AP: CHAPTER 24: THE ORIGIN OF SPECIES ANSWERS 1. A SPECIES is a population or group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring. 2. PATTERNS OF SPECIATION ANAGENESIS – accumulation of changes that gradually transform a given species into one with different characteristics CLADOGENESIS – splitting of a gene pool into two or more separate pools, each giving rise to one or more new species. This is the only pattern of speciation that increases the diversity and number of different species. 3. Reproductive isolation and/or geographical isolation, which causes a reduction in gene flow. 4. BARRIERS THAT CAUSE ISOLATION PREZYGOTIC BARRIERS (“before zygote”) Barrier that prevents mating between species or hinders fertilization of the egg if different species attempt to mate Temporal Isolation – species breed during different times of the day, season, years, therefore games cannot mix Habitat Isolation – when two species occupy different habitats within the same area, therefore they will rarely encounter one another (e.g. garter snakes, 1 species lives in water, the other lives on land) Behavioural Isolation – Courtship rituals that attract mates and other behaviours that are unique to a species (e.g. blue footed boobies) Mechanical Isolation – Morphological differences prevent successful mating (e.g. snails of Japan) Gamete Isolation – Sperm of one species may not be able to fertilize the egg of another species. (e.g. sperm cannot survive in the reproductive tract of females, mechanisms prevent sperm from penetrating egg’s outer membrane) POSTZYGOTIC BARRIERS (“after zygote”) Barrier which prevents zygote from developing into a viable, fertile adult. Hybrid Inviability (or Reduced Viability) – Genes of different parent species may interact and impair hybrid’s development (e.g. hybrids are weak, frail or die) Hybrid Sterility – when hybrids produced are sterile even if vigorous. Occurs when chromosomes of parent species are different in number and/or structure, therefore meiosis does not produce normal gametes (e.g. horse + donkey = mule) 5. MODES OF SPECIATION ALLOPATRIC SPECIATION (“other country”) – gene flow disrupted when population is dived into geographically isolated subpopulations. When the environment changes and these populations come into contact, they cannot interbreed, which means they are now two different species. SYMPATRIC SPECIATION (“same country”) – geographically overlapping populations that still experience speciation due to other barriers (e.g. habitat, temporal, mechanical, behavoural isolation) 6. Harris’ Antelope Squirrel and the White-Tailed Antelope Squirrels demonstrate allopatric speciation because are separated by the Grand Canyon. This causes the gene pools to change and experience “forces” of evolution differently (e.g. mutations, sexual selection, etc.) until they are no longer able to interbreed, even if they were to come into contact with one another. 8. ADAPTIVE RADIATION – the emergence of numerous species from a common ancestor introduced into a new environment. Each newly emerging form specializes to fill an ecological niche. E.g. Galapagos finches. Island chains encourage adaptive radiation because many (like the Hawaiian Islands) were created by volcanoes. When these formed, they became gradually populated by organisms that traveled from other areas. This opened up ecological niches for these animals, which provided many opportunities for natural selection and therefore a splitting of the gene pool and thus the emergence of numerous species. 9. Reproductive isolation (without geographical isolation) and sexual selection are two intrinsic factors that result in sympatric speciation. 10. POLYPOLOIDY – mutation that occurs resulting in an extra set of chromosomes (normally cells have 2 sets – diploid). HORSE 64 chromosomes (32 pairs) + DONKEY 62 chromosomes (31 pairs) = MULE 63 chromosomes (has one unpaired chromosome during meiosis therefore mule is infertile). Because mules cannot interbreed with either a horse or donkey, they are considered to be a different species than a horse and donkey. And, because horses and donkeys cannot produce fertile offspring, they are both considered to be a different species. 13. Bottleneck effect and sexual selection demonstrate sympatric speciation. The drying of the lake caused a bottleneck effect. This drying formed new ecological niches for the “new species” of cichlids. 14. RATE OF SPECIATION GRADUALISM – species descend from a common ancestor slowly and diverge more in their morphology as they acquire new adaptations. PUNCTUATED EQUILIBRIUM – a new species changes most as it buts from “parent” species, and then changes little for the rest of its existence (stasis). 15. MICROEVOLUTION – explains how adaptations evolve in populations; is confined to a single gene pool MACROEVOLUTION – explains how a new species originates and develops; describes the divergence of gene pools 17. The Mollusk eye shows that complex structures (i.e. the human eye and the Mollusk eye) have evolved in increments from much simpler versions (i.e. photoreceptor cells) that performed the same basic function (e.g. vision). The eye of the Small Mollusk uses photoreceptors still because it is adequate for its survival and reproduction, therefore it has not evolved a more complex eye like ours. 19. EVOLUTIONARY TRENDS CONVERGENT EVOLUTION – when unrelated species occupy the same environment or habitat experience the same selective pressures and show similar adaptations. ANALOGOUS TRAITS – traits that have the same function, but different structures E.g. a whale looks like a fish because they both evolved in the same environment, but the underlying bone structure of the whale reveals a common ancestry to mammals, not to fish. PARALLEL EVOLUTION – describes two related species that have made similar evolutionary adaptations after the divergence from a common ancestor E.g. marsupial mammals of Australia and the placental mammals of North America – there are striking similarities between them because they share a common ancestor and evolved in similar environments CO-EVOLUTION – two evolutionary set of adaptations of two interacting species. All-predator prey relationships are an example. E.g. the milkweed plant contains poisons that deter herbivores from eating them. The butterfly lays its eggs in the weed and absorbs the poisonous chemicals from the plant. They store the poison in their tissues. The poison, which is present in the adult butterfly, makes the butterfly toxic to any animal that eats it.