1. SPECIATION and PATTERNS OF EVOLUTION
READ 8.2 & 8.3 XI

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

6. 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

7. 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

8. 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

9. 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

12. 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

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

15. 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)

16. Hawthorn flies lay eggs in small fruits of hawthorn trees, introduction of apple trees in 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

17. 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

18. 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

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

21. 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

23. 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

24. Same defensive spine function

25. 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