D4.1 Natural selection Flashcards
What is natural selection?
The process in which diff environments influences survival & reproduction of organisms due to presence of variations or adaptations.
What are the key components of Darwin’s theory of natural selection?
- Variations are seen among organisms in a pop. Heritable or genetic variations are passed on to offspring.
- Due to overproduction, there is competition for resources leading to a struggle for existence.
- In struggle for existence, organisms with traits that are better suited to environment survive & reproduce. This is often referred to as ‘survival of fittest’.
- Organisms that survive pass on these variations to their offspring.
- Over a period of time, frequency of occurrence of favourable variations in pop increases.
- Natural selection eventually causes pop to become better adapted to its environment.
What does the forces of natural selection operate on?
Forces of natural selection operate continuously on genetic variations present in a population.
How does isolation cause genetic variation which leads to natural selection?
Isolation of these pops by geographical or other barriers prevents interbreeding. In isolated pops, environmental differences cause natural selection to favour diff traits. Over generations, accumulation of differences happens to such an extent that isolated pops are considered as separate species.
What is an example where isolation has created genetic variation?
For instance, diff species of Darwin’s finches seen in Galapagos islands have originated from one ancestral species. Isolation & differing environments led to evolution of several similar-looking species with distinct beak shapes.
What do mechanism of natural selection enable? And over what period of time?
Thus, mechanism of natural selection enables evolutionary change. Over a period of bn of yrs, natural selection has led to amazing biodiversity that exists on Earth today.
What is biodiversity?
Biodiversity refers to diversity of life on Earth & encompasses genetic diversity, species diversity & ecosystem diversity
What is genetic variation?
Genetic variation refers to differences in genomes among individuals of same species, & is an essential prerequisite for natural selection.
What are the two reasons for genetic variation?
Genetic variation arises due to mutation & sexual reproduction
What are mutations?
Mutations are errors in copying genetic info during DNA replication, resulting in alleles or many alternate forms of a gene. Thus, mutation introduces new alleles into a pop, resulting in variation.
What are the impacts of mutations?
Most mutations are neutral to an organism; however, some could be harmful & some beneficial. E.g. a chance mutation in a pop of bacteria could make a few individuals resistant to a particular antibiotic. Following exposure to antibiotic, resistant bacteria are more likely to survive, reproduce & pass on favourable trait.
What do mutations in gametes lead to?
It is important to note that only mutations in cells that produce gametes can be inherited. In other cells of body, these mutations would be ‘dead-end’ mutations & have no value in introducing variation into pop
What are the two ways that sexual reproduction introduces genetic variation in a population?
- Formation of gametes involves crossing over & independent assortment of homologous chromosomes, both of which result in unique allele combinations.
- In addition, random fertilisation leads to unique & new combinations of alleles of both parents. In a pop of many reproducing individuals, alleles are mixed again & again each time reproduction occurs, resulting in endless variation.
What is Malthus’ concept around population growth?
Malthus was an economist who stated, in his Essay on Principle of Population, that growth of human pop was exponential whereas increase in food production happened in a linear manner. In other words, food produced would not meet demands of pop, resulting in poverty, famine & even wars
What was Darwin’s concept of population growth?
Darwin stated that, in nature, there is a tendency towards overproduction – plants & animals can produce far more offspring than can possibly survive. E.g. many species of fish lay thousands of eggs. Oysters can lay between 60 & 80 mn eggs at a time. In both these cases, however, only a fraction survives up to adulthood
What do all organisms need? What are these called? And what do they determine?
All living organisms need resources – food, water, shelter, space &, even, mates. These factors are termed limiting factors, as they determine carrying capacity of environment.
Define: Limiting factor
Carrying capacity
Limited factor: A resource in environment that can significantly affect a population’s size if it becomes limited.
Carrying capacity: Maximum pop size of a species that can be sustained by a given environment.
What does overproduction lead to?
Overproduction leads to competition for these limited resources. Organisms who are ‘fit’ – that is, have favourable variations that enable them to meet these needs – survive & reproduce. Others die or produce fewer offspring. This keeps pop size relatively stable. In other words, overproduction & subsequent competition for resources leads to differential survival & differential reproduction, thereby promoting natural selection. Over time, offspring of survivors make up a larger proportion of pop. Pop will now be better adapted to environment & may look entirely diff from ancestral pop. Eventually this would result in evolution of a new species.
What is selective pressures?
