4.4 - variation and evolution Flashcards
Variation def?
the differences between organisms of the same species
Organisms differ in phenotype for 3 reasons?
1) have a different genotype
2) same genotype but different epigenetic modifications
3) different environments
If difference is due to the DNA sequence or epigenetics?
variation is heritable
( inherited )
In A- sexual organisms?
variation = increased by mutation
in sexually reproducing organisms
there = 4 mechanisms for variations?
1st mechanism for variation?
crossing over between homologous chromosomes during prophase I of meiosis
2nd mechanism?
Random distribution of chromosomes during metaphase I of meiosis
3rd mechanism?
random distribution of chromatids at metaphase II of meiosis
4th mechanism?
the mixing of 2 different parental genotypes at fertilisation
Non heritable?
variation= imposed by the environment but not transmitted to the offspring
Discontinuous variation?
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
Continuous variation?
Isolation and Speciation?
species
- a group of phenotypically similar organisms that can interbreed to produce fertile offspring
reproductive isolation def?
prevention of reproduction or gene flow between breeding groups within a species
allopatric def?
the evolution of a new species from demes isolated in different geological locations
sympatric speciation?
evolution of a new species from demes sharing the same geographical location
demes?
sub groups within a population that may breed more frequently with each other than the rest of the population
Reproductive isolation?
prezygotic?
gametes are prevented from fusing so a zygote is never formed
postzygotic?
gametes fuse and a zygote forms but the offspring are sterile
such as a zebronkey or a mule
5 types of prezygotic?
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
Morphological isolation?
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
Gametic isolation?
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
Reasons why gametes may not fuse?
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
