Anatomy and Evolution

Species definitions; ramifications of natural selection; competitive exclusion; sexual selection; optimality; sex ratios; altruism; game theory; socio-biology; testable theories.

Introduction

Aims

This course aims to convey an understanding and appreciation of:

* evolutionary theory and classification;

* the importance of primatology in the study of the human condition;

* the variety in extant and extinct primates;

* current thinking on the origins of humans;

* the gaps and controversies in our knowledge and understanding of primate evolutionary history;

Objectives

At the end of this course, the students should be able to:

* discuss evolutionary theory and perform phylogenetic reconstructions using a variety of techniques;

* discuss the origins, anatomy, behaviour and relationships of the various groups of extant and extinct primates;

* discuss and critically analyze the various theories concerning primate (human and non-human) evolution;

* recognize and discuss a variety of primate material (animals, skeletons, fossils).

Tinbergen Why's

1. Survival value or function

2. Causation

3. Development

4. Evolutionary history

e.g. Why do starlings sing? (1) To attract mates to breed. (2) Increasing day-length changes hormone levels. Or even, because of the way air flows through the syrinx. (3) Starlings have learnt to sing from their neighbours. (4) Song has evolved from simpler songs in ancestral bird species.

1 & 4 are "Ultimate" causes, 2 & 3 are proximate.

Species Definition

Buffon (1707-1788), "Les espèce sont les seuls êtres de la nature."

Morphological

Classical taxonomy

Davis and Heywood (1963), "assemblages of individuals with morphological features in common and separable from other such assemblages by correlated morphological discontinuities in a number of features"

lumpers and splitters

Numerical (Phenetic) Taxonomy

large number of characters all given equal numerical weight, then use various statistical techniques to obtain measures of similarity

weighting (e.g. chromosomal changes versus petal colour)

John Locke (1689), "the boundaries of the species, whereby men sort them, are made by men."

Biological

1. Isolation species concept

a. k. a. THE biological species concept

Buffon (1707-1788), "We should regard two animals as belonging to the same species if, by means of copulation, they can perpetuate themselves and preserve the likeness of the species; and we should regard them as belonging to different species if they are incapable of producing progeny by the same means."

Reproductive Isolation

1. Do populations in the same locality normally fertilize each other?

2. Should cross-fertilization occur, are the hybrids viable and fertile?

2. Recognition species concept

Paterson (1985) - changes the focus from isolating mechanisms to promoting mechanism (behaviour, anatomy and physiology that aid mating). "Species are defined as the most inclusive population of individual biparental organisms which share a common fertilisation system."

3. Evolutionary species concept

Simpson (1961), "an evolutionary species is a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies."

Templeton (1989), "a species consists of a population or group of populations that share a common evolutionary fate through time."

Species held together not merely by gene flow, but also developmental, genetic and ecological constraints.

Problems

Only 1 and 2 actually provide an objective mechanism for deciding whether an organism is in a particular species or not. And both of them have problems with sex.

Too little sex

Asexual organisms, parthenogenetic organisms, self and sib-mating species.

Fossils

Too much sex

Syngameons - species groups with naturally occurring hybridizations. Quite common among plants, but also mammals. e.g. Coyotes and Wolves

And practically, there is an awful lot of testing to be done - experimental breeding etc.

Solutions

No, not really. There are some combination evolutionary and recognition systems (e.g. Templeton's Cohesion species concept), but it all boils down to one expert's opinion against another (and that is the definition of a genus!).

As an expert, my opinion (for what it is worth) is that there is no such biological entity as a species and that they are just convenient nameplates that we set up for good practical reasons in a vast, multi-dimensional sea of variation. One might describe this as mega-lumping!

Ramifications of natural selection

Darwin's Theory

Summary from Kreb's and Davis 1993

Origin of Species

1. Individuals within a species differ in their morphology, physiology and behaviour (variation).

2. Some of this variation is heritable; on average, offspring tend to resemble their parents more than other individuals in the population.

3. Organisms have a huge capacity for increase in numbers; they produce far more offspring than give rise to breeding individuals. This capacity is not realised because the number of individuals within a population tends to remain more or less constant over time. Therefore, there must be competition between individuals for scarce resources such as food, mates and places to live.

4. As a result of this competition, some variants will leave more offspring than others. These will inherit the characteristics of their parents and so evolutionary change will take place by natural selection.

5. As a consequence of natural selection organisms will come to be adapted to their environment. The individuals that are selected will be those best able to find food and mates, avoid predators and so on.

Selfish Gene

1. All organisms have genes which code for protein synthesis. These proteins regulate the development of the nervous system, muscles and structure of the individual and so determine its behaviour.

2. Within a population many genes are present in two or more alternate forms, or alleles, which code for slightly different forms of the same protein. These will cause differences in development and so there will be variation within a population.

