Evolutionary Anthropology - Macroevolution

Macroevolution

• Macroevolution, Speciation, and Phylogenetics are the main topics.

• Assigned readings for Week #6 are available on Quercus.

Speciation

• Speciation continues.

• Allopatric Speciation:

Other forms of speciation without geographic isolation:

• Parapatric speciation: speciation between subpopulations of a larger population, with hybrid zones being common.

• Sympatric speciation: speciation from a single species within the same geographic area.

Barriers to Gene Exchange:

• Many phenotypic differences can create barriers to gene exchange.

• These barriers can occur:

  • Premating

  • Postmating post prezygotic

  • Postzygotic

• Once barriers are in place to stop gene flow, other evolutionary forces take effect.

Microevolutionary Processes:

• Evolutionary Forces:

  • Natural selection:

    • Non-random (dependent on environment)

    • Increases or decreases variation within populations

    • Increases or decreases variation between populations

  • Mutation:

    • Random

    • Increases variation within populations

    • Increases variation between populations

  • Genetic drift:

    • Random

    • Decreases variation within populations

    • Increases variation between populations

  • Gene flow:

    • Random or non-random

    • Increases variation within populations

    • Decreases variation between populations

Mode and Tempo of Evolution

• Gradualism (Darwin):

  • Slow and gradual process.

  • The fossil record is expected to show smooth species transitions.

• Punctuated equilibrium (Eldredge and Gould):

  • Long periods of stasis, punctuated by rapid change.

  • Rapid speciation happens at the edges of a species' range.

  • Results in “gaps” in the fossil record.

• Speciation and extinction rates are roughly equal over long periods.

• There is a lot of variation over time among different groups.

• High turnover: new species continually form and replace extinct species.

• Mass extinction leads to high rates of evolution.

• It creates widespread ecological opportunities that can be exploited by surviving lineages.

• Example: Mammals diversified after the K-Pg Extinction.

• An evolutionary trend is defined as a persistent temporal change in a characteristic of a lineage or clade.

• It is not necessarily progress.

• Strongly directional trends are rare; most species show fluctuating or meandering changes.

• However, some well-documented trends exist.

Cope’s Rule:

• A well-documented evolutionary trend in mammals.

• Compared ancestor and descendant species pairs and found a bias toward size increases.

• Descendants are larger on average.

Organismal Complexity:

• Progression from simple to complex in lineages.

• Life started off very simple and gets increasingly complex over time.

• For example, prokaryotes and simple microscopic organisms existed for a long time; animals appeared at 800 Ma, and bilateral mammals diversified at 250 Ma.

• How to characterize complexity:

  • Number and diversity of parts

  • Number of cell types

• To study the tempo and mode of evolution, a phylogenetic tree is required for most macroevolutionary studies.

Systematics

• Systematics: Biological classification and reconstructing evolution.

• Includes all activities involved in the study of the diversity and origins of living and extinct organisms.

• Systematics = Taxonomy + Phylogenetics

• Steps, as defined by Uluutku and Wood, 2023:

  1. Identification and comparison

  2. Species-level classification

  3. Phylogenetic reconstruction

  4. Classification above the species level

  • Alpha taxonomy (steps 1 & 2)

  • Beta taxonomy (step 3)

Identification and Comparison:

• Identification of a new specimen:

  • In zoology or paleontology, assess what broad group it belongs to and what anatomical region it represents.

  • For example, is it a primate?

• Comparison with other specimens:

  • Compare the specimen with appropriate extant and fossil taxa, then assign it to an existing group or a novel group.

  • Does it belong to a known species, or does it represent a new species?

  • It is not always easy to tell due to high levels of variation within populations.

Species-Level Classification:

• Recognizing groups as species by applying a species concept (e.g., Biological species concept, Phylogenetic species concept).

• Giving them a binominal name (Genus species).

Linnaean System of Taxonomy:

• Binominal nomenclature was developed by Carolus Linnaeus (Systema Naturae, 1758).

• An organism is assigned two names: a generic name (genus) and a specific name (species).

• Italicized or underlined; only the genus is capitalized.

• For example:

  • Homo sapiens or Homo sapiens

• Uses phenotypic or genetic information to make inferences about the relationships between taxa (singular = taxon).

• Results in a hypothesis of relationships, represented by a tree diagram.

Evolutionary or Phylogenetic Relationships:

• Uses the results of phylogenetic reconstruction to allocate species to taxonomic ranks above the species level (Genus, family, class, etc.).

