Williams, W. Hamilton and Richard Dawkins, stems directly from population-genetic reasoning; indeed, important aspects of gene's eye thinking were already present in Fisher's writings Okasha Proponents of the gene's eye view argue that genes are the real beneficiaries of the evolutionary process; genotypes and organisms are mere temporary manifestations. Gene's eye thinking has revolutionised many areas of evolutionary biology in the last thirty years, particularly in the field of animal behaviour; but in many ways it is simply a colourful gloss on the conception of evolution implicit in the formalisms of population genetics.
Population genetics raises a number of interesting philosophical issues. One such issue concerns the concept of the gene itself. As we have seen, population genetics came into being in the s and s, long before the molecular structure of genes had been discovered.
In these pre-molecular days, the gene was a theoretical entity, postulated in order to explain observed patterns of inheritance in breeding experiments; what genes were made of, how they caused phenotypic changes, and how they were transmitted from parent to offspring were not known. Today we do know the answers to these questions, thanks to the spectacular success of the molecular genetics ushered in by Watson and Crick's discovery of the structure of DNA in The gene has gone from being a theoretical entity to being something that can actually be manipulated in the laboratory.
The relationship between the gene of classical pre-molecular genetics, and the gene of modern molecular genetics is a subtle and much discussed topic Beurton, Falk and Rheinberger eds.
But it is clear that the two concepts do not have precisely the same extension; not every molecular gene is a classical gene, nor vice-versa. Some theorists go further than this, arguing that what molecular biology really shows is that there are no such things as classical genes. Whatever one's view of this debate, it is striking that virtually all of the central concepts of population genetics were devised in the pre-molecular era, when so little was known about what genes were; the basic structure of population-genetic theory has changed little since the days of Fisher, Haldane and Wright.
This reflects the fact that the empirical presuppositions of population-genetic models are really quite slim; the basic presupposition is simply the existence of hereditary particles which obey the Mendelian rules of transmission, and which somehow affect the phenotype.
Therefore, even without knowing what these hereditary particles are made of, or how they exert their phenotypic effects, the early population geneticists were able to devise an impressive body of theory.
That the theory continues to be useful today illustrates the power of abstract models in science. This leads us to another facet of population genetics that has attracted philosophers' attention: the way in which abstract models, that involve simplifying assumptions known to be false, can illuminate actual empirical phenomena.
Idealized models of this sort play a central role in many sciences, including physics, economics and biology, and raise interesting methodological issues. In particular, there is often a trade-off between realism and tractability; the more realistic a model the more complicated it becomes, which typically limits its usefulness and its range of applicability. This general problem and others like it have been extensively discussed in the philosophical literature on modelling e.
Godfrey-Smith , Weisberg , Frigg and Hartmann , and are related to population genetics by Plutynski The simplest population-genetic models assume random mating, non-overlapping generations, infinite population size, perfect Mendelian segregation, frequency-independent genotype fitnesses, and the absence of stochastic effects; it is very unlikely and in the case of the infinite population assumption, impossible that any of these assumptions hold true of any actual biological population.
More realistic models, that relax one of more of the above assumptions, have been constructed, but they are invariably much harder to analyze. This question is taken up by Morrison in relation to Fisher's early population-genetic work.
Another philosophical issue raised by population genetics is reductionism. It is often argued that the population-genetic view of evolution is inherently reductionistic, by both its critics and its defenders. This is apparent from how population geneticists define evolution: change in gene frequency.
Implicit in this definition is the idea that evolutionary phenomena such as speciation, adaptive radiation, diversification, as well as phenotypic evolution, can ultimately be reduced to gene frequency change.
But do we really know this to be true? This is a large question, and is related to the issues discussed in section 4. The question can be usefully divided into two: i can microevolutionary processes explain all of evolution? These changes typically involve the substitution of a gene for its alleles, of exactly the sort modelled by population genetics.
Authors such as Gould and Eldredge , for example, have argued persuasively that macro-evolutionary phenomena are governed by autonomous dynamics, irreducible to a microevolutionary basis. Philosophical discussions of this issue include Sterelny , Grantham and Okasha Setting aside the reducibility of macro to micro-evolution, there is still the issue of whether an exclusively population-genetic approach to the latter is satisfactory.
Some reasons for doubting this have been discussed already; they include the complexity of the genotype-phenotype relation, the fact that population genetics treats development as a black-box, and the idealizing assumptions that its models rest on. Another point, not discussed above, is the fact that population genetics models are deliberately silent about the causes of the fitness differences between genotypes whose consequences they model Sober , Glymour For example, in the simple one-locus model of section 3.
To fully understand evolution, the ecological factors that lead to these fitness differences must also be understood. While this is a valid point, the most it shows is that an exclusively population-genetic approach cannot yield a complete understanding of the evolutionary process.
