D2 Species and Speciation Flashcards
Define allele frequency
The proportion of an allele within a population
Define gene pool
All the alleles present in an interbreeding population
Outline how natural selection results in a change in allele frequency in a population’s gene pool over a number of generations
- By natural selection, alleles encoding for beneficial adaptations will result in a survival advantage and lead to improved reproduction
- These alleles are consequently more likely to be inherited and thus the population’s gene pool will change over generations
Discuss limitations of the definition of ‘species’
- Certain organisms (e.g. bacteria) produce asexually and therefore they do not interbreed
- Some organisms are only known through fossil records, making it impossible to ascertain breeding capacity
- ‘Sibling species’ look identical but don’t interbreed
- Some distinct species can interbreed and produce fertile offspring
- Geographically isolated organisms may never come into contact, meaning there is no information regarding their ability to interbreed
State three barriers to gene pools
Temporal isolation, behavioural isolation, mechanical isolation
Describe temporal isolation
Occurs when two species mate or flower at different times
E.G. Different frog species live in the same pond but breed at different times
Describe behavioural isolation
Occurs when two species respond to different specific courtship patterns
E.G. Some crickets are morphologically identical but only respond to species specific mating songs
Describe mechanical isolation
Occurs when genital differences prevent copulation (animals) or when flowers are pollinated by different animals (plants)
E.G. Galagos have distinctly shaped genitalia that will only fit other members from the same species
Explain how polyploidy can contribute to speciation
- Polyploidy is a condition in which an organism has more than two complete sets of chromosomes in all somatic cells
- It is far more common in plant species as theylack separate sexes and are capable of asexual reproduction (self-pollination)
- It may occur as a result of the failure of a meiotic cell to undergo cytokinesis (so chromosome replication occurs minus cell division)
- Consequently gametes are diploid (2n) and resulting offspring are tetraploid (4n)
- Because tetraploid offspring can no longer mate with diploid organisms(triploid offspring tend to be infertile), speciation has occurred
Outline allopatric speciation
Caused by geographical separation of populations
Populations occupy different geographic areas
E.G. Adaptive radiation of Galapagos finches
Outline sympatric speciation
Involves a reproductive, temporal or behavioural separation
Populations occupy the same geographical areas
Example: Polyploidy in wheat strains
State a similarity between allopatric and sympatric speciation
Both involve the formation of a new speciesvia isolation of the gene pool from an existing species
Define speciation
The formation of new and distinct species in the course of evolution by the splitting of an existing species
Outline the process of adaptive radiation
- Adaptive radiation describes a rapid evolutionary diversification of a single ancestral lineage
- It occurs when members of a single species occupy a variety of niches with different environmental selection pressures
- Consequently, members evolve different morphological adaptations as a result of natural selection
- Adaptive radiation results in speciation (many species from an ancestral line) and may be further enhanced by reproductive isolation
- An example of adaptive radiation can be seen in the variety of beaks seen in the Galapagos finches
Outline convergent evolution
Different ancestor
Converge to produce analogous structures
Species appearance becomes more similar over time
Species are unrelated (genetically different)
Example: Wings in insects, birds and bats
Outline divergent evolution
Common ancestor
Diverge to produce homologous structures
Species appearance becomes more different over time
Species are closely related (share genetic homology)
Example: Pentadactyl limb structure in vertebrates
Outline gradualism as a pace of evolution
- Continuous change at aconstant paceover a long period of time
- Arises as a result of the gradual accumulation of mutations / variations
- Examples in the fossil record of gradual change with intermediate forms support this theory
- An example is the evolution of the modern horse (gradual change in size and foot structure with changing habitat)
Outline punctuated equilibrium as a pace of evolution
- Evolution proceedsrapidly in burstsfor short periods of time, intermittent with long periods of stability
- In periods of stability organisms become well suited to the environment, with natural selection acting to maintain characteristics
- Equilibrium is punctuated by a rapid environmental change (e.g. volcanic eruption, meteor impact) which leads to directional selection
- Gaps in the fossil record and the lack of intermediate forms for many species support this theory
- Strata in the fossil record that show the appearance of many new species following a mass extinction support this theory
Define transient polymorphism
- A situation when there are two alleles in a gene pool (polymorphic) and one allele is gradually replacing another
- This is due to a strong environmental selective pressure causing directional selection to eliminate one allele
Outline an example of transient polymorphism
Antibiotic resistance in bacteria
Define balanced polymorphism
- A situation when there are two alleles in a gene pool (polymorphic) and the frequency of the two alleles in not changing
- Occurs when selective pressures promote the coexistence of the two alleles (i.e. heterozygous advantage) causing a stabilising selection
Outline an example of balanced polymorphism
- Sickle cell anaemia is controlled by a single gene mutation
- Individuals homozygous for the sickle cell allele have abnormally shaped red blood cells that cause anaemia
- Individuals homozygous for the normal blood cell allele are highly susceptible to malarial infection
- Malaria parasite is less successful at infecting sickle-shaped blood cells
- In areas where malaria is common, heterozygous individuals experience an advantage over either form of homozygote
- These individuals are more likely to survive and reproduce, leading to a balance in the frequency of the two alleles (heterozygous advantage)
