Advanced Higher/ A-Level Biology course notes. Concise notes covering the topic of evolution. Notes are easy to follow, highlight key-points, and give supportive question examples where appropriate.
2 a Drift 31 Selection
Evolution is the change over time in the proportion of individuals in a population
differing in one or more inherited traits
During evolution changes in allele frequency occur through the non random processes
of natural selection and sexual selection and the random process of genetic drift
MV T AT l NS
Di erent versions of inherited traits have di erent DNA base sequences as a result of genetic
mutation. If a mutation results in a base sequence which codes for a di erent amino acid in a
polypeptide, then the mutation has produced a novel allele - a new version of the gene. Other
mutations produce new combinations of alleles in the organisms.
There are three possible e ects of mutations on the evolutionary tness of an individual:
◦Neutral mutations - have no e ect on the evolutionary tness of the individual
◦Harmful mutations - reduce the individual’s evolutionary tness
◦Bene cial mutations - are bene cial to the individual’s evolutionary tness
NA T V R AL S E L E C T 1 ON
Natural selection acts on genetic variation in a population
It is a non random process which selects individuals based on their survival chances
and thus increases their chances of passing on their alleles
Alleles that are beneficial increase survival so are more likely to be passed on to the
next generation their frequency will increase
Natural selection results in the non random increase in the frequency
of
advantageous alleles and the non random decrease in the frequency of deleterious
alleles
Natural selection summary
Individuals with variations
that are bettersuited to The alleles for the
Populations produce more their environment tend to Mgmt.BR
favoured traits are
offspring than the
MgmthhB
survive longer and produce inherited so they are
environment can support more offspring breeding to more likely so become
pass on those alleles that more frequent in
conferred an advantage to subsequent generations
the next generation
, S E XV AL S E L E C T 1 ON
sexual selection is the non random process involving the selection of alleles that
increase the individual's chances of mating and producing offspring
Sexual selection
may lead to
SEXUAL DIMORPHISM
Sexual selection can be due to MALE MALE RIVALRY and FEMALE CHOICE
MALE MALE RIVALRY
In many sexually dimorphic in competitive male male rivalry can increase
species success
the access
of males to females for mating through conflict real or ritualised
Large size strength or weaponry increase access to females through conflict and
these are fitness characteristics that will be advantageous to offspring
Examples of weaponry antlers tusks horn Musk Ox
FEMALE CHOICE
In other species female choice drives the evolution of conspicuous markings structures or
behaviours in males as females assess the fitness of males
The male displays tend to reveal HONEST SIGNALS characteristics that allow
potential mates to assess the genetic quality or fitness of the males
Good phenotypic quality is an indicator of favourable alleles that increase the
chances of survival of offspring and low parasite burden suggesting a
healthy individual
Examples peacock's plumage t peahen's dull plumage
SEXUAL DIMORPHISM
Sexual dimorphism is defined as differences in characteristics between the two sexes of
the same species other than differences in the sex organs
Typically differences are in terms of size and colouration
In most cases of sexual dimorphism the male is either larger more heavily
armoured or has more conspicuous markings structures or behaviours
Females are generally inconspicuous males have more conspicuous markings structures
or behaviours
Some species have REVERSED SEXUAL DIMORPHISM in which the female is larger or
more brightly coloured than the male Eg Praying Mantis
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