This does not really threaten the traditional view that population genetics is fundamental to evolutionary theory. A final suite of philosophical issues surrounding population genetics concerns causation. Evolutionary biology is standardly thought of as a science that yields causal explanations: it tells us the causes of particular evolutionary phenomena Okasha The basis for this way of speaking is obvious enough.
If the frequency of gene A in a population increases from one generation to another, and if the population obeys the rules of Mendelian inheritance, then as a matter of logic one of three things must have happened: organisms bearing gene A must have outreproduced organisms without I ; organisms bearing gene A must have migrated into the population II ; or there must have been mutation to gene A from one of its alleles III. It is straightforward to verify that if none of I - III had happened, then the frequency of gene A would have been unchanged.
Note that case I covers both selection and random drift, depending on whether the A and non-A organisms reproduced differentially because of their genotypic difference, or by chance. Despite this argument, a number of philosophers have objected to the idea that evolutionary change can usefully be thought of as caused by different factors, including natural selection e.
Matthen and Ariew , Walsh The status of these objections is a controversial matter; see Reisman and Forber , Brandon and Ramsey and Sarkar for critical discussion. There is an important difference between drift on the other hand and the other three factors on the other.
This is because mutation, selection and migration are directional; they typically lead to a non-zero expected change in gene frequencies Rice p. Random drift on the other hand is non-directional; the expected change due to drift is by definition zero.
As Rice points out, this means that mutation, selection and migration can each be represented by a vector field on the space of gene frequencies; their combined effects on the overall evolutionary change is then represented by ordinary vector addition. But drift cannot be treated this way, for it has a magnitude but not a direction. However this line of argument is specific to random drift; it does not generalize to all the factors that affect gene frequency change.
A related consideration is this. It is straightforward to verify that at least one of these three factors must have operated, if gene frequencies in a population change.
It seems unproblematic to regard these three factors as causes of evolution. Rather, what we mean is that the differential reproduction was not the result of systematic differences in how well the genotypes were adapted to the environment. To conclude, it is unsurprising to find so much philosophical discussion of population genetics given its centrality to evolutionary biology, a science which has long attracted the attention of philosophers.
The preceding discussion has focused on the most prominent debates surrounding population genetics in the recent philosophical literature; but in fact population genetics is relevant, at least indirectly, to virtually all of the topics traditionally discussed by philosophers of evolutionary biology. The Origins of Population Genetics 2. The Hardy-Weinberg Principle 3. Population-Genetic Models of Evolution 3.
Population Genetics and its Critics 5. The Origins of Population Genetics To understand how population genetics came into being, and to appreciate its intellectual significance, a brief excursion into the history of biology is necessary.
Population Genetics and its Critics The basic models of classical population genetics, expounded in the previous sections, have been around for nearly a century; they derive from the work of Fisher, Haldane and Wright in the s.
Philosophical Issues in Population Genetics Population genetics raises a number of interesting philosophical issues. Bibliography Amundson, R. Beurton, P. Bowler, P. Reisman, K. Brandon, R. Hull and M. Ruse eds. The Cambridge Companion to the Philosophy of Biology , 66— Bromham, L. Carroll, S. Crow, J. Darwin, C. Dawkins, R. Dietrich, M. Dunn, L. Edwards, A. Eldredge, N. Ewens, W. Falconer, D. Fisher, R. Frigg, R. Zalta ed. Gillespie, J. Glymour, B.
Godfrey-Smith, P. Gould, S. Grantham, T. Griffiths, P. Haldane, J. Hardy, G. Hartl, D. Kimura, M. Lewontin, R. Mayr and W. Provine eds. Lynch, M. Matthen, M. Maynard Smith, J. Millstein, R. Morrison, M. Moss, L. Okasha, S. Beebee, C. Hitchcock and P. Menzies eds. Pigliucci, M. The four most important evolutionary forces, which will disrupt the equilibrium, are natural selection, mutation, genetic drift , and migration into or out of a population.
A fifth factor, nonrandom mating, will also disrupt the Hardy-Weinberg equilibrium but only by shifting genotype frequencies, not allele frequencies. In nonrandom mating, individuals are more likely to mate with like individuals or unlike individuals rather than at random. Since nonrandom mating does not change allele frequencies, it does not cause evolution directly. Natural selection has been described. If natural selection acts against the allele, it will be removed from the population at a low rate leading to a frequency that results from a balance between selection and mutation.
This is one reason that genetic diseases remain in the human population at very low frequencies. If the allele is favored by selection, it will increase in frequency. Genetic drift causes random changes in allele frequencies when populations are small. Genetic drift can often be important in evolution, as discussed in the next section. Finally, if two populations of a species have different allele frequencies, migration of individuals between them will cause frequency changes in both populations.
