Evolution, Natural Selection,and Species Origination

Evolution - all the changes that have transformed life on Earth from its earliest beginnings to the diversity that characterizes it today.Taken on a grand scale to mean the gradual appearance of all biological diversity, from the earliest microbes to the enormous variety of organisms alive today.

Evolution has been and continues to be such a monumental concept because its study illuminates biology at every level including the molecular, microscopic, and macroscopic.

The Historical Road Leading to Darwin’s Ideas on

Descent with Modification

In ancient times, several Greek philosophers proposed that life gradually evolved, but Aristotle (384-322 B.C.), who had more influence on early Western science, viewed species as unchanging.

Through his observations, Aristotle devised the scala naturae (or “scale of nature”) on which he arranged life-forms according to their increasing complexity.

This idea that each organism had its allotted place persisted well into the 1700s with scientists now seeing species as having been perfectly designed by the Creator to fulfill a particular role/purpose. One such scientist was Carolus Linnaeus (1707-1778) who sought to classify life’s diversity.

Linnaeus founded the branch of biology concerned with naming and classifying organisms, known as taxonomy. Although he subscribed to the unchanging view of species, Linnaeus recognized similarities among organisms and adopted a nested classification system in which similar species are grouped into increasingly more general categories (unlike the hierarchical linearity of the scala naturae).

Additionally, Linnaeus developed the two-part system of naming organisms according to genus and species (binomial nomenclature) that is still in use today.

Even though Linnaeus did not think that the observations concerning the resemblance of species implied evolutionary kinship, a century later, his work would aid in Darwin’s arguments for evolution.

Influence of Paleontology on Darwin:

Paleontology - the study of fossils; developed largely by Georges Cuvier (1769-1832).

Cuvier observed that fossils were more dissimilar from current life as one examined deeper (older) layers of the sedimentary strata and that from one stratum to the next, new species appeared while others disappeared.

Still, Cuvier strongly disavowed any support for an idea of gradual evolutionary change; rather, he speculated that each boundary between strata was reflective of a catastrophe (catastrophism), accounting for the demise of species alive at the time with subsequent repopulation by species immigrating from other areas not affected by the catastrophe.

In contrast to the view of catastrophism, James Hutton (geologist, 1769-1832) subscribed to a concept of gradualism (profound change occurs through the cumulative effect of slow but continuous processes), proposing that Earth’s geologic features could be explained by gradual mechanisms currently operating in the world.

Charles Lyell (geologist, 1797-1875) incorporated Hutton’s thinking and derived a theory of uniformitarianism, offering that the same geological processes are operating today as in the past, and at the same rate.

The ideas of Huton and Lyell greatly influenced Darwin’s thinking - however, Darwin was not the first to apply the principle of gradualsim to biological evolution.

The Lamarckian Theory of Evolution:

Jean Baptiste de Lamarck (1744-1829) was a biologist who developed a comprehensive model concerning how life evolves:

He explained gradual evolutionary descent based on two principles -

1) An idea of use and disuse, where the components of an organism’s anatomy that are used extensively become larger and stronger, while those that are not used deteriorate.

2) The belief in the inheritance of acquired characteristics in which an organism could pass the modifications from the above principle to its offspring.

We now know this reasoning to be faulty, but his thoughts deserve merit as they recognized the influence of gradual evolutionary change over time as the best explanation regarding his observations.

Darwin (1809-1882) and the Origin of Species:

In 1831, Charles Darwin sets out as part of the crew on the HMS Beagle for a voyage around the world.

As the ship’s naturalist, he collects thousands of plant and animal specimens.

Upon the Beagle’s stop at the Galapagos Islands, Darwin observes several kinds of finches seeming to be different species, but quite similar in appearance. The most striking differences among the finches were their beaks, each one adapted for a specific diet.

It would not be until several years after his return home (in 1836) from the voyage that Darwin realized that an explanation for such adaptations was critical to understanding evolution.

In 1844, Darwin composed an essay on the origin of species and natural selection as the mechanism of evolution, but refrained from having it published in anticipation of the controversy it would cause.

In 1858, Darwin receives a manuscript from Alfred Wallace, another naturalist who has developed a theory of natural selection similar to Darwin’s.

Darwin is now spurred to quickly finish his book, On the of Origin of Species by Means of Natural Selection…, and has it published in 1859.

Darwin and Wallace are both credited with the development of the theory concerning natural selection, but Wallace, a great admirer of Darwin’s, agreed that Darwin should be recognized as the primary scientist.

In publishing his theory, Darwin developed two main ideas:

1) Evolution explains life’s unity and diversity and

2) Natural selection is a cause for adaptive evolution

Darwin saw unity in life with all organisms having descended from a common ancestor that lived in the far distant past - when descendants of that ancestral organism spread forth into various habitats (over millions of years), they accumulated adaptations that best suited them to cope with their environments.

As stated earlier, Linnaeus saw that some organisms resemble each other more closely than others, but had not identified this as a result of evolution. Still, his taxonomic scheme largely fit with Darwin’s theory. By Darwin’s reasoning, Linnaeus’ classification system reflected organismal history with life at various taxonomic levels related through descent from a common ancestor.

Natural Selection:

How the heck does this work & how does it explain adaptation?

