Populations and evolution Flashcards
Population
A group of organisms of the same species that occupies a particular space at a particular time and that can potentially interbreed
Gene pool
All the alleles of all the genes of all the individuals in a population at a given time
Allele frequency
The number of times an allele occurs within the gene pool
What is used to predict allele frequency
hardy-Weinberg principle
Hardy-Weinberg principle
Mathematical model which predicts that allele frequency won’t change from generation to generation
What assumptions is the hardy Weinberg principle based off
-mating is random
-no natural selection occurs
-no mutations
-no gene flow/genetic drift
-population size is infinite
What factors is variation due to in populations of species
-genetic
-environmental influences
What genetic factors influence variation
-Mutations
-Meiosis
-random fertilisation of gametes
mutations
these sudden changes to genes and chromosomes may be passed on to the next generation
meiosis
This special form of nuclear division produces new combinations of alleles before they are passes into the gametes
random fertilisation of gametes
In sexual reproduction this produces new combinations of alleles and the offspring are therefore different from parents
Describe variation due
to environmental
influences
Environmental factors affect the way in which an
organisms genes are expressed, the gene sets the limits
but the environment determines where within this the
organism lies.
What are examples of environmental influences in plants
pH, climatic conditions, availability of food. Environmental
variation affects characteristics controlled by polygenes
(more than one gene)
Why is it hard to distinguish between the effects of environmental and genetic factors
because they combine to produce differences in individuals
Selection pressures
environmental factors that limit the population of a species
What do selection pressures include
Predation, disease and competition
Define evolution in terms of alleles
change in the allele frequencies in a population.
what factors does evolution depend on
-organisms produce more offspring than can be supported by the available supply of food
-there is genetic variety within the populations of all species
-A variety of phenotypes that selection operates against
What strategy has been evolved by many species to ensure that sufficiently large populations survive to breed?
high reproductive rates
How do species with low reproductive rates ensure survival of the species
high levels of parental care and therefore lower death rates
Explain why the allele frequency of subsequent populations is different from the previous
organisms better adapted for their environment survive and pass on the favourable alleles
Explain the role of overproduction of offspring in natural selection.
The death rate for organisms is high. Organisms compensate for this by overproducing. The vast numbers of offspring allow enough organisms to survive despite the high death rate due to predation, competition for food etc.
Explain the role of variation in natural selection.
Variation is important in natural selection as environmental conditions change over time. A wide range of genotypes and therefore phenotypes in a population will mean some have the characteristics to survive in most new sets of circumstances.
Name the 3 types of selection
directional, stabilising, disruptive
Describe how stabilising selection affects the characteristics of a population
selection against extremes. Preserves average phenotype by favouring average individuals
Describe how directional selection affects the characteristics of a population
selects for one extreme phenotype. Changes phenotype of a population in one direction
Describe how disruptive selection affects the characteristics of a population
selects for extreme phenotypes and against the mean
What is the impact of stabilising selection on evolutionary change?
it eliminates the chance of evolutionary change
Under what environmental conditions does stabilising selection occur
those that are constant for long periods of time
Under what environmental conditions does directional selection occur?
those that change
What happens to the mean on a normal distribution curve when environmental change occurs?
it moves to the left or right
Under what environmental conditions does disruptive selection occur?
those that have two distinct forms (eg high temperature and low temperature)
Which type of selection is most likely to lead to speciation?
Disruptive
Explain how selection affects allelic frequencies.
Allelic frequency is the number of times an allele occurs within the gene pool. This is affected by selection which in turn is affected by environmental conditions.
Explain how new species are formed.
New species are formed by speciation - the evolution of new species from existing ones. New species form when there is reproductive separation and then genetic change due to natural selection.
Explain how populations can become geographically isolated.
Geographic isolation is any physical barrier between two populations that prevents them interbreeding. This may include; oceans, rivers, mountain ranges, deserts. What constitutes a geographical barrier is determined by the nature of the organisms being separated
speciation
evolution of new species from existing one. Members of a species are reproductively separated from other species
genetic drift
change in allele frequency in a population due to chance not selection
Allopatric speciation
-geographical isolation
-reproductive isolation/separate gene pools
-different environmental conditions
-variation due to to mutation in different populations
-advantageous alleles passed on
-eventually different species cannot interbreed to produce fertile offspring
sympatric speciation
-occurs in the same habitat/reproductively isolated
-random mutations influence reproductive behaviour
-individuals will not reproduce together
-different alleles passed on change in frequency of alleles
-disruptive selection
-eventually different species cannot interbreed to produce fertile offspring
why is genetic drift important only in small populations
-each individual represents a larger proportion of the population
-greater chance of an allele being lost from the population
-small populations have less variation and therefore less chance of adapting to changing conditions
