4.4 - variation and evolution

Question 1

Variation def?

Answer 1

the differences between organisms of the same species

Question 2

Organisms differ in phenotype for 3 reasons?

Answer 2

1) have a different genotype
2) same genotype but different epigenetic modifications
3) different environments

Question 3

If difference is due to the DNA sequence or epigenetics?

Answer 3

variation is heritable
( inherited )

Question 4

In A- sexual organisms?

Answer 4

variation = increased by mutation
in sexually reproducing organisms
there = 4 mechanisms for variations?

Question 5

1st mechanism for variation?

Answer 5

crossing over between homologous chromosomes during prophase I of meiosis

Question 6

2nd mechanism?

Answer 6

Random distribution of chromosomes during metaphase I of meiosis

Question 7

3rd mechanism?

Answer 7

random distribution of chromatids at metaphase II of meiosis

Question 8

4th mechanism?

Answer 8

the mixing of 2 different parental genotypes at fertilisation

Question 9

Non heritable?

Answer 9

variation= imposed by the environment but not transmitted to the offspring

Question 10

Discontinuous variation?

Answer 10

some characteristics have only a small number of possibilities
5 or 6 digits
moths can be speckled or dark
there is no intermediate type, so characteristics - discrete
each of the characteristics= controlled by a single gene
( monogenic)
environment has no influence on discontinuous variation in terms of the genes expression

Question 11

Continuous variation?

Question 12

Isolation and Speciation?

Question 13

species

Answer 13

• a group of phenotypically similar organisms that can interbreed to produce fertile offspring

Question 14

reproductive isolation def?

Answer 14

prevention of reproduction or gene flow between breeding groups within a species

Question 15

allopatric def?

Answer 15

the evolution of a new species from demes isolated in different geological locations

Question 16

sympatric speciation?

Answer 16

evolution of a new species from demes sharing the same geographical location

Question 17

demes?

Answer 17

sub groups within a population that may breed more frequently with each other than the rest of the population

Question 18

Reproductive isolation?

Question 19

prezygotic?

Answer 19

gametes are prevented from fusing so a zygote is never formed

Question 20

postzygotic?

Answer 20

gametes fuse and a zygote forms but the offspring are sterile
such as a zebronkey or a mule

Question 21

5 types of prezygotic?

Answer 21

geographical isolation
a physical barrier such as a mountain, river or a desert prevents interbreeding
if the demes are in separate areas, the speciation is called allopatric
African elephants
behavioural isolation
animals have mating rituals and courtship displays that are only recognised by members of the same species
for example - the male grasshopper vibrates its wings against its hind legs, creating a song
if the female does not recognise the song, she wont mate with the male
but if the male is prevented from singing and another species makes a song, she will mate
as the individuals share a habitat, it is sympatric speciation

Question 22

Morphological isolation?

Answer 22

genitalia of males and females must be complementary for sperm to be transferred successfully
a reproductive barrier can be mechanical isolation
exoskeleton of insects are rigid, so mating can be difficult
they share a habitat so this is sympatric speciation

Question 23

Gametic isolation?

Answer 23

gametes in an open environment such as a body of water are more likely to meet gametes of another species
in a third of cases, a viable interspecies hybrid is produced

Question 24

Reasons why gametes may not fuse?

Answer 24

molecules in a gamete membrane
prevent fusion
chemo attractants from female gametes are only recognised by male gametes of the same species
pollen of a flowering plant can germinate on the stigma of another species but no pollen tube grows
(sympatric)
seasonal isolation
reproductive organs of different demes mature at different times of the year
(sympatric)
in the same place

Question 25

Post zygotic isolation?

Answer 25

hybrid in viability
fertilisation occurs but the genes of the parents are incompatible so the embryo cannot develop

Question 26

Hybrid sterility?

Answer 26

embryo forms but the chromosomes are unable to pair at prophase 1 of meiosis so gametes in the offspring cannot form so the hybrid is sterile
female donkey x male horse = mule

male donkey x female horse = Hinny
Mules and Hinnies are sterile
a horse has 64 chromosomes whereas the donkey has 62
but mules and hinnies has 63 so cannot form homologous pairs at meiosis

Question 27

Darwin’s observations ?

Answer 27

membrane of a population show variation
individuals in a population have the potential to produce large numbers of offspring yet the number of adults = constant from generation to generation
resources are always limited

Question 28

Darwin’s reductions

Answer 28

there is a struggle for existence
those that survive reproduce
as they must have has characteristics for success
if environmental conditions change
the features needed to survive must change
so natural selection is a continuous process