5.2 Natural Selection Flashcards
Who proposed the theory of natural selection?
The theory of natural selection was posited by Charles Darwin (and also Alfred Wallace) who described it as ‘survival of the fittest’
What species is most likely to survive, in terms of natural selection?
According to this theory, it is not necessarily the strongest or most intelligent that survives, but the ones most responsive to change
In response to what conditions does natural selection occur (5)?
inherited variation
competition
selection
adaptations
evolution
How does natural selection occur in relation to inherited variation?
Inherited Variation – There is genetic variation within a population which can be inherited
How does natural selection occur in relation to competition?
Competition – There is a struggle for survival (species tend to produce more offspring than the environment can support)
How does natural selection occur in relation to selection?
Selection – Environmental pressures lead to differential reproduction within a population
How does natural selection occur in relation to adaptations?
Adaptations – Individuals with beneficial traits will be more likely to survive and pass these traits on to their offspring
How does natural selection occur in relation to evolution?
Evolution – Over time, there is a change in allele frequency within the population gene pool
What is the summary of natural selection (mnemonic)
Inherited variation exists within the population
Competition results from an overproduction of offspring
Environmental pressures lead to differential reproduction
Adaptations which benefit survival are selected for
Genotype frequency changes across generations
Evolution occurs within the population
Mnemonic: ICE AGE
What does natural selection require?
Natural selection requires variation among members of a species in order to differentiate survival (variation needed for selection)
In what forms can variation arise?
This variation can manifest as either discontinous (distinct classes) or continuous (range across a characteristic spectrum)
What are the 3 main mechanisms in which variation may occur in an individual?
mutations
meiosis
sexual reproduction
How do mutations lead to variation?
Mutations – Changing the genetic composition of gametes (germline mutation) leads to changed characteristics in offspring
How does meiosis lead to variation?
Meiosis – Via either crossing over (prophase I) or independent assortment (metaphase I)
How does sexual reproduction lead to variation?
Sexual reproduction – The combination of genetic material from two distinct sources creates new gene combinations in offspring
What is a gene mutation?
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
What is formed by mutations?
New alleles are formed by mutation
What 3 types of gene mutations are there?
Gene mutations can be beneficial, detrimental or neutral
What are beneficial mutations?
Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
What are detrimental mutations?
Detrimental mutations truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait
What are neutral mutations?
Neutral mutations have no effect on the functioning of the specific feature (silent mutations)
What does crossing over involve? (meiosis)
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
Where does the exchange of genetic material occur in meisosis?
The exchange of genetic material occurs between non-sister chromatids at points called chiasmata
What does crossing over lead to in terms of the 4 chromatids? meiosis
As a consequence of this recombination, all four chromatids that comprise the bivalent will be genetically different
What will the chromatids consist of in meiosis?
Chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
What will offspring with recombinant chromosomes have?
Offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent
What is random in meiosis?
When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random
What is independent assortment?
The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
What is the total number of combinations that can occur in gametes?
The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
How many different gametes can humans produce?
Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (223) by random orientation
How many different gametes can humans produce WITH crossing over?
If crossing over also occurs, the number of different gamete combinations becomes immeasurable
What does the fusion of two haploid gametes result in?
The fusion of two haploid gametes results in the formation of a diploid zygote
What can a zygote do to form an embryo?
This zygote can then divide by mitosis and differentiate to form a developing embryo
How does random fertilisation occur?
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes
Due to random fertilisation what does offspring show?
This means that individual offspring will typically show variation despite shared parentage
How are random twins formed?
Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses
What is the Malthusian dilemma?
The Malthusian dilemma was proposed by English clergyman Thomas Malthus who identified that populations multiply geometrically (i.e. exponential progression), while food resources only increase arithmetically (i.e. linear progression)
What is the simplified form of the Malthusian dilemma?
In other words, species tend to produce more offspring than the environment can sustainably support
How is competition therefore created?
If left to follow course, a stable population will inevitably outgrow its resource base, leading to competition for survival
How will the population grow with an abundance of resources?
When there is an abundance of resources, a population will grow according to its biotic potential (exponential J-curve)
What happens when more offspring are produced?
With more offspring, there are less resources available to other members of the population (environmental resistance)
What can too many offspring lead to?
This will lead to a struggle for survival and an increase in the mortality rate (causing population growth to slow and plateau)
What is the Malthusian dilemma key to?
This concept is central to Darwin’s understanding of ‘survival of the fittest’ – any trait that is beneficial for competitive survival will be more likely to be passed on to offspring according to natural selection
What are adaptations?
Adaptations are features of organisms that aid their survival by allowing them to be better suited to their environment
What are 5 categories for adaptations?
structural
behavioural
physiological
biochemical
developmental
What are structural adaptations?
Structural: Physical differences in biological structure (e.g. neck length of a giraffe)
What are behavioural adaptations?
Differences in patterns of activity (e.g. opossums feigning death when threatened)
What are physiological adaptations?
Variations in detection and response by vital organs (e.g. homeothermy, colour perception)
What are biochemical adaptations?
