Summary
Summary 17 Selection and Evolution
- Institution
- CIE
Everything you need to know about "Selection and Evolution" for A-level Biology CIE
[Show more]
Connected book
Book Title:
Author(s):
- Edition:
- ISBN:
- Edition:
Seller
Follow
Content preview
Evolution10 March 2022 09:40
Variation in Populations
• Genetic variation may be introduced by:
- Independent assortment of chromosomes in metaphase I
- Crossing over between homologous chromosomes in prophase I
- Random mating between organisms
- Random fusion of gametes
- Mutation
• Environmental factors will also influence the phenotype, but they will not be passed onto the offspring
• These differences can be measured as quantitative differences (eg, mass, height), or as qualitative differences (eg, ABO blood group)
• These two types of variation tend to be caused by different patterns of genetic inheritance
Continuous Variation:
• Tends to be polygenic, eg, many genes have a combined effect on a single phenotype
• This leads to a continuous range of phenotypes (phenotypes exhibit a 'normal distribution')
• These genes have the same, often additive, effect
• Therefore different alleles at a single locus have small effects on the phenotype
• Environmental factors can influence the phenotype
• Eg, height, birth mass, number of stomata on a leaf
Discontinuous Variation:
• Phenotypes within populations have distinct categories
• They tend to be controlled by a single or a small number of genes
• Different alleles at a single gene locus can have large effects on the phenotype
• Different genes can have quite different effects on the phenotype
• Environment has less/no influence on phenotype
• Eg, blood type, albinism, tall and dwarf pea plants
Environment and Phenotype
• The phenotype can be affected by the interaction between the genotype of an individual and the environment
• Eg, many genes for height are inherited, but these only give the potential to reach a certain height. A lack of protein when young may result in stunted
growth
• Plants will show a variety of phenotypes, depending on their growing conditions
• Eg, plants lacking in magnesium will be unable to make chlorophyll and therefore will have yellowish leaves
• In Himalayan rabbits, temperature controls the gene for pigment production
• At low temperatures the gene is expressed and therefore the pigment is made, causing blacking fur
• In crocodiles, high temperatures of 32°C-34°C result in males, low temperatures of 28°C-30°C yield females. At intermediate temperatures over a narrow
range, both males and females are produced in varying proportions
• In humans, UV light triggers the production of melanin, a pigment found in skin. This reduces the risk of skin cancer
T-Test
• The t-test is applied to sample sizes of more than five and fewer than 30 sets of normally distributed data
• It provides a way of measuring the overlap between two data sets
• A large value of 't' means there is little overlap, so a highly significant difference between the two data sets
• Eg, an ecologist was investigating a woodland habitat and comparing the widths of ivy leaves growing in the shade with those growing in full sunlight
• Leaf widths were measured, but because the distance of the leaf along the stem also had an effect on leaf size, only the fourth leaf on every branch was
measured
•
• The null hypothesis assumes any differences under investigation has arisen by chance, so the null hypothesis is that there is no difference in size between sun
and shade leaves
• The role of the statistical test is to determine whether or not to accept or reject the null hypothesis
• If we reject it, it means we are confident that the difference between the sizes of the two types of leaf is statistically significant
• Once a value of t has been calculated, and the t value in this case is 2.1, we determine the degrees of freedom for the two samples
• (12 + 12) - 2 = 22
• We look down the column of significance levels (p) at the 0.05 level until we reach the line corresponding to the correct degrees of freedom
• The critical value at this point is 2.08
• Since the calculated t value (2.1) exceeds the critical value at the 0.05 level of significance, it indicates that there is a less than 0.05 or 5% probability that the
difference between the two samples is due to chance
• Therefore we can reject the null hypothesis and conclude that the difference between the two samples is significant
Natural Selection
Observations:
• Organisms produce more offspring than needed to replace themselves
• Established populations tend to stay the same size
• Members of the same species tend to resemble their parents, but also show variation amongst themselves
Deductions:
• There is a struggle for existence in the competition for resources, many will die
• Those organisms best suited to their environment will survive and reproduce. Their offspring will tend to inherit their beneficial characteristics
Competition for Resources:
• Normally as a population increases, various environmental factors come into play to keep numbers limited
• These factors may be biotic, caused by other living organisms, such as predators, competition for food, pathogens
• The factors may also be abiotic, water supply or nutrient levels in the soil
• As population numbers increase, overcrowding may occur, increasing the passing on of diseases
• Predators may move into the area one they've realised there's food there
• Fights are more likely to break out
• Food will become short in supply
• Nesting/burrowing space will become limited
• The result will be that of all the organisms born in a population, many will die from lack of food, predation and disease, leaving only a small proportion of
young to grow to adulthood and pass on their alleles
• Population growth may slow
• The population may even decrease if the pressure of environmental factors is sufficiently great
• Only when the number of individuals has fallen considerably will the numbers be able to grow again
• Over a period of time, population numbers may therefore oscillate about a mean level
• Gause's Law, two species competing for the same resource cannot coexist if all other ecological factors are constant
Evolution by Means of Natural Selection:
• There is variation within every population
• With rabbits, it may be that coat colour plays a role
• Normal rabbits are agouti in colour
• A few will have an allele combination which makes them white
• This will make them stand out from the rest and they are more likely to be picked off by predators
• The chances of a white rabbit in the wild passing on its alleles therefore becomes rare
• The term 'fitness' is often used to describe how well organisms are adapted to their environment
• Fitness is the capacity of an organism to transmit its genotype to its offspring
• Being eaten by a predator is an example of selection pressure
• Selection pressure increases the chances of some alleles being passed on and decreases the chances of others
• The effect of selection pressures on the frequency of alleles in a population is called natural selection
