10.3 Speciation

Question 1

what is the gene pool?

Answer 1

represents the sum total of alleles for all genes present in a sexually reproducing population

Question 2

what does a large gene pool indicate?

Answer 2

high amounts of genetic diversity, increasing the chances of biological fitness and survival

Question 3

what does a small gene pool indicate?

Answer 3

low amounts of genetic diversity, reducing biological fitness and increasing chances of extinction

Question 4

what can gene pools be used to determine?

Answer 4

allele frequency (the proportion of a particular allele within a population)

Question 5

what is evolution?

Answer 5

the cumulative change in the heritable characteristics of a population across successive generations

Question 6

what is required for evolution to occur?

Answer 6

allele frequencies change within the gene pool of the population

Question 7

what are the 5 mechanisms of change?

Answer 7

• mutation - a random change in the genetic composition of an organism due to change in the DNA base sequence
• gene flow - the movement of alleles into/out of a population as a result of immigration/emigration
• sexual reproduction - sex can introduce new gene combos and alter allele frequency it mating is assortive
• genetic drift - the change in the composition of a gene pool sue to random chance event
• natural selection - the change in the composition of as a result of differentially selective environment pressures

Question 8

what is genetic drift?

Answer 8

the change in the composition of a gene pool as a result of change or random events

Question 9

what is the difference in genetic drift between smaller and large populations?

Answer 9

• it will occur faster and more significantly in smaller populations (change has big effect on gene pool)
• less affected by random events in larger population (maintain more stable allele frequencies with low genetic drift)

Question 10

what are 2 mechanisms by which small populations may change?

Answer 10

population bottlenecks and founder effect

Question 11

when do bottlenecks occur?

Answer 11

when an event reduces population size by order of magnitude (- >50%)

Question 12

what is a bottleneck? (2)

Answer 12

• may result from natural occurrences (fires, floods) or human induces (overhunting)
• the surviving population has less genetic variability to a higher level of genetic drift
• as surviving members begin to repopulate, the newly developing gene pool will be divergent to the original

Question 13

when does founder effect occur?

Answer 13

small group breaks away from a larger population breaks away from a larger population to colonise a new territory

Question 14

what is the founder effect? (2)

Answer 14

• the as the small group of population subset does not have the same degree of diversity as the lager population, it is subject to more genetic drift
• this new colony increases in size, its gene pool will no longer be representative of the original gene pool

Question 15

what do allele frequencies represent?

Answer 15

the prevalence of a particular allele in a population, as a proportion of all alleles for that gene

Question 16

what can changes in allele frequency reflect?

Answer 16

random process (genetic drift)
differential process (natural selection)

Question 17

what is the difference between the founder effect and population bottlenecks?

Answer 17

original populations remains largely intact

Question 18

what is natural selection?

Answer 18

change in the composition of a gene pool in response to a differentially selective environmental pressure

Question 19

what is stabilising selection? (2)

Answer 19

• when an intermediate phenotype is favoured at the expense o both phenotypic extremes
• result in removal of extreme phenotypes (phenotypic distribution becomes centrally cluster to reflect homogeneity)

Question 20

when does stabilising selection occur?

Answer 20

when environmental conditions are stable and competition is low

Question 21

what is an example of stabilising selection?

Answer 21

human birth weights
(too large = birthing complications)
(too small = rick of infant mortality)

Question 22

what is directional selection?

Answer 22

• where one phenotypic extreme is selected at the cost of the other phenotypic extreme
• causes phenotypic distribution to clearly shift in one direction (towards the beneficial extreme)

Question 23

what typically occurs after directional selection?

Answer 23

will typically be followed by stabilising selection once optimal phenotype has been normalised

Question 24

what is an example of directional selection?

Answer 24

antibiotic resistance in bacterial populations

Question 25

what is disruptive selection?

Answer 25

• where both phenotypic extremes are favoured at the expense of the intermediate phenotypic ranges
• causes the phenotypic distribution to deviate from the centre and results in a bimodal spread

Question 26

when does disruptive selection occur? (2)

Answer 26

• when fluctuating environmental conditions favour the presence of 2 different phenotypes
• continued separation of phenotypic variant may eventually split the population into 2 distinct sub-populations (speciation)

Question 27

what is an example of disruptive selection?