Differences in molecular composition of cells and enzyme functions (e.g. blood groups, lactose tolerance)
What are developmental adaptations?
Variable changes that occur across the life span of an organism (e.g. patterns of ageing / senescence)
What basis do biological adaptations have?
Biological adaptations have a genetic basis (i.e. encoded by genes) and may be passed to offspring when the parents reproduce
What type of organism is more likely to survive, in terms of adaptations?
Organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass on these genes
What happens to an organism not adapted?
Organisms without these beneficial adaptations will be less likely to survive long enough to reproduce and pass on their genes
Therefore what do adaptations result in in terms of reproduction?
Hence adaptations result in differential reproduction within a species, allowing for natural selection to occur
What can the variation in a population be considered to be, due to alleles?
The variation that exists within a population is heritable (i.e. genetic) and determined by the presence of alleles
How may alleles be passed down?
These alleles may be passed from parent to offspring via sexual reproduction
What may alleles encode for?
Alleles encode for the phenotypic polymorphisms of a particular trait and may be beneficial, detrimental or neutral
What can beneficial alleles do?
Beneficial alleles will better equip the organism to survive and hence produce more offspring (encodes beneficial adaptations)
What can detrimental alleles do?
Detrimental alleles will harm the survival prospects of an organism, leading to fewer viable offspring
What will neutral alleles do?
Neutral alleles will not affect the organisms survival prospects
What will natural selection do to allele frequency?
Due to natural selection, the proportion of different alleles will change across generations (evolution)
How do beneficial alleles affect reproductive prospects?
As beneficial alleles improve reproductive prospects (more offspring), they are more likely to be passed on to future generations
How do detrimental alleles affect reproductive prospects?
Conversely, detrimental alleles result in fewer offspring and hence are less likely to be present in future generations
How may environmental conditions change allele frequency?
If environmental conditions change, what constitutes a beneficial or detrimental trait may change, and thus the allele frequencies in a population are constantly evolving
What is adaptive radiation?
Adaptive radiation describes the rapid evolutionary diversification of a single ancestral line
When does adaptive radiation occur?
It occurs when members of a single species occupy a variety of distinct niches with different environmental conditions
What does adaptive radiation result in?
Consequently, members evolve different morphological features (adaptations) in response to the different selection pressures
Where can adapative radiation be seen?
An example of adaptive radiation can be seen in the variety of beak types seen in the finches of the Galapagos Islands
How do finches on Galapagos Island show adaptive radiation?
These finches have specialised beak shapes depending on their primary source of nutrition (e.g. seeds, insects, nuts, nectar)
What inhabits Daphne Major?
Daphne Major is a volcanic island that forms part of the archipelago that is collectively referred to as the Galapagos Islands
It is the native habitat of a variety of bird species known as Darwin’s finches (subfamily: Geospizinae)
What do Darwin’s finches demonstrate?
Darwin’s finches demonstrate adaptive radiation and show marked variation in beak size and shape according to diet
What beaks do birds that feed on seeds have?
Finches that feed on seeds possess compact, powerful beaks – with larger beaks better equipped to crack larger seed cases
What changed the frequency of beaks?
In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection
What do dry conditions lead to?
Dry conditions result in plants producing larger seeds with tougher seed casings
What specific change occurred to beaks due to the flood?
Between 1976 and 1978 there was a change in average beak depth within the finch population
Finches with larger beaks were better equipped to feed on the seeds and thus produced more offspring with larger beaks
what are antibiotics?
Antibiotics are chemicals produced by microbes that either kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria
what are antibiotics used to treat?
Antibiotics are commonly used by man as a treatment for bacterial infections (not effective against viral infections)
How may antibiotic resistance arise in a bacterial colony?
In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation
- What causes the gene for antibiotic resistance to be passed down?
When treated with antibiotics, the resistant bacteria will survive and reproduce by binary fission (asexual reproduction)
- What causes a decrease in competition?
The antibiotic resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotic)
- What process apart from reproduction can pass on the gene for antibiotic resistance?
Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
- What has caused the gene to increase in frequency?
he introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)
What is an example of an increase in antibiotic resistance?
An example of antibiotic resistance in bacteria can be seen in the evolution of Staphylococcus aureus (Golden staph)
What can golden staph cause?
Golden staph can cause infections to the skin (lesions and boils) as well as more serious infections (pneumonia, meningitis)
Historically, how was golden staph treated?
Historically, these infections were treated using the antibiotic methicillin
What is the name of bacteria resistant to methicillin?
Bacterial strains developed that were resistant to this antibiotic (methicillin-resistant Staphylococcus aureus – or MRSA)
How did MRSA strains increase?
These strains proliferated while susceptible strains died out (methicillin-sensitive Staphylococcus aureus – or MSSA)
Where are MRSA infections most common?
MRSA infections are now especially present in hospitals and nursing homes, where the use of methicillin was most common
What is the solution to golden staph?
Medical practitioners now prescribe alternate antibiotic agents to treat infections caused by Staphylococcus aureus