3. There will be competition between alleles of a gene for a particular site (locus) on the chromosome.

4. Any allele that can make more surviving copies of itself than its alternative will eventually replace the alternative form in the population. Natural selection is the differential survival of alternative alleles.

The Competitive Exclusion Principle

From Begon, Harper and Townsend 1990, "if two competing species exist in a stable environment, then they do so as a result of niche differentiation, i.e. differentiation of their realised niches. If, however, there is no such differentiation, or it is precluded by the habitat, then one species will eliminate or exclude the other. Thus exclusion occurs when the realised niche of the superior competitor completely fills those parts of the inferior competitor's fundamental niche which the habitat provides."

Classical example (Park 1962) using two species of flour beetle (Tribolium confusum and Tribolium castaneum). These animals were kept together in simple containers of flour, at a variety of different temperatures and humidities. In all cases, only one species survived, though which one depended on the exact conditions. However, if the available habitat was made more heterogeneous by the provision of small tube segments to act as refuges, it was possible to get both beetle species of beetle living in the same container.

However, there are problems in using this theory. It is essentially unprovable. It often works, makes good sense and fits in nicely with theory, but there are always situations where species appear to co-exist with no apparent niche differentiation - in these cases the principle would indicate that the differentiation has simply not yet been identified. Establishing positively that there is niche differentiation is often difficult and it is impossible to prove its absence.

Sexual Conflict

Anisogamy: females produce large, immobile, food-rich gametes (eggs); males produce small, mobile gametes (sperm). This is almost universal in multi-cellular organisms (metazoa). Because females put more resources into offspring than males, this leads to males (in general) competing to exploit female investment.

Sex ratios

Males can potentially fertilize many females. Among mammals, polygamy is probably the commonest mating system. So why, in almost all populations is the sex ratio 1:1, even when males do nothing but fertilize females?

Fisher (1930) explains this observation. Consider a population with 20 females to every male. A male thus has 20 times the reproductive success of a female. If a gene promoting sons enters the population, it will rapidly spread, and the sex ratio will fall towards 1 to 1. Conversely, in a population where males are 20 times as common as females, a gene increasing the number of females will spread rapidly. The rarer sex is always at an advantage, and so the only stable sex ratio is 1 to 1.

This should actually be refined to talk about investment, rather than numbers. If males are more costly to produce (because they are larger for example, and require more food), then the sex ratio will be skewed in favour of the cheaper to produce females so that the return per unit investment is the same.

Now, the old adage about "the exception proves the rule" can usefully be applied here (note: this uses the archaic meaning of prove, which is "to test"). So, to test this explanation about 1:1 sex ratios, we look at examples where it isn't true.

One good example is the mite Acarophenox (Hamilton 1967). In this case, there is no competition between males for females - the eggs hatch inside the mother and one son mates with his 20 or so sisters and dies before anyone is born. Clearly, in this case, there is no advantage to the mother to produce more sons since it wouldn't change her genetic contribution to the next generation. This is an example of local mate competition.

Although on average, the population should have a sex ratio of 1 to 1, this is not the case for individuals. In many animals, males compete strongly for females, and a few successful males are very successful and many males have no offspring at all. Whereas among females, reproductive success is much less varied. It follows from this, that if a mother is in particularly good condition and hence likely to produce particularly good offspring, then she is likely to have more grandchildren if she produces male offspring. On the other hand, if she is in poor condition, then the safer option is to produce females children. This has been born out by work on red deer (Clutton-Brock et al. 1984).

Sexual Selection

This combination of (in general) greater female investment in offspring and a 1 to 1 sex ration means that males generally compete for females, and this brings us back to some anatomy. Male competition for females is often extreme, and there are two classic ways that this shows up.

In animals where there seems to be little female choice in meeting, males tend to compete directly with each other for females. This often involves direct conflict, or perhaps more ritualised mock fighting (see Maynard Smith 1982 for a discussion of the theories behind this). This leads for strong selection for combative anatomical features - horns in ungulates, canine teeth in primates. It also shows up in rather more subtle ways. Many invertebrates have rather bizarre mechanisms for sperm competition - scoops on their penises to remove a rival's sperm, or cement like secretions to seal up a female's genital openings. There are various mammals with barbed penises that lock into the female's vagina so that the mating pair become locked together for an extended period to ensure fertilisation by that male's sperm.

In other animals, there is a great deal of female choice. Males cannot force females to mate with them, so they have to persuade the females instead. This is usually done by indicating to the female that the particular male is of high quality and will therefore be likely to father high quality offspring. Or, in animals where there is a significant amount of male parental care, it may be by showing the female that the male is a good provider.

