Evolutionary Patterns, Rates and Trends Chapter 22 / 20

Natural Selection Results in adaptations of a population to the biotic and abiotic environment Evolution by Natural Selection requires: Variation Inheritance Differential adaptiveness Differential reproduction

Speciation Speciation process by which one species splits into two or more explains the features shared between organisms due to inheritance from their recent common ancestor forms a conceptual bridge between microevolution and macroevolution Microevolution consists of changes in allele frequency in a population over time Macroevolution refers to broad patterns of evolutionary change above the species level 3

Speciation & Natural Selection Natural selection can lead to speciation New biological species evolve Changes in a gene pool allele and genotypic frequencies Speciation can also occur as a result of other micro-evolutionary processes Genetic drift Founder effect Bottleneck effect Mutation

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 populations together genetically 5

(a) Similarity between different species Figure 22.2a The biological species concept is based on the potential to interbreed rather than on physical similarity (part 1: similarity) (a) Similarity between different species 6

(b) Diversity within a species Figure 22.2b The biological species concept is based on the potential to interbreed rather than on physical similarity (part 2: diversity) (b) Diversity within a species 7

Reproductive Isolation Existence of biological barriers that impede two species from producing viable, fertile offspring Arises as a by-product of genetic change Speciation is the attainment of reproductive isolation Cornerstone of the biological species concept Can be classified by whether barriers act before or after fertilization

Pre-Zygotic Isolating Mechanisms 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

Pre-Zygotic Isolating Mechanisms Habitat Isolation Two species encounter each other rarely, or not at all, because they occupy different habitats Temporal Isolation reproduce at different times of the year Mechanical Isolation animal genitalia or plant floral structures are incompatible Behavioral Isolation courtship behaviors

Reproductive Isolation Albatross courtship

Pre-Zygotic Isolating Mechanisms Gamete Isolation sperm of 1 species may not be able to survive in the reproductive tract of another species Polyploidy occurs in plants can’t reproduce with plants that have polyploidy

Post Zygotic Isolating Mechanisms Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult by Reduced hybrid viability / Zygote mortality Genes of the different parent species may interact and impair the hybrid’s development or survival Reduced hybrid fertility Even if hybrids are vigorous, they may be sterile Hybrid breakdown Some first-generation hybrids are fertile when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

Mechanisms of Speciation Speciation can occur in two ways Allopatric speciation Sympatric speciation 14

Allopatric speciation: forms a new species while Figure 22.5 Two main modes of speciation Allopatric speciation: forms a new species while geographically isolated. (a) (b) Sympatric speciation: a subset forms a new species without geographic separation. 15

Allopatric Speciation Allopatric speciation (other country) gene flow is interrupted when a population is divided into geographically isolated subpopulations the definition of a geographic barrier depends on the ability of a population to disperse a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen 16

Allopatric Speciation Separate populations may evolve independently through mutation, natural selection, and genetic drift Reproductive isolation may arise as a result of genetic divergence For example, mosquitofish in the Bahamas comprise several isolated populations in different ponds 17

body shape that enables rapid bursts of speed Under low predation: Figure 22.6 Reproductive isolation as a by-product of selection Under high predation: body shape that enables rapid bursts of speed Under low predation: body shape that favors long, steady swimming 18

Allopatric speciation on an archipelago Models of Speciation Allopatric speciation on an archipelago

Sympatric Speciation Sympatric Speciation (same country) speciation takes place in populations that live in the same geographic area occurs when gene flow is reduced between groups that remain in contact through factors including Habitat differentiation Sexual selection Polyploidy

Polyploidy Polyploidy presence of extra sets of chromosomes due to accidents during cell division much more common in plants than in animals Autopolyploid an individual with more than two chromosome sets, derived from one species the offspring of matings between autopolyploids and diploids have reduced fertility 21

Polyploidy Allopolyploid a species with multiple sets of chromosomes derived from different species Allopolyploids cannot interbreed with either parent species 22

Parapatric Speciation: Hybrid zones region in which members of different species mate and produce hybrids Hybrids are the result of mating between species with incomplete reproductive barriers Hybrids often have reduced fitness compared with parent species 23

B. variegata-specific allele Distance from hybrid zone center (km) Figure 22.11 Fire-bellied toad range Fire-bellied toad, Bombina bombina Hybrid zone Yellow-bellied toad range 0.99 Hybrid zone 0.9 Figure 22.11 A narrow hybrid zone for Bombina toads in Europe B. variegata-specific allele Frequency of 0.5 Yellow-bellied toad range Fire-bellied toad range Yellow-bellied toad, Bombina variegata 0.1 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km) 24

Parapatric Speciation BULLOCK’S ORIOLE BALTIMORE ORIOLE HYBRID ZONE

Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes: Reinforcement occurs when hybrids are less fit than the parent species Natural selection strengthens (reinforces) reproductive barriers, and, over time, the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than for allopatric species 26