As it happens, there is no population in which one or more of these processes are not operating, so populations are always evolving, and the Hardy-Weinberg equilibrium will never be exactly observed. However, the Hardy-Weinberg principle gives scientists a baseline expectation for allele frequencies in a non-evolving population to which they can compare evolving populations and thereby infer what evolutionary forces might be at play.
The population is evolving if the frequencies of alleles or genotypes deviate from the value expected from the Hardy-Weinberg principle. Darwin identified a special case of natural selection that he called sexual selection. Sexual selection occurs in two ways: through male—male competition for mates and through female selection of mates. Sometimes the competition is for territory, with females more likely to mate with males with higher quality territories. Female choice occurs when females choose a male based on a particular trait, such as feather colors, the performance of a mating dance, or the building of an elaborate structure.
In some cases male—male competition and female choice combine in the mating process. In each of these cases, the traits selected for, such as fighting ability or feather color and length, become enhanced in the males. For example, colorful feathers or an elaborate display make the male more obvious to predators. Evolution by natural selection arises from three conditions: individuals within a species vary, some of those variations are heritable, and organisms have more offspring than resources can support.
The consequence is that individuals with relatively advantageous variations will be more likely to survive and have higher reproductive rates than those individuals with different traits.
The advantageous traits will be passed on to offspring in greater proportion. Thus, the trait will have higher representation in the next and subsequent generations leading to genetic change in the population.
Population genetics is a theoretical framework for describing evolutionary change in populations through the change in allele frequencies. Population genetics defines evolution as a change in allele frequency over generations.
In the absence of evolutionary forces allele frequencies will not change in a population; this is known as Hardy-Weinberg equilibrium principle.
However, in all populations, mutation, natural selection, genetic drift, and migration act to change allele frequencies.
Which scientific concept did Charles Darwin and Alfred Wallace independently discover? If a person scatters a handful of plant seeds from one species in an area, how would natural selection work in this situation? The plants that can best use the resources of the area, including competing with other individuals for those resources, will produce more seeds themselves and those traits that allowed them to better use the resources will increase in the population of the next generation.
Unless an evolutionary force is acting upon the population, the population would carry the same genes at the same frequencies generation after generation, and individuals would, as a whole, look essentially the same. Skip to content Chapter Evolution and Its Processes. Which of the following situations will lead to natural selection? Through shades of blue and green the same visualization shows the population structure over the last decades up to You see that in each subsequent decade the population pyramid was fatter than before — in each decade more people of all ages were added to the world population.
If you look at the green pyramid for you see that the narrowing above the base is much less strong than back in ; the child mortality rate fell from 1-in-5 in to fewer than 1-in today. In comparing and we see that the number of children born has increased — 97 million in to million today — and that the mortality of children decreased at the same time.
If you now compare the base of the pyramid in with the projection for you see that the coming decades will not resemble the past: According to the projections there will be fewer children born at the end of this century than today.
The base of the future population structure is narrower. We are at a turning point in global population history. Between and today, it was a widening of the entire pyramid — an increase of the number of children — that was responsible for the increase of the world population. As global health is improving and mortality is falling, the people alive today are expected to live longer than any generation before us.
This is now happening at a global scale. For every child younger than 15 there were 1. Richer countries have benefited from this transition in the last decades and are now facing the demographic problem of an increasingly larger share of retired people that are not contributing to the labor market. In the coming decades it will be the poorer countries that can benefit from this demographic dividend.
The change from to today and the projections to show a world population that is becoming healthier. When the top of the pyramid becomes wider and looks less like a pyramid and instead becomes more box-shaped, the population lives through younger ages with very low risk of death and dies at an old age.
The demographic structure of a healthy population at the final stage of the demographic transition is the box shape that we see for the entire world for The world population has grown rapidly , particularly over the past century: in there were fewer than 2 billion people on the planet; today there are 7.
The change in the world population is determined by two metrics: the number of babies born, and the number of people dying. The line chart shows the same data, but also includes the UN projection until the end of the century.
It is possible to switch this chart to any other country or world region in the world. In around 55 million people died. The world population therefore increased by 84 million in that year that is an increase of 1. Again it is possible to switch this chart to any other country or world region in the world. How do we expect this to change in the coming decades? What does this mean for population growth? Population projections show that the yearly number of births will remain at around million per year over the coming decades.
It is then expected to slowly decline in the second-half of the century. As the world population ages , the annual number of deaths is expected to continue to increase in the coming decades until it reaches a similar annual number as global births towards the end of the century.
As the number of births is expected to slowly fall and the number of deaths to rise the global population growth rate will continue to fall. This is when the world population will stop to increase in the future. Population growth is determined by births and deaths and every country has seen very substantial changes in both: In our overview on how health has changed over the long run you find the data on the dramatic decline of child mortality that has been achieved in all parts of the world.
