Understanding Evolution: Macroevolution & Major Transitions

Evolution is a fascinating topic that allows you to look at life through the lens of deep time. ‘Deep time’ are vast intervals that cannot be experienced in normal everyday life and it is studied through the science disciplines of geology and paleontology.

But what is ‘evolution’? Is it a fact? Is it a theory?

There are many variants on the definition of evolution, but in the most concise definition, evolution is change over time. More importantly, it is the observable change over time on generations. Changes in a single human individual is not evolution, but developmental. Evolution is concerned with changes on a population.

Evolution is often divided into microevolution and macroevolution. Microevolution refers to change within a species. Species are a group of organisms with shared traits, but a condensed definition of species is a closed gene pool. Macroevolution is when a species changes into another species – a process known as speciation.

Evolution is both a fact and theory.  Across the scientific disciplines, evolution without an argument is a fact. The ‘fact’ stems from the overwhelming amount of evidence of the fossil records, transitional fossils, molecular clock, genetics, and so forth, which also includes facts from laboratory experiments and data from the observable real-world. The ‘theory’ part deals with how we think the fact of evolution has occurred.

In this post, we will be concerned with macroevolutionary transitions.

Macroevolution transitions are that of where some great leap happened, such as a novel mean of adapting to a radical environment. For some examples, think about transitions from sea to the land, the land to the air, the land back to the sea, or salt water to fresh water. Think about leaps in habitats that also forced additional leaps in reproduction.  For example, think of the development on land of both plant seeds and enclosed animal eggs as adaptation to arid environments. Another example is live birth by dolphins, which is an adaptation to marine environments.  Fruits with surrounding seeds that entice hungry animals is an adaptation to enhance plant reproduction.

In turn, some major evolutionary transitions actually changed environments, thus driving additional evolutionary transitions. Think of how the evolution of land plants changed landscapes, the evolution of coral reefs changed seascapes, and evolution flying pollinating insects that changed landscapes.

We can see these sorts of transitions all around us. For example, although modern seals and walruses are adapted to spend most of their lives in marine environments, they are descended from land dwelling animals.  An opposite example are penguins which are in transition from terrestrial to marine environments. They descended from birds that originally flew in the air. They now use their flying ability to great advantage when flying under the water.

Here are some major evolutionary transitions:

  • Eukaryotic cells – the first major transition which happened more than 2 billion years ago.
  • Multi-celled animals – single cell eukaryotes evolved to become multi-celled animals ca. 600-550 ma
  • Skeletons – animals went from softbodied to hardbodied with the development of skeletons and other mineralized parts, whether these evolved with shells, bones or teeth.  Animals with a backbone show up in the fossil record ca. 530 ma.
  • Life on land – This is an interesting transition that happened ca. 500 – 400 ma. Here, plants, fungi, and animals all evolved to live on land that descended from organisms that originally lived in marine environments.
  • Vertebrate origins – Vertebrate also became four-legged and started to walk on land ca. 400 ma.
  • Insect flight and coevolution with seed plants – While vertebrates were beginning to walk on four legs, insects also began to appear ca. 400 – 350 ma. The appearance of insects nearly coincided with the transition of plants with seeds and the first forest.
  • First eggs – The evolution of enclosed animal eggs happened ca. 340 – 310 ma. This was a huge transition. This freed reptiles from watery environments and led to the later evolution of mammals and dinosaurs, as well as flying and swimming reptiles.
  • First flowers – The evolution of flowering plants happened ca. 130 -125 ma. This changed the world as we know it. Not coincidentally, this major transition happened with an evolution of insects into pollinating insects.
  • Mammal origins and evolution – Their evolution originally from reptiles happened ca. 230 ma. Later, they underwent additional evolutionary leaps, such as from egg-laying to giving live birth, and from hoofed land-dwellers to whales.
  • Primate and human origins –  There are several important transitions involved in their overall transitions; from tree dwelling primates to the eventual evolution of humans.

Four Factors that are Responsible for Macroevolutionary Change*

Geographic Isolation

Geographic isolation refers to how populations of a species may become isolated from one another. This include a physical barrier like a mountain chain, a river, or an ocean, that splits a species into different places.  Geographic isolation is not restricted to physical barriers, it simply may be an unfavorable habitat between two populations that prevent them from mating with each other. With enough time, geographic isolation can result in a second factor responsible for macroevolutionary change: genetic drift.

Genetic Drift

This means that a species has drifted enough, genetically speaking, from its ancestral population that it then became a different species or even diversified into many species. We sometimes call this adaptive radiation. It is important to note that while we are speaking of genetic drift in a macroevolutionary sense, genetic drift can also operate on the microevolution level without directly attributing to a new species. Genetic drift on this level concerns itself with fluctuations in allele frequencies on a population due to chance.  One example are the Amish people of Eastern Pennsylvania. They are a closed population that originated from a small number of German immigrants. They inherited rare concentrations of gene mutations from the German founders (hence, the founder effect) that is still active in their population. These mutations causes a number of disorders such as polydactyly (extra fingers) and forms of dwarfism. Because Amish people tend to marry within their population, the recessive genes have a high chance to come together during meiosis which requires two copies of the gene to trigger the disorders.

Environmental Change

Environmental change can be local or global. For example, lowering of sea levels during a time of global cooling would be advantageous for those organisms adapted to cool environment, or to an expansion of land environment.

Mass Extinctions

Mass extinctions are recognized as a factor in macro evolution. There has been 5 mass extinctions (a 6th is currently hypothesized) in the geologic past well before humans showed up. These extinctions occurred 440 ma, 360 ma, 250 ma, 200 ma, and 65 ma. We now suspect that these extinction events while also taking out a lot of species, also led to the opening of habitats and resources for those species that survived. One example is the evolution of dinosaurs from ancestral reptiles that occurred after an extinction event 200ma. Extinctions may have hastened some evolutionary transitions.

These four factors could be summarized by a little phrase coined by Charles Darwin: Natural Selection.  After all, as environments change or mass extinctions take place, organisms in the right place, with the right stuff (genetically speaking), gets selected to pass on those genes to the next generation.  Selection also includes the ‘artificial selection’ that we humans have done through the selective breeding and modification of domestic animals.

*Although we speak of four factors/natural selection that drive macroevolutionary change, it is important to note that there are other roles that attribute to evolutionary changes. We’ll examine a bit of these roles to understand how far science has progressed since Charles Darwin proposed the hypothesis of descent with modification via his 1859  book Origins of Species, which later turned into evolutionary theory.

New science roles that emerged since Darwin’s time include plate tectonics, which helps explain how populations have become geographically isolated in the geologic past.  It also helps to explain how volcanism might have altered the atmosphere, how mountain building and how other earth processes change environments or even influence mass extinctions. Developmental biology  examines how genes code for the development of growing organisms and how these genes might be switched on or off during evolution. Ecology is sophisticated science that looks closely and systematically at how organisms and communities interact with one another and how physical factors in their environments affect and guide their evolution. Modern genetic studies also examine genetic similarity and even help to explain sources of variation. One of the tools that are used in genetic studies include the molecular clock (the general idea  is using calculated rates of change in RNA and DNA to estimate when major transitions may have happened.)

Time-permitting, I will come back to expand on a web series regarding evolution.