SPECIATION and PATTERNS OF EVOLUTION READ 8.2 & 8.3 http://www.youtube.com/watch?v=1zOWYj59B XI

SPECIATION Microevolution: changes in gene (allele) frequencies and phenotypic traits within a population Speciation: the formation of new species

SPECIATION What is a species? Includes all the members of a population that can interbreed under natural conditions Individuals of different species cannot interbreed under natural conditions and are described as being reproductively isolated from one another Populations of different species don’t exchange genetic information Some species look very different from each other, or can be indistinguishable Biologists have come up with a variety of methods to help distinguish species

SPECIATION Modes of speciation Although a species can evolve under many different circumstances, the process always includes evolution of distinct features that isolate new species reproductively Mechanisms for reproductive isolation For a new species to form, individuals from the original species must evolve to become reproductively isolated from the remainder of the population and they must establish a new interbreeding population Reproductive isolating mechanism: any biological factor that prevents the two populations from interbreeding when living in the same region

SPECIATION Can operate in many ways Breeding season, physical or behavioural traits, habitat preferences, incompatibility of gametes Prezygotic mechanisms: isolating mechanisms that prevents interspecies from mating and fertilization Postzygotic mechanism: prevents maturation and reproduction of offspring from interspecies reproduction Usually very subtle

Behavioural isolation MECHANISM DESCRIPTION Prezygotic Behavioural isolation Different courtship and other mating clues to find and attract a mate Temporal isolation Breed at different times of the year Ecological isolation Occupy different habitats within a region Mechanical isolation Morphological features differ making two species incompatible Gametic isolation Male gametes may not be recognizable and fertilize an egg of a different species Postzygotic Zygotic mortality Mating and fertilization are possible; zygote is unable to develop properly Hybrid invariability Hybrid develops but dies before birth, or, if born, doesn’t survive to maturity Hybrid infertility Hybrid offspring remain healthy and viable but are sterile http://www.youtube.com/watch?v=FhVi4Z6CjZk&feature=fvsr

SPECIATION Allopatric speciation: formation of a new species as a result of evolutionary changes following a period of geographic isolation Once they are physically separated they can’t exchange genetic information Over time, they will become less alike Any mutation in one population is not shared with the other Any differences in one of the environments will lead to a different natural selection There is a good chance that reproductive isolation has occurred

SPECIATION (Allopatric cont.) Can happen by: Isolated islands Mountain ranges may form Continental drift Glaciers

SPECIATION Sympatric speciation: When individuals within a population become genetically isolated from the larger population Can happen because of direct human action A single mutation can be the reason from sympatric speciation (if two individuals happen to have the same mutation and can only mate with each other)

Hawthorn flies lay eggs in small fruits of hawthorn trees, introduction of apple trees in 1800-1850 caused some of these flies to lay eggs in apple trees This is also disruptive selection Still small amount of interbreeding Can occur by chance through mutation in plants. Could have one mutation and can’t breed with the rest of the population so it breeds asexually with itself and the new offspring can breed with each other

SPECIATION (sympatric cont) Polyploidy can result in sympatric speciation Mutations causing polyploidy double the number of chromosomes They produce fertile offspring with each other but when they mate with the original population, they produce sterile offspring May be hybridized when mating with other polyploids Gametes carries two copies of each chromosome and act as typical homologous pairs Mix of chromosomes from original two species This hybrid can form a new species – but will be sterile when crossed with either of the two parents Humans: agricultural expansion and construction of roads

Eastern gray frog is a tetraploid different from its doploid cope’s gray tree frog Chromosomes of both species are a near perfect match the difference being tetraploid due to a polyploid mutation

PATTERNS of EVOLUTION Adaptive radiation: relatively rapid evolution of a single species into many new species, filling a variety of formerly empty ecological niches

PATTERNS of EVOLUTION Divergent evolution: large-scale evolution of a group into many different forms Lead to two predictable outcomes: Competition between species is minimized as new species diverge Given enough time, new species continue to evolve until most available resources are used Result is an overall increase in biodiversity

PATTERNS of EVOLUTION Convergent evolution: evolution of similar traits in distantly related species Two species are places under similar selective pressure Most obvious when comparing different geographical regions Result in similar features evolving in very distantly related organisms Two predicted outcomes: Natural selection traits will favour the evolution of similar traits in similar environments While some traits will converge in form or function, each species will retain other features that provide evidence of their distinct evolutionary past

Same defensive spine function

PATTERNS of EVOLUTION Coevolution: A process in which once species evolves in response to the evolution of another species “evolutionary arms race” One or both species may become increasingly dependent on the other A threat to once species may be a threat to the other Extinction of one may lead to the extinction of the other Pronounced in symbiotic relationships