Selection pressures are factors (environmental conditions placed on pop) that lead to differential survival or reproduction, which in turn cause a change in the genetic composition of a pop. Selection pressure could include both density-dependent & density-independent factors
What are density-dependent factors?
Density-dependent factors are factors that affect size of pop & depend on density of pop in a given area. Picture a pop of carp living in a pond with access to a fixed supply of food. As long as pop density is low, there will be enough food available for carp. However, as number of carp in pond increases, competition for food may result in starvation & death of some of carp. Pop density becomes too great for fixed amount of food to support.
What are example of density-dependent factors?
- Availability of food
- Availability of water, shelter & other resources
- Presence of predators
- Spread of pathogens
- Finding of mate
What are density-independent factors?
Density-independent, abiotic factors are physical factors that affect size of pop, irrespective of pop density. Factors such as availability of oxygen, temperature & even natural disasters like wildfires, hurricanes & volcanic eruptions would affect all individuals in a pop, regardless of their density. Returning to carp example: a decrease in dissolved oxygen levels of water would affect all carp in pond, irrespective of how many carp are present in pond, & would lead to an overall decline in their pop.
What are example of density-independent factors?
- Natural disasters like wildfires or volcanic eruptions
- Temp
- CO2 & O2 levels
- Pollution
What does variation of individual lead?
Variation in individuals leads to differences in their ability to survive & reproduce allowing populations to adapt over time. These differences form basis of natural selection.
What is biological fitness in terms of evolution?
Biological fitness = ability of an organism to reproduce & pass on its genetic material to its offspring.
What factors determine biological fitness?
- Environment in which individual (genotype) lives. Fitness of individuals varies as environmental conditions change, & need not be same across environments. E.g. fittest genotype among polar bears today may not be fittest genotype in future as more & more ice melts.
- Survival value of an individual. Survival value refers to traits that enable individuals to survive & reach reproductive age. These could include mechanisms to avoid predators or higher disease resistance.
- Reproductive potential of an individual. Capacity of an individual to produce offspring is known as reproductive potential. It is determined by traits such as ability to attract mates, production of viable gametes & parental care. E.g. individuals with a brighter plumage may have greater chances of attracting mates & thereby produce more offspring.
What is an example of biological fitness?
Both brown beetles & green beetles live on rotting wood & have common aerial predators. Brown beetles can blend in while green beetles stand out & are easily spotted by predators. Fitness of brown beetles is higher as they have a greater chance of surviving & reproducing.
Why does competition occur? And what are the different types?
Competition occurs when organisms belonging either to same species or to diff species compete for same resources. When individuals of same species compete with each other for resources such as food, water, space, sunlight or mates, it is known as intraspecific competition. On other hand, when individuals belonging to different species compete, it is called interspecific competition.
What are two example of intraspecific competition?
- Two trees of same species, growing in close proximity to each other, compete for water, nutrients & sunlight. One may out-compete other by growing taller or by increasing spread of leaf canopy.
- Dance of adders is seen between rival males to establish their supremacy & attract females. ‘dance’ continues until one of them slithers away defeated or out-competed.
What is intraspecific competition dependent on?
Intraspecific competition is density dependent. As pop size increases & reaches carrying capacity of environment, competition for resources intensifies. Individuals with higher fitness have a greater chance of acquiring resources & reproducing than individuals with lower fitness
What does intraspecific competition lead to?
- a decrease in the growth rate of pop as weaker or less fit individuals begin to die.
- traits possessed by individuals with higher fitness value being passed on to offspring & becoming more prevalent in pop over a period of time. This in turn causes the pop to be better adapted to environment &, in long run, could lead to emergence of a new species
Thus, intraspecific competition paves way for natural selection.
What two features must traits favoured by natural selection have?
- Trait is beneficial to organism & increases chance of their survival & reproduction in a particular environment.
- Trait is heritable. In other words, characteristic is encoded in genetic material so that it can be passed on to offspring. Skin colour, colour of fur & shape of ears are examples of heritable traits.
What are acquired traits?
Acquired traits are traits acquired in lifetime of an individual due to environmental factors. These traits are often physical or behavioural & are reflected only in phenotype of an organism. If these phenotypic characteristics do not have corresponding genotypic alterations – that is, are not encoded in base sequence of DNA – they cannot be passed on to the offspring. Thus, acquired traits are not heritable
What is an example of an acquired trait?
