Evolution - 5.2 Natural Selection Flashcards
variation across a range - GRADE
A species may GRADE (how gradual change) in phenotype over a geographic area
variation across a range - CLINE
Continuous gradual change = CLINE, often occours along the length of a country or continent
All the populations are of the same species IF
they are capable of interbreeding
Genetic compatibility of a species decreases as..
the genetic divergence between the distanced populations increase
Reproductively isolated =
two populations will diverge to an extent where they can no longer interbreed if returned to a shared environment
Speciation =
when 2 populations can no longer interbreed and produce fertile, viable offspring. They are considered to be separate species.
Charles Darwin (1859)
produced the first convincing case for evolution = THE ORIGIN OF SPECIES
Argued that new species developed from ancestral ones by NATURAL SELECTION
Theory = SURVIVAL OF THE FITTEST
Alfred Russel Wallace
developed a theory of natural selection independently of Darwin, but, Darwin supported the theory more extensively and receives most of the credit
Darwin’s conclusions (4):
1) all organisms have a high reproductve rate = more offspring than the environment can handle/limited resources = struggle for existence
2) among the offspring these is inherited variation in all characteristics, some adapted more to the environment than others (eg mutations)
3) Those organisms possess FAVOURABLE VARIATIONS survive long enough to have offspring and pass of the favourable characterists
4) over a period of time each successive generation will be better adapted to the environment = the fittest will survive
–> eventually evolution will occur. and new species can be born
ICE AGE
I-nherited variation exists within the population
C-ompetition results from an overproduction of offspring
E-nvironmental pressures lead to differential reproduction
A-daptations which benefit survival are selected for
G-enotype frequency changes across generations
E-volution occurs within the population
Understandings
Natural selection can only occur if there is variation among members of the same species
Mutation, meiosis and sexual reproduction cause variation between individuals in a species
Adaptations are characteristics that make an individual suited to its environment and way of life
Species tend to produce more offspring than the environment can support
Individuals that are better adapted tend to survive and produce more offspring while the less well adapted tend to die or produce fewer offspring
Individuals that reproduce pass on characteristics to their offspring
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
(MORE) The process of natural selection occurs in response to a number of conditions (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 these traits on to their offspring
- Evolution – Over time, there is a change in allele frequency within the population gene pool
There are three main mechanisms by which genetic variation between individuals in a species may occur:
1) Mutations – Changing the genetic composition of gametes (germline mutation) leads to changed characteristics in offspring
2) Meiosis – Via either crossing over (prophase I) or independent assortment (metaphase I)
3) Sexual reproduction – The combination of genetic material from two distinct sources creates new gene combinations in offspring
Genetic variation = mutations
A gene mutation is a change in the nucleotide sequence of a section of DNA coding for a specific trait
New alleles are formed by mutation
Gene mutations can be beneficial, detrimental or neutral
Beneficial mutations change the gene sequence (missense mutations) to create new variations of a trait
Detrimental mutations 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 feature (silent mutations)
Genetic variation = Meiosis (1)
Meiosis promotes variation by creating new gene combinations via either crossing over or independent assortment
- Crossing Over
Crossing over involves the exchange of segments of DNA between homologous chromosomes during prophase I
The exchange of genetic material occurs between non-sister chromatids at points called chiasmata
As a consequence of this recombination, 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
Genetic variation = Meiosis (2)
Independent Assortment
When homologous chromosomes line up in metaphase I, their orientation towards the opposing poles is random
The orientation of each bivalent occurs independently, meaning different combinations of maternal / paternal chromosomes can be inherited when bivalents separate in anaphase I
The total number of combinations that can occur in gametes is 2n – where n = haploid number of chromosomes
Humans have 46 chromosomes (n = 23) and thus can produce 8,388,608 different gametes (223) by random orientation
If crossing over also occurs, the number of different gamete combinations becomes immeasurable
Genetic variation = sexual reproduction
Sexual Reproduction
The fusion of two haploid gametes results in the formation of a diploid zygote
This zygote can then divide by mitosis and differentiate to form a developing embryo
As meiosis results in genetically distinct gametes, random fertilisation by egg and sperm will always generate different zygotes
This means that individual offspring will typically show variation despite shared parentage
Identical twins are formed after fertilisation, by the complete fission of the zygote into two separate cell masses
High reproductive rates = COMPETITION =
species tend to produce more offspring than the environment can sustainably support
If left to follow course, a stable population will inevitably outgrow its resource base, leading to competition for survival
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 other 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)
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
Genetic adaptations - structural
Structural: Physical differences in biological structure (e.g. neck length of a giraffe)
genetic adaptations - Behavioural
Behavioural: Differences in patterns of activity (e.g. opossums feigning death when threatened)
genetic adaptations - physiological
Physiological: Variations in detection and response by vital organs (e.g. homeothermy, colour perception)
genetic adaptations - biochemical
Biochemical: Differences in molecular composition of cells and enzyme functions (e.g. blood groups, lactose tolerance)
genetic adaptations - developmental
Developmental: Variable changes that occur across the life span of an organism (e.g. patterns of ageing / senescence)
likelihood of living w/ and w/o genetic adaptations
Organisms with beneficial adaptations will be more likely to survive long enough to reproduce and pass on these genes
Organisms without these beneficial adaptations will be less likely to survive long enough to reproduce and pass on their genes
