 48

is the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation

Answer 49

genetically and reproductively isolated from the pre-existing species population

Answer 50

mutations that lead to the incompatibility of gametes or sex organs, or differences in breeding behaviour. When two populations are reproductively isolated, they can also be said to be genetically isolated from each other, meaning that they do not exchange genes with each other in the production of offspring

Answer 51

Changes in the allele frequencies of isolated populations are not shared so they evolve independently of each other; this can lead to the formation of two groups that are no longer successfully able to interbreed and that are said to be separate species **the formation of new species** in this way is known as speciation ## Footnote The evolution of a new species can take a very long time and many generations For organisms with a short generation time (such as bacteria), evolution of new species can be observed far more quickly

Answer 52

DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species The differences between the nucleotide sequences (DNA) of different species can provide a lot of information: The more similar the sequence the more closely related the species are Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Answer 53

DNA found in the nucleus, mitochondria and chloroplasts of cells can be sequenced and used to show evolutionary relationships between species The differences between the nucleotide sequences (DNA) of different species can provide a lot of information: The more similar the sequence the more closely related the species are Two groups of organisms with very similar DNA will have separated into separate species more recently than two groups with less similarity in their DNA sequences DNA sequence analysis and comparison can also be used to create family trees that show the evolutionary relationships between species

Answer 54

DNA is extracted from the nuclei of cells taken from an organism DNA can be extracted from blood or skin samples from living organisms or from fossils The extracted DNA is processed, analysed and the base sequence is obtained The base sequence is compared to that of other organisms to determine evolutionary relationships The more similarities there are in the DNA base sequence, the more closely related (in that the less distant the species separation) members of different species are ## Footnote In 2005, the chimpanzee genome was sequenced, and when compared to the human genome it was discovered that humans and chimpanzees share almost 99% of their DNA sequences, making them our closest living relatives In 2012, the sequencing of the bonobo genome also revealed that humans and bonobos also share 98% of their genome (with slight differences to the differences seen in chimpanzees)

Answer 55

When analysing DNA from the mitochondria is is important to remember that: - A zygote only contains the mitochondria of the egg and none from the sperm so only maternal mitochondrial DNA is present in a zygote - There is no crossing over that occurs in mtDNA so the base sequence can only change by mutation - The lack of crossing over in mtDNA has allowed scientists to research the origins of species, genetic drift and migration events - It has even been possible to estimate how long ago the first human lived and where - Mitochondrial Eve is thought to have lived in Africa ~200,000 years ago - The estimation of this date relies on the molecular clock theory which assumes there is a constant rate of mutation over time - The greater the number of differences there are between nucleotide sequences, the longer ago the common ancestor of both species existed - The molecular clock is calibrated by using fossils and carbon dating - A fossil of a known species is carbon-dated to estimate how long ago that organism lived - This mtDNA of this species is then used as a baseline for comparison with the mtDNA of other species Although for your exams you should say that only maternal mitochondrial DNA can be passed on or inherited by the zygote, recent research suggests that paternal mDNA may also be present in zygotes

Answer 56

speciation: the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation Genetic isolation between the new population and the pre-existing species population is necessary for speciation

Answer 57

- Two groups of a species are separated by a geographic barrier - Two groups of species are reproductively isolated but still living in the same area (experiencing similar environmental selection pressures)

Answer 58

- occurs as a result of geographical isolation - It is the most common type of speciation - A species population splits into one or more groups which then become separated from each other by geographical barriers - The barrier could be natural like a body of water, or a mountain range - It can also be man-made (like a motorway) - This separation creates two populations of the same species who are isolated from each other, and as a result, no genetic exchange can occur between them - If there is sufficient selection pressure or genetic drift acting to change the gene pools within both populations then eventually these populations will diverge and form separate species - The changes in the alleles/genes of each population will affect the phenotypes present in both populations Over time, the two populations may begin to differ physiologically, behaviourally and morphologically (structurally) ## Footnote Example of Allopatric Speciation in Trees Imagine there is a population of trees that are all one species A new mountain range forms that divides the population into two The natural barrier prevents the two groups from interbreeding, so there is no gene flow between them The two populations experience different selection pressures and genetic drift Over thousands of years the divided populations form two distinct species that can no longer interbreed

Answer 59

- takes place with no geographical barrier - A group of the same species could be living in the same place but in order for speciation to take place there must exist two populations within that group and no gene flow occurs between them - Something has to happen that splits or separates the population: - Ecological separation: Populations are separated because they live in different environments within the same area For example, soil pH can differ greatly in different areas. Soil pH has a major effect on plant growth and flowering -Behavioural separation: Populations are separated because they have different behaviours For example differences in feeding, communication or social behaviour ## Footnote Example of Sympatric Speciation in Fish A species of fish lives in a lake Some individuals within the population feed on the bottom while others remain higher up in the open water The different feeding behaviours separates the population into different environments Behavioural separation leads to ecological separation The separated groups experience different selection pressures Long jaws are advantageous for bottom-feeding whereas shorter jaws are advantageous for mid-water feeding Over time natural selection causes the populations to diverge and evolve different courtship displays They can no longer interbreed; they are separate species

selection and evolution Flashcards

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