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CHAPTER 17 - SELECTION AND EVOLUTION
GENETIC VARIATION
-the variation in the DNA sequence in each of our genomes
GENE FLOW (GENE MIGRATION)
-introduces new genes into a population as organisms migrate into a new environment
-new gene combinations are made possible by the availability of new alleles in the gene pool
CONTINUOUS & DISCONTINUOUS VARIATION
DISCONTINUOUS VARIATION
DEFINITION : controlled by alleles of a single gene or a small number of genes. The
environment has little effect on this type of variation
-qualitative differences
-clear distinguishable categories
-no normal distribution
-you either have the characteristics or you do not
-eg ; ABO blood group : A, B ,AB ,O
CONTINUOUS VARIATION
DEFINITION : the combined effect of many genes and affected by environmental influences
-quantitative differences
-no distinguishable categories
-eg ; individual heights or masses : may be small and difficult to distinguish
, EVOLUTION
Definition : Changes in heritable traits that occurs within populations over many generations
over a long period of time
MAIN CONDITIONS OF EVOLUTION
1)Natural selection depends on the environment
2)Natural selection acts on existing heritable variation
3)Heritable variation comes from random mutations
4)Natural selection and evolution of species (speciation)
NATURAL SELECTION
Definition : occurs when alleles that enhance the fitness of the individuals that carry them
rise in frequency
-Individuals with characteristics best suited to their environment are more likely to survive,
finding food, avoiding predators and resisting disease.
-These individuals are more likely to reproduce and pass their genes on to their children
THE PROCESS OF EVOLUTION BY NATURAL SELECTION
1)Populations / a species shows genetic variation
2)Selection pressure such as predation, competition, disease take place
3)The best adapted individuals survive and reproduce, passing on their advantageous alleles
to the offspring
4)There will be an increase in allele frequencies
SELECTION PRESSURE
DEFINITION : external agents which affect an organism’s ability to survive in a given
environment
-the effects of selection pressure on the frequency of alleles in a population
Example;-green beetles tend to get eaten by birds and survive to reproduce less often than
brown beetles do. The surviving brown beetles have brown baby beetles because traits have
a genetic basis. Therefore, the more advantageous trait allows the beetle to have more
offspring and become more common in the population. Eventually, all beetles will be brown.
TYPES OF SELECTION
1)STABILISING SELECTION
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