Evolution - 5.2 Natural Selection

Question 1

variation across a range - GRADE

Answer 1

A species may GRADE (how gradual change) in phenotype over a geographic area

Question 2

variation across a range - CLINE

Answer 2

Continuous gradual change = CLINE, often occours along the length of a country or continent

Question 3

All the populations are of the same species IF

Answer 3

they are capable of interbreeding

Question 4

Genetic compatibility of a species decreases as..

Answer 4

the genetic divergence between the distanced populations increase

Question 5

Reproductively isolated =

Answer 5

two populations will diverge to an extent where they can no longer interbreed if returned to a shared environment

Question 6

Speciation =

Answer 6

when 2 populations can no longer interbreed and produce fertile, viable offspring. They are considered to be separate species.

Question 7

Charles Darwin (1859)

Answer 7

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

Question 8

Alfred Russel Wallace

Answer 8

developed a theory of natural selection independently of Darwin, but, Darwin supported the theory more extensively and receives most of the credit

Question 9

Darwin’s conclusions (4):

Answer 9

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

Question 10

ICE AGE

Answer 10

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

Question 11

Understandings

Answer 11

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

Question 12

(MORE) The process of natural selection occurs in response to a number of conditions (5)

Answer 12

• 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

Question 13

There are three main mechanisms by which genetic variation between individuals in a species may occur:

Answer 13

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

Question 14

Genetic variation = mutations

Answer 14

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)

Question 15

Genetic variation = Meiosis (1)

Answer 15

Meiosis promotes variation by creating new gene combinations via either crossing over or independent assortment

1. 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

Question 16

Genetic variation = Meiosis (2)

Answer 16

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

Question 17

Genetic variation = sexual reproduction

Answer 17

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

Question 18

High reproductive rates = COMPETITION =

Answer 18

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

Question 19

Genetic adaptations - structural

Answer 19

Structural: Physical differences in biological structure (e.g. neck length of a giraffe)

Question 20

genetic adaptations - Behavioural

Answer 20

Behavioural: Differences in patterns of activity (e.g. opossums feigning death when threatened)

Question 21

genetic adaptations - physiological

Answer 21

Physiological: Variations in detection and response by vital organs (e.g. homeothermy, colour perception)

Question 22

genetic adaptations - biochemical

Answer 22

Biochemical: Differences in molecular composition of cells and enzyme functions (e.g. blood groups, lactose tolerance)

Question 23

genetic adaptations - developmental

Answer 23

Developmental: Variable changes that occur across the life span of an organism (e.g. patterns of ageing / senescence)

Question 24

likelihood of living w/ and w/o genetic adaptations

Answer 24

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

Question 25

Allele frequencies

Answer 25

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

Question 26

Adaptive radiation

Answer 26

describes the rapid evolutionary diversification of a single ancestral line

Question 27

Adaptive radiation
- further breakdown

EXAMPLE = BEAKS OF FINCHES

Answer 27

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)

Question 28

Daphne Major

Answer 28

Daphne Major is a volcanic island that forms part of the archipelago that is collectively referred to as the Galapagos Islands

Question 29

Daphne Major - example with regard to Darwin’s studies

Answer 29

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

Question 30

Antibiotic Resistance

Answer 30

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)

Question 31

Antibiotic Resistance - in bacterial colonies

Answer 31

In a bacterial colony, over many generations, a small proportion of bacteria may develop antibiotic resistance via gene mutation

Question 32

antibiotic resistant bacteria - process / steps breakdown

Answer 32

When treated with antibiotics, the resistant 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 antibiotic)
—>
Antibiotic resistant bacteria may also confer resistance to susceptible strains by transferring plasmids via bacterial conjugation
—>
The introduction of antibiotic (selection pressure) has caused the antibiotic resistance gene to become more frequent (evolution)

Question 33

example of antibiotic resistance in bacteria

Answer 33

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

Question 34

Gene pool =

Answer 34

All of the alleles that are present in the population

Question 35

key contributors to genetic variance: Mutations =

Answer 35

add new alleles to the population

(by mutagens where the genetic code is not copied properly)

Question 36

key contributors to genetic variance: Inheritance patterns

Answer 36

alleles combine to produce unique phenotypes, including the effect of co-dominance, incomplete dominance, lethal alleles and multiple alleles

Question 37

key contributors to genetic variance: Sexual reproduction

Answer 37

genes are a combination of the 2 parents

Question 38

key contributors to genetic variance: Meiosis =

Answer 38

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

Question 39

key contributors to genetic variance: Crossing over

Answer 39

homo. chromosomes tangle and recombine during meiosis - changes the combinations of linked alleles

Question 40

Thomas Malthus ideas:

Answer 40

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)

Question 41

Adaptations =

Answer 41

characteristics that make an individual suited to its environment and way of life

Question 42

the success of gene

Answer 42

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

Question 43

Development of melanistic insects (peppered moths) in polluted areas

Answer 43

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

Question 44

Antibiotic resistance = simplified

Answer 44

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