Speciation and Macroevolution

The Biological Species Concept

• Biologists compare morphology (shape and structure), physiology, biochemistry, and DNA sequences when grouping organisms.

• The biological species concept states which groups of organisms should be considered a species.

  • It 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.

    • Viable means the offspring can live to adulthood and mate

    • Fertile means that the offspring is capable of producing offspring at all. 

      • E.G. Mules are the offspring of donkeys and horses. The mule is viable and lives to adulthood, but it is non-fertile and cannot produce its own offspring. 

• Gene flow between populations of the same species holds the populations together genetically.

Reproductive Isolation

• Reproductive isolation is the existence of biological barriers that impede two species from producing viable, fertile offspring.

• Reproductive barriers can be classified by whether they act before or after fertilization, termed prezygotic and postzygotic barriers.

Prezygotic Barriers

• 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

Prezygotic Barriers

• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

  • EX: Apple maggot flies and blueberry maggot flies occur in the same geographic areas, but the apple maggot fly feeds and mates on apples, while the blueberry maggot fly feeds and mates on blueberries. 

• Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

  • EX: The geographic ranges of the western spotted skunk and eastern spotted spunk overlap, but the western one mates in late summer and the eastern one mates in late winter. 

• Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers

  • EX: Blue-footed boobies, inhabitants of the Galapagos, mate only after a courtship display unique to their species. Part of the display is a high-step that calls the female’s attention to the male's bright blue feet. 

• Mechanical isolation: When mating is attempted, morphological differences prevent its successful completion.

  • EX: The shells of two species of snails spiral in different directions preventing the snails’ genital openings from aligning and completing the mating attempt. 

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

  • For instance, sperm may not be able to survive in the reproductive tract of the other species, or biochemical mechanisms may prevent penetrating the membrane surrounding the other species’ eggs.

  • EX: It is difficult for gametes of different species of sea urchin, such as the red and purple sea urchins, to fuse because  proteins on the surfaces of the eggs and sperm bind very poorly to each other

Postzygotic Barriers

• Postzygotic barriers prevents the hybrid fertilized egg from developing into a viable, fertile adult by

  • 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 or survival

  • EX: Some salamander subspecies occasionally hybridize, but most of the hybrids do not complete development, and those that do are frail and die quickly.

• Reduced hybrid fertility: Even if hybrids are strong and survive, they may be sterile.

  • EX: the hybrid offspring of a male donkey and female horse is a mule, which is sterile.

• 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 weak or sterile.

  • EX: strains of cultivated rice have accumulated different recessive alleles at two different genes. Hybrids between them are fertile, but plants in the next generation that carry too many of these recessive alleles are small and sterile.

  • Technically, these rice strains are still considered the same species, but are in the process of being separated by postzygotic barriers (one day their offspring will not be fertile).

Limitations of the Biological Species Concept

• The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

• There are other species concepts that can be applied to these groups:

  • The morphological species concept defines a species by structural features.

  • The ecological species concept views a species in terms of its ecological niche.

  • The phylogenetic species concept defines a species as the smallest group of individuals that share a common ancestor, a single branch on a phylogenetic tree.

Intro to Speciation

• Speciation is the process by which one species splits into two or more species

  • Reproductive isolation is the driving force that often leads to speciation.

• Speciation explains the features shared between organisms due to inheritance from their recent common ancestor

• Speciation 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 on a scale larger than the species level

• Speciation can occur in two ways

  • Allopatric speciation: gene flow is interrupted when a population is divided into geographically isolated subpopulations

  • Sympatric speciation: gene flow is reduced between groups in a population that are still in contact. 

Allopatric Speciation

• When populations become isolated, they may begin evolving independently through mutation, natural selection, and genetic drift.

  • This could lead to reproductive isolation.

• EX: the mosquitofish in the Bahamas live in several isolated populations in different ponds

  • Populations in ponds with high predation rates have evolved different body shapes from populations in “low-predation” ponds

• Evidence of Allopatric Speciation: 

  • Reproductive barriers can develop in lab populations that are experimentally isolated and subjected to different environmental conditions

    • EX: isolated lab populations of fruit flies raised on different diets evolve to digest their food source more efficiently and prefer to mate with partners adapted to the same food source

  • Allopatric speciation has also been observed in nature

    • EX: sister species of snapping shrimp (Alpheus) began to diverge 9 to 3 million years ago when they became isolated by the formation of the Isthmus of Panama 

Sympatric Speciation

• In sympatric speciation, speciation takes place in populations that live in the same geographic area and are not physically separated.

• Caused by factors such as:

  • Polyploidy

  • Habitat differentiation

  • Sexual selection

Sympatric Speciation: Polyploidy

• Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division. 

  • Polyploidy is much more common in plants than in animals

• Sometimes polyploidy produce successful offspring that are fertile. The offspring can reproduce with one another but not with the original parent species. 

  • This is an immediate way that a new species can form. 

• EX: three diploid species of goats beard plant (Tragopogon) have interbred to produce two new tetraploid species (species containing 4 sets of chromosomes).

Sympatric Speciation: Habitat Differentiation

• Sympatric speciation can also result from the appearance of new ecological niches.

  • EX: the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees.

