H2 Biology 26 - Biological Evolution

descent w/ modification from a common ancestor

small-scale evolutionary changes below species level which is change in allele frequency of population over time

large-scale evolutionary changes @/above species level, results in creation of new species

GV results in diff. alleles in individuals of same species, hence differences in characteristics >

variations in characteristics subjected to selection pressures from

environment >

natural selection takes place/ there is differential reproductive success in population & indivs who are better adapted to environment @ selective advantage >

indivs w/ favourable characteristics survive to maturity, reproduce & pass down favourable alleles to offspring

1. molecular & anatomical homology suggest common ancestry -

from e.g. anatomical homologous feature suggests species descended from common ancestor w/ basic form of that feature >

molecular homology; DNA nucleotide/a.a. seq. of homologous genes b/w organisms suggest species descended from common ancestor w/ original seq.

2. cumulative changes in molecular seq & anatomical structures show 'descent w/ mod' -

descendant species modified through NS & changes in allele freq over long period of time

1. evidence for 'descent w/ mod'

species closely related tend to be found in same region

2. evidence for environmental pressures driving NS, similar ecological niches in distant regions occupied by unrelated but similar species

process that occurs when a single species evolves into several species occupying diff ecological niches due to NS

differentiation of a homologous structure to perform variety of functions

similar ecological niches in distant regions occupied by unrelated but similar-looking species due to similar selection pressures

process by which one or more species arise from previously existing species

transfer of alleles from one population to another due to migration of fertile individuals

allopatric speciation >

geographical isolation, GB disrupts GF b/w 2 popns resulting in reproductive isolation >

diff environmental conditions on either side of barrier leads to diff selection pressures >

over successive gens, gene pool diff becomes very large, 2 populations cannot interbreed to produce fertile, viable offspring

sympatric speciation >

same geographical area, RB caused by behavioural(unique mating rituals, preferences) /physiological isolation(unique internal fns of diff sub popns) >

prevents interbreeding as GF disrupted, over successive gens diff species formed

Biological Species Concept

group of organisms capable of interbreeding, producing fertile, viable offspring

+ organisms can be studied to see if they interbreed & produce fertile, viable offspring

- defn doesn't apply to asexually reproducing organisms, extinct species whose breeding behaviour cannot be observed

Ecological Species Concept

group of organisms sharing same ecological niche

+ competitive exclusion principle - every organism has a niche, no 2 species can share niche

- defn doesn't apply to unrelated species occupying similar niche

- many animals elusive, time consuming & difficult to determine niche

Morphogical Species Concept

group of organisms sharing similar body shape & structural features

+ every organism has morphological features easily studied if specimen available

- defn makes it difficult to determine degree of difference required to separate species & what features should be used to distinguish the differences

- some organisms superficially similar but diff evolutionary origins

Genetic Species Concept

group of genetically compatible organisms, genetically isolated from other groups

+ easy to measure genetic similarity through DNA analysis, quickly carried out; useful to distinguish small/morphologically similar organisms

- human judgment determines how much differences suffice to separate species

Phylogenetic Species Concept

smallest group of organisms, share most recent common ancestor, distinguished from other such groups

+ avoid wrongly classifying organisms based on superficial morphological similarities by comparing characteristics based on common ancestry/homology

- difficult to determine most recent common ancestor as fossil evidence rare

classification is organisation of species according to shared characteristics, may not take into account evolutionary rs >

however phylogeny is organisation of species to show evolutionary rs

1. unambiguous & objective, ATCG easily recognised & not dependent on subjective judgments, observations involving qualitative differences

2. quantifiable & can be converted to numerical form, open to statistical analysis

3. homologous regions of DNA from diff species provides many points of comparison as each nucleotide serves as comparison point

1. no recombination/crossing over in mitochondrial genome, differences only from mutations

2. non-coding mitochondrial control region of mtDNA has rapid mutation rate, allows accumulation of diff in DNA seq, allow better differentiation b/w closely related species

3. multiple mitochondria, multiple copies of mtDNA in each cell; easier to obtain

4. mt chromosomes are circular, subjected to less degradation by exonucleases