Evolution

Evidence for evolution

Homology, fossil record (Paleontology), biogeographical evidence (analogous), direct observations

Homology

similarity due to common ancestry, diverge in different environments with divergent evolution

anatomical, early embryonic development, vestigial structures, molecular and biochemical

Homologous structures

Anatomical resemblances that represent variations on structure in common ancestor

Fundamental similarities, too numerous to be coincidental

Different functions, different selection pressures

Early embryonic development

embryo of different species look similar despite distant relation

Vestigial structures

no more function/primary function lost

reduced in size and form, similar to functioning structures in other organisms

Molecular and Biological homology

universal genetic code shared by all organisms

conserved DNA and protein sequences (e.g. % of same amino acids in hemoglobin of different organisms)

Paleontological evidence

Transitional fossils - mixed features of 2 lineages

Sedimentary layers - deeper = longer ago

Biogeographical evidence

where organisms live now and historically, patterns of distribution of living and fossil species

Homologous divergence: closely related but different species, slightly different habitats

Analogous convergence: distantly related but similar species, similar habitat

similar environmental challenges and selection pressures, evolved independantly

Direct observation

Natural selection favouring a certain trait

Microevolution

change in allele frequency in given population, short term (a few generations)

minor changes, caused by mutation, migration, selection and chance events

Macroevolution

broad pattern of evolution across species, long term

profound changes in species, consequence of macroevolution and speciation

Hardy Weinberg equilibrium

non-evolving population, shuffling alleles by gene recombination has no effect on overall gene pool

no mutation, random mating, no natural selection, no gene flow, extremely large population size (no genetic drift)

Genetic variation

Mutation - random changes in genes or chromosomes

Gene recombination - mixing genes via independent assortment of chromosomes, crossing over of homologous chromosomes, random fertilisation of eggs by sperm

Preserving genetic variation

Diploidy - hidden recessive alleles in heterozygous are not expressed, so not selected

Balanced polymorphism (Heterozygous advantage and Frequency dependent selection)

Neutral variations - big reservoir of raw material for natural selection

Mutation

molecular alteration in genetic material

must occur in gamete to have evolutionary significance

meiosis - chromosome carrying mutation assorts - 50% chance of passing allele to offspring

may be neutral, harmful or beneficial

Gene flow (migration)

exchange of genes (new alleles) between 2 populations

across geographical barriers - 2 populations become more homogeneous

need fertile individual and possible inter-mating

Genetic drift (general)

random change in allele frequency, more significant in smaller populations

can cause harmful effects to be fixed

lead to less genetic variation within population, genetic divergence across population

large gene pool = more individuals = buffer against fluctuations

Genetic drift (causes):

Founder’s effect - small number of individuals colonise new area (isolated from mother population)

allele frequency in individual different from mother population = large change in allele frequency

less diversity as rare alleles easily lost

Bottleneck effect - disaster drastically reduces population

does not represent genetic makeup initially (allele under/over represented or eliminated)

Natural selection

Existing variation in individuals in species

Individuals of species compete for resources (food, space)

Some competition lead to death of some individuals while others survive

Individuals with advantageous variation more likely to survive and reproduce

Types of natural selection

Stabilising selection - for moderate traits

Directional selection - for one extreme trait

Disruptive selection - for both extremes

Natural selection vs genetic drift

non-random vs random

due to environment changes vs not

directional (for/against particular trait) vs non

generate adaptive traits vs non

survival of the fittest vs important allele may be deleted

all the time but slower, more obvious during gradual environmental change vs all the time but obvious during sudden environmental change

Non-random mating

Assortative mating and sexual selection

Assortative mating

choosing mates with similar phenotypes

Disruptive selection: extreme traits favoured over moderate

Artificial selection: humans choose favourable traits

Sexual selection

males compete for mate, lead to sexual dimorphism (secondary sex characteristics)

Intrasexual selection

competition between males for access to female

may select for phenotypes not favoured by natural selection, mating is a strong selection pressure

Intersexual selection

females pick mates to increase survival of offspring, compare secondary sex characteristics in males

Good gene hypothesis - choose mates by traits that improve survival

Run-away hypothesis - choose mates by traits that improve appearance (may decrease chance of survival)

Speciation

formation of new species by transformation of one species to another or splitting of ancestral species into 2 descendant species

Biological, morphological, ecological, phylogenetic

Biological species concept

populations of the same species can interbreed and produce viable fertile offspring

gene flow between populations, reproductive isolation with other species

Limitations: fossils and asexual organisms, some distinct species can still interbreed and produce viable fertile offspring

Reproductive isolation mechanisms

Geographical isolation (rivers, mountains, etc.)

Ecological isolation (different types of habitats)

Temporal isolation (different breeding season or time)

Behavioral isolation (different mating rituals, colours, calls, etc)

Mechanical isolation (body structure prevents mating)

Prevent gamete fusion (e.g. sea urchins)

Reduced hybrid fertility (hybrid infertile)

Reduced hybrid viability (genetic incompatibility, frail offspring or aborted at embryonic stage)

Hybrid breakdown

Morphological species concept

substantial consistent structural features

easy to apply, can use with fossils and asexual organisms

not testable, subjective, breaks down with convergence, cryptic species, hybridisation and intermediate phenotypes

Ecological species concept

exploiting different ecological niches, natural selection to adapt to niche

works for hybridisation and asexual species

hard to define niche independently of species

Phylogenetic species concept

unique derived features and shared ancestry (using phylogenetic tree)

can apply to asexual organisms

require more information to construct tree identifying monophyletic groups

Allopatric speciation

geographic isolation from parent population, gene flow interrupted

independent mutations, natural selection and genetic drift lead to divergent evolution

reproductive isolation due to genetic divergence

Adaptive radiation

Rapid evolution of many diversely adapted species (occupying different niche) from common ancestor

Driven by competition

Sympatric speciation

without geographical isolation, due to autopolyploidy, sexual selection for similar traits or habitat differentiation to exploit different niches

Reconstructing phylogeny

By morphological data, shared characteristics, molecular data

Morphological data

phenotypic similarities due to shared ancestry

Homology - shared ancestry, complex similar structures, fossil evidence

Analogy - convergent evolution, structures evolved independantly

Shared characteristics

Shared ancestral character - originated in ancestor, shared by many clades

Shared derived character - evolutionary novelty unique to particular clade

Compare derived traits of ingroup to outgroup (older most recent common ancestor with ingroup, no derived traits)

Phylogenetic tree

Monophyletic group (clade) - common ancestor and all descendents

Paraphyletic group - descendant left out

Polyphyletic group - common ancestor and descendants left out

Root - MRCA of all

Basal taxon - diverge early, originate near root

Polytomy - unresolved divergence

Molecular data

compare sequences coding for certain proteins

large molecular datasets generated relatively quickly, easily converted to numerical form for analysis

Taxonomic categories

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species