Albia Dugger • Miami Dade College

Chapter 18Organizing Information

About Species

18.1 Bye Bye Birdie

• Over millions of years, unique forms and behaviors evolved in many different lineages of Hawaiian finches – the Hawaiian honeycreepers

• Variations in traits allowed the birds to exploit special opportunities presented by their island habitats

• Polynesians arrived on the Hawaiian Islands around 1000 A.D. – followed by Europeans in 1778

The Hawaiian Honeycreepers

• By 1778 at least 43 honeycreeper species that had thrived on the Hawaiian islands before humans arrived were extinct

• Conservation efforts began in the 1960s, but 26 more species have since disappeared – today, 35 of the remaining 68 species are endangered

• They are pressured by invasive, non-native species of plants and animals, and by rising global temperatures that allow disease-bearing mosquitoes to invade higher-altitude habitats

Endangered: The Palila

Endangered: The Akekee

Extinct: The Poouli

18.2 Phylogeny

• Evolutionary history can be reconstructed by studying shared, heritable traits

• Phylogeny is the evolutionary history of a species or a group of species – a kind of genealogy that follows a lineage’s evolutionary relationships through time

Characters

• Each species bears evidence of its own unique evolutionary history in its characters

• A character is any heritable physical, behavioral, or biochemical feature that can be measured or quantified

• Examples: Number of segments in a backbone, the nucleotide sequence of ribosomal RNA

Table 18-1 p296

Traditional Classification

• Traditional classification groups organisms based on shared characters, such as feathers in birds

• Traditional classification does not always reflect phylogeny – species that appear very similar are not necessarily closely related

Evolutionary Classification

• Evolutionary biologists try to pinpoint the source of shared characters: a common ancestor

• Common ancestry is determined by derived traits – characters present in a group, but not in that group’s ancestors

• A group whose members share one or more defining derived traits is called a clade – a monophyletic group consisting of an ancestor with a derived trait, and all of its descendants

Cladistics

• Making hypotheses about evolutionary relationships among clades is called cladistics

• Parsimony analysis is used to find the simplest and most likely evolutionary pathway – the one in which defining derived traits evolved the fewest number of times

Parsimony Analysis

Parsimony Analysis

Parsimony Analysis

Cladograms

• Cladistic analysis produces a cladogram – an evolutionary tree that diagrams evolutionary trends and patterns

• Data from an outgroup (a species not closely related to any member of the group) may be included to “root” the tree

• Each line represents a lineage, which may branch into two lineages at a node – a common ancestor of two lineages

• Every branch on a cladogram is a clade; the two lineages that emerge from a node are sister groups

Figure 18-3 p297

earthworm

multicellular with a backbone, legs, and hair

multicellular with a backbone and legs

multicellular with a backbone

multicellular

human

mouse

lizard

tuna

earthworm

human

mouse

lizard

tuna

ANIMATED FIGURE: Interpreting a cladogram

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Take-Home Message: How do evolutionary biologists study life’s diversity?

• Evolutionary biologists study phylogeny to understand how all species are connected by shared ancestry

• A clade is a monophyletic group whose members share one or more derived traits; cladistics is a method of making hypotheses about evolutionary relationships among clades

• Cladograms and other evolutionary tree diagrams are hypotheses based on our best understanding of the evolutionary history of a group of organisms

18.3 Comparing Form and Function

• Physical similarities are often evidence of shared ancestry, but sometimes a trait evolves independently in two groups

• In many cases, comparative morphology can be used to unravel evolutionary relationships

Morphological Divergence

• Homologous structures are similar body parts in separate lineages that evolved in a common ancestor

• Homologous structures may be used for different purposes, but the same genes direct their development

• Change from the body form of a common ancestor is an evolutionary pattern called morphological divergence

• Example: Vertebrate forelimbs vary in size, shape, and function, but are alike in structure

Figure 18-4 p298

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Morphological Convergence

• Analogous structures are body parts that look alike but did not evolve in a shared ancestor – they evolved independently in lineages subject to the same environmental pressures

• The independent evolution of similar body parts in different lineages is called morphological convergence

• Example: Bird, bat, and insect wings all perform the same function, but are derived from different structures

Morphological Convergence

Take-Home Message: What does comparative morphology reveal about phylogeny?

