5.2 Natural Selection Flashcards
what is the theory of natural selection and who is it by?
- charles darwin
- it is not necessarily the strongest or most intelligent that survives but ones most responsive to change
what is the process of natural selection? (5)
- inherited variation- there is genetic variation within a population which can be inherited
- competition - there is a struggle for survival (species tend to produce more offspring than the environment can support)
- selection - environmental pressures lead to differential reproduction within a population
- adaptations - individuals with beneficial traits will be more likely to survive and pass there traits on to their offspring
- evolution - over time, there is a change in allele frequency within the population gene pool
what are the key components of natural selection? (6)
Inherited variation - exists within the population
competition - results from an overproduction of offspring
environmental pressures - lead to different reproduction
adaptations - which benefit survival are selected for
genotype frequency - changes across generation
evolution - occurs within the population
what are the 2 types of variation?
discontinuous - distinct classes
continuous - range across a characteristic spectrum
what are 3 main mechanisms by which genetic variation between individuals? (3)
- mutation - changing the genetic composition of gametes (germline mutation) leads to change characteristics in offspring
- meiosis - via crossing over (P1) or independent assortment (M1)
- sexual reproduction - the combination of genetic material from 2 distinct sources creates new gene combinations in offspring
what is a gene mutation? (2)
a change in the nucleotide sequence of a section of DNA coding for a specific trait
- new alleles are formed by mutation
what three things can gene mutations be?
- beneficial - change the gene sequence (missense mutation) to create new variations of a trait
- detrimental - truncate the gene sequence (nonsense mutations) to abrogate the normal function of a trait
- neutral - mutations have no effect on the functioning of the specific features (silent mutations)
how does meiosis promote variation?
by creating new gene combinations via crossing over or independent assortment
how does crossing over promote variation through meiosis? (2)
- involves the crossing over of segments of DNA between homologous chromosomes during (P1)
- the exchange of genetic material occurs between non-sister chromatids at chismata
what is the consequence of crossing over? (3)
- all four chromatids that comprise the bivalent will be genetically different
- chromatids that consist of a combination of DNA derived from both homologous chromosomes are called recombinants
- offspring with recombinant chromosomes will have unique gene combinations that are not present in either parent
what is independent assortment? (3)
- when homologous chromosomes line up in M1, their orientation towards the opposing poles is random
- the orientation of each bivalent occurs independently
- so different combinations of maternal/paternal chromosomes can be inherited when bivalents separate in A1
how can you work out the total number of combinations that can occur in gametes?
2^n (n = haploid number of chromosomes)
how do you work out the total number of combinations if crossing occurs?
- the number of different gamete combinations becomes immeasurable
what is sexual reproduction?(2)
- the fusion of 2 haploid gametes resulting in the formation of a diploid gamete
- the zygote then divides by mitosis and differentiate to form a developing embryo
why will random fertilisation by egg and sperm always generate different zygotes? (2)
- due to meiosis resulting in genetically distinct gametes
- Individuals will show variation despite shared parentage
why will populations lead to competition for survival?
a stable population will inevitably outgrow its resource base
what is the process of competition for survival? (3)
- when there is an abundance of resources a population will grow according to its biotic potential (exponential J-curve)
- with more offspring there are less resources available to members of the population (environmental resistance)
- this will lead to a struggle for survival and an increase in the mortality rate ( causing population growth to slow and plateau)
why is competition for survival essential to Darwin’s -survival of the fittest?
- any trait that is beneficial for competition survival will be more likely to be passed on to offspring according to natural selection
what is the definition of adaptations?
- features of organisms that aid their survival by allowing them to be better suited to their environment
what different ways may adaptations be classified? (5)
- Structural - physical differences in biological structures
- Behavioural - differences in patterns of activity
- Physiological - variation in detection and response by vital organs
- Biochemical - differences in molecular compositions of cells and enzyme functions
- Developmental - variable changes that occur across the life span of an organism
why may biological adaptations be passed onto offspring when parent reproduce?
- they have a genetic basis
- organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass of these genes
- organisms without these beneficial adaptations will be less likely to survive long enough to reproduce and pass on their genes
what do adaptations result in?
differential reproduction within a species (allowing for natural selection to occur)
what do alleles encode for?
phenotypic polymorphisms of a particular trait that may be beneficial, detrimental or neutral
what do beneficial, detrimental or neutral traits mean?
- beneficial alleles will better equip the organisms to survive and hence produce more offspring
- detrimental alleles will harm the survival prospects of an organisms leading to fewer viable offspring
- neutral alleles will not affect the organism survival prospects
how does natural selection change the proportion of different alleles? (2)
- as beneficial alleles improve reproductive prospects they are more likely to be passed on to future generations
- detrimental alleles result in fewer offspring and are less likely to be present in future generations
what constitutes beneficial or detrimental traits and why are allele frequencies constantly evolving in a population? (1)
environmental conditions
what does adaptive radiation describe?
the rapid evolutionary diversification of a single ancestral line
how does adaptive radiation occur? (2)
- when members of a single species occupy a variety of distinct niches with different environmental conditions
- members evolve different morphological features in response to the different selection pressures
what is an example of adaptive radiation? (2)
variety of beak types in the finches of the Galapagos islands
- they have specialised beak shapes depending on their primary source of nutrition
what are antibiotics?
chemicals produced by microbes that kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria
In a bacterial colony over many generation, a small proportion of bacteria may develop antibiotics via gene mutation, and what may occur as a result of this is antibiotics are used? (4)
- when treated with antibiotics, the resistance bacteria will survive and reproduce by binary fission (asexual reproduction)
- the antibiotic-resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotics)
- antibiotic-resistant bacteria may also give resistance to susceptible strains by transferring plasmids via bacterial conjunction
- the introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)
How has the evolution of antibiotic-resistant golden staph (Staphylococcus aureus) illustrated antibiotic resistance? (6)
- golden staph caused infections on skin as well as other infections
- these infections were treated using antibiotic methicillin
- bacterial strains developed were resistant to this antibiotic (MRSA)
- these strains proliferated while susceptible strains died out
- MRSA infections are now especially present in hospitals and nursing homes where methicillin was commonly used
- medical practitioners now prescribe alternate antibiotic agents to treat infections caused by golden staph
