D4.1 Flashcards
Natural Selection (34 cards)
Natural Sexual Selection
What is Selective Pressure?
An environmental factor that can influence the success of a population.
Usually a factor that favours one variation over another in that environment so plays a crucial role in natural selection.
Can be both an abiotic factor such as temperature or biotic factors, such as food avilability.
Can also be anthropogenic in cause such as exposure to pesticides and antibiotics.
Natural and Sexual Selection
What is meant by ‘Fitness’ iin evolutionary terms?
An organisms’s biological fitness relates to their ability to pass on their genes.
Generally linked to survival as living long enough to reproduce is crucial for genetic contribution rather than age at death.
Fitness is related to having adaptations suited for one’s environment.
Natural and Sexual Selection
What is sexual selection?
A process similar to natural selection, in that some organisms are more succesful at producing offspring, leading to the increase in their genetic attributes in their population over time.
In sexual selection though, the increased number of offspring is unrelated to survival and solely related to the ability to attract a mate to reproduce with.
Natural and Sexual Selection
What is meant by ‘Antibiotic resistance?’
Natural selection can be directly observed with antibiotic resistance, as it doesn’t require a change in alleles/ phenotypes over many generations.
Not being killed by antibiotics is a genetic trait is some bacteria.
When the colony is exposed to antibiotics, the non-resistant bacteria die and the resistant bacteria have a selective advantage and survive to reproduce.
Due to the rapid reproductive spped of the bacteria and the significane of the selective pressure, this can be observed in real time in laboratories.
Natural and Sexual Selection
How is darwin theory a ‘paradigm shift?’
- Before Darwin’s theory on inheritance of aquired traits, Llmarck’s theory was tge oaradign through which we understood evolution and change
- Darwin’s theory highlighted that evolution is driven only by heritable traits and changed the paradign through which evolutionary change is still viewed.
Natural and Sexual Selection
Cause of genetic variation - mutation
- Many mutations have a negative or deleterious (causing harm) impact on the phenotype of an individual since most proteins are structured precisely for their function.
- Many mutations have no impact because they are in a non-coding region or they are silent, coding for the same protein.
- But there are mutations that simply give rise to a new phenotype and add variation.
- These variations may end up driving natural selection if they are positive in an environment.
Natural and Sexual Selection
Causes of genetic variation - meiosis
- Gamates produced by individuals are all unique from one another, due to TWO process that occur during Meiosis.
1) Independeent Assortment: relates to random combinations of chromosomes (as gametes recieve only one of each.)
2) Crossing over: this swaps genetic material between the pairs first. Can create many more possibilities of genetic variation of the gametes.
- Both of these processes cause new COMBINATIONS of alleles (not new alleles themselves.)
- New combinations sometimes produce a new phenotypic of physical variation in offspring.
Natural and Sexual Selection
Causes of genetic variation - sexual reproduction
- Once the unique gametes have been created, it is totally random as to which one is fertilised during reproduction.
- Meaning that a shuffling of many diverse gametes occurs every time offspring are made, ensuring even siblings are always unqiue.
Natural and Sexual Selection
Carrying capacity and natural selection
- One of the requirements for natural selection is overproduction of offspring so that there is competition for survival.
- The competition ensures that only the more ‘fit’/ better adpated members of the population survive –> and leads to the evolution of more positive traits in a populaiton.
- If the population is well under the carrying capacity and all individuals survive to reproduce –> the population will not evolve by natural selection.
Natural and Sexual Selection
Abiotic factors and natural selection
- The abiotic factors of an ecosystem often act as selective pressure.
- Extreme abiotic conditions (eg: high/low tempreatures) can limit the survivability of a species and provide an opportunity for the differential success of those with adaptations suited to those factors.
Natural and Sexual Selection
Acquired versus heritable traits.
- An acquired trait is one that an individual obtains during their lifetime, often as a result of behaviour. (eg: nutritious can cause variation in colouration of falmingoes, but is not related to any genetic differences.
- Heritable traits are those that are encoded in our DNA. Thus, they are something with are born with. Darwin’s theory higlighted that only heritable traits can be passed on to offspring and therefore drive evolution.
Natural and Sexual Selection
Sexual selection and birds of paradise
- Male and female birds of paradise look entirely different due to sexual selection.
- Females have dark colours for camoflaging.
- Males have big bright feathers, that must require the male to have healthy nutrition. Therefore, male birds with brighter feathers are advertising their wellbeing and are more attractive to females for mating.
- Over time this has been sexually selected, until all males of the species became colourful while females remained effectively camouflaged.
Natural and Sexual Selection
John Endler’s Guppy Experiment
- His work was to investigate whether the presence of predators would influence the colouration of guppies living on the island of Trinidad.
- Male (guppies) have genes for bright colours and there is a lot of genetic variation for the extent and pattern of colouration.
- Duller colours would provide camoflauge but brighter colours attracted more mates.
- In both field and lab pond work, he found that after 15 generations the presence of predatory fish resulted in less colouration in the guppies.
- AKA: in the absence of predators the sexual selection prodcued colourful males but in the presense of predators colour-less fish survived better.