For instance, a person who works out at a gym may have an impressive set of muscles, but these would not be passed on to their offspring. Beautiful pink colour of feathers of flamingos depends on pigments present in their diet &, hence, is an acquired trait.
What is sexual selection?
Sexual selection is a special case of natural selection, where focus is on finding a mate & reproducing. Selection for traits which may not confer a survival advantage to organism, but may provide a mating advantage.
Sexual selection could be intrasexual selection or intersexual selection
What is intrasexual selection?
Intrasexual selection refers to competition between individuals of one sex to mate with opposite sex. Intrasexual selection is usually male–male competition to mate with a female, though female–female competition is also seen. Contests of strength seen in male deer & male elephant seals are examples of intrasexual selection.
What intersexual selection?
Intersexual selection refers to individuals of one sex, often females, choosing an individual of opposite sex as a mate. This in turn exerts strong selection pressures on characteristics of opposite sex. Examples include courtship dances, mating calls, elaborate plumage & large genitalia.
Why are males selected in intersexual selection?
In intersexual selection, mates are selected on physical & behavioural characteristics they possess, which are often taken as markers of overall fitness. In males, this takes form of size, strength or elaborate ornamentation.
What is example of intersexual selection?
Birds of paradise are a prime example of intersexual selection. Found in forests of Papua New Guinea, there are close to 40 species of birds, all very diff from one another. Males are flamboyant & showy, & come in a variety of body shapes, sizes & colours. E.g. ribbon-tailed Astrapia (Astrapia mayeri) has white tail feathers nearly a metre in length. Females are generally more subdued with greys & browns. They aren’t ones that have to attract a mate in this species! Courtship ritual of males is elaborate, often consisting of spreading of feathers, bird song & special dance moves, all in an effort to entice a female. Female selects most impressive male, ensuring that his genes pass on to next generation. In a way, females decide how males evolve.
What did birds of paradise allow Darwin to realise?
Darwin realised that elaborate adaptations in animals were less to do with daily survival & more due to sexual selection. Sexual selection leads to differences between sexes & is thought to be chief cause of sexual dimorphism in animals
How did Endler model sexual vs natural selection?
Stage 1: Endler built artificial ponds & stocked them with guppies. There was considerable variation among male guppies at this point. Endler calculated mean number of spots on male guppies. Guppies were left to breed for 6 months. It is important to note that in these 6 months, guppies were able to reproduce several times. At end of 6 months, Endler noticed that there was an increase in mean number of spots on male guppies. This suggested that highly spotted males were more attractive to females & hence had more offspring. This was sexual selection in action.
Stage 2: Endler now divided pop into 3 groups & placed them in three separate ponds (A, B & C).
Pond A, he did not add any other fish. Pond B, he added Rivulus, a genus of fish that feeds sporadically on juvenile guppies & is probably least dangerous of all guppy predators. Pond C, he added pike cichlids, a type of fish that are voracious predators of guppies. He left pop in 3 ponds alone for nearly 20 months. During this period, guppies bred several times. He then carried out his final analysis & found that in ponds A & B mean number of spots had increased, while in pond C mean number of spots had decreased. He concluded that in ponds A & B, in absence of predators, sexual selection favoured males with spots. Pop, within a short period of time, had evolved into one of brightly coloured males. In pond C, natural selection favoured individuals with drab colours (who could hide from predator)
& pop, over same period of time, had evolved into one of dull-coloured males.
What is evolution?
Evolution is often defined as a change in genetic composition of a pop over time.
What is a population?
A pop is defined as a group of interbreeding organisms belonging to same species living in a given area.
What is the gene pool?
Gene pool is sum total of all alleles of all genes present in a pop.
Why is a gene pool an indicator of genetic variation?
Gene pool = indicator of genetic variation that exists in a pop. A large gene pool indicates extensive genetic variation. This in turn indicates a greater ability of pop to adjust & adapt to changes in the environment. E.g. when a pathogen is introduced into a large pop, there is a greater chance that some individuals already have a version of a gene that enables them to resist pathogen, survive & reproduce. On the other hand, a small gene pool indicates lower genetic variation. This in turn makes pop less able to adapt when faced with environmental changes.
What is the allele frequency?
Allele frequency = relative frequency of a particular allele (in comparison to other alleles of same gene) in a pop. Its value can range from 0 (meaning that it is not present in any individuals) to 1 (meaning that it is present in all individuals) & can be expressed as a decimal, fraction or percentage.
How is the allele frequency calculated?