This female choice is thought to have led to some of the more elaborate examples of male plumage and display. Fisher (1930) again noticed this. It is postulated that initially, having, for example a long tail, was selected for by females because it indicated that a particular male was of high quality - better at flying for example. Therefore, because males with longer tails find more mates and are better at flying, genes for longer tails spread through the population. However, there will come a point when having too long a tail is deleterious - but this needs to be balanced against the strong selection pressure applied by females choosing longer tailed males. Depending on the degrees of these selection pressures, you can end up with birds with ridiculously long tails (peacocks for example) that are enormously inconvenient, simply due to female choice (See Bateson 1983 for discussion of these issues).

Summary

A little bit of philosophy

Problems of species definition and solutions

Evolution by natural selection and the survival of the fittest

Important implications

Where next?

Phylogenetic Reconstruction

Phenetics; cladistics; practical reconstruction; homology; parallel evolution; morphology versus biochemistry.

Bibliography

Library

 AUTHOR(S)       :Krebs, J. R. Davies,  N. B.  1952-
 TITLE           :An introduction to behavioural ecology / J.R. Krebs, N.B.
                  Davies / drawings by Jan Parr
 IMPRINT         :Oxford : Blackwell Scientific Publications, 1993

 TITLE           :Speciation and its consequences / edited by Daniel Otte and
                  John A. Endler
 IMPRINT         :Sunderland, Mass. : Sinaver Associates, 1989

 AUTHOR(S)       :Gould, Stephen Jay
 TITLE           :The panda's thumb : more reflections in natural history
 IMPRINT         :Harmondsworth : Penguin Books, 1983

 AUTHOR(S)       :Sneath,  Peter H. A.
                  Sokal,  Robert R.
 TITLE           :Numerical taxonomy : the principles and practice of numerical
                  classification / [by] Peter H. A. Sneath [and] Robert R. Sokal
 IMPRINT         :San Francisco : W. H. Freeman, [1973]
 SERIES          :A Series of books in biology

 AUTHOR(S)       :Dawkins, Richard 1941-
 TITLE           :The selfish gene
 IMPRINT         :Oxford 1976

 AUTHOR(S)       :Fisher, Sir Ronald A. 1890-1962
 TITLE           :The genetical theory of natural selection
 IMPRINT         :Oxford : The Clarendon Press, 1930

 AUTHOR(S)       :Begon, Michael Harper,  John L.
                  Townsend,  Colin R.
 TITLE           :Ecology : individuals, populations, and communities / Michael
                  Begon, John L. Harper, Colin R. Townsend
 EDITION         :2nd ed.
 IMPRINT         :Boston Oxford : Blackwell Scientific Publications, 1990

 AUTHOR(S)       :Darwin, Charles 1809-1882
 TITLE           :The origin of species by means of natural selection or the
                  preservation of favoured races in the struggle for life
 EDITION         :6th ed.
 IMPRINT         :London : John Murray, 1872

 AUTHOR(S)       :Clutton-Brock, T. H. Guinness,  F. E.
                  Albon,  S. D.
 TITLE           :Red deer : behavior and ecology of two sexes / T.H.
                  Clutton-Brock, F.E. Guinness, S.D. Albon / with original
                  drawings by Priscilla Barrett
 IMPRINT         :Edinburgh : Edinburgh University Press, c1982
 SERIES          :Wildlife behavior and ecology

 AUTHOR(S)       :Smith, John Maynard 1920-
 TITLE           :Evolution and the theory of games
 IMPRINT         :Cambridge : Cambridge University Press, 1982

 TITLE           :Mate choice / edited by Patrick Bateson
 IMPRINT         :Cambridge : Cambridge University Press, 1983

Others

Strickberger M. W., 1990. Evolution. Jones and Bartlett Publishers, Boston.

Buffon, 1753. Natural History.

Locke J., 1689. An essay concerning human understanding.

Simpson G. G., 1961. Principles of animal taxonomy. Columbia University Press, New York.

Davis P. H., Heywood V. H., 1963. Principles of angiosperm taxonomy. Van Nostrand, Princeton, N. J..

Paterson H. E. H., 1985. The recognition concept of species. In Species and Speciation. Vrba E. S. (ed.). Transvaal Museum Monograph No. 4, Pretoria, 21-29.

**Templeton A. R., 1989. The meaning of species and speciation: a genetic perspective. In peciation and its consequences. Otte D., Endler J. A. (eds.). Sinaver Associates, Sunderland, Mass.

*Park T., 1962. Beetles, competition and populations. Science, 138, 1369-1375.

*Gould S. J., Lewontin R. C., 1979. Spandrels of San Marco and the Panglossian paradigm - a critique of the adaptionist program. Proceedings of the Royal Society, Series B, 205, 581-598.

* Hamilton W. D., 1967. Extraordinary sex ratios. Science 156, 477-488.

[**] Book available in the library

[*] Journal available in the library