Variation & Evolution- Unit 4.4 Flashcards
What is the definition of variation?
Small differences in phenotype existing between SAME species
What are reasons for differences in phenotypes?
-Different genotypes
-Same genotype but different epigenetic modifications
-Different environments
What is heritable variation?
Differences in phenotypes that are inherited.
In asexual reproduction, heritable variation can only be increased by mutation.
In sexual reproduction, heritable variation is increased when genotype from one parent combines with the other.
How does sexual reproduction create variation? (3)
- Mixing 2 parental genotypes during cross fertilisation.
- Random distribution of chromosome during metaphase | and || of meiosis.
- Crossing over of homologous chromosomes during prophase | of meioisis. (same gene sequence, length, etc)
Describe non-heritable variation:
-Variations results from environmental factors (diet/exercise, light/temperature) with low evolutionary significance as they ARE NOT passed down.
What is discontinuous variation?
Normally controlled by a single gene that may have two alleles or more. (monogenic) (blood group is an example as you must be A, B, AB or O)
What is continuous variation?
Influenced by combined effect of many genes. (polygenic) (height)
What are the phenotypes like is discontinuous variation?
-Characteristics are discreet- in on group or other.
-Monogenic
-No environmental influence on gene expression
What are the phenotypes like in continuous variation?
-Characteristics not discreet- can be in multiple groups/combo.
-Polygenic
-Large environmental influence on gene expression
What is a mutation?
Random change in DNA
New combinations of alleles are created
It is rare a useful mutation occurs
Name examples of selection pressures:
-Space for nesting
-Overcrowding
-Predation
-Temperature
-Human impact
-Food/water supply
Define the term population genetics:
The behaviour of genes/alleles of an entire population
Describes proportions of different alleles in whole gene pool. (allele frequencies)
Define the term gene pool:
All the alleles of all the genes of the whole population of a species/organism that results in variation.
A gene pool is constantly changing.
What is genetic drift?
Variations in gene frequency in populations that occur by chance.
(if an individual with a certain gene (1/10) does not reproduce for some reason, the gene frequency is reduced by 10%)
What is evolution?
A change in the average phenotype over time
What happens if evolution is really severe?
The phenotype has changed so much that they have become so different so can no longer reproduce with each other, like they have become slightly different species. (tiger,lion) THIS IS CALLED SPECIATION.
What is the founder effect?
When few individuals have become isolated from the original population, colonise a new habitat, they become founder members. As they are a small sample of the original, they, by chance, may have different allele frequencies. This founder population may undergo genetic drift and undergo speciation.
Name the 3 types of natural selection:
-Stabilising selection
-Directional selection
-Disruptive selection
What is stabilising selection?
Eliminates extreme phenotypes from the population.
In a stable environment, the middle/average phenotypes are selected in greater numbers than the extreme ones. This will continue until most individuals have the same value/phenotype/alleles.
STABILISING SELECTION PREVENTS CHANGE as average is favoured.
What is directional selection?
Occurs during a changing environment where an extreme phenotype has become advantageous, meaning the average phenotype changes overtime as those with an extreme allele are favoured, causing speciation.
What is disruptive selection?
The average phenotype is not favoured as it provides no advantage, so the extremes on either end are selected. (2 peaks on graph)
2 distinct forms emerge from this (rare) and will cause speciation.
It is the opposite of stabilising selection.
What are demes?
Breeding sub-units where populations of one deme tend to reproduce in that deme, rather than with those in other demes.
What happens if a deme is isolated?
Gene flow in and out is prevented. (isolating mechanism)
The deme is considered reproductively isolated. If this continues over generations, the allele frequencies change and mutations occur.
Eventually, speciation would occur, meaning that if other demes were reunited, they could not interbreed.
What is a zygote?
Fertilised ovum
What is pre-zygotic reproductive isolation?
Gametes are prevented from fusing so zygote DOES NOT form
What is post-zygotic reproductive isolation?
Gametes fuse, zygote is formed but is sterile so the species do not merge. (mule)
What is geographical isolation? (causes allopatric speciation)
When a physical barrier has separated demes, such as a river.
What is sympatric speciation?
No physical barriers have separated the demes. Reproductive isolation has usually occurred.
Name some pre-zygotic causes of sympatric speciation? (4)
-One sub-species fails to attract other sub-species.
-Sex organs incompatible.
-Gametes prevented from meeting. (sperm fails to reach ovum/survive in oviduct)
-Reproductive organs mature during different seasons.
What are some post-zygotic causes of sympatric speciation? (2)
Hybrid inviability- fertilisation occurs but zygote fails to develop.
Hybrid sterilisation- Offspring unable to produce gametes due to sets of chromosomes from parents unable to pair at prophase | of meiosis, and so they cannot breed themselves.
What is overproduction?
When a population reproduces more than it needs to to replace the parents. Since most populations have stable numbers, there must be a certain number that do not survive. This could be due to competition, lack of space, etc.