• In morphological divergence, a body part inherited from a common ancestor becomes modified differently in different lines of descent (homologous structures)

• In morphological convergence, body parts that appear alike evolved independently in different lineages, not in a common ancestor (analogous structures)

18.4 Comparing Biochemistry

• The kind and number of biochemical similarities among species are clues about evolutionary relationships

Molecular Clocks

• A molecular clock is used to estimate how long ago two lineages diverged by comparing DNA or protein sequences

• Over time, neutral mutations that have no effect on survival or reproduction accumulate at a constant rate

• The accumulation of neutral mutations in the DNA of a lineage act as a molecular clock

• The number of differences between genomes can be used to estimate the relative times of divergence

DNA and Protein Sequence Comparisons

• Some essential genes are highly conserved (their DNA sequences have changed very little over evolutionary time) – other genes are not conserved at all

• Comparing the nucleotide sequence of a gene or the amino acid sequence of a protein can provide evidence of an evolutionary relationship

• Generally, two species with many identical proteins are likely to be close relatives – the number of amino acid differences give us an idea of evolutionary relationships

Comparison of an Amino Acid Sequence

DNA Comparisons

• DNA from nuclei, mitochondria, and chloroplasts can be used in nucleotide comparisons

• Mitochondria are inherited intact from a single parent, usually the mother – any differences in mitochondrial DNA sequences between maternally related individuals are due to mutations, not genetic recombination during fertilization

Comparison of a DNA Sequence

Cladogram Based On DNA Sequence

Take-Home Message: How does biochemistry reflect evolutionary history?

• Mutations change the nucleotide sequence of a lineage’s DNA over time

• Lineages that diverged long ago have more differences between their DNA and amino acid sequences than do lineages that diverged more recently

18.5 Comparing Patterns of Development

• Similar patterns of embryonic development are an outcome of highly conserved master genes

• A mutation in a master gene typically halts development

Similar Forms in Plants

• Homeotic genes encode transcription factors that determine details of body form during embryonic development

• Example: A floral identity gene, Apetala1, affects petal formation across many different lineages – it is likely that this gene evolved in a shared ancestor

Developmental Comparisons in Animals

• The embryos of many vertebrate species develop in similar ways – directed by the very same genes

• Differences are brought about by variations in expression patterns of master genes that govern development

• Example: All vertebrates go through a stage in which they have four limb buds, a tail, and a series of somites – divisions of the body that give rise to a backbone

Comparison of Vertebrate Embryos

Hox Genes

• Hox genes are homeotic genes of animals

• The pattern of expression of Hox genes determines the identity of particular zones along the body axis

• Hox genes occur in clusters on a chromosome, in the order in which they are expressed in a developing embryo

• Example: Legs develop wherever the antennapedia gene is expressed in an embryo

Expression of the Antennapedia Gene

Vertebrate Hox Genes

• In vertebrates, expression of the Hoxc6 gene causes ribs to develop on vertebrae of the back – not the neck or tail

• The Dlx gene encodes a transcription factor that signals embryonic cells to form buds that give rise to appendages

• Hox genes suppress Dlx expression in all parts of an embryo that will not have appendages

Expression of the Hoxc6 Gene

Persistent Juvenile Features

• A chimpanzee skull and a human skull appear quite similar an early stage

• As development continues, both skulls change shape as different parts grow at different rates

• A human adult skull is proportioned more like the skull of an infant chimpanzee than the skull of an adult chimpanzee

• Human evolution involved changes that caused traits typical of juvenile stages to persist into adulthood

Proportional Changes During Skull Development: Chimpanzee

proportions in infantadult

Proportional Changes During Skull Development: Human

proportions in infant adult

Persistent Juvenile Traits in Salamanders

• In axolotls, external gills and other larval traits persist into adulthood

Take-Home Message: Why are similarities in development indicative of shared ancestry?

• Similarities in patterns of development are the result of master genes that have been conserved over evolutionary time

• Some differences between closely related species are a result of master gene mutations that change the rate or onset of development

ANIMATED FIGURE: Mutation and proportional changes

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18.6 Applications of Phylogeny Research

• Studies of phylogeny reveal how species relate to one another and to species that are now extinct

• We use information about phylogeny to understand how to preserve the species that exist today

Conservation Biology

• The reservoir of genetic diversity among Hawaiian honeycreepers is diminishing along with its numbers

• Lowered diversity means the group as a whole is less resilient to change, and more likely to suffer species losses

• Deciphering honeycreeper phylogeny can tell us which ones are most valuable in terms of preserving genetic diversity

Diversity of Hawaiian Honeycreepers

Evolutionary Relationships Among Honeycreepers

Conservation Biology (cont.)

• Cladistics analyses are also used to correlate past evolutionary divergences with behavior and dispersal patterns of existing populations

• Example: A cladistic analysis of mitochondrial DNA sequences suggests that blue wildebeest populations are genetically less similar than they should be

• Using a combination of data, conservation biologists can recommend measures to improve gene flow

Medical Applications

• Researchers study the evolution of infectious agents by grouping their biochemical characters into clades

• Example: A phylogenetic analysis of H5N1 influenza virus isolated from pigs showed that the virus “jumped” from birds to pigs at least three times since 2005, and that one group had acquired the potential to be transmitted among humans

Take-Home Message:

How is studying phylogeny useful?

• Phylogeny research is yielding an ever more specific and accurate picture of how all life is related by shared ancestry

• Among other applications, phylogeny research can help us preserve species in danger of becoming extinct, and to understand the spread of infectious diseases