Evolution and Speciation TEST NOTES

Patterns of Evolution: Divergent Evolution Convergent Evolution Co-Evolution Adaptive Radiation Punctuated Equilibrium Phyletic Gradualism   Divergent Evolution: Ancestral species evolve into 2 or more species Become specialised to occupy different niches Involves formation of a large number of species from ancestral species Due to the availability of many niches Associated with Homologous Structures Different functions Same ancestral origins Evolve differently Exposed to different selection pressures Exposed to different ecological niches   Convergent Evolution: 2 or more unrelated species evolve to resemble each other Develop similar features in response to similar niche requirements Associated with Analogous Structures Different ancestral origins Similar functions Similar appearances Evolve similarly Exposed to similar selection pressures Exposed to similar ecological niches   Adaptive Radiation: Different groups able to occupy a wide variety of different ecological niches recently become available A form of divergent evolution The rapid evolution of a number of different species from the same ancestral origin   Co-Evolution: 2 different species ecologically intimate Influence each other’s evolution Evolutionary changes in one species That species acts as a selection pressure on another Vice Versa Has 3 forms Predator-Prey Predator and prey evolve together Predator evolves whatever necessary to eat prey Speed, camouflage, heightened sense, immunity to prey’s poison, own poison Prey evolves whatever necessary to avoid the predator Speed, camouflage, heightened senses, own poison, thorns Mutualistic A relationship develops because of selection pressures Develop adaptations for the benefit of each other’s niche requirements - gain survival advantages Mutual benefits for both species Greater reproductive success Pass on their favoured alleles Parasitic Parasite causes harm Takes nutrition from the host Lifecycles in or on the host Lives outside the host (external parasites) Lives inside the host (internal parasites)   Phyletic Gradualism: Evolutionary model that occurs as a slow steady process Organisms change and develop slowly over time Small periodic changes in the gene pool Natural selection gradually changes the average features of species  Removes less fit individuals Increases fit individuals Not concentrated in lineage-splitting evolution Have non-straight lines in morphology graph Single species gradually change into new species - original goes extinct   Punctuated Equilibrium: Evolutionary pattern Connected to speciation events E.g RIM's Species undergoing very little evolution for long periods of time Before suddenly evolving rapidly Statis (straight line) Lack of substantial change over millions of years Happens to small populations that have become isolated from original species   Causes of Punctuated Equilibrium: Mutations Change in environment   Mutations - Punctuated Equilibrium: Mutations in the genes of few individuals cause a rapid change Very small population Chances of evolution increase rapidly Individuals evolve independently Exposed to different selection pressures Different alleles selected for and passed on Gene pool drastically changes in small time Sudden divergence   Change in environment - Punctuated Equilibrium: Stage 1: statis Organism living, dying Stage 2: isolation A sudden change in environment e.g drop in sea level forms lake A small number of population of organism isolated Stage 3:  selection pressures  Small population experiences strong selection and rapid change New environment formed - new selection pressures Stage 4: no preservation  

Processes of Evolution: Natural Selection   Natural Selection: Organisms with structural, physiological, behavioural features More likely to survive Reproductive success Better suited to the environment Contribute more alleles to the gene pool Pass favourable traits to next generation Organisms have adaptations Enables survival Selected for by the environment Increased chance of survival Increased likelihood of passing favourable genes to next generation   The mutation causes a new allele to form and increases the allele diversity. Mutations provide the "raw material" upon which the mechanisms of natural selection can act.  If the mutation increases fitness, the individuals with this adaptive phenotype are more likely to reproduce and pass on the favourable allele. The favourable allele will increase in frequency within the population over time.   Types of Evolution: Stabilising Selection Intermediate phenotype selected for - has the highest fitness Two or more extreme phenotypes selected against Reduces diversity within gene pools Weak intraspecific competition cause disruptive selection Weak selection pressures cause disruptive selection Disruptive Selection Two or more extreme phenotypes selected for - has the highest fitness Intermediate phenotype selected against Increases diversity within gene pools Strong intraspecific competition cause disruptive selection Strong selection pressures cause disruptive selection Leads to speciation Splits population into two groups with distinct phenotypes Directional Selection One extreme phenotype selected for Other phenotypes selected against Shifts allele frequency over time towards the selected phenotype Advantageous alleles increase in frequency Increased fitness of individuals with those phenotypes (reduces competition)  

Polyploidy: When an organism has more than two complete sets of chromosomes in its somatic cells Occurs due to non-disjunction  Chromosomes are not pulled apart correctly (by spindle fibres) during cell division (meiosis or mitosis) Gamete cells have extra pairs of chromosomes Results in 2n gametes  If these 2n gametes fuse - a tetraploid (4n) may result Results in instant speciation   Allopolyploidy: Results from the mating between 2 different but closely related species Hybrid is formed and may be sterile Does not have homologous pairs of chromosomes Cannot form gametes   Autopolyploidy: One species contributes all the chromosomes     Polyploidy is where an organism has more than two homologous sets of chromosomes in their somatic cells. The polyploids are fertile because they have homologous pairs / even number of chromosomes so can form viable gametes. Polyploidy can be caused by the spindle pulling an incorrect number of chromosomes to one end. This nondisjunction may result in 2n gametes. If two 2n gametes fuse, a tetraploid (4n) may result. Postzygotic RIMs such as hybrid infertility / hybrid sterility may keep polyploids isolated. E.g. tetraploid with 18 chromosomes in gametes + hexaploid with 27 chromosomes in gametes results in hybrid with 45 chromosomes, which cannot form viable gametes because the chromosomes cannot pair at meiosis because some have no homologues. Changes in morphology resulting from polyploidy contribute to habitat / niche differentiation. These new physiological tolerances allow them to exploit novel niches. Different habitats / niches have different selection pressures such as water availability which may lead to (sympatric) speciation. For example, polyploids have lower stomata density and thicker epidermis which reduces water loss / transpiration and maintains turgor. The wider xylem reduces the chances of air blockage in drought conditions, which gives an adaptive advantage in these conditions and allows photosynthesis to be maintained.           1) The definition of polyploidy  2) For hybrids to form, parents need to be closely related. 3) Hybrid infertility (sterility) is a result of NOT having enough chromosomes from either parent. Inheriting only 1 set from each parent means that homologous chromosomes cannot form. 4) In order for hybrids to become fertile, one of two things needs to happen: - self-fertilisation occurs or - non-disjunction event occurs which involves the duplication of chromosome numbers Both of these occurrences result in chromosomes being able to pair up (homologous pairs) 5) In order for a hybrid to become fertile, homologous chromosomes from each parent are required. 6) Why does this result in a new species? No longer is this hybrid able to reproduce with original species to produce fertile offspring due to an incorrect number of chromosomes.    

Speciation: Allopatric Speciation Sympatric Speciation RIM's   Allopatric Speciation: Formation of 2 different species One population divides by a physical barrier Two or more geographically isolated populations form Prevents gene flow between populations Subject to different selection pressures  Selection pressures favour phenotypes suited to each environment The allele frequency of favourable alleles increase over time   Sympatric Speciation: Formation of 2 different species In the same geographical area without prior geographical isolation Diverged from a common ancestor Occupy separate niches in that area and rarely come into contact Prevents gene flow between the species due to competition avoidance Subject to different selection pressures