  • Because of having different niches, the gene flow between these two groups of flies is being reduced and may lead to reproductive isolation.

Sympatric Speciation: Sexual Selection

• Sexual selection can drive sympatric speciation, causing groups of individuals in a population to only mate with individuals with certain phenotypes.

• This reduces the gene flow between these groups and can ultimately lead to reproductive isolation and the production of new species. 

  • EX: Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria, which now contains as many as 600 species of cichlids. 

Speciation and Diversity 

“Speciation results in diversity of life forms” -AP Biology CED pg 141

• Increased species diversity allows for more genetic variation and therefore  better survival against changes in the environment, including disease and natural disaster. 

• This prevents extinction of life on Earth. 

What is a Common Ancestor?

• A common ancestor is a species of organism from which one or more new species evolves.

• All living organisms share a distant common ancestor as evidenced by the fact that all organisms share the same fundamental molecular and cellular features.

  • I.e. all use DNA For storing genetic material, all use glycolysis, etc. 

• All living Eukaryotes share a distant common ancestor as evidenced by the fact that they all

  • Have membrane bound organelles

  • Contain linear chromosomes

  • Have genes that contain introns.

• How closely related two species are depends on how recently they shared a common ancestor. 

  • More recent common ancestor = more related to each other

  • Less recent common ancestor = less related to each other. 

• Common ancestry is illustrated in diagrams called cladograms.

  • AKA phylogenetic trees. 

Phylogenetic Trees

• A phylogenetic tree, or cladogram, is a branching tree diagram that represents evolutionary relationships among organisms. 

• The pattern of branching in a phylogenetic tree reflects how species evolved from a series of common ancestors.

• The line leading into a group represents a common ancestor, up to the point at which it splits, indicating a speciation event.

Cladograms vs Phylogenetic Trees

Cladograms

• Grouping of organisms based on shared characteristics present in more than one species.

• Length of the branches does not give any time scale for evolution. 

Phylogenetic Trees

•  Length of the branches represents evolutionary time scale and amount of change.  

• Grouping of organisms based on shared characteristics OR genetic sequence similarity. 

Homologies

• The arrangement of species on a tree is determined by how many homologies are shared between the species. 

  • A homology is a phenotypic similarity (morphological or biochemical trait) or genetic similarity (DNA or protein sequences) found between two species due to shared ancestry. 

  • Homologies can be found in both living and fossilized species. 

• Organisms with similar morphologies or DNA sequences are likely to be more closely related than organisms with different structures or sequences.

• Homologies can also be referred to as shared derived characteristics. 
  When constructing a phylogenetic tree, biologists need to distinguish whether a similarity is the result of homology or analogy.

• Homology is similarity due to shared ancestry, while analogy is similarity due to convergent evolution

  • Convergent evolution occurs when similar environmental pressures and natural selection produce similar (analogous) adaptations in organisms from different evolutionary lineages. 

Common Ancestry 

• The relatedness of any two groups on the tree is shown by how recently two groups had a common ancestor.

  • Species who are the most closely related are ones that have the most recent common ancestor. Meaning that they were more recently the same species. 

• Species who share a recent common ancestor are expected to share more homologies between them than species who have a more decent common ancestor. 

• Phylogenetic trees allow scientists to predict features of ancestors and their extinct descendants based on the features of closely related descendants that are still alive today.

• EX: Scientist can infer characteristics of dinosaurs based on shared characters in modern birds and crocodiles.

  • Birds and crocodiles share several features: four-chambered hearts, song, nest building, and egg brooding.

  • These characteristics likely evolved in a common ancestor and were shared by all of its descendants, including dinosaurs

Maximum Parsimony

• Scientists can never be sure of finding the best tree in a large data set

• They narrow possibilities by applying the principle of maximum parsimony

  • Maximum parsimony assumes that the tree that requires the fewest evolutionary events (appearances of shared derived characters) is the most likely.

• Computer programs are used to search for trees that are parsimonious

• The best hypothesized phylogenetic tree fits the most data: morphological, molecular, and fossil

• Phylogenetic hypotheses are modified when new evidence arises.

Anatomy of a Cladogram/Phylogenetic Tree

Node 

• Represents the most recent common ancestor of the species branching from it. 

• The branching from the node represents a speciation event. 

How to read a phylogenetic tree 

• Same way as a cladogram

• Sometimes they will put a timeline on under the tree and ask you to estimate the age of the most recent common ancestor of two species. 

Limitations of a Phylogenetic Tree

• Phylogenetic trees show how species have

evolved from a series of common ancestors; 

they do not necessarily depict phenotypic 

similarity.

• It should not be assumed that a species 

evolved from the species next to it on a tree.

• EX: we can’t assume humans evolved 

from mice or vice versa.

• Phylogenetic trees do not generally indicate 

when a species evolved or how much change occurred in a lineage

• EX: humans did not evolve 50 mya, but we started evolving separately from mice at that point. 

How to generate a cladogram/phylogenetic tree

• First: Check to see if it is giving you similarities or differences

  • If differences, remember that more differences=less related

• Second: Figure out which species is the outlier. (Has the most differences from ALL the other species)

• Third: Look at the chart and figure out which species are closely related to each other. 

• Fourth: Use this information and logic to place everything on the provided chart.