Chapter 24 – The Origin of Species

Macroevolution Macroevolution: evolutionary change above the species level, the formation of new taxonomic groups

What is a Species? Biological Species Concept (Ernst Mayr) - a population or group of populations whose members have the potential to interbreed and produce viable, fertile offspring (genetic exchange is possible and that is genetically isolated from other populations)

Reproductive Barriers Biological factors that impede production of viable, fertile hybrids. Both prezygotic and postzygotic barriers. Prezygotic barriers impede mating or hinder fertilization if mating does occur Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult

Prezygotic Barriers

Prezygotic Barriers Habitat isolation: 2 species that occupy different habitats within the same area will rarely, if ever, come into contact with each other Example: 2 garter snakes (genus Thamnophis) live in same area but (a) lives in water while (b) lives on land.

Prezygotic Barriers Temporal Isolation: species that breed during different times of the day, seasons, or years; prevents gamete mixing Example: habitats of the the eastern spotted skunk (c) and the western spotted skunk (d) overlap but (c) mates in late winter while (d) mates in late summer.

Prezygotic Barriers Behavioral Isolation: 2 species have different courtship rituals thereby attracting different mates Example: the blue footed boobies – specific dance, bird calls, insect mating dances Video

Prezygotic Barriers: mating attempt Mechanical Isolation: when mating is attempted, but morphological differences prevent successful mating Example: 2 species of snail whose shells spiral in different directions, prevent genital openings from being aligned Gametic Isolation: one species cannot fertilize the eggs of another species Example: common in sea creatures who discharge their gametes into the water column, like sea urchins and sea anemones

Postzygotic Barriers Reduced hybrid viability: genes of different parent species may interact and impair the hybrid’s development; offspring usually die in the embryonic stage of development Example: salamanders in genus Ensatina can hybridize, but hybrids do not complete development, to frail.

Postzygotic Barriers Reduced hybrid fertility: hybrid are sterile Example: male donkey (i) and female horse (j) = a mule (k) which are sterile

Postzygotic Barriers Hybrid breakdown: first generation hybrids are viable and fertile, but the second generation offspring are feeble or sterile Example: cultivated rice, hybrid offspring are fertile (l, left and right), but the F2 generation offspring with too many localized recessive alleles are small and sterile (l, center)

Biological Species Concept There are some limitations with the biological species concept: No method to measure reproductive success of fossils or asexual organisms Difficult to determine ability of one organism to mate with another Other definitions of a species: Morphological species concept – uses structural features Palentological species concept – morphology of fossils Ecological species concept – ecological niche Phylogenetic species concept – genetic history None of these are perfect, but a combination of each (especially biological and morphological) is most accepted.

Modes of Speciation Must have some type of geographic separation of populations for speciation to occur; 2 different modes 1 – Allopatric Speciation 2 – Sympatric Speciation

Allopatric Speciation “other country” or “other homeland” Populations segregated by a geographical barrier Can result in adaptive radiation Example: white-tailed antelope squirrel and Harris’s antelope squirrel – separated by Grand Canyon

Sympatric Speciation “same country” Speciation takes place in overlapping populations Can result from various mechanisms including chromosomal changes (polyploidy in plants) and non-random mating that reduces gene flow (cichlids). Pundamilia pundamilia Pundamilia nyererei

Adaptive Radiation The evolution of many diversely adapted species from a common ancestor

Tempo of Speciation Gradual model – gradual divergence from a common ancestor Punctuated model – changes in short bursts, mostly when it buds from a parent species, with long stretches of little change Helped explain the non-gradual appearance of species in the fossil record

Hybrid Organism Assignment Identify 10 different hybrid organisms. For each organism identify The mother and father species The post zygotic barrier that causes them to be a hybrid and not all one species One interesting fact about the hybrid or mating pair You may identify/classify an additional 2 hybrids for extra credit Assignment due on

Speciation Speciation: the origin of new species Anagenesis (phyletic evolution): accumulation of heritable changes Cladogenesis (branching evolution): budding of new species from a parent species that continues to exist (basis of biological diversity)