The Origin of Species Mom, Dad… There’s something you need to know… I’m a MAMMAL!

“That mystery of mysteries…” Darwin never actually tackled how new species arose… In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species Animation: Macroevolution 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 Both in space and time, we seem to be brought somewhat near to that great fact —that mystery of mysteries— the first appearance of new beings on this Earth.

So…what is a species? defined by Ernst Mayr Biological species concept defined by Ernst Mayr population whose members can interbreed & produce viable, fertile offspring Distinct species: songs & behaviors are different enough to prevent interbreeding Humans re so diverse but considered one species, whereas these Meadowlarks look so similar but are considered different species. Meadowlarks Similar body & colorations, but are distinct biological species because their songs & other behaviors are different enough to prevent interbreeding Eastern Meadowlark Western Meadowlark

Not the only “species” Other species definitions: Morphological Ecological Paleological Species is a Latin word meaning “kind” or “appearance” Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations Gene flow between populations holds the phenotype of a population together The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) Other species concepts emphasize the unity within a species rather than the separateness of different species The morphological species concept defines a species by structural features It applies to sexual and asexual species but relies on subjective criteria The ecological species concept views a species in terms of its ecological niche It applies to sexual and asexual species and emphasizes the role of disruptive selection The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

How and why do new species originate? Species are created by a series of evolutionary processes populations become isolated geographically isolated reproductively isolated isolated populations evolve independently Isolation allopatric geographic separation sympatric still live in same area In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations The definition of barrier depends on the ability of a population to disperse Separate populations may evolve independently through mutation, natural selection, and genetic drift Regions with many geographic barriers typically have more species than do regions with fewer barriers In allopatric speciation, geographic isolation restricts gene flow between populations Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations Even if contact is restored between populations, interbreeding is prevented In sympatric speciation, speciation takes place in geographically overlapping populations In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species Sympatric speciation can result from polyploidy, natural selection, or sexual selection

Allopatric speciation Fig. 24-6 Allopatric speciation A. harrisi A. leucurus Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

Allopatric speciation Fig. 24-7 Allopatric speciation Mantellinae (Madagascar only): 100 species Rhacophorinae (India/Southeast Asia): 310 species Other Indian/ Southeast Asian frogs 100 80 60 40 20 1 2 3 Figure 24.7 Allopatric speciation in frogs Regions with many geographic barriers typically have more species than do regions with fewer barriers Millions of years ago (mya) 1 2 3 India Madagascar 88 mya 65 mya 56 mya

Fig. 24-8 Reproductive isolation between populations generally increases as the distance between them increases Allopatric speciation 2.0 1.5 Degree of reproductive isolation 1.0 0.5 Figure 24.8 Variation in reproductive isolation with distance between populations of dusky salamanders Barriers to reproduction are intrinsic; separation itself is not a biological barrier 50 100 150 200 250 300 Geographic distance (km)

Allopatric speciation Fig. 24-9 EXPERIMENT Allopatric speciation Initial population Some flies raised on starch medium Some flies raised on maltose medium Mating experiments after 40 generations RESULTS Female Female Starch Starch Starch Maltose population 1 population 2 Figure 24.9 Can divergence of allopatric populations lead to reproductive isolation? Starch population 1 Starch 22 9 18 15 Male Male Maltose 8 20 12 15 Starch population 2 Mating frequencies in experimental group Mating frequencies in control group

Polyploidy Sympatric speciation Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division An autopolyploid is an individual with more than two chromosome sets, derived from one species Polyploidy is much more common in plants than in animals Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

Sympatric speciation 2n = 6 4n = 12 2n 4n Failure of cell Fig. 24-10-3 Sympatric speciation 2n = 6 4n = 12 2n 4n Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Offspring with tetraploid karyotypes may be viable and fertile. Fig 24.10 Sympatric speciation by autopolyploidy in plants

Sympatric speciation An allopolyploid is a species with multiple sets of chromosomes derived from different species

Species B 2n = 4 Unreduced gamete with 4 chromosomes Unreduced gamete Fig. 24-11-4 Species B 2n = 4 Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Hybrid with 7 chromosomes Meiotic error Normal gamete n = 3 Viable fertile hybrid (allopolyploid) 2n = 10 Figure 24.11 One mechanism for allopolyploid speciation in plants Normal gamete n = 3 Species A 2n = 6

Habitat Differentiation Sympatric speciation Sympatric speciation can also result from the appearance of new ecological niches For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

Fig. 24-13 A hybrid zone is a region in which members of different species mate and produce hybrids EUROPE Fire-bellied toad range Hybrid zone Fire-bellied toad, Bombina bombina Yellow-bellied toad range Yellow-bellied toad, Bombina variegata 0.99 0.9 Fig 24.13 A narrow hybrid zone for B. variegata and B. bombina in Europe A hybrid zone can occur in a single band where adjacent species meet Hybrids often have reduced fitness compared with parent species The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region Allele frequency (log scale) 0.5 0.1 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km)

Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: Strengthening of reproductive barriers Weakening of reproductive barriers Continued formation of hybrid individuals

Isolated population diverges Fig. 24-14-4 Isolated population diverges Possible outcomes: Hybrid zone Reinforcement OR Fusion Gene flow Hybrid OR Figure 24.14 Formation of a hybrid zone and possible outcomes for hybrids over time The reinforcement of barriers occurs when hybrids are less fit than the parent species Over time, the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species If hybrids are as fit as parents, there can be substantial gene flow between species If gene flow is great enough, the parent species can fuse into a single species Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone In cases where hybrids have increased fitness, local extinctions of parent species within the hybrid zone can prevent the breakdown of reproductive barriers Barrier to gene flow Population (five individuals are shown) Stability

PRE-reproduction barriers Obstacle to mating or to fertilization if mating occurs geographic isolation ecological isolation temporal isolation Prezygotic barriers block fertilization from occurring by: Impeding different species from attempting to mate Preventing the successful completion of mating Hindering fertilization if mating is successful 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

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

Mechanical isolation: Morphological differences can prevent successful mating Bradybaena with shells spiraling in opposite directions

Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

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 Ensatina hybrid

Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile Donkey Mule (sterile hybrid) Horse

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 Hybrid cultivated rice plants with stunted offspring (center)

Figure 24.4 Reproductive barriers Prezygotic barriers Postzygotic barriers Habitat Isolation Temporal Isolation Behavioral Isolation Mechanical Isolation Gametic Isolation Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Individuals of different species Mating attempt Viable, fertile offspring Fertilization (a) (c) (e) (f) (g) (h) (i) (l) (d) (j) (b) Figure 24.4 Reproductive barriers (k)

Rate of Speciation Current debate: Does speciation happen gradually or rapidly? Gradualism Charles Darwin Charles Lyell Punctuated equilibrium Stephen Jay Gould Niles Eldredge Niles Eldredge Curator American Museum of Natural History

Gradualism Gradual divergence over long spans of time assume that big changes occur as the accumulation of many small ones

Punctuated Equilibrium Rate of speciation is not constant rapid bursts of change long periods of little or no change species undergo rapid change when they 1st bud from parent population

Stephen Jay Gould (1941-2002) Harvard paleontologist & evolutionary biologist punctuated equilibrium prolific author popularized evolutionary thought Stephen Jay Gould (September 10, 1941 – May 20, 2002) was a New York-born American paleontologist, evolutionary biologist, and historian of science. He was the most influential and widely read writer of popular science of his generation. He served as a member of the faculty at Harvard University beginning in 1967. Toward the end of his life he served as the Alexander Agassiz Professor of Zoology at that university. He helped Niles Eldredge develop Eldredge's theory of punctuated equilibrium 1972, wherein evolutionary change occurs relatively rapidly in comparatively brief periods of environmental stress, separated by longer periods of evolutionary stability. According to Gould, this overthrew a key tenet of neo-Darwinism; according to most evolutionary biologists, the theory was an important insight but merely modified neo-Darwinism in a way fully compatible with what had been known before. Gould became widely known through his popular science essays in Natural History magazine, collections of essays like The Panda's Thumb and The Flamingo's Smile, and extended studies like Wonderful Life and others.Gould was an emphatic advocate of evolution and wrote prolifically on the subject, trying to communicate his understanding of contemporary evolutionary theory to a wide audience. A recurring theme in his writings is the history and development of evolutionary (and pre-evolutionary) thinking. His early research involved the study of the fossil record of snails (detailed in one of his essays). He was also an enthusiastic baseball fan and made frequent references to the sport (including an entire essay) and a very wide range of other topics.

1. A biologist discovers two populations of wolf spiders whose members appear identical. Members of one population are found in the leaf litter deep within the woods. Members of the other population are found in the grass at the edge of the woods. The biologist decides to designate the members of the two populations as two separate species. Which species concept is this biologist most closely utilizing? Ecological Biological Morphological Pluralistic genealogical Answer: a Source: Barstow - Test Bank for Biology, Seventh Edition, Question #28 34

2. The formation of a land bridge between North and South America about three million years ago should have resulted in which of the following? I. allopatry of marine populations that were previously sympatric II. sympatry of marine populations that were previously allopatric III. sympatry of terrestrial populations that were previously allopatric I only II only III only I and II I and III Answer: e Source: Barstow - Test Bank for Biology, Seventh Edition, Question #33 35

A. Geographical isolation B. Behavioral isolation For each of the following situations, choose the best answer from the following types of isolating mechanisms: A. Geographical isolation B. Behavioral isolation C. Mechanical isolation D. Temporal isolation E. Reproductive isolation Two species of rats live on different islands. Refers to all species that adhere to the biological definition. Two species of fruit fly have different courtship rituals. The pollen from a Dogwood tree can not penetrate the pine cone of a Douglas Fir. 3. A 4. E 5. B 6. C