Allele frequency can be calculated if you know diff genotypes associated with allele in a pop.
Why can frequency of alleles change?
Frequency of alleles in gene pool can change due to both natural selection & chance or random events.
What is genetic drift?
Genetic drift = change in allele frequencies in gene pool of a pop due to chance events. Impact of genetic drift is greater if pops are small and/or geographically isolated from each other.
What are the two situations that genetic drift can occur?
Genetic drift can occur in two situations: bottleneck effect & founder effect.
What is the bottleneck effect?
At times, events such as earthquakes or a tsunami can lead to a sudden decrease in pop. Handful of survivors left behind may not represent genetic diversity of original pop. While pop may rebound & increase in numbers, reduction in genetic variation makes them less adaptable, as seen in black-footed ferrets. This is known as bottleneck effect.
What is an example of bottleneck effect?
E.g. in 1800s, hunting of northern elephant seals (Mirounga angustirostris) reduced their pop to a mere 20 individuals. Today, conservation efforts have led to an increase in their numbers to over 30 000 individuals. Yet, gene pool of entire pop is more or less gene pool of random survivors. In other words, genetic make-up of current pop comprises mainly of alleles of survivors, indicating a loss in genetic diversity. For instance, killing of individuals could have led to loss of many alleles and/or changed frequency of others. Similarly, as pop recovers & increases in number, certain other alleles may become more common or over-represented in gene pool.
What is the founder effect?
Suppose a small subset of pop breaks away from larger pop to colonise a new area. Pop that branches off may not be an exact genetic representation of original population. Genetic variation could be lower. Less frequent (or rarer) genes may be under-represented or absent altogether while others may be over-represented. As pop increases, composition of gene pool would be very diff from the original pop. In other words, genotypes & phenotypes may be very diff.
What is an example of founder effect?
In 1652, a shipload of Dutch immigrants landed in South Africa. They were founders of Afrikaner pop of South Africa. The ship contained individuals suffering from a genetic disorder known as Huntington’s disease, symptoms of which normally appear only after age of 40. Today Afrikaner pop has a higher than usual incidence of Huntington’s disease, with most cases being traced back to founders, indicating an increase in frequency of harmful allele.
What is neo-Darwinism?
Darwin introduced natural selection as a mechanism that leads to evolution. However, at time Darwin established his theory, he like many others had no real understanding of genetics. Mendel had yet to publish results of his experiments on pea plants. So while Darwin stated that natural selection acted on heritable traits, he was unable to explain mechanism of how heritable traits passed on from one generation to next – that is, how variation arose in pop & how heritable traits disappeared & reappeared. Modern synthesis or neo-Darwinism is combined work of many scientists & tries to explain natural selection on foundation of Mendelian genetics.
What does the neo-Darwinism theory state?
- Evolution is change in genetic composition of gene pool of a pop. Neo-Darwinism clarifies that phenotype is largely a product of genotype. Natural selection, therefore, increases frequency of beneficial alleles (& reduces frequency of harmful alleles), resulting in microevolution.
- Apart from natural selection, other factors such as genetic drift & gene flow can bring changes in gene frequency of a pop.
- Mutation is ultimate source of genetic variation.
- Thus, microevolution results in changes in frequency of alleles in a pop over many generations. These changes gradually add up & over a long period of time lead to large changes such as formation of a new species (macroevolution).
What are the three types of natural selection?
- Stabilising selection
- Directional selection
- Disruptive selection
What is stabilising selection?
Stabilising selection favours average individuals with intermediate forms of trait & eliminates the extreme forms. As selection favours moderate phenotype, over a period of time, most of individuals look similar to each other, with a loss in genetic diversity. For instance, studies have shown that high & low birth weights in babies are often associated with increased mortality rates. Babies with average birth weights thus have a selective advantage.
What is directional selection?
Directional selection favours one extreme form of trait over all other forms of trait. Directional selection is seen when environment changes. Peppered moth simulation activity that you did earlier represents directional selection. A typical example is that of giraffes, where change in environment gave a selective advantage to giraffes with long necks (that is, extreme variation of trait was selected). Over a period of time, this gave rise to populations with longer necks. Drug resistance in bacteria is another example of directional selection, where selection favours individuals carrying genes for antibiotic resistance.
What is disruptive selection?
