Speciation: How Species Form

Lesson 6

SPECIATION• Microevolution: changes in allele frequencies and

phenotypic traits within populations and species; can result in new species.

• Speciation: The evolutionary formation of new species.

• Species: members of groups or populations that interbreed or have the ability to interbreed with each other under natural conditions.– reproductively isolated from other groups– evolve independently

Reproductive Isolation• Whether or not different species interbreed successfully in the

wild can be difficult to examine.

• Differentiating species is mainly done by physical appearance/morphology, however, subspecies (or races) are morphologically distinct subpopulations that can interbreed.

• Where morphology is not possible, behaviour or reproductive isolating mechanisms are used.

• A reproductive isolating mechanism is any behavioural, structural or biochemical trait that prevents individuals of different species from reproducing successfully together.

Means of Reproductive Isolation:PREZYGOTIC (prevention of interspecies mating or

fertilization)

• Prevention of mating:

– Ecological isolation: different habitat, niches,

– Temporal isolation: different reproductive cycles (timing of day, seasons)

– Behavioural isolation: different signals

Which type is this?

Means of Reproductive Isolation:

• Prevention of fertilization: – Mechanical isolation: structural differences

in reproductive organs (arthropod, flowers)– Gametic isolation: molecular recognition of

sperm and egg (water animals)

Different penis structures of damselflies

Means of Reproductive Isolation:

• POSTZYGOTIC (prevent maturation and reproduction in offspring from interspecies reproduction)

– Zygotic Mortality: different species can mate but no embryos develop to maturity,

– Hybrid Inviability: baby hybrids aren't viable, don't live long

– Hybrid Infertility: baby hybrid viable but not fertile.

Reproductive isolation and Speciation

• Reproductive isolation may lead to speciation.

• The gene pool is isolated, any mutation and selection that occurs is no longer shared;

• any significant evolutionary changes that occur in either population (new or old) will result in the formation of separate species.

In class work

• Pg 363, Q 13-18

Modes of Speciation• Sympatric Speciation: evolution of populations

within the same geographic area into separate species. (ex. grey tree frogs)

• Chromosomal changes (plants) and non-random mating (animals) alter gene flow

• More common in plants

• Polyploidy (3 or more sets of chromosomes) can lead to speciation

Modes of Speciation

• Allopatric Speciation: evolution of populations into separate species as a result of geographic isolation. Ex. water, canyon, mountain range, human construction (dams, highways, canals)

• Darwin’s finches pg 336

In class work

• Pg 365, Q 19-24

Adaptive Radiation• Adaptive radiation is an increase in the

morphological (what they look like) and ecological (where they live) diversity of a species eventually resulting in the formation of new species.

Adaptive Radiation

• It usually occurs very rapidly when a species colonizes a new environment (Darwin’s Finches) OR by survivors after a massive extinction event (Mammals after the extinction of the Dinosaurs).

Adaptive Radiation

• Darwin’s Finches: An ancestral finch population got blown off the mainland of South America onto the Galapagos Islands. Over time that finch species evolved to fulfill all the niches on the islands and thereby give rise to the variety of finches seen on the islands.

• Mammals after the extinction of the Dinosaurs: With Dinosaurs out of the way, mammals were able to grow bigger and fill all the niches vacated by the larger reptiles, which explains the wide diversity of forms we see in mammals today.

Adaptive Radiation• It supports evolution by showing that

groups of organisms (i.e. mammals) are all related to each other and came from a common ancestor that inhabited new environments and evolved to adapt to these environments.

• All mammalian forelimbs contain the same bones which shows that they all evolved from a common ancestor.

Divergent and Convergent Evolution

• Divergent– Species that were once similar diverge or become

increasingly distinct– Eg/ Darwin’s finches

• Convergent– A pattern of evolution in which similar traits arise

because different species have independently adapted to similar environmental conditions

– Eg/ bats and birds

CONVERGENT EVOLUTIONCONVERGENT EVOLUTION• SpeciesSpecies from different evolutionary branchesevolutionary branches may

come to resemble one another if they live in very very similar environments.similar environments.

• Example:Example:1.1. Sidewinder (Mojave Desert) andSidewinder (Mojave Desert) and

Horned Viper (Middle East Desert)Horned Viper (Middle East Desert)

CONVERGENT EVOLUTION - EXAMPLE

• 1.1. Ostrich (Africa) and Emu Ostrich (Africa) and Emu (Australia).(Australia).

Macroevolution• Large scale evolutionary change significant enough to

warrant the classification of groups into genera or even higher-taxa level.

• For example

– the separation of eubacteria and archaebacteria.

– Cambrian explosion – rapid speciation and diversification in the animal kingdom for about 40 million years starting 565 mya. Early arthropods, echinoderms, molluscs, primitive chordates

• Burgess Shale in B.C.

• 2 major theories for macroevolution / the rate of evolution:

Theory of Gradualism• The accumulation of many small and ongoing changes

and processes.– When new species first evolve, they appear very similar to

the originator species and only become more distinctive, as natural selection and genetic drift act on both species.

– One would expect then to find many transitional species in the fossil record.

– This is explained by an incomplete fossil record, and the possibility that intermediate forms were not preserved.

Theory of Punctuated Equilibrium

• Eldredge and Gould (1972) rejected the explanation of the incomplete fossil record and proposed the Theory of Punctuated Equilibrium

• Relatively rapid spurts of change followed by long periods of little or no change.– Species evolve very rapidly in evolutionary time– Speciation usually occurs in small isolated

populations and thus intermediate fossils (transitional species) are very rare.

– After the initial burst of evolution, species do not change significantly over long periods of time.

Which one do we use today?• Today both theories are needed to understand the fossil

record.• It is widely accepted that both gradual and rapid evolutionary

processes are at work.• Example:

– Before a major extinction event, an environment may be host to many well adapted species and evolutionary change would be slow.

– An environmental crisis resulting in extinction of many species would leave many niches empty.

– Surviving species have many new opportunities and experience disruptive selection, evolving into many species which fill the niches.

– Once they become well adapted, stabilizing selection kicks in again and more gradual change occurs.

Consequences of Human Activities

• Human-made barriers may prevent gene flow between the split populations

• Isolated populations may undergo adaptive radiation

• Severely fragmented populations may eventually die out if there is insufficient genetic diversity

• E.g. giant panda in China

Speciation and Mass Extinctions

• Five major mass extinctions have been identified

In class work

• Pg 370, Q 25-30

• Final Evolution Quiz Wed May 16

• Evolution unit test Wed May 23

• Assignment, pg 375, due Wednesday May 23