Allele frequencies
The variation that exists within a population is heritable (i.e. genetic) and determined by the presence of alleles
These alleles may be passed from parent to offspring via sexual reproduction
Alleles encode for the phenotypic polymorphisms of a particular trait and may be beneficial, detrimental or neutral:
Beneficial alleles will better equip the organism to survive and hence produce more offspring (encodes beneficial adaptations)
Detrimental alleles will harm the survival prospects of an organism, leading to fewer viable offspring
Neutral alleles will not affect the organisms survival prospects
Due to natural selection, the proportion of different alleles will change across generations (evolution)
As beneficial alleles improve reproductive prospects (more offspring), they are more likely to be passed on to future generations
Conversely, detrimental alleles result in fewer offspring and hence are less likely to be present in future generations
If environmental conditions change, what constitutes a beneficial or detrimental trait may change, and thus the allele frequencies in a population are constantly evolving
Adaptive radiation
describes the rapid evolutionary diversification of a single ancestral line
Adaptive radiation
- further breakdown
EXAMPLE = BEAKS OF FINCHES
It occurs when members of a single species occupy a variety of distinct niches with different environmental conditions
Consequently, members evolve different morphological features (adaptations) in response to the different selection pressures
An example of adaptive radiation can be seen in the variety of beak types seen in the finches of the Galapagos Islands
These finches have specialised beak shapes depending on their primary source of nutrition (e.g. seeds, insects, nuts, nectar)
Daphne Major
Daphne Major is a volcanic island that forms part of the archipelago that is collectively referred to as the Galapagos Islands
Daphne Major - example with regard to Darwin’s studies
It is the native habitat of a variety of bird species known as Darwin’s finches (subfamily: Geospizinae)
Darwin’s finches demonstrate adaptive radiation and show marked variation in beak size and shape according to diet
Finches that feed on seeds possess compact, powerful beaks – with larger beaks better equipped to crack larger seed cases
In 1977, an extended drought changed the frequency of larger beak sizes within the population by natural selection
Dry conditions result in plants producing larger seeds with tougher seed casings
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
Antibiotic Resistance
Antibiotics are chemicals produced by microbes that either kill (bactericidal) or inhibit the growth (bacteriostatic) of bacteria
Antibiotics are commonly used by man as a treatment for bacterial infections (not effective against viral infections)
Antibiotic Resistance - in bacterial colonies
In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation
antibiotic resistant bacteria - process / steps breakdown
When treated with antibiotics, the resistant bacteria will survive and reproduce by binary fission (asexual reproduction)
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The antibiotic resistant bacteria will flourish in the absence of competition from other strains of bacteria (killed by antibiotic)
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Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
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The introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)
example of antibiotic resistance in bacteria
evolution of Staphylococcus aureus (Golden staph)
Golden staph can cause infections to the skin (lesions and boils) as well as more serious infections (pneumonia, meningitis)
Historically, these infections were treated using the antibiotic methicillin
Bacterial strains developed that were resistant to this antibiotic (methicillin-resistant Staphylococcus aureus – or MRSA)
These strains proliferated while susceptible strains died out (methicillin-sensitive Staphylococcus aureus – or MSSA)
MRSA infections are now especially present in hospitals and nursing homes, where the use of methicillin was most common
Medical practitioners now prescribe alternate antibiotic agents to treat infections caused by Staphylococcus aureus
Gene pool =
All of the alleles that are present in the population
key contributors to genetic variance: Mutations =
add new alleles to the population
(by mutagens where the genetic code is not copied properly)
key contributors to genetic variance: Inheritance patterns
alleles combine to produce unique phenotypes, including the effect of co-dominance, incomplete dominance, lethal alleles and multiple alleles
key contributors to genetic variance: Sexual reproduction
genes are a combination of the 2 parents
key contributors to genetic variance: Meiosis =
sex cells are made in which the segregation of alleles and the independent assortment of chromosomes means that it is rare for any two gametes to be the same
key contributors to genetic variance: Crossing over
homo. chromosomes tangle and recombine during meiosis - changes the combinations of linked alleles
Thomas Malthus ideas:
he identified =
populations grow/multiply geometrically/exponentially
while
food recourse only increase arithmetically/linear
==> leads to competition and a struggle for survival (–> survival of the fittest)
Adaptations =
characteristics that make an individual suited to its environment and way of life
the success of gene
Bad genes:
reducing the survival and reproductive success of phenotypes poorly suited to the prevailing conditions, their alleles become less common in the gene pool
Positive genes:
enhancing the survival and reproductive success of phenotypes well suited to the prevailing conditions; their alleles become more common in the gene pool
== therefore, only a small number of the individuals remain in the gene pool to contribute their genes to the next generations
Development of melanistic insects (peppered moths) in polluted areas
Peppered moth = Biston betularia
- grey mottled form
- dark melanic form
= before industrial revolution, the trees were white = white moths blended and weren’t killed. during IR = coal = soot = trees became dark = black moths survived and white/grey ones died, now less coal = less soot = more white trees = less black moths, more white virations
Antibiotic resistance = simplified
before antibiotics, it was up to the human immune system to address the infection - if they did not kill it, they would be killed. As antibiotics developed and was initially used on bacteria - the majority of the bacteria would die but it was possible for one or two to survive - these surviving bacteria would reproduce and their offspring would gain the mutations/gene to be resistant to that specific antibiotic, this process would continue down the generations
= antibiotic resistance
