Big Idea 1 Notes – Ch. 24 – The Origin of Species
The biological species concept • Species is a Latin word meaning “kind” or “appearance” • Remember, a species is a group of organisms that can reproduce fertile offspring • If we can’t get a new species by just breeding two different species together, how do they occur? Similarity between different species. Diversity within a species.
Speciation, the origin of new species, is at the focal point of evolutionary theory • Evolutionary theory must explain how new species originate and how populations evolve
Microevolution consists of adaptations that evolve within a population, confined to one gene pool • Macroevolution refers to evolutionary change above the species level
Two basic patterns of evolutionary change: • Anagenesis (phyletic evolution) transforms one species into another • Cladogenesis (branching evolution) is the splitting of a gene pool, giving rise to one or more new species
Reproductive Isolation • Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile hybrids • Two types of barriers: prezygotic and postzygotic
Prezygotic barriers impede mating or hinder fertilization if mating does occur: • Habitat isolation • Temporal isolation • Behavioral isolation • Mechanical isolation • Gametic isolation
Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers • Flycatchers (birds) are found in different habitats in the same area. One species prefers open woods and farmland; one frequents beech trees; one is found in alders, one in conifer woods, and one in willowy thickets.
Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Individuals of different species LE 24-4a Mating attempt Fertilization TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION HABITAT ISOLATION Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult Reduced hybrid fertility Reduced hybrid viability Hybrid breakdown Viable, fertile offspring Fertilization REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers • Courtship rituals are only understood by species that share the same communication system
Mechanical isolation: Morphological differences can prevent successful mating • the genital organs of different species are incompatible • The flowers of black sage and white sage are structurally different and are pollinated by different species of insects. In this example, each insect species pollinates flowers of only one of the sage species. Therefore, interbreeding does not occur.
Gametic isolation: the egg and sperm of different species are incompatible. • Gametic isolation is particularly important in aquatic environments because many aquatic animals release their gametes into the water, where fertilization takes place.
LE 24-4aa Prezygotic barriers impede mating or hinder fertilization if mating does occur Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Individuals of different species Mating attempt Fertilization TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION HABITAT ISOLATION
Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: • Reduced hybrid viability • Reduced hybrid fertility • Hybrid breakdown
Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development (lowered potential to survive compared to parents)
Reduced hybrid fertility:when two different species mate with one another and produce NONFERTILE offspring. • The reason for this is that the 2 species have different chromosome numbers, so when the offspring go to perform meiosis, their chromosomes cannot segregate in a manner that allows them to be fertile. • The classic example is when a horse is mated to a donkey, producing a mule. Both the horse and the donkey are obviously fertile, but the MULE is not, because it has a different chromosome number.
Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile • an interspecies hybrid is viable and fertile, but subsequent generations harbour detrimental genetic abnormalities.
Postzygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult LE 24-4ab Hybrid breakdown Reduced hybrid fertility Reduced hybrid viability Viable, fertile offspring Fertilization REDUCED HYBRID VIABILITY REDUCED HYBRID FERTILITY HYBRID BREAKDOWN
Limitations of the Biological Species Concept • The biological species concept does not apply to • Asexual organisms • Fossils • Organisms about which little is known regarding their reproduction
Other Definitions of Species • Morphological: defines a species by structural features • Paleontological: focuses on morphologically discrete species known only from the fossil record • Ecological: views a species in terms of its ecological niche • Phylogenetic: defines a species as a set of organisms with a unique genetic history
Speciation can take place with or without geographic separation • Speciation can occur in two ways: • Allopatric speciation • Sympatric speciation
LE 24-5 Allopatric speciation Sympatric speciation
Allopatric (“Other Country”) Speciation • In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations • One or both populations may undergo evolutionary change during the period of separation • To determine if allopatric speciation has occurred, reproductive isolation must have been established
LE 24-6 A. leucurus A. harrisi
Sympatric (“Same Country”) Speciation • In sympatric speciation, speciation takes place in geographically overlapping populations • No physical barrier dividing the population • Usually caused by mutations or a change in behavior
Polyploidy • Polyploidy is presence of extra sets of chromosomes due to accidents during cell division • It has caused the evolution of some plant species • An autopolyploidis an individual with more than two chromosome sets, derived from one species (potato) • An allopolyploid is a species with multiple sets of chromosomes derived from different species (wheat)
Offspring with tetraploid karyo- types may be viable and fertile—a new biological species. Failure of cell division in a cell of a growing diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue. LE 24-8 Gametes produced by flowers on this tetraploid branch are diploid. 2n 2n = 6 4n = 12 4n
Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes LE 24-9 Hybrid with 7 chromosomes Viable fertile hybrid (allopolyploid) Meiotic error; chromosome number not reduced from 2n to n Species A 2n = 4 2n = 10 Normal gamete n = 3 Normal gamete n = 3 Species B 2n = 6
Habitat Differentiation and Sexual Selection • Sympatric speciation can also result from the appearance of new ecological niches • In cichlid fish, sympatric speciation has resulted from nonrandom mating due to sexual selection • the fish have genetic variation, and some of this variation affects the fishes' ability to see different colors. Some fish have genes that enable them to see blue light better, while other fish have a red light advantage. Because of the differential penetration of light into the lake, fish with gene versions sensitizing them to blue light have an advantage in shallower waters because they can better find food and spot predators there, while fish tuned to red light have an advantage in deeper waters. So in different parts of the fishes' habitat, different color-sensitivity genes are favored by natural selection. Over many generations, if the fish don't move too much within their range, blue sensitivity will evolve to be more common among fish living near the surface and red sensitivity will become more common among fish living further down the slope.