• This system was also developed by Carolus Linnaeus and has been elaborated upon since.

• It is a hierarchical system.

Classification Above the Species Level:

• Suborder

• Hypoorder

• Infraorder

• Superfamily

• Subfamily

• Subspecies

Linnaean System of Taxonomy:

• Kingdom: Animalia

• Phylum: Chordata (vertebrates)

• Class: Mammalia

• Order: Primates

• Family: Hominidae (hominids)

• Genus: Homo

• Species: Homo sapiens

Phylogenetic Trees

• One of the most fundamental concepts of evolution is that species share a common origin and have subsequently diverged through time.

• Evolutionary trees show the divergence or ‘branching’ of lineages from a common ancestor.

• All life forms share common ancestry

• The crucial evolutionary information of a tree is the branching order.

  • topology

• An evolutionary tree is a hypothesis of relationships: everything is related, but it’s about degrees!

Taxonomy and Phylogenetics:

• Organisms are named based on the hierarchical pattern of descent resulting from evolution.

• Closely related organisms are classified together into named groups.

• Cladograms depict sister-taxon relationships.

• In a chronogram (traditionally called a phylogenetic tree), branch lengths are scaled according to time.

• In a cladogram, the branch lengths have no meaning.

• Now all are referred to as phylogenetic trees.

Types of Phylogenetic Groups:

• Monophyletic group (also known as a clade): a group that contains an ancestor and all of its descendants.

• Paraphyletic group: a group that contains an ancestor and only some of its descendants.

• Polyphyletic group: a group that contains descendants of more than one common ancestor and does not contain the ultimate ancestor of all of the taxa in the group.

• Monophyletic group = ancestor and all descendants.

• Paraphyletic group = ancestor and only some descendants.

• Polyphyletic group = unnatural grouping based on convergent characteristics.

• "Pongidae" = paraphyletic group

• Hominidae = monophyletic group/clade

• Group of taxa that are all descended from a recent common ancestor.

Building Phylogenetic Trees

1. Data

   • Characters: the organism attributes under consideration (features, traits).

   • Character states: the particular values that can be taken by different individuals for specific characters.

2. A model of evolution: These are hypotheses about how characters evolve. They take a mathematical and statistical form.

• Select features that are homologous in the taxa of interest.

• Homology is a character (trait) shared by two or more taxa due to common ancestry.

• Some features that look similar are not shared due to common ancestry but through convergent evolution.

• Homoplasy: a trait shared by two or more taxa that has evolved independently.

• We want to select homologous features.

• But we also need to look for features that are unique to some taxa within the group of interest (shared and derived).

• Synapomorphies

• Synapomorphy: a feature that exhibits states that have been modified relative to the common ancestor and is shared by some but not all taxa.

• Shared features can also be ancestral (inherited from a common ancestor) = symplesiomorphies.

• Features can also be uniquely derived = autapomorphies.

• These are NOT information for phylogenetic analysis.

• Symplesiomorphy: A character that has not been modified relative to the form seen in the common ancestor. NO HAIR

• Autapomorphy: Diagnostic characteristic of one taxon.

Determine Synapomorphies of Primates:

• Group #1:

  • Have body hair / fur

  • Have four limbs

  • Give birth to live young nurse young with milk

• Group #2:

  • Grasping hands and feet

  • Postorbital bar/wall

  • Larger brains

• So, we need features that represent homologies and that are synapomorphies.

• How do we choose features?

  • In paleontology: careful examination of morphology, typically of skeletal material

• Example: Sivapithecus, a fossil ape.

• Develop a matrix of characters.

• Convert to a numerical matrix

Methods for Inferring a Tree from Character Data:

• Distance: find the tree that accounts for estimated evolutionary distances (phenetics).

• Maximum parsimony: find the tree that requires the minimum number of changes to explain the data.

• Likelihood: find the tree that maximizes the statistical likelihood of the data.

• Bayesian: find the tree that is correct given the data, statistical priors, and likelihood.

• Develop a matrix of characters.

• Convert to a numerical matrix.

Why Build Phylogenetic Trees?

• Helps us understand evolutionary relationships and to more accurately name and classify organisms.

• Questions of tempo and mode

• Diversification

• Rate of evolution

• Evolutionary trends

• Helps to explain when (at least in a relative sense) certain traits evolved among organisms.

• Can explain whether certain traits are homologous or convergent.

• Reconstruct biogeography.

• Reconstruct ancestors.