Ch. 24 – The Origin of Species

What is a Species? Speciation – the origin of new species The biological species concept defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to produce viable, fertile offspring, but who cannot produce viable, fertile offspring with members of other species Eastern and Western Meadowlarks

How does speciation occur?

Isolation of a Species Reproductive Isolation – biological species concept states that each species is isolated by factors (barriers) that prevent interbreeding Prezygotic Barriers – impede mating between species or hinder fertilization of ova Habitat Isolation Behavioral Isolation Temporal Isolation Mechanical Isolation Gametic Isolation

Prezygotic Barriers Habitat Isolation – two species living in different habitats within the same area will not encounter one another Example: garter snakes in the same genus that live in the same area, but one lives mainly in water and one on land; they will most likely NOT mate.

Prezygotic Barriers Behavioral Isolation – Special signals used by a species to attract mates Examples: fireflies recognize signals of their own species, frogs have calls that only attract their own species, and birds have songs that only attract their own species Blue-Footed Boobies

Prezygotic Barriers Temporal Isolation – Species that breed during different times of the day, season, or year Example: Similar frog species that do not interbreed

Prezygotic Barriers Mechanical Isolation – Structural differences in genitalia or flowers Example: flowers whose pollen is carried by insects to flowers of other species; insects whose copulatory organs may not fit together, so no sperm would be transferred

Prezygotic Barriers Gametic Isolation – male and female gametes fail to attract each other or are unviable Example: aquatic species who release sperm and egg into the water where the eggs are fertilized. Gamete recognition does not occur and the sperm are unable to fertilize the egg. Similar to the mechanism used by flowers to recognize sperm (pollen) of their own species.

Postzygotic Barriers If a sperm cell from one species does fertilize an ovum of another species, then postzygotic barriers usually prevent the hybrid zygote from developing into a viable, fertile adult Reduced Hybrid Viability Reduced Hybrid Fertility Reduced Breakdown

Postzygotic Barriers Reduced Hybrid Viability – hybrid zygotes fail to develop or fail to reach sexual maturity Example: Some species of frogs in the genus Rana may interbreed, however the offspring usually do not complete development and those that do are frail.

Postzygotic Barriers Reduced Hybrid Fertility – hybrid fails to produce functional gametes Example: mules – a hybrid of a donkey and a horse. Mules cannot backbreed with either species

Postzygotic Barriers Hybrid Breakdown – offspring of hybrids have reduced viability or fertility Example: different cotton species can produce fertile hybrids, but breakdown occurs in the next generation when offspring of the hybrids die as seeds or grow into weak and defective plants

Reproductive Barrier Summary

Three Modes of Speciation Allopatric Speciation Adaptive Radiation Sympatric Speciation Polyploidy

Allopatric Speciation Geographic Barrier Interbreeding between two resulting populations is prevented Barriers include rivers to mountain ranges Gene flow is initially interrupted because they are separated in space Gene flow then diverges

Allopatric Speciation Likelihood of allopatric speciation increases with small, isolated populations; Ex. Galapagos Islands The barrier’s effect depends on the ability of the organisms to move about The gene pool can be changed dramatically by genetic drift and natural selection

Allopatric Speciation

Allopatric Speciation Antelope Squirrels Interbreeding cannot occur

Does Speciation Always Occur with Allopatric Conditions? Allopatric conditions could lead to reproductive isolation Evolutionary divergence of the ring species Adaptive radiation of the island species Origin of prezygotic barriers (ex. Fruit flies) Origin of postzygotic barriers between distant populations of the monkey flower The isolated population’s gene pool evolves by genetic drift and natural selection; reproductive isolation evolves as a byproduct

An Example of Adaptive Radiation on Island Chains - Species diverge as populations specialize for each set of geographical and ecological conditions

“Lobster Claw Syndrome”

Sympatric Speciation Formation of a new species without the presence of a geographic barrier Gene flow is reduced due to chromosomal changes and nonrandom mating

Sympatric Speciation In plants Autopolyploids have more than 2 chromosome sets Allopolyploids are hybrids of two different species with more than 2 chromosome sets

Sympatric Speciation Autopolyploidy in plants Allopolyploidy in plants

Sympatric Speciation In animals Genetic factors could cause “daughter” populations to become dependent on resources other than the parent population Ex. Cichlids in Lake Victoria show sexual dimorphism and females tend to mate only with males who have their same coloration although they are able to mate with both morphs. Over time the two cichlid populations may diverge into two species

Divergent Evolution Two or more species originate from a common ancestor. Results from Allopatric speciation, Adaptive Radiation and/or Sympatric speciation

From Speciation to Macroevolution Microevolution – change over generations in a population's allele frequencies (by genetic drift and natural selection) Speciation – population’s genetic divergence from its ancestral population that results in reproductive isolation. Macroevolution – Level of change evident over the time scale of the fossil record

Morphological Transformations The evolution of complex structures from simpler versions (ex. eyes) Exaptations – structures that evolve in one context, but become co-opted for another function (ex. birds hollow bones must have had some adaptation on the ground)

Punctuated Equilibrium Model Paleontologists rarely find gradual transitions of fossil forms. They often observe species appearing as new forms rather suddenly in a layer of rock, persisting essentially unchanged, and then disappearing from the fossil record as suddenly as they appeared.

Punctuated Equilibrium Model Punctuated Equilibrium – species diverge in spurts of relatively rapid change. Long periods of stasis punctuated by episodes of speciation.

Species Selection Species that endure the longest and reproduce the most offspring, determine the direction of major evolutionary trends Evolution is never-ending, it is still going on today Evolution is a response to interactions between organisms and their current environments. If conditions change, an evolutionary trend may cease or even reverse itself.