evolution simple definition by Dobzhansky
a change in the frequency of alleles within a gene pool
what does population genetic theory state
that the frequency of alleles and genotypes in a population will remain constant unless acted upon by non-mendelian processes e.g. mechanisms of evolution (change in environment, mutation etc)
relationship between allele frequency (haploid gametes) and genotypic frequency (diploid individuals)
Natural selection acts on..
variation in a population
evoltion occrs because of
changes in allele freqeuncies
what leads to speciation
divergence within populations
the fittest genotypes vary from..
pop. to pop. due to differences in eniornment
selection acts on phenotype...
influencing allele frequency
4 postulate of evolution by NS
1.there is variety within species (due to mutations creating new alleles)
2. some of the variations are passed on to offspring (genes are passed on to offspring intact and often independenlt of other genes)
3. more offpsring are produced than can survive
4. survival and reproduction are not random: individuals with the highest reproductive success are those with the most favoruable variation ( with alleles and allelic combinations that best adapt them to their environmet- fitness)
what are the units of selection that survive and reproduce, or dont?
drift is the chance dfference in transmission of alleles, leading to fluctuations in allele freqeuncy.
-most strongly affects rare allelss and has greater influence on rare alleles than selection
-primary mechanisms for increasing rare recessive and is respnsbible for changing freqeuncies of neutral mutations (where selection is not important)
where is selection not important
genetic drift may..
in haploid species genotype=
genotypic variation promotes..
selection will operate to remove less fit variant, or increase vsatiant with greater fitness
in diploid organisms genetic combinations are
disrupted during meiosis --> only continuity is the transmission of allels
invasion of rare mutants (frw of rare recessive alleles in pop. is set by (u)
-new alleles are continously created at a rate (u) by mutation, new mutations ar eintially rare, and msot are recessive. rare, recessive alleles are mostly present in heterozygotes, which makes sleection aganst them negligible.
-However dominant alleels are always expressed and so if they are freqeunt, selection agains them will remove recessive alleles (even if beneficial)
-genetic drift will contribute to rare allele loss
types of selection
natural selection in whcih egenitc diversity decreases as the pop. stabilizes on a particular trait value- most common and the opposite of disruptive selection.
-favours individuals with intermediate phenotypic values
example of stabilizing selection
E.g. A classic example of this is human birth weight. Babies of low weight lose heat more quickly and get ill from infectious disease more easily, whereas babies of large body weight are more difficult to deliver through the pelvis. Infants of a more medium weight survive much more often. For the larger or smaller babies, the baby mortality rate is much higher.
occurs under environmental change- where populations may migrate to new aread with diff enviornemntal pressures e.g. climate chainge - bear with dark fur (where these is less fur) ae ore likelt to survive and not be found by hunters, therefre allelss for dark hair will increase and be passed onto further generations
change in pop. genetics in which extremme values for a trait are favoured over intermediate values--> variance of the trait increases and the pop. is divided int two distinct groups --> main force driving sypatric speciation
biology, fitness landscapes or adaptive landscapes (types of Evolutionary landscapes) are used to visualize the relationship between genotypes and reproductive success. It is assumed that every genotype has a well-defined replication rate (often referred to as fitness).
Natural selection and the adaptive landscape
natural selection tends to move a pop. towards the peak of the hills. When te environment changes the adaptive peaks shift and the pop. follow a never ending evolutionary journey
a species is often defined as a group of organism capable of interbreeding and producing fertile offspring.
-speciation is the development of a new species from another species
-speciation can be allopatric or sympatric
geopgrpahical speciation,w here pop. of the same species become isolated from eachothe to the extent that gene flow stops
process through which new species evolve froma single species while inhabiting the same geographic land
speciation is reinforced by
- behavioral choices
- spatial constrains
- temporal isolation
- mechanical incompatibility
negatives of sexual selection
powerful enough to produce features that are harmful to the individual
e.g. extravagant and colourful tail feathers are likely to also attract predators
natural selection in which an apparently disadvantageous characteristic (especially altruistic behaviour) increases in the population due to increased survival of individuals genetically related to those possessing the characteristic.
Changes in gene frequency across genertions driven by interactions between individuals-- by helping your relatives eeven at a cost to yourself, you may be promoting the success and transmission of genes that you share in common
the more closely two animals are related..
the higher the prob. that they share some identical genes and therefore the more closely their interests coincide
further explanation of kin selection
Kin selection occurs when an animal engages in self-sacrificial behaviour that benefits the genetic fitness of its relatives. The theory of kin selection is one of the foundations of the modern study of social behaviour.
although their own genes may not be passed on their sacrificial behaviour will mean their relatives who have the same genes as them will be able to pass theirs on because they can survive
he reformulatd the definition of fitness as the number of an individuals alleles in the next generation.
This led to the notion of inclusive fitness i.e. an individuals combined representation in the gene pool of the next generation
special case of natural selection- sexual selection acts on an organisms ability to obtain or successfully copulate with a mate. Selection makes many organism go to extreme lenght for sex e.g. peakcocks maintain elaborate tails, elephants fight over territory, fruit flies perform dances etc
r- genetic relatedness to the recipient of the alturist act de
B- additional reproducitve benefit gained by the recipient of the altruistic act
C- the reprdoctive cost to the individual performing the act
C must be larger than rB to be showing altuiristic behaviour
fluctuating selection can
select for rare alles
can be used in Hamiltons rule
-coefficient of relatedness
where n= connection removed from self
'i would lay my life down for 2 brothers or 8 cousins'
is a sharp reduction in the size of a population due to environmental events (such as earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide)--> dramatically decreasing variation in alleles
Cultural isolation, as well as colonization, can result in founder effects. Amish populations in the United States have grown from an initial group of about 200 immigrants, dating back to the mid-1700s. Because they have remained culturally and reproductively isolated from non-Amish Americans, they show considerable uniformity. The Amish today are often studied for their genetic uniformity, as well as certain recessive conditions. Geneticists believe that just one or two of the initial 200 Amish carried a recessive allele for Ellis-van Creveld syndrome (short limbs, extra fingers, and heart anomalies), yet through genetic drift, the isolated Amish population now has the highest incidence of this syndrome in the world
freqeuncy dependent selection
e.g. when mutations causes some snails to have rare alleles producing an unusual phenotype, which means they are predated less.
Their alleles will be passed on.
However when alleles become more common predators will learn.
Alleles only have advantage when they are rare
heterozygous disease which can increase fitness
e.g. sickle cell
those heterozygous have the highest fitness (1) since they are immune to malaria. those without the disease have a lower fitness (0.8)
random chnage in alllele frequency due to chance.
- doesnt produce adaptations
- smaller populations ar emore effected due to statistics
when does genetic drift occur
when random factors impact the frequency of alleles in subsequent generations of a populations
e.g. in a population of 3 beatles (2 red and 1 green) where the green and red beatles mate and by chance only red offspring are produced, after the first generation has died out there will only ever be red beatles