Neo-Darwinism and Patterns of Natural Selection
What is a Gene Pool?
- consists of all of the genes and their different alleles, present in a population.
- When we are talking about gene pools in terms of natural selection for a trait, we would be talking about what alleles are present for that one gene in the one population we are studying.
- This is a way of tracing the genetic basis to the physical variation we see.
Neo-Darwinism and Patterns of Natural Selection
What is meant by ‘allele’ frequency?
- After establishing which alleles are present in a population for a gene, we then look at how common each are.
- The allele frequency is the proportion of the total alleles that each individual allele occupies.
- Usually given as a decimal and you can multiply by 100 to get the percent.
- As heterozygotes and homozygous dominant genotypes (Bb and BB) often have the SAME phenotype (they look physically the same), this information is not often as easily self-evident.
Neo-Darwinism and Patterns of Natural Selection
What does it mean to say that a trait is polymorphic?
- Poly means ‘many’, morph means ‘form’, thus polymorphic means ‘many forms’.
- Refers generally to a trait that not only has two variants, but many different genetic variations.
- Can be at a phenotypic level, as a result of multiple genes contributing to the phenotype.
- Polmorphism can also be referred to for a single gene, this would give rise to more than two alleles and resulting in variations.
- The term SNP refers to a ‘single nucleotide polymorphism’, for more than two variations at that base.
Patterns of Natural Selection
DIRECTIONAL Selection.
- For when a trait is continous (such as weight, height, skin colour) meaning there are many phenotypes between two extremes (such as incredibly tall/ fat and incredibly skinny/ short), there are more different patterns natural selection can follow
- In Directional Selection, one extreme phenotype has an advantage and so the population shifts towards that extreme.
- There will still be many different levels of the phenotype but all of them more more to the favoured extreme.
Neo-Darwinism and Patterns of Natural Selection
Patterns of Natural Selection
STABILISING Selection.
- In other cases when there is a continuum of phenotypes for a trait, natural selection can favor the AVERAGE and NEITHER extreme.
- Stabilising selection is when natural selection actually shortens the contoniuum, with less individuals at either extreme, and more in the average.
- This will actually reduce the variation in the population and reduce the possibilty of speciation.
Neo-Darwinism and Patterns of Natural Selection
Patterns of Natural Selection
DISRUPTIVE SELECTION
- Either extreme phenotype (very short/tall, skinny/fat, dark/light coloured) is preferrable (works better) to the average.
- Each extreme phenotype provides a specific advantage (often in a different nice), which the average holds no advantage.
- As a result, the distribution will change –> many individuals at each extreme, very few in the average range.
- This CAN be the start of speciation –> two extremes offering different niches and stopping reproduction. Their evolving towards the extremes is already an example of differntial selection.
Neo-Darwinism and Patterns of Natural Selection
How does geographic isolation impact allele frequency?
- Geographic isolation is crucial for speciation, as it stops the flow of genes/ prevents gene flow.
- When gene flow is removed, evolution can favor one allele in one location and another in the other.
- As a result, over time the allele frequencies differ in the two locations.
- There are many examples of this in human populations.
- One example is the HLA genes found on chromosome 6 that create cell surface antigens that tell the immune system not to attack. This gene is polymorphic –> the common variants vary significantly by geographic location. HLA compatibility is crucial for organ transplant.
Neo-Darwinism and Patterns of Natural Selection
What is unique about Neo-Darwinism?
- Although Darwin understood that only heritable traits can be inherited by offspring, but, at the time of his work, DNA as the source of heritable material had yet to be discovered.
- When we now take Darwin’s understanding and add to it what we now understanding about the role of DNA and alleles and how they are specifically inherited, this updated theory of natural selection is referred to as Neo-Darwinism or Modern Synthesis.
- This is the model we now work from.
Neo-Darwinism and Patterns of Natural Selection
Comparing and Contrasting natural and artificial selection.
- Human interference can lead to evolution of species.
- If humans breed organisms with specfic traits we desire, this gives rise to change by artificial selection.
- If our actions just add selective pressure that impacts the survival of other organsisms (such as the exposure to antibiotics or pesticides), this is an example of natural selection.
- Both forms of selection lead to a change in allele frequences of other species due to human influence.
Hardy Weinberg Equilibrium
What is Genetic Equilibrium?
- Genetic Equilibrium is the opposite of evolution.
- If evolution is defined as ‘a change in inhertiable traits/allele frequencies over a period of time’, then equilibrium is no change in allele frequencies over time.
- Usually looked at with reference to one trait, thus one gene.
- The work on genetic equilibrium is attributed to mathematician Hardy and Weinberg, –> created a mathematical model TO ASSES ALLELE FREQUENCES.
- They acknowledge five conditions that must be met for a gene to STAY IN EQUILIBRIUM.
Hardy Weinberg Equilibrium
Conditions for Hardy-Weinberg Equilibrium
LARGE POPULATION SIZE
- For a gene to remain stable.
- There must be a large population size.
- In small populations each individual has too much impact and chance events tha could accidently kill a few individuals, can shift the gene pool of the whole population.
- This change in allele frequences due to chance events in small populations is called GENETIC DRIFT.