Ernst Mayr (contemporary evolutionary biologist) has synopsized Darwin’s theory into 3 inferences based on 5 observations:

Observation #1 - For any species, pop. sizes would increase exponentially if all offspring reproduced successfully.Observation #2 - However, pops. tend to remain stable in size, not counting seasonal fluctuations.Observation #3 - Resources are limited.Inference #1 - Production of more individuals than the environment can sustain leads to an existence struggle among individuals of a pop. with only a fraction of their offspring surviving each generation.Observation #4 - Members of a pop. vary extensively in their characteristics; no 2 individuals are exactly alike.Observation #5 - Much of this variation is heritable.

Inference #2 - Survival depends in part on inherited traits. Individuals whose inherited traits give them a high probability of surviving and reproducing in a given environment have higher fitness (an advantage) and are likely to leave more offspring than less fit (disadvantaged) individuals.Inference #3 - This unequal ability of individuals to survive and reproduce will lead to a gradual change in a pop., with favorable characteristics accumulating over generations.

Summary:•Natural selection is the differential success in reproduction among individuals that vary in their heritable traits. These reproductive differences emerge as each individual interacts with its environment.•Over time, natural selection can increase the adaptation of organisms to their environment.•If an environment changes over time, or if individuals of a species move to a new environment, natural selection may result in adaptation to these new conditions, sometimes giving rise to new species in the process.

3 important caveats to remember about natural selection:

1) Although natural selection occurs through interactions between individual organisms and their environment, individuals do not evolve; the smallest unit that can evolve is a population.

2) Natural selection can amplify or diminish only heritable traits - that is, traits that are passed from organisms to their offspring.

3) Environmental factors vary from place to place and from time to time. A trait that is favorable in one situation may be useless - or even detrimental - in different circumstances.

Darwin’s theory endures due to its ability to explain so many different types of observations including:

Anatomical and molecular homologies (similarities in characteristics resulting from a shared ancestry) that match patterns in space (biogeography) and time (the fossil record).

Remember, the term “theory” is not used in science as it is in colloquial terms where its meaning is more like a hypothesis. Evolutional theory has withstood the skepticism of science and tests of experimentation, enduring as the most logic/plausible explanation for our observations in biology today.

By attributing the diversity of life to natural processes, Darwin gave biology a sound, scientific basis.

•Speciation, the origin of new species, is at the focal point of evolutionary theory

•Evolutionary theory must explain how new species originate and how populations evolve

•Microevolution consists of adaptations that evolve within a population, confined to one gene pool

•Macroevolution refers to evolutionary change above the species level, for example:

- Appearance of major new changes/features in organisms

- Impact of mass extinctions on diversity of life

Reproductive IsolationReproductive Isolation

Absence of gene flow is one contributing factor Absence of gene flow is one contributing factor to speciation to speciation

Reproductive isolation is the existence of Reproductive isolation is the existence of biological factors (barriers) that impede two biological factors (barriers) that impede two species from producing viable, fertile hybridsspecies from producing viable, fertile hybrids

Two types of barriers: prezygotic and Two types of barriers: prezygotic and postzygoticpostzygotic

Figure 24.4Figure 24.4

Prezygotic barriers impede mating or hinder fertilization if mating does occur

Postzygotic barriers prevent a hybrid zygote fromdeveloping into a viable, fertile adult

REDUCED HYBRIDVIABILITY

REDUCED HYBRIDFERTILITY

HYBRID BREAKDOWN

HABITAT ISOLATION TEMPORAL ISOLATION BEHAVIORAL ISOLATION MECHANICAL ISOLATION GAMETIC ISOLATION

Reducedhybrid

viability

FertilizationViable,fertile

offspring

Reducedhybridfertility

Hybridbreakdown

Matingattempt

Gameticisolation

Fertilization

Mechanicalisolation

Behavioralisolation

Temporalisolation

Habitatisolation

Individualsof

differentspecies

Speciation can occur in two ways:Speciation can occur in two ways: Allopatric speciationAllopatric speciation Sympatric speciationSympatric speciation

In allopatric (“other country”) speciation, gene In allopatric (“other country”) speciation, gene flow is interrupted or reduced when a population flow is interrupted or reduced when a population is divided into geographically isolated is divided into geographically isolated subpopulationssubpopulations

One or both populations may undergo One or both populations may undergo evolutionary change during the period of evolutionary change during the period of separationseparation

In sympatric (“same country” speciation, In sympatric (“same country” speciation, speciation takes place in geographically speciation takes place in geographically overlapping populationsoverlapping populations

Allopatric speciation Sympatric speciation

Allopatric and Sympatric Allopatric and Sympatric Speciation: A SummarySpeciation: A Summary

In allopatric speciation, a new species forms In allopatric speciation, a new species forms while geographically isolated from its parent while geographically isolated from its parent populationpopulation

In sympatric speciation, a reproductive barrier In sympatric speciation, a reproductive barrier isolates a subset of a population without isolates a subset of a population without geographic separation from the parent speciesgeographic separation from the parent species

Concept 24.3: Macroevolutionary changes can Concept 24.3: Macroevolutionary changes can accumulate through many speciation eventsaccumulate through many speciation events

Macroevolutionary change is cumulative change Macroevolutionary change is cumulative change during thousands of small speciation episodesduring thousands of small speciation episodes

Most novel biological structures evolve in many Most novel biological structures evolve in many stages from previously existing structuresstages from previously existing structures

Many large evolutionary changes may have been Many large evolutionary changes may have been associated with mutations genes that regulate associated with mutations genes that regulate development. development.

Genes that program development control the Genes that program development control the rate, timing, and spatial pattern of changes in an rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adultorganism’s form as it develops into an adult