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very similar organisms who can interbreed in nature and produce healthy, fertile offspring, genetic similarity



an interbreeding group of organisms of the same species in the same geographic location at the same time



A change, over time, to the DNA composition that generally results in a different phenotype


Variation in a single gene

Polymorph- 'many forms,' coded by one gene that has two or more alleles (can be co-dominant)
Monomorph- 'one form,' all members of the population are identical in terms of a particular trait
Variation is discontinuous- discrete forms, non-overlapping (e.g. either vestigial or wild type), clear demarcation/discrete/distinguished


Variation due to many genes

Continuous- polygenic inheritance, trait controlled by polygenes, variants can be distributed across a continuous range


(autopolyploidy or allopolyploidy)

Polyploidy- three or more full sets of chromosomes (triploids, tetraploid, hexaploidy), ploid; sets of chromosomes
• Autopolyploidy- from same species
• Allopolyploidy- additional sets from other species



chromosome number differs from the standard number of chromosomes
• Trisomy- three copies of a particular chromosome (usually lethal, loss of function of genes)
• Monosomy- only one copy of a particular type of chromosome (not favourable, chromosome imbalance)


Gene pool and allele frequency

the sum of all genes and all of their alleles within a specific population/entire species
Allele frequency- the percentage of one allele of a gene compared to all the other alleles of the same gene
(an allele is a variance of a gene)


Natural selection (4 steps)

Natural selection- selecting agent favours one phenotype more likely to survive and reproduce, and therefore a selection advantage/higher fitness value.
1. Pre-existing variation that is genetically determined
2. A selecting agent makes one form more likely to survive and produce more fertile offspring
3. Selection agent continues over a number of successive generations
4. Change in allele frequency is established


Gene flow

movement of individuals from one population to another. When they interbreed, they share alleles, and may introduce new alleles to that population
Also affects allele frequency in the new and old population


Genetic drift (bottleneck, founder)

unpredictable change of the gene pool (natural disaster), reduces variation, unrepresentative
The smaller the population, the greater the potential impact of genetic drift
Doesn't favour an allele over another

• Bottleneck events- when the generation after the few survivors may, by chance, be an unrepresentative sample of the gene pool of the original population
○ Size of population is reduced for at least a generation (from drought, natural disaster, disease)
○ Loss of alleles (favourable and unfavourable), may increase risk of extinction
• Founder effects- when a small, unrepresentative sample of the population leaves to colonise a new region
○ When a new colony, of few members, separates from a larger population
○ Reduced genetic variation, and likely not an original sample of the whole gene pool



process of formation of a new species


Examples of pre-mating and post-mating isolation mechansims

keep different species separated and prevent interbreeding or gene flow

Pre: Time they are active, barrier, mating behaviours not recognised, location, anatomy
Post: incompatibility of gametes, zygote mortality, inviability of zygote, hybrid offspring is sterile


Allopatric speciation

Allopatric speciation- different homeland (clearing of habitats, river course changes, uplift of mountains)
• Original population is separated
• Accumulation of gene and trait differences, takes thousands of years for multicellular, bacteria- 10 years

• Species living under different environments are subject to different selection factors
• New alleles, through mutation, are likely to be different among the two populations
• Therefore, these varying populations, over time, become increasingly different in structure, physiology, and behaviour, forming two different species
• Either offspring don't survive or are sterile


Phyletic evolution and Cline

Phyletic evolution- linear evolution of one species into a new one
Cline- a gradual change in features within a species, not quite yet a different species


Gene mutation
Mutagenic agents
Point mutations

Gene mutation- alters base sequence, or the location of a gene is changed
Mutagenic agents (radiation, UV and X-rays, chemicals, pesticides and heavy metals, some viruses)
Point mutations affect a single base, by substitution, insertion or deletion



• Silent mutations- base substitution but doesn't change the protein encoded by the gene
• Nonsense mutation- base substitution that codes for a stop signal instead of an amino acid
• Missense mutation- base substitution that results in the alteration of one amino acid (conservative if mutated polypeptide can carry out its normal function, non-conservative if it can't carry out its function)
• Frameshift- single base insertion or deletion that alters the triplet where it was inserted/removed and every triplet after that point


Chromosomal mutations

Happen due to errors in crossing-over during meiosis (deletion/duplication of part of a chromosome, translocation between non-homologous chromosomes, or inversion) or mutagenic agents


Selective breeding

human intervention of breeding together organisms with desirable qualities (aesthetic appeal or size) so the traits are passed on and become common.
May be disadvantageous in terms of survival and reproduction
Decreases variation


Examples of selective breeding

Artificial insemination- collecting semen from a stud, introducing it artificially to females of the same species
MOET (multiple ovulation and embryo transfer)- cows and ewes are injected with FSH and GnRH to produce multiple eggs and make them all nature at the same time, embryos that have undergone 6-7 days of development are removed and transplanted into other females (surrogate mothers) or frozen
Sex selection can also occur through fluorescing a semen sample and separating them based on containing X or Y
Oestrus synchronisation- all sexually mature females undergo oestrus within a predictable time frame

Reduces genetic variation (genetic differences in a population)