Variation and Selection OCR A2 Biology

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Biology F215
Variation and Selection
Key Words
Key Term Definition
Speciation due to organisms of a species being separated by geographical barriers so
Allopatric speciation that over time members of the two populations become so different that they cannot
interbreed and are considered to be two different species.
Organisms with beneficial characteristics are selected by humans and allowed to
Artificial selection
reproduce, passing on the alleles for these characteristics to their offspring.
Biological species The biological species concept is a group of similar organisms that can interbreed and
concept produce fertile offspring.
Clade A monophyletic taxonomic group; that is, a single ancestor and all its descendants.
Cladistics A method of classifying living organisms based on their evolutionary ancestry.
Genetic variation, also called quantitative variation, where there is a wide range of
Continuous variation phenotypic variation within the population. There are no distinct categories. It is
controlled by many genes.
A type of natural selection in which the allele and genotype frequency within a
Directional selection population changes because there is a change in the environment that favours one
extreme phenotype. Leads to evolution.
Discontinuous Also called qualitative variation. Genetic variation where there are distinct phenotypic
variation categories. Usually controlled by one gene.
A type of natural selection in which the allele and genotype frequencies change
Disruptive selection because there is a selection pressure favouring either extreme phenotype. May lead to
The combined action of biotic and abiotic factors that limits the growth of a population.
The process of gradual change in the inherited traits passed from one generation to the
next within a population. It results in the formation of new species.
Total genetic information possessed by the reproductive members within a population
Gene pool
of organisms.
The change in allele frequency in a population, as some alleles pass to the next
Genetic drift generation and some disappear. This causes some phenotypic traits to become rarer
or more common.
Genome All the genetic information within an organism/cell.

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Genotype Alleles present within cells of an individual, for a particular trait/characteristic.
The concept that both genotype frequencies and gene frequencies will stay constant
from generation to generation, within a large interbreeding population where mating is
random, there is no mutation and no selection or migration.
Isolating mechanism Mechanism that divides populations of organisms into subgroups.
Monogenic Characteristic coded for by one gene.
A monophyletic group is one that includes an ancestral organism and all its descendent
Mechanism for evolution.…read more

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What is variation?
Variation is the differences in phenotype between individuals of the same or different species
o Phenotype is the observable characteristics of an organism
What is discontinuous variation?
Qualitative differences between phenotypes
o Discrete categories with no intermediate phenotypes
Individuals may be evenly or unevenly distributed between the
Usually due to action of one gene with few alleles that
is little influenced by the environment
o An example is human blood grouping
What is the genetic basis of discontinuous variation?
The Characteristic…read more

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What is the genetic basis for continuous variation?
Characteristic controlled by two or more genes (polygenic characteristic)
o Each gene contributes to the phenotype
Different alleles have a small effect on the phenotype
An example is human eye colour
How do the variations in the environment and genes contribute to the variation
in phenotype?
They contribute to the variations in phenotype by the following formula
o VP = VG + VE
VP = variation in phenotype
VG = variation in genotype
VE = variation due…read more

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What is a population?
A population is a group of individuals of the same species that can interbreed
o Populations change in size due to birth, death and migration
All the alleles in a population represents the gene pool
Individuals have genomes and Populations have gene pools
What are allele frequencies and what factors may change these frequencies?
Proportion of different alleles in a gene pool may change over time
What will changes these frequencies?
o Population size
o Natural Selection
Individuals with favourable alleles…read more

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Put into practise
o The frequency of cystic fibrosis (genotype ff) in the UK is currently approximately 1 birth in 2000.
Estimate the proportion of people in the UK that are carriers of the allele for cystic fibrosis.
1 in 2000 is recessive = q2 = 1/2000 = 0.0005 so: q = Ö0.0005 = 0.022
p = 1 - 0.022 = 0.978
o Heterozygotes = 2pq = 2 ´ 0.978 ´ 0.022 = 0.043
The frequency of carriers in the UK population is 4.…read more

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In natural selection, the environment selects the individuals that survive and reproduce
o These selection pressures act on individual organisms with different alleles
This changes the frequency of alleles in a population
What types of natural selection are there?
Graph section 1 is stabilising selection which reduces variation but does not change the mean.…read more

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What is genetic drift?
Genetic drift in a small population is
when a chance event may cause a
large change in allele frequencies
o Even large populations may
pass through bottlenecks,
when population levels fall
What may produce
such a small
A natural disaster e.g.…read more

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What are isolating mechanisms?
Isolating mechanisms are when a population becomes split into sub groups by an isolating mechanism that
prevents individuals interbreeding
o The allele frequencies in each sub-group will change independently
This is due to mutation and natural selection
And may lead to speciation
Examples of this are:
o Geographic
o Seasonal
o Reproductive
What is speciation?
This is the process of forming a new species
o It increases biodiversity
Sympatric speciation (same place)
o Occurs between groups in the same environment
Allopatric…read more

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Mechanisms exist that can prevent two populations from interbreeding, even though they exist in the same
o Biological isolation
Biological isolation prevents two populations from interbreeding, even though they exist in the
same environment
Mutations occur independently
Selection pressures differ
Allele frequencies change and gene pools diverge
o Speciation occurs
Biological isolation mechanisms
May be pre-zygotic
o Prevent mating and fertilisation taking place
May be post-zygotic
o Fertilisation takes place but the zygotes/adults are less viable or the hybrid
offspring are infertile e.g.…read more


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