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[h] IB DP Biology HL D4.1 Natural selection Flashcards
[q] D 4.1.1 Natural selection as the mechanism driving evolutionary change
[a] Natural selection – the mechanisn that drives a species to evolve into another species or for a species to remain the same species but still evolve (changes in the species just occur). Aka evolutaionary change.
– organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success.
Darwin’s theory of evolution is based on natural selection.
Evolutionary change – the heritable change in populations and species over time, due to mechanisms such as natural selection, random genetic drift, and sexual selection.
[q] D 4.1.2 Roles of mutation and sexual reproduction in generating the variations on which natural selection acts
[a] The only way that there are variations to be selected from or against is for there to be mutations and sexual reproduction.
Variations are generated by:
– mutations creating different alleles.
– and sexual reproduction mixing up the different alleles and passing them on.
If ever individual of a species was identical there can not be any natural selection or evolutionary.
[q] D 4.1.3 Overproduction of offspring and competition for resources as factors that promote natural selection
[a] Theory of evolution by natural selection depends on:
– more offspring produced than what a environment can support
– and that those organisms have to compete for resources
Carrying capacity – how many of a species an environment can support.
– based on all the biotic and abiotic factors that control how many of a species can live in an environment.
– example of resources: predators, food, nesting, etc.
If there is nothing limitimg a population (it is smaller than what the environment can support) then having one characteristic over another will not make a organism be selected.
[q] D 4.1.4 Abiotic factors as selective pressures
[a] Abiotic factors of an ecosystem that can provide selective pressure:
Density independent:
– high/low temperatures
– natural disasters
– the number of the species present does not matter.
Density dependent:
– based on how many of the species is present.
[q] D 4.1.5 Differences between individuals in adaptations, survival, and reproduction as the basis for natural selection
[a] Individuals that are best adapted have the highest probability of survival, thus are able to pass on their genes.
Intraspecific competition – competition between individuals of the same species.
Survival value/fitness – a measure of how well a certain phenotype is in a population.
Reproductive potential of a genotype:
– some have a larger potential than others.
– based on the phenotype that the genotype creates.
[q] D 4.1.6 Requirement that traits are heritable for evolutionary change to occur
[a] A trait has to be heritable for evolutionary change to occur.
– This is where Lamarck was wrong.
– If a trait is not heritable, it can not be a part of natural selection.
[q] D 4.1.7 Sexual selection as a selection pressure in animal species
[a] Sexual selection – a form of natural selection in which the reproductive success of an individual results in more offspring compared to others in the population who do not have as much success in finding a mate.
Individuals choose who to mate with.
In animal species, sexual selection is a selection pressure that drives the evolution of a population.
Physical/behavioral traits attract animals to one another.
– these traits are signs of fitness.
– example: in a lot of bird species the males are very colorful (the most colorful males are the ones who reproduce the most/are choosen as a mate the most);
females are attracted to the fact that they have been able to survive for so long while having very bright/flashy colors – overall ability to survive is good.
[q] D 4.1.8 Modeling of sexual and natural selection based on experimental control of selective pressure
[a] Endler’s guppy experiment (case study of natural selection in the color patterns in guppies)
– wanted to see if the presence of predators modified the bright colors seen in guppies.
– carried out in two ways: in the field (in the streams of Trinidad) and in 10 artificial ponds in a greenhouse at Prinnceton University, USA. (where more variables could be cntrolled than in nature).
– in both the field and in the greenhouse, some populations of guppies where kept with predators while others were not.
– hypothesis: guppies in pools protected from predatory fish would be ornate colors (because there is no threat of their survival);
while guppies in pools where predators where presnt would be less colorful (because there ability to hide would lead to better chances of survival).
– independent variable: presence of predators (one of the selective pressures on male guppies)
[q] D 4.1.9 Concepts of the gene pool
[a] Gene pool – all the genetic information present in the reproducing memebrs of a population at a given time.
– all the possible genes (and their alleles) in an interbreeding population.
Inbreeding – when closely related organisms mate wit each other.
– inbreeding narrows the gene pool.
Size of gene pool:
– a population that shows substantial variety in its traits has a larger gene pool.
– a population whose memebers show little variation has a small gene pool.
[q] D 4.1.10 Allele frequencies of geographically isolated populations
[a] Allele frequency – a measure of the proportion of a specific version of a gene in a population.
– expressed as a proportion or a percentage.
Geographically isolated populations can have different allele frequencies.
