4.4 Variation & Evolution Flashcards
Define variation
Differences in the phenotype of a species
Describe the heritable variation
- mutations in DNA base sequence or epigenetic modification
- crossing of chiasmata at meiosis I, random assortment, gamete fusion
Discontinuous variation
- no intermediates
- monogenic
- minimal environmental influence
Continuous variation
- ranges ie height
- polygenic
- environment has an impact
Define selection pressures and list some examples
Environmental factors that can alter the frequency of alleles in a population when they are limiting
- nesting site availability, population density, predation, temperature, human impact
Define polymorphic
Occurrence of more than one phenotype in a population that cannot be accounted for by mutation alone
Define natural selection
The increased chance of survival and reproduction of organisms with phenotypes suited to their environment, e enhancing the transfer of favourable alleles from one generation to the next
Define the founder effect
The loss of genetic variation in a new population established by a very small number of individuals from a larger population
Define the Hardy-Weinberg principle
- in ideal conditions, allele and genotype frequencies in a population are constant from generation to generation
p^2 + 2pm + q^2 = 1
Draw and explain stabilising selection
- average phenotype provides great advantage than either extreme
Draw and explain directional selection
- one extreme phenotype is selected against
Draw and explain disruptive/diversifying selection
- average does not provide advantage so extremes are selected for
What are the conditions for Hardy-Weinberg
- diploid organisms with equal allele frequencies in both species
- random mating and sexual reproduction
- large populations
- no mutations or selections, and no emigration or immigration
Define a species
A group of phenotypically similar offspring that can interbreed and produce fertile offspring
What are the ways that new species arise?
- polyploidy
- isolating individuals
What is meant by a deme?
Sub group of a population
Define reproductive isolation
The prevention of reproduction and gene flow between breeding groups within a species
Briefly explain how speciation occurs
- demes are isolated for many generations
- undergo changes in allele frequency and accumulate so many mutations that they can no longer interbreed with the original population
Differentiate between pre-zygotic and post-zygotic isolation
- in pre-zygotic, gametes are prevented from fusing so the zygote never forms
- in post-zygotic, gametes fuse and the zygote forms but the organism is sterile and cannot form gametes, so species don’t merge
Define allopatric speciation
Evolution of a new species from demes isolated in different geographical locations
Define sympatric speciation
The evolution of new species from demes sharing a geographical location
Describe hybrid inviability
- fertilisation occurs but incompatibility between genes of parents prevents embryo development
Describe hybrid sterility
- embryo of 2 diff species can develop
- if chromosomes are not sufficiently similar, they cannot pair at prophase 1 so gametes cannot occur
- sterility prevents gene flow so species remain distinct
Describe hybrid breakdown
- F1 hybrids are fertile but the F2 are sterile
- incompatibility in nuclear genes and genes in mitochondria or chloroplasts
Define evolution
When changes in allele frequency cause the average phenotype produced by a population to change
What factors alter allele frequency?
- competition
- predation
- disease
Describe natural selection
- offspring are produced with slight variation
- there is competition, disease and predation
- those that have a selective advantage are better suited and therefore survive
- they are able to reproduce and pass their advantageous alleles to offspring
—> alleles are selected for
What is meant by adaptive traits?
- features used in an original form
What is meant by exaptations?
- structures that have had their original use modified
—> ie feathers used for body heat are now for flying
What are some observations of evolution?
- it is not a direct process going from simple to complex - it’s completely contingent on the enviro
- some structures do not appear to be ideally suited to their current function
When 2 different species produce offspring, why might they be sterile?
- chromosomes are not homologous
- cannot form bivalents in prophase 1
- no meiosis
- gametes can’t be produced