• Natural selection tends to keep things the same, this is stabilising selection
• So agouti rabbits are the best adapted rabbits to survive predation, so the agouti allele remains the most common
• As long as nothing changes, this will remain the situation
• There is an optimum birth weight for human babies
• Those with heavier or lighter birth weights are at a disadvantage and so less likely to survive
• This means that the alleles for particularly heavy or light babies are eliminated from the population
• Hence the population is stabilised
Directional Selection:
• If a new environmental factor or a new allele appears, then allele frequencies may change
• This is directional selection
• In this case, the majority of existing forms may no longer be best suited to the environment
• An example of this is the development of antibiotic resistance in bacteria
• A resistant bacterium has no selective advantage under normal conditions
• However, if exposed to antibiotics, those which carry a gene for resistance will survive and there will be an abrupt change in the genome of the population
• Bacteria only have one chromosome, so any mutation has an immediate effect on phenotype
• Antibiotic resistance often occurs on plasmids
• Plasmids are frequently transferred from one bacterium to another, even between different species
• So it is possible for resistance to arise in one species of bacterium and then be passed on to another
• The more we use antibiotics, the greater the selection pressure we put on bacteria to evolve resistance to them
Disruptive Selection:
• This is where particular environmental conditions favour the extremes of a phenotypic range over the intermediate phenotype
• As a result, the gene pool may be split in two and it is entirely possible that new species may arise
• A good example is the colonisation of plants by mine waste tips
• These often contain high concentrations of metals such as copper and lead
• Most plants can't grow on these mine waste sites
• However, some grasses have colonised the area so it seems that they must have developed resistance to toxic metals and can flourish where others can't
• Grasses are wind pollinated so breeding between resistant and non-resistant plants occurs
• When the resulting seeds land on the ground and start to germinate, disruptive selection may occur
• Both the non-resistant seeds trying to germinate on the toxic soil and the resistant seeds trying to germinate on the uncontaminated soil fail to reproduce
• The result is increasing divergence of populations into two different varieties of grass plants
Balancing Selection:
• Directional and stabilising selection tends to reduce variation
• Balancing selection has the opposite effect
• It is a process which maintains multiple alleles in a gene pool
• It does this at frequencies well above that due to mutation
• A good example of this is the sickle cell trait:
- A faulty allele HbS of the gene coding for the production of beta-globin in haemoglobin can produce sickling of red blood cells
- People who are homozygous for this allele have sickle cell anaemia
- This is severe and often lethal in its effects
- The possession of two faulty alleles clearly puts people at a severe selective disadvantage
- They are less likely to survive and reproduce
- Yet the frequency of this allele is very high in some parts of the world
- In some parts of East Africa, nearly 50% of the babies born are carriers of the allele and 14% are homozygous
- The parts of the world where the sickle cell allele is most common are also the parts of the world where malaria is most prevalent
- Malaria is the major source of illness and death in many parts of the world
- It has been found that people who are heterozygous for the sickle cell allele HbA HbS are much less likely to suffer from a serious attack of malaria than
people who are homozygous HbA HbA
- In malaria areas, heterozygotes are at a selective advantage as they do not suffer from sickle cell anaemia
- So both alleles remain in the population if malaria is an important environmental factor
- In non-malaria areas, the HbS allele has almost been completely removed from the population
Genetic Drift
• The examples we've just looked at show the effect of non-random processes on the allele frequencies of a population
• However, allele frequencies may also change due to a random process known as genetic drift
• This change occurs by chance because only some of the organisms of each generation reproduce
• It is most obvious when a small number of individuals are separated from the main population
• They form only a small sample of the original population so are unlikely to have the same allele frequency as the population as a whole
• Further genetic drift in this small population will alter the allele frequency even more and evolution of this population may be in a different direction from
the main portion of the population
• This process, which occurs in recently isolated small populations is called the founder effect
Population Genetics
• This is the study of genes in populations
• A population is a group of individuals of a species living in a defined area and able to reproduce to generate fertile offspring
• A population of snails in a garden may live near a compost heap in a smallish area, whereas a population of thrushes may occupy the same garden part of the
time, some other gardens and the neighbouring fields
• So the area occupied by a population depends not only on environmental factors but also on the size and mobility of the organism
The benefits of buying summaries with Stuvia:
Guaranteed quality through customer reviews
Stuvia customers have reviewed more than 700,000 summaries. This how you know that you are buying the best documents.
Quick and easy check-out
You can quickly pay through credit card for the summaries. There is no membership needed.
Focus on what matters
Your fellow students write the study notes themselves, which is why the documents are always reliable and up-to-date. This ensures you quickly get to the core!
Frequently asked questions
What do I get when I buy this document?
You get a PDF, available immediately after your purchase. The purchased document is accessible anytime, anywhere and indefinitely through your profile.
Satisfaction guarantee: how does it work?
Our satisfaction guarantee ensures that you always find a study document that suits you well. You fill out a form, and our customer service team takes care of the rest.
Who am I buying these notes from?
Stuvia is a marketplace, so you are not buying this document from us, but from seller Zubaid. Stuvia facilitates payment to the seller.
Will I be stuck with a subscription?
No, you only buy these notes for £2.99. You're not tied to anything after your purchase.
Can Stuvia be trusted?
4.6 stars on Google & Trustpilot (+1000 reviews)
67096 documents were sold in the last 30 days
Founded in 2010, the go-to place to buy revision notes and other study material for 14 years now