– they are not interbreeding, therefore mutations, differential rates, and etc. can occur.
– example of isolated populations having different gene ratios: ABO blood typing amongst different populations of humans.
[q] D 4.1.11 Changes in allele frequency in the gene pool as a consequence of natural selection between individuals according to differences in their heritable traits
[a] Allele frequencies can change as a result of natural selection.
– if one allele has an advantage then it can become a higher frequency in the population.
allele = heritable trait
[q] D 4.1.12 Differences between directional, disruptive, and stabilizing selection
[a] Populations of members of the same species (and thus the same gene pool) can be stopped from reprodcuing together because there is an insurmountable barrier between them.
– barriers can be geographical, temporal, behavioural or related to the infertility caused by hybridization.
Directional selection – takes place when one extreme phenotype is favoured over all other phenotypes by natural selection.
– the frequency of one phenotype is seen to increase over time, whereas the other phenotype decreases.
– example: if a drought occurs and the plants that make the little seeds that finches who have dainty bills can eat die off, then the finches whose bills can break the hard seeds will survive and the finches with dainty bills will die off.
[q] D 4.1.12 Differences between directional, disruptive, and stabilizing selection (continued)
[a] Disruptive selection – when two extreme phenotypes are favoured by natural selection, rather than one intermediate phenotype.
– can be an advantage (ex. could provide a better chance of survival in places where food sources are vaiable).
– if the differences caused are too extreme and the two populations occupy different niches, it is possible for speciation to occur.
– example: in a species of invertebrate that has body shades that go from very light to very dark, both the very light and very dark will survive by living on different parts of the sandy beach that best fit their coloring (along the ocean floor or within the rocks).
Stabilizing selection – when one phenotype is favoured over two extreme phenotypes.
– when the average is selected.
– example: birth weight in humans. Babies that are born too small can lose heat too easily and may die, whereas babies being born too large can lead to complications during childbirth and the death of the mother or the baby.
All of these types of selection change allele frequencies.
– any alleles that do not led to the favoured phenotype(s) will not be selected for and will make up less of the next generation.
– any of the alleles that are favoured will be more popular.
[q] D 4.1.13 Hardy-Weinberg equation and calculations of allele or genotype frequencies
[a] Hardy-Weinberg equation and calculations of allele or genotype frequencies:
p + q = 1 (frequency of the alleles)
p^2 + 2pq + q^2 = 1 (frequency of the genotypes)
Dominant allele = p
Recessive allele = q
homozygous dominant = p^2
heterozygous = 2pq
homozygous recessive = q^2
If you know the frequency of the HOMOZYGOUS RECESSIVE, you can figure out the frequency of the dominant allele.
Examples:
– freckles
– PTC tasting
[q] D 4.1.14 Hardy-Weinberg conditions that must be maintained for a population to be in genetic equilibrium
[a] Hardy-Weinberg conditions that must be maintained for a population to be in genetic equilibrium:
– No gene mutations
– Mating is random
– No immigration or emigration
– Population large enough for chance events to alter frequencies
– Natural selection does not favor one phenotype
*If an allele frequency is calculated based on Hardy-Weinberg then compared to previous calculations and the frequency is not the same/has to had changed, one of these conditions had to have not been maitained.
Genetic equilibrium – the condition where a gene pool is not changing in frequency across generations.
[q] D 4.1.15 Artificial selection by deliberate choice of traits
[a] Artificial selection – the process by which humans breed organisms to increase what we see as desirable characteristics.
– is performed by selective breeding.
– when specific traits are slected for.
Artificial selection leds to an artificial change in allele frequencies.
[q] Linking Questions
[a] 1. How do intraspecific interactions differ from interspecific interactions?
2. What mechanisms minimize competition?
[q] Variation
[a] Natural Phenomenon.
Not always seen, as variation can be genotypic, or phenotypic
Causes:
– Mutation, Any change to the DNA sequence
-Meiosis, Produce gametes containing unique combinations of alleles, increasing genetic variation of a species
-Sexual Reproduction, Combination of gametes results in a zygote with even more increased variation
[q] Adaptation
[a] Unconscious adaptations made populations rather than individuals, as if their characteristic fits, they will have a better life.
[q] Overproduction of Offspring
[a] Rare phenomenon in which parents produce too many offspring, meaning that those with the best features will survive, and be more likely to reproduce, creating natural selection
[q] Inheritance
[a] Individuals that do reproduce pass on characteristics to their offspring.