Adaptive Radiation • Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities
Studying the Genetics of Speciation • The explosion of genomics (a discipline in genetics that sequences, assembles, and analyzes the function and structure of genomes) is enabling researchers to identify specific genes involved in some cases of speciation
The Tempo of Speciation • The fossil record includes many episodes in which new species appear suddenly in a geologic stratum, persist essentially unchanged through several strata, and then apparently disappear • Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibriumto describe periods of apparent stasis punctuated by sudden change • The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence
Macroevolutionarychanges can accumulate through many speciation events • Macroevolutionary change is cumulative change during thousands of small speciation episodes
Evolutionary Novelties • Most novel biological structures evolve in many stages from previously existing structures • Some complex structures, such as the eye, have had similar functions during all stages of their evolution
Changes in Rate and Timing • Heterochrony is an evolutionary change in the rate or timing of developmental events and is used for comparison of different organisms • For example, the timing of the development of fins in one fish species can be heterochronic to that of another resulting in different shapes or sizes of fins and other body parts. • It can have a significant impact on body shape
Allometric growth refers to different growth rates of parts of a particular organism. • Allometric growth is the proportioning that helps give a body its specific form • This can refer to changes in growth rate over time and explains why babies have a bigger head in proportion to limbs than an adult. • As a human grows, the growth rates of the head and limbs are different. • Different allometric patterns contribute to the contrasting shapes of human and chimpanzee skulls • in chimpanzee fetuses brain and head growth starts at about the same developmental stage and present a growth rate similar to that of humans, but end soon after birth. Humans, on the contrary, continue their brain and head growth several years after birth
LE 24-15b Chimpanzee adult Chimpanzee fetus Human adult Human fetus Comparison of chimpanzee and human skull growth
Paedomorphosis – the retention by an organism of juvenile or even larval traits into later life. • The rate of reproductive development accelerates compared with somatic development • The sexually mature species may retain body features that were juvenile structures in an ancestral species Ground-dwelling salamander Tree-dwelling salamander
Changes in Spatial Pattern • Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts • Homeotic genescause the development of specific structures in plants and animals • Homeoticgenes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged
The products of one class of homeotic genes called Hox genes • Hox genes provide positional information in the development of fins in fish and limbs in tetrapods • Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes
LE 24-18 Chicken leg bud Region of Hox gene expression Zebrafish fin bud
Hypothetical vertebrate ancestor (invertebrate) with a single Hox cluster First Hox duplication LE 24-19 Hypothetical early vertebrates (jawless) with two Hox clusters Second Hox duplication Vertebrates (with jaws) with four Hox clusters
Evolution Is Not Goal Oriented • The fossil record often shows apparent trends in evolution that may arise because of adaptation to a changing environment
Recent Equus Hippidion and other genera Pleistocene Nannippus Pliohippus Neohipparion Hipparion Pliocene Sinohippus Megahippus LE 24-20 Callippus Archaeohippus Merychippus Miocene Anchitherium Hypohippus Parahippus Miohippus Oligocene Mesohippus Paleotherium Epihippus Propalaeotherium Pachynolophus Orohippus Eocene Key Grazers Hyracotherium Browsers
According to the species selection model, trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics • The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular phenotype