What causes variation in phenotype between individuals in a population?
Genetic variation - mutations (primary source), meiosis, random fertilisation of gametes during sexual reproduction
Environmental variation - environment influences how genes are expressed, genes set limits and environment influences where organism lies within limits can be shown on normal distribution curves
Selection pressures
Environmental factors that limit population
Eg predatation, disease, competition
Selective advantage
Organisms whose phenotypes are better suited to immediate environment
Selective disadvantage
Organisms whose phenotypes are less suited to immediate environment
Natural selection
Variation in species due to different alleles
Selection pressures (predatation, disease, competition) create struggle for survival
Due to variation some individuals better adapted to selection pressures
Causes differential levels of survival and reproductive success
Individuals with phenotype increasing survival more likely to survive and reproduce and pass on advantageous allele to offspring
Greater proportion of next generation inherit advantageous allele
Frequency of advantageous allele in gene pool increases from generation to generation
Why is genetic diversity beneficial?
Environmental conditions constantly changing changes selection pressures
Greater genetic diversity means some will have allele combination needed to survive
Variation provides potential for population to evolve and adapt in new circumstances
What happens to population size when unchecked?
Grows exponentially
Beneficial for natural selection as higher intraspecific competition
Those with selective advantage more likely to pass on beneficial alleles to offspring
Evolution
Change in allele frequencies in population over time
Natural selection is one way evolution occurs
Types of natural selection
Stabilising
Directional
Disruptive
Stabilising selection
Occurs when environment isn’t changing
Reduces variation / range of possible phenotypes
Individuals with alleles towards middle of range more likely to survive and reproduce - acts against both extremes
Repeated over many generations so increasing proportion of population becomes well adapted to environment
Maintains best adapted phenotypes in population
Range/SD reduced but mean/median/mode unaltered
Directional selection
Operates in changing environments
Individuals with alleles for single extreme phenotype more likely to survive and reproduce
Reduction in range of variation in population and progressive shift in mode
Disruptive selection
Occurs when environment favours more than one phenotype
Individuals with alleles for extreme phenotypes at either end of range more likely to survive and reproduce
Characteristics towards middle of range lost - opposite of stabilising selection