Answer 27

proliferation of black or white moths in regions of sharply contrasting colour extremes

Question 28

when does reproductive isolation occur?

Answer 28

when barriers prevent 2 populations from interbreeding, keeping gene pools separate

Question 29

what are 2 main categories of reproductive isolation?

Answer 29

• prezygotic isolation - occurs before fertilisation can occur (no offspring are produced)
• postzygotic isolation - occurs after fertilisation (offspring are either not viable or infertile)

Question 30

what can prezygotic isolation barriers be? (4)

Answer 30

temporal, behavioural, geographic/ecological, mechanical

Question 31

what can postzygotic isolation barriers be? (3)

Answer 31

inviability, infertility, breakdown of hybrid organisms

Question 32

what is speciation?

Answer 32

evolutionary process that result in the formation of a new species from a pre-existing species

Question 33

when does speciation occur?

Answer 33

when reproductive isolation mechanisms prevent 2 breeding organims from producing fertile, viable offspring

Question 34

what are 2 basic mechanisms via which speciation occurs?

Answer 34

allopatric speciation -> geographical isolation
sympatric speciation -> reproductive isolation

Question 35

what is allopatric speciation?

Answer 35

when a geological barrier physically isolated populations of an ancestral species

Question 36

what is the process of allopatric speciation? (2)

Answer 36

• 2 populations begin to evolve separately as a result of cumulative mutation, genetic drift and natural selection
• eventually 2 populations reach a degree of genetic divergence whereby thy can no longer interbreed (speciation)

Question 37

what is sympatric speciation?

Answer 37

the divergence of species within the sma geological location (without physical barrier)

Question 38

what is the process of sympatric speciation?

Answer 38

• may result from the reproductive isolation of 2 populations as a result of genetic abnormalities
• typically a chromosomal error may arise which prevents successful reproduction with any organism lacking the same error

Question 39

what is sympatric speciation most commonly caused by?

Answer 39

meiotic failure
- meiotic cells fail to undergo cytokinesis, chromosomal number will double in the gamete (eg. diploid instead of haploid)
- result in offspring that have additional sets of chromosomes (polyploidy)
- speciation will result if the polyploid offspring are viable and fertile but cannot interbreed with the original parent population

Question 40

Why do fertile offspring typically require 2 polyploid parent? (Unless allopolyploidy occurs)

Answer 40

Because reproduction with the original parent population results in offspring with an uneven number of chromosome sets
Eg. Diploid gamete + haploid gamete = infertile triploïdies zygote

Question 41

why is polyploidy more common in plants species? (3)

Answer 41

as they may lack separate sexes or can reproduce asexually
- self-pollination -> many plant species possess both male and female reproductive parts (monoeclious) and hence self fertilise
- asexual reproduction - infertile polyploids can still reproduce asexually via vegetative propagation

Question 42

why may polyploid crops be desirable to farmers? (2)

Answer 42

• allows for the production of seedless fruits
• they will typically grow larger and demonstrate improved longevity and disease resistance (hybrid vigour)

Question 43

what is an example of speciation?

Answer 43

the genus allium
- it is comprised of monocotyledonous flowering plants and includes onions, garlic, chives and leeks
- many of these species polyploidy has occurred resulting in reproductively isolated population with distinct phenotypes

Question 44

what 2 model ways can evolution via speciation occur? (2)

Answer 44

• phyletic gradualism
• punctuated equilibrium

Question 45

what is phyletic gradualism? (3)

Answer 45

speciation generally occurs uniformly via steady and gradual transformation of whole lineages
- speciation is seen as smooth and continuous process (big changes result from many cumulative small changes)
- supported by fossil of horse (many intermediate forms connecting the ancestral species to the modern equivalent

Question 46

what is punctuated equilibrium?

Answer 46

species remain stable for long periods before going abrupt and rapid change (speciation)
- speciation is seen as a periodic process (big changes occur suddenly followed by long periods of no change)
- supported by general lack of transitional fossils (could be explained by rare and irregular conditions required for fossilisation)