Natural selection therefore increases the frequency of favourable characteristics in a population, meaning individuals will be better adapted, results in a change of species
[q] Process – Natural Selection
[a] 1. Variation exists within any population of organisms
2.Organisms with suitable features for environment will survive and reproduce, whereas others will not.
3. Organisms that are able to survive can reproduce and pass on their heritable characteristics to their population
4. Population will become more suited to the environment
5. Natural selection acts to increase the frequency of characteristics that make individuals better adapted
[q] Differential Survival and Reproduction
[a] Individuals are better adapted tend to survive and reproduce, while the less adapted tend to die or produce fewer offspring.
[q] Progressive Change
[a] Natural selection increases the frequency of characteristics that make individuals better adapted and decreases the frequency of other characteristics leading to changes within the species
[q] Galapagos Finches
[a] 14 different species unevenly distributed in Galapagos Islands
They evolved to adapt to particular food source, with large beaks appearing for large seeds, and small beaks appearing for small seeds, which allowed the various finch populations to survive side by side.
[q] Geospiza Fortis
[a] Could live off both small and large seeds because of variation in beak size.
1977, a drought occurs, causing a shortage smaller seeds, and the birds with longer and narrower beaks survived, as they could survive on large seeds.
El Nino of 1982/83 caused massive rainfall, meaning the G.fortis population increased due to ample small seed availability.
With the return of the drought in 1987, only a small cohort of birds were still breeding, but they were not a random sample of the earlier Geospiza population.
[q] Antibiotic resistance
[a] Antibiotics provide a selection pressure and because of the variation in a population.
Some bacteria will be resistant to the antibiotic, meaning that if the resistance is heritable, it will pass into future generations, as well as other cells via plasmid transfer.
Bacteria may also gain resistance through a misuse of antibiotics, as they have been exposed, and can develop resistance.
Natural selection benefits the individual and not necessarily the species.
Additional higher level: although epigenetic tags could be inherited in gametes and thus passed to offspring (gene imprinting), they do not change DNA sequences and thus far are not regarded as having a role in evolutionary change
Mutations drive the genetic variation which natural selection acts on.
Sexual reproduction also contributes to variation within a species through crossing over in prophase I of meiosis and random/independent assortment of homologous pairs in metaphase I.
Density-dependent factors arise from biological phenomena like competition between populations, competition, disease, and predation.
For instance, species have the tendency to overproduce offspring beyond what the environment can provide in terms of resources.
Thus, natural selection selects those adapted to hotter temperatures.
For many species, these traits do not help the organism adapt to its environment, and expediate quite a lot of energy despite natural selection being responsible for their development.
For example, colorful plumage in male birds of paradise have evolved to attract female mates by indicating desirable traits in the male.
In any case, evolutionary theory shows that in events of sexual selection, three types of genes will all increase in frequency together: genes for a male “indicator” trait reflecting that he has good genes, genes that make a female prefer that indicator trait, and the “good” genes in males whose presence is reflected by the indicator.
1. Stabilizing selection: the intermediate phenotype is chosen as the population stabilizes on a single trait, which decreases genetic diversity due to homogeneity.
o (i.e. human birth weight is a type of stabilizing selection since large babies caused pregnancy complications and light babies were less likely to survive, so the phenotype stabilized in the middle at around 7lb)
2. Directional selection: one of two phenotypic extremes within the spectrum is selected for, causing the allele frequency to continuously shift towards one ‘direction’.
o (i.e. during the industrial revolution, air pollution caused tree bark to turn into a black color, so black peppered moths were more likely to survive than white ones due to camouflage)
3. Disruptive (diversifying) selection: when two extreme/divergent phenotypes are favored over intermediate ones, increasing genetic diversity and heterogeneity.
o (i.e. big male elephants can mate by brute force and small ones can sneak into the big males’ territory and mate whereas medium-sized elephants are too big to sneak in and too small to overtake the large elephants in a battle)
• no mutations
• no gene flow (individuals do not enter or exit the population)
• random mating (individuals do not favor some characteristics over others)
• no natural selection
This state of equilibrium can be attributed either to the entire genome or to one specific allele.
If one of the genotype frequencies is known, the allele frequencies can be calculated using the same equations.
Not only does allele frequency interest biologists, but genetic frequency as well, which describes the frequency of a specific genotype (combination of alleles i.e. homozygous pp/qq or heterozygous pq) within a population.
• 2pq = frequency of individuals who are pq heterozygous
• q2 = frequency of individuals who are qq homozygous
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