Fusion when parent species fuses into a single species may occur if hybrids are as fit as parents, allowing substantial gene flow between species Figure 22.13 Fusion: the breakdown of reproductive barriers 27

hybrids continue to be produced over time Stability hybrids continue to be produced over time 28

Gene flow Population Barrier to gene flow Figure 22.12-1 Formation of a hybrid zone and possible outcomes for hybrids over time (step 1) Population Barrier to gene flow 29

Isolated population diverges. Gene flow Population Barrier to Figure 22.12-2 Formation of a hybrid zone and possible outcomes for hybrids over time (step 2) Population Barrier to gene flow 30

Isolated population diverges. Hybrid zone Gene flow Population Figure 22.12-3 Formation of a hybrid zone and possible outcomes for hybrids over time (step 3) Population Barrier to gene flow Hybrid individual 31

Isolated population diverges. Possible outcomes: Hybrid zone Reinforcement Fusion Gene flow Figure 22.12-4 Formation of a hybrid zone and possible outcomes for hybrids over time (step 4) Population Barrier to gene flow Hybrid individual Stability 32

Evolutionary tree diagram Evolutionary Trees Evolutionary tree diagram

Gradual Model Speciation model in which species emerge through many small morphological changes that accumulate over a long time period

Punctuated Model Speciation model in which most changes in morphology are compressed into brief period near onset of divergence

Phylogeny The scientific study of evolutionary relationships among related species Practical applications Allows predictions about the needs or weaknesses of one species on the basis of its known relationship to another The discipline of systematics classifies organisms and determines their evolutionary relationships

TAXONOMY Early taxonomists classified all species as either plants or animals Later, five kingdoms were recognized: Monera (prokaryotes) Protista Fungi Plantae Animalia 37

TAXONOMY More recently, the three-domain system has been adopted Archaea Eubacteria single celled prokaryotes Eukarya dominated by multicellular organisms consists of Fungi, Plants and Animals The three-domain system is supported by data from many sequenced genomes

Euglenozoans Forams Diatoms Ciliates Domain Eukarya Red algae Green algae Land plants Amoebas Fungi Animals Nanoarchaeotes Archaea Domain Methanogens COMMON ANCESTOR OF ALL LIFE Thermophiles Figure 20.20 The three domains of life Proteobacteria (Mitochondria)* Chlamydias Spirochetes Domain Bacteria Gram-positive bacteria Cyanobacteria (Chloroplasts)* 39

Binomial System Devised by Carl von Linneas Each species has a two-part Latin name First part is generic Second part is specific name 40

Higher Taxa Kingdom Phylum Class Order Family Genus Species 41

Figure 20.3 Linnaean classification 42

Cladistics Cladistics classifies organisms by common descent (a) Monophyletic group (clade) (b) Paraphyletic group (c) Polyphyletic group A A A 1 1 B Group I B B Group III C C C D D D E E Group II E 2 2 F F F G G G Cladistics classifies organisms by common descent A clade is a group of species that includes an ancestral species and all its descendants 43

A valid clade is monophyletic, signifying that it consists of the ancestor species and all its descendants A 1 B Group I C D Figure 20.10a Monophyletic, paraphyletic, and polyphyletic groups (part 1: monophyletic) E F G 44

A paraphyletic grouping consists of an ancestral species and some, but not all, of the descendants A B C D Figure 20.10b Monophyletic, paraphyletic, and polyphyletic groups (part 2: paraphyletic) E Group II 2 F G 45

A polyphyletic grouping consists of various taxa with different ancestors A 1 B Group III C D Figure 20.10c Monophyletic, paraphyletic, and polyphyletic groups (part 3: polyphyletic) E 2 F G 46

Shared Ancestral and Shared Derived Characters In comparison with its ancestor, an organism has both shared and different characteristics A shared ancestral character is a character that originated in an ancestor of the taxon A shared derived character is an evolutionary novelty unique to a particular clade 47

Phylogenetic Trees with Proportional Branch Lengths In some trees, the length of a branch can reflect the number of genetic changes that have taken place in a particular DNA sequence in that lineage Drosophila Lancelet Zebrafish Frog Chicken Human Mouse 48

In other trees, branch length can represent chronological time, and branching points can be determined from the fossil record Drosophila Lancelet Zebrafish Frog Figure 20.13 Branch lengths can indicate time Chicken Human Mouse PALEOZOIC MESOZOIC CENOZOIC 251 65.5 Present 542 Millions of years ago 49

Patterns of Change in a Lineage Cladogenesis Branching pattern Lineage splits, isolated populations diverge Anagenesis No branching Changes occur within single lineage Gene flow throughout process