And in our coverage of fertility you find the data and research on how modern socio-economic changes — most importantly structural changes to the economy and a rise of the status and opportunities for women — contributed to a very substantial reduction of the number of children that couples have. But declining mortality rates and declining fertility rates alone would not explain why the population increases.
If they happened at the same time the growth rate of the population would not change in this transition. What is crucial here is the timing at which mortality and fertility changes. It is shown in the schematic figure. It is a beautifully simple model that describes the observed pattern in countries around the world and is one of the great insights of demography. If fertility fell in lockstep with mortality we would not have seen an increase in the population at all.
The demographic transition works through the asynchronous timing of the two fundamental demographic changes: The decline of the death rate is followed by the decline of birth rates.
This decline of the death rate followed by a decline of the birth rate is something we observe with great regularity and independent of the culture or religion of the population. The chart presents the empirical evidence for the demographic transition for five very different countries in Europe, Latin America, Africa, and Asia.
In all countries we observed the pattern of the demographic transition, first a decline of mortality that starts the population boom and then a decline of fertility which brings the population boom to an end. The population boom is a temporary event. In the past the size of the population was stagnant because of high mortality, now country after country is moving into a world in which the population is stagnant because of low fertility.
Perhaps the longest available view of the demographic transition comes from data for England and Wales. In , Anthony Wrigley and Roger Schofield 11 published a major research project analyzing English parish registers—a unique source that allowed them to trace demographic changes for the three centuries prior to state records.
As far as we know, there is no comparable data for any other country up until the mid-eighteenth century see the following section for Sweden , where recordkeeping began in The chart shows the birth and death rates in England and Wales over the span of nearly years.
As we can see, a growing gap opens up between the birth and death rate after , creating a population explosion. Statistics Sweden, the successor of the Tabellverket, publishes data on both deaths and births since recordkeeping began more than years ago.
These records suggest that around the year , the Swedish death rate started falling, mainly due to improvements in health and living standards, especially for children.
Yet while death rates were falling, birth rates remained at a constant pre-modern level until the s. During this period and up until the first half of the 20th century, there was a sustained gap between the frequency of deaths and the frequency of births. It was because of this gap that the Swedish population increased. The following visualization supports these observations. The visualization presents the birth and death rate for all countries of the world over the last 5 decades.
Countries per continent can also be highlighted by hovering and clicking on them in the legend on the right side of the chart. By visualising this change we see how in country after country the death rate fell and the birth rate followed — countries moved to left-hand-side first and then fell to the bottom left corner.
Today, different countries straddle different stages of the model. Most developed countries have reached stage four and have low birth and death rates, while developing countries continue to make their way through the stages. There are two important relationships that help explain how the level of development of a country affects its population growth rates:. Combining these two relationships, we would expect that as a country develops, population growth rates decline. Generally, this is true.
Over the last two decades we have seen declining population growth rates in countries at all stages of development. In the average woman on the planet had 5 children. The first panel in this chart shows this fundamental change. The total fertility rate at which a population replaces itself from one generation to the next is called the replacement fertility rate. If no children died before they grew up to have children themselves the replacement fertility rate would be 2. Because some children die , the global replacement fertility rate is currently 2.
Why then is global population growth not coming to an end yet? The number of births per woman in the reproductive age bracket is only one of two drivers that matter here. The second one is the number of women in the reproductive age bracket. If there were few women in the reproductive age bracket the number of births will be low even when the fertility rate is high.
At times when an increasing share of women enter the reproductive age bracket the population can keep growing even if the fertility rate is falling. The second chart in this panel shows that the population growth over the last decades resulted in increasingly larger cohorts of women in the reproductive age bracket. As a result, the number of births will stay high even as the number of births per woman is falling.
This is what the bottom panel in the chart shows. According to the UN projections, the two drivers will cancel each other out so that the number of births will stay close to the current level for many decades.
The number of births is projected to change little over the course of this century. In the middle of the 21st century the number of births is projected to reach a peak at million and then to decline slowly to million births by The coming decades will be very different from the last. How close we are to peak child we looked at in a more detailed post. Population momentum is one important driver for high population growth. But it of course also matters that all of us today live much longer than our ancestors just a few generations ago.
Life expectancy is now twice as long in all world regions. In all of this it is important to keep in mind that these are projections and how the future will actually play out will depend on what we are doing today. Population momentum is driven by the increasingly large cohorts of women in the reproductive age bracket. And this is when global population growth will come to an end.
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You cannot download interactives. In the mids, Charles Darwin famously described variation in the anatomy of finches from the Galapagos Islands. Alfred Russel Wallace noted the similarities and differences between nearby species and those separated by natural boundaries in the Amazon and Indonesia. Independently they came to the same conclusion: over generations, natural selection of inherited traits could give rise to new species.
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