Disruptive selection is when both extreme forms of trait are favoured over intermediate forms of the trait. E.g. imagine a population of mice living in an area with sandy beaches and dark-coloured rocks. Fur colour of mice ranges from cream to tan to dark brown. In sandy areas, cream-coloured mice have an advantage, while in rocky areas, dark brown mice have an advantage from aerial predators. However, tan-coloured mice are at a disadvantage as they can be easily spotted in both areas. Thus, selection favours extreme forms of phenotype & eliminates intermediate forms. Disruptive selection acting over a period of time could eventually lead to formation of a new species.
What do al three type of natural selection lead?
All three types of selection lead to changes in frequency of alleles. Over a period of time, significant changes in composition of gene pool of a population could lead to emergence of new species.
What is the Hardy-Weinberg principle?
Hardy–Weinberg principle states that in a stable pop, frequency of alleles would remain constant generation after generation, provided certain conditions are met. This law is mathematically expressed in form of a binomial equation.
What is the Hardy-Weinberg equation?
Let us consider a single gene with two alleles, R & r. Combinations of these alleles would result in three genotypes: RR, Rr & rr. By convention, frequency of R is denoted by p & frequency of r by q. In a gene pool, p & q represent 100% of alleles of a specific gene.
Therefore, this can be expressed as:
p + q = 1 (that is, 100%)
Now square both sides of the equation:
(p + q)2 = 12
Expand out the brackets to give:
p2 + 2pq + q2 = 1
In the equation:
p2 represents frequency of individuals homozygous for one allele, i.e. frequency of genotype RR
q2 represents frequency of individuals homozygous for one allele, i.e. frequency of genotype rr
pq represents frequency of heterozygous individuals, i.e. frequency of genotype Rr.
What does Hardy-Weinberg equations help calculate?
Thus, Hardy–Weinberg equation helps to calculate genotype frequencies of homozygous dominant individuals, heterozygous individuals + homozygous recessive individuals.
What is the Hardy-Weinberg equilibrium?
Hardy–Weinberg equilibrium refers to a state of genetic equilibrium where frequency of alleles remains same generation after generation.
What conditions does the Hardy–Weinberg equilibrium stands true?
- There are no mutations. Mutations bring in new alleles & disrupt equilibrium of frequencies of alleles.
- There is random mating. In most pop, non-random mating is norm as reproduction involves ‘selecting’ a mate. This in turn leads to sexual selection.
- Natural selection cannot occur.
- No genes should enter or leave pop. Breeding between two pops transfers new alleles, resulting in gene flow, & disrupts Hardy–Weinberg equilibrium. Hybridisation, especially in plants, can also introduce new genes into pop.
- Pop must be large. In small pops, genetic drift or chance events cause changes in allele frequencies.
What is a population if they meet Hardy-Weinberg equilibrium?
Population at Hardy–Weinberg equilibrium is at genetic equilibrium & is not evolving. However, in most natural populations none of these conditions are met. For instance, mating is non-random, mutations are frequent, pops migrate resulting in gene flow & natural selection acts on favourable traits. Changes in frequency of alleles indicate evolution at work.
What is artificial selection?
For hundreds of years, long before ppl knew about concept of natural selection, farmers, animal breeders & horticulturists had been selecting traits to be represented in next generation. This process is known as artificial selection or selective breeding.
How is artificial selection similar to natural selection?
Artificial selection is similar to natural selection except that it is perpetuated by humans. Humans select organisms with desirable traits, from high yield for crop products or milk to disease resistance & faster growth rate. This is followed by breeding for several generations until favourable traits manifest strongly in offspring. Selection of some traits over others leads to a change in frequency of alleles. As a result, descendant plants & animals are genetically diff from ancestral wild pop.
What are example for artificial selection?
- Dogs evolved from wolves due to domestication. Breeds that we have today – such as Great Danes, terriers & chihuahuas – are a result of selective breeding.
- Many crop plants seen today – including wheat, maize & rice – are products of artificial selection. Traits such as higher number of kernels per ear of corn, insect resistance & shorter stems to withstand wind & rain have been selected.
- Cattle have been selectively bred for centuries to increase their milk & meat production.
What is the difference between artificial selection and ‘unintentional’ evolution?
It is important to note that artificial selection is intentional selection of desirable traits by humans. It should not be confused with ‘unintentional’ evolution that results from human activity. Continued use of antibiotics has resulted in a tremendous selection pressure on bacteria & led to evolution of antibiotic-resistant organisms. A similar case is seen in pesticide-resistant insects. Here, evolution is result of natural selection